METHOD FOR DETERMINING AN ACTUAL VALUE AND / OR AN ACTUAL VALUE RANGE OF AT LEAST ONE CONDITION VARIETY OF A FLUID IN A FLUID FLOW BY MEANS OF AT LEAST ONE INDICATOR PARTICLE, METHOD FOR OPERATING A FLUID-FLYING DEVICE AND DEVICE FOR DETERMINING THE ACTUAL VALUE OF AT LEAST ONE CONDITION VARIETY.

Description

[0001] The invention relates to a method for determining an actual value and / or an actual value range of at least one state variable of a fluid in a fluid flow by means of at least one indicator particle introduced into the fluid, wherein the at least one indicator particle is provided and designed for an irreversible change in an indicator property of the indicator particle when a certain indicator value of the at least one state variable is present in the fluid flow, and / or as a unique function of the actual value after a certain period of time has elapsed following the introduction of the indicator particle into the fluid, wherein the indicator particle is detected at a detection point, the indicator property of the indicator particle is evaluated and the actual value and / or the actual value range of the state variable upstream of the detection point is deduced from the indicator property.The invention further relates to a method for operating a fluid-carrying device and a device for determining the actual value and / or the actual value range of the at least one state variable.

[0002] For example, the prior art is known from publication GB 2510862 A. This describes a velocity measurement based on imaging techniques. The so-called Defocused Digital Particle Image Velocimetry (DDPIV) method is used. This involves introducing particles into a fluid, where the particles are detected at a measurement point and used to determine the velocity field of the fluid. To additionally determine the temperature of the fluid at the measurement point, thermoliquid crystals can be added to the fluid.

[0003] Furthermore, patent US 2002 / 0044590 A1 discloses a method for generating a temperature measurement for a batch or continuous stream of material, the method comprising: (a) providing a particle with a signal that changes at a predetermined temperature; (b) introducing the particle provided in step (a) into the batch or continuous stream; and (c) detecting a signal change from the particle in order to generate a temperature measurement for the batch or continuous stream.

[0004] Document US 2013 / 0122301 A1 also shows a pressure-sensitive indicator particle with which a state variable of a fluid flow can be determined.

[0005] The object of the invention is to propose a method for determining the actual value and / or the actual value range of at least one state variable of the fluid in the fluid flow, which has advantages over known methods, in particular also enabling insights into the state variable for such locations or areas of a fluid-carrying device which are not accessible for direct measurement at these locations or in these areas.

[0006] According to the invention, this is achieved by a method for determining the target value of at least one state variable of the fluid with the features of claim 1. It is provided that the indicator particle is designed and configured for an irreversible change in an indicator property of the indicator particle when a specific indicator value of the at least one state variable is present in the fluid flow, wherein the state variable is a normal stress and / or a shear stress of the fluid, and the indicator particle has a base body with a particle shell enclosing a cavity, wherein a reference pressure is present in the cavity, and the particle shell is designed and configured to irreversibly change the indicator property in the form of its shape and / or to tear when the normal stress deviates from the reference pressure by a specific pressure difference, and / or wherein the particle shell is designed and configured toupon deviation of the shear stress from a reference stress, the indicator property is irreversibly altered in the form of its shape and / or it tears, and / or the indicator particle is designed and configured for an irreversible change in an indicator property of the indicator particle as a unique function of the actual value after a certain period of time following its introduction into the fluid, wherein a sensor material providing the indicator property is present on the base body and is sensitive to state variables, wherein the base body is covered by a protective covering so that the base body or the sensor material is only exposed to the fluid after the certain period of time following its introduction into the fluid has elapsed.

[0007] The method is used, for example, to operate a device for determining the actual value or the actual value range and / or to operate a fluid-carrying device, in particular for controlling and / or regulating the device. Within the framework of the method, the actual value and / or the actual value range of the state variable is to be determined, namely at a point located upstream of the actual measurement point. The point or area at which or in which the actual value and / or the actual value range is determined, or is to be determined, can also be referred to as the test point or test area.

[0008] The actual value refers to the real value of the state variable at the point of investigation. The actual value range, on the other hand, describes the area within which the actual value lies. The actual value range can be a closed or semi-closed mathematical interval. Thus, the actual value range is either closed on both sides or only on one side, or bounded by a specific limit value. In the case of a closed interval, the actual value range is bounded on one side, for example, below, by a first limit value, and on the other side, for example, above, by a second limit value, where the first and second limit values ​​are different from each other. In the case of a semi-open interval, the actual value range is bounded only by the first limit value or the second limit value and is open in the direction of the other limit value.Using the actual value range allows the actual value to be narrowed down to a specific range. It should be noted at this point that whenever the actual value is mentioned in this description, the actual value range is always implied, either additionally or alternatively, even if it is not explicitly stated.

[0009] The determination of the actual value and / or the actual value range is generally carried out using at least one indicator particle. This particle is introduced into the fluid at an introduction point located upstream of the point of investigation in the fluid flow. The indicator particle travels along a path through the fluid flow from the introduction point, past the point of investigation, to the detection point where the indicator particle is evaluated. Since the path of the indicator particle in the fluid flow and its time-dependent position along this path are initially unknown after its introduction into the fluid flow, it may be necessary to determine the path using suitable methods in order to pinpoint the location of the point of investigation. Subsequently, the actual value and / or the actual value range can be assigned to the point of investigation.

[0010] Whenever this description refers to the indicator particle, the corresponding statements always apply to the at least one indicator particle and vice versa. This also applies to the state variable, with corresponding statements applying to the at least one state variable and vice versa. The respective statements are identical in this respect. Likewise, several indicator particles can be used in the process. In this case, the statements for the indicator particle and the at least one indicator particle are preferably applicable to several of the indicator particles or to each of the several indicator particles.

[0011] The relevant explanations can always be used as a supplement and analogous reference.

[0012] The method can be used to determine the actual value and / or the actual value range of exactly one state variable or to determine actual values ​​and / or actual value ranges of several state variables. In the latter case, the specifications for the state variable or the at least one state variable are preferably transferable to each of the several state variables. In a first embodiment, the indicator particle is designed such that it changes its indicator property depending on certain properties of the fluid, i.e., as soon as the state variable of the fluid exhibits the specified indicator value. When introduced into the fluid at the point of introduction, the indicator particle possesses a specific property that changes as soon as the indicator particle reaches a location in the fluid where the state variable exhibits the specified indicator value.The changing property is, for example, one of the following: size, shape, weight, color, reflectance, refractive index, material composition, material structure, or the like. Thus, if the indicator value is present in the fluid along the path of the indicator particle, the size, shape, weight, color, reflectance, refractive index, and / or material structure of the indicator particle will change permanently and irreversibly. Naturally, several of the aforementioned properties can change as soon as the state variable at the location where the indicator particle is currently situated exhibits the indicator value. Material composition refers to the composition of the material from which the indicator particle is at least partially composed, in particular its chemical composition.The material structure describes, for example, a crystalline structure or crystal structure of the indicator particle or the material.

[0013] In a second embodiment, the indicator property changes depending on, or in other words, as a unique function of, the actual value and / or the actual value range, namely at the end of a specific time interval after the indicator particle has been introduced into the fluid. This means that the indicator property initially remains constant after the indicator particle has been introduced into the fluid, regardless of the fluid's state variable or its actual value and / or actual value range, i.e., over the entire specified time interval. Only at the end of this time interval does the indicator particle react to the state variable, and then only once. The indicator property is thus changed depending on, or as a unique function of, the state variable or its actual value and / or actual value range, and subsequently—preferably—remains constant.The time interval is implemented, for example, by means of a protective shell for the indicator particle. The indicator particle is preferably designed such that the change in the indicator property after the time interval has elapsed is dependent on, or a unique function of, the actual value of the state variable present at that time. Of course, it is also possible to combine the first and second embodiments in such a way that the indicator particle is designed such that the change in the indicator property according to the first embodiment occurs when the specific indicator value of the at least one state variable is present in the fluid flow, with the change only being released after the specified time interval has elapsed.

[0014] Particularly preferably, a third embodiment of the indicator particle, which is otherwise preferably based on and further developed from the first embodiment, may be provided for an irreversible change in the properties of the respective surface element or volume element, with a state-variable-value-sensitive indicator property, which occurs progressively over time across the particle surface or volume. It may be provided that the indicator property changes when a specific indicator value is present, or that the indicator property changes as a function of, or as a unique function of, the actual value of the state variable in the fluid.For example, it is provided that several sensor elements or sensor areas are formed on the base body, wherein a first part of the sensor elements or sensor areas has a first indicator value or a first indicator property, and a second part of the sensor elements or sensor areas has a second indicator value or a second indicator property that differs from the first indicator value. Wherever the term "indicator property" is used in this description, it can be understood, for example, as an indicator value. The first indicator property then corresponds to the first indicator value, and the second indicator property to the second indicator value.For example, the first part of the sensor elements or sensor areas consists of one sensor material, and the second part of the sensor elements or sensor areas consists of a different sensor material. The sensor elements or sensor areas can be designed in such a way that they undergo a chemical surface reaction as soon as they come into contact with the fluid; in particular, a surface reaction dependent on the actual value takes place.

[0015] Naturally, any number of parts or areas can be implemented, differing in their indicator properties. For example, the parts or areas can be designed to exhibit the same indicator property, but the property change occurs after different exposure times. This means that along the path of the fluid flow, the property changes of the parts or areas occur at different times, depending on the indicator value or as a unique function of the actual value of the state variable in the fluid at the respective time. By detecting the indicator particle, it is possible to determine at what time or where on the path of the indicator particle the determined actual value of the state variable was present, or on which portion of the path or during which period the indicator particle was exposed to the actual value range defined by the indicator value.

[0016] The frequency with which changes in the indicator properties of the different sensor elements or sensor areas are perceived over time can be referred to as the progression rate. In any case, it can be provided – but this is purely optional – that the sensor elements or sensor areas are covered by protective shells of varying thicknesses, so that the progression rate is ultimately determined by the protective shell and its thickness.

[0017] The change in property is irreversible, meaning that a single occurrence of the specific indicator value of the state variable and / or the elapse of a time period is sufficient to permanently effect the property change. This occurs from an initial indicator property, corresponding to the indicator particle's property upon introduction into the fluid, to a modified indicator property, which the indicator particle assumes after the presence of the indicator value and / or after the elapsed time period. This means that even if the state variable subsequently deviates from the indicator value, the indicator particle's property remains modified and does not revert to its original state.Thus, in the case of an indicator particle of the first embodiment, the presence of a specific indicator value in the fluid flow along the path between the injection point and the detection point can be determined simply by detecting the indicator particle and evaluating its indicator properties. For example, this makes it possible to easily investigate the presence of adverse flow effects in the fluid flow significantly downstream of where they actually occur. For instance, with a suitable design of the indicator particle, the method can be used to determine whether cavitation occurs in the fluid-carrying device, which is designed, for example, as a pump, particularly a jet pump, or in the fluid flow present in the fluid-carrying device, specifically by means of the first embodiment of the indicator particle.Cavitation is characterized by the collapse of cavitation bubbles and the resulting high pressure. Accordingly, pressure is used as the state variable to check for cavitation, and the indicator value is chosen such that the indicator particle reacts to cavitation, and in particular to high pressure, with a reversible change in its properties. If the indicator particle exhibits its original indicator properties at the detection point, i.e., downstream of the test point, it can be assumed that no cavitation is present or that the pressure along the path has not exceeded the indicator value. If, however, the altered indicator properties are present, this indicates the presence of cavitation or a pressure higher than the indicator value.

[0018] The presence of a specific indicator value means, in particular, that the actual value of the state variable corresponds to the indicator value or at least lies within a range—also referred to as the actual value range—which is bounded by the indicator value, for example, below or above. The indicator value thus unilaterally delimits a value interval, which is otherwise open. As soon as the actual value lies within this value interval, the condition for the irreversible change of the indicator property is fulfilled. The indicator property therefore changes irreversibly, preferably, when the actual value of at least one state variable equals or exceeds the indicator value, or alternatively, when it equals or exceeds the indicator value—that is, when it lies within the actual value range defined by the indicator value.However, it can also be understood that the indicator value refers to the value interval itself, meaning that the indicator value is defined as an indicator value range. This indicator value range is bounded both above and below. The specific indicator value is present as soon as the actual value of the state variable lies within this indicator value range, resulting in an irreversible change in the property, which persists even if the actual value falls outside the indicator value range. Whenever this description refers to the actual value, it implicitly includes, either additionally or alternatively, the actual value range, so the corresponding statements also apply to it, even if it is not explicitly mentioned.

[0019] To determine the actual value and / or the actual value range, the indicator particle is detected at the detection point. This can be done in any way, although non-intrusive methods, such as optical or imaging techniques, are preferred. The indicator particle's properties are evaluated during or after detection. This evaluation can be performed directly or indirectly. In the former case, the indicator property is preferably measured during the detection of the indicator particle. However, it is also possible to determine the indicator property indirectly, namely by measuring a property that is different from but dependent on the indicator property, from which the indicator property is then subsequently inferred.

[0020] After evaluating the indicator property, this is used to determine the actual value or actual value range of the state variable present at the point of investigation, and thus upstream of the detection point. Because the actual value or actual value range at the point of investigation is initially unknown, and the indicator particle can, for example, only represent two different states that describe the state variable along the entire flow path or trajectory, it may be necessary to introduce several different indicator particles with different indicator values ​​into the fluid, detect them at the detection point, and evaluate the respective indicator property in order to actually arrive at the actual value or actual value range of the state variable at the point of investigation.The described acquisition of actual values ​​or actual value ranges of the state variable of a fluid flow on the path or trajectory of an indicator particle corresponds to a Lagrange description of the fluid flow.

[0021] To determine the actual value or actual value range of the fluid's state variable at a specific point or in a specific region of the fluid flow (so-called Eulerian description of the fluid flow) using the described basic procedure with one or more indicator particles (Lagrangian description), it may be necessary to create indicator particles of the aforementioned embodiments, including different embodiments, as well as a representative computer model of the fluid flow and the trajectories of the indicator particles embedded within it (so-called digital twin). This will be discussed in more detail below.

[0022] The detection of the indicator particle can, in principle, be carried out in any way, as long as the indicator property is directly or at least indirectly captured. For example, the indicator particle can be detected using the PIV method (Particle Image Velocimetry) or the PTV method (Particle Tracking Velocimetry). In these cases, in addition to the indicator property and thus the actual value or actual value range of the state variable at the detection point, the velocity or velocity field of the fluid or fluid flow at the detection point can be determined. Furthermore, the position and velocity vector of the indicator particle can be determined. The PIV method is based on an Eulerian approach to fluid flow, whereas the PTV method uses a Lagrangian approach.Of course, it is also possible to detect the indicator particles without (also) detecting the velocity field and / or the position and velocity of the indicator particle.

[0023] The described procedure has the advantage that determining the actual value or actual value range is also possible at inaccessible points in the fluid-carrying system. This is achieved by inferring the actual value or actual value range of the fluid's state variable at the upstream testing point from the state of the indicator particle at the detection point (i.e., its indicator property). Given this significant advantage, it is acceptable that the actual value or actual value range determined using the described method at the testing point may be subject to a larger error than with a direct measurement at the testing point.It can also be accepted that, due to the only indirect determination of the actual value or actual value range, several different indicator particles, i.e., indicator particles with different indicator values ​​and / or time intervals until a change in the indicator property as a unique function of the actual value of the state variable and / or indicator particles of different designs, may have to be used until the actual value, i.e., the value of the state variable along the track, or the actual value range, can be determined with an acceptable error.It can also be accepted that, for the determination of the actual value or the actual value range of the state variable at a specific, spatially defined point of investigation in the fluid flow (Euler's description), a representative computer model of the fluid flow and the trajectories of the indicator particles placed therein (so-called digital twin) is required in order to deduce the actual value or the actual value range of the previously spatially defined point of investigation from the recorded indicator properties of the indicator particles (Lagrangian description).

[0024] According to a first embodiment of the invention, the state variable is a normal stress and / or a shear stress of the fluid, and the indicator particle has a base body with a particle shell enclosing a cavity, wherein a reference pressure is present in the cavity, and the particle shell is designed and configured to irreversibly change its indicator property, in the form of its shape, and / or to tear when the normal stress deviates from the reference pressure by a certain pressure difference, and / or wherein the particle shell is designed and configured to irreversibly change its indicator property, in the form of its shape, and / or to tear when the shear stress deviates from a reference stress. The normal stress here describes the fluid pressure.

[0025] The indicator particle should therefore be designed in such a way that it enables the determination of the fluid pressure, specifically at the point of investigation and / or along the path of the indicator particle from the point of introduction to the point of measurement. For this purpose, the indicator particle has a base body, which in turn has a particle shell that initially encloses the cavity in a fluid-tight manner. The particle shell completely surrounds the cavity. For example, the cavity has a volume of at least 50%, at least 75%, or at least 90% of the total volume of the base body. Before the indicator particle is introduced into the fluid, at least in the first variant, the reference pressure, which corresponds in particular to the indicator value, is present in the cavity.

[0026] When the indicator particle is immersed in the fluid, a force acting on the particle shell results from the pressure difference between the instantaneous pressure or normal stress at the location of the indicator particle and the reference pressure. If this force exceeds a certain threshold, i.e., if the fluid pressure deviates from the reference pressure by a specific pressure difference, the particle shell is expected to rupture and / or change shape, both processes being irreversible. Rupture of the particle shell primarily involves the bursting of the base body or the indicator particle itself. The change in the shape of the particle shell occurs, for example, as an irreversible reduction in the volume of the base body or the cavity.For example, the indicator particle or particle shell is designed in such a way that, upon deformation or rupture, pressure equalization occurs between the cavity and the fluid, so that subsequently the same, or at least nearly the same, fluid pressure is present in the cavity as in the fluid. The deformation or rupture of the particle shell can be detected and evaluated particularly easily.

[0027] In a second variant, the shear stress in the fluid is evaluated. If the shear stress deviates from the reference stress, particularly by a certain difference, irreversible deformation or rupture of the shell occurs. The material of the particle shell is selected based on the size of the indicator particle, the thickness of the particle shell, the reference pressure, and the pressure difference or shear stress difference between the shear stress and the reference stress. The size of the indicator particle can be in the micrometer and / or nanometer range. A polymer, a metal, a metal alloy, carbon, or another suitable element can be used as the base material. A combination of these materials, i.e., the use of at least two of them for the indicator particle or the base material, is also possible.

[0028] According to a second embodiment of the invention, a sensor material providing indicator properties is present on the base body. This sensor material is sensitive to state variables, such as fluid pressure, fluid temperature, or fluid concentration. The sensor material is applied to the base body. The base body can be configured as described above and may include the cavity and the particle shell enclosing it. However, the base body can be configured in any way when using the sensor material, for example, it can also be solid.

[0029] The sensor material is selected to be sensitive to the state variable, meaning its indicator properties change when a specific indicator value of the state variable is present. In other words, the sensor material is state-variable sensitive or state-variable value sensitive. Furthermore, the sensor material is designed or selected such that this change in properties is irreversible. The state variable evaluated by the sensor material can be, for example, fluid pressure, fluid temperature, fluid concentration, a substance concentration, normal stress, or shear stress. Other state variables can, of course, also be evaluated with a suitable sensor material. The use of this sensor material allows for a particularly simple design of the indicator particle and also facilitates the evaluation of the indicator properties.

[0030] As a general rule, a preferred sensor material is one that permanently changes one or more of its measurable properties depending on an external state variable. This at least one property can be optical reflectivity or refractive index, electrical resistance, electrical capacitance, photoluminescence, or chemical composition. Suitable materials include, in particular, multilayer porous photonic silicon crystals, thermochromic pigments, and self-organizing isotropic or anisotropic nanostructures, especially when stimulated by various external state variables.

[0031] In the second embodiment of the invention, it is further provided that the base body, in particular the sensor material, is covered by a protective shell that is biodegradable by the fluid, so that the base body or the sensor material is only exposed to the fluid for a certain period of time after being introduced into the fluid. The protective shell enables a particularly precise spatial evaluation of the state variable or an evaluation or delimitation of the actual value or the actual value range. It covers the base body at least partially or even completely. Preferably, it extends at least partially or completely over the sensor material, if the sensor material is present. The protective shell is designed such that it is degraded by the fluid, dissolves in the fluid, or degrades itself as soon as it is placed in or exposed to the fluid.

[0032] The degradation preferably occurs at a known and essentially constant rate, so that the time interval after which the fluid acts on the base body or the sensor material can be directly inferred from the thickness of the protective shell. Particularly preferably, in such a design, the sensor material is selected such that it only allows the property change immediately upon contact with the fluid. This property change is, in turn, sensitive to the state variable or value of the state variable. Thus, once the protective shell has degraded, exposing the sensor material to the fluid, the property change occurs according to the actual value or range of values ​​present at the time of contact with the fluid. This enables a particularly precise determination of the actual value of the state variable or a particularly precise localization of the point of investigation.

[0033] A further development of the invention provides that the at least one indicator particle is part of a plurality of indicator particles introduced into the fluid, wherein the indicator value of the state variable corresponds to a first indicator value for a first subset of the indicator particles and to a second indicator value for a second subset of the indicator particles, and / or the time interval, in particular until a change in the indicator property as a unique function of the actual value of the state variable, corresponds to a first time interval for the first subset of the indicator particles and to a second time interval for the second subset of the indicator particles. In principle, any number of indicator particles can be introduced into the fluid, for example, at least partially at the point of introduction. The at least one indicator particle mentioned above constitutes one of the multiple indicator particles and is therefore not present in addition to them.The indicator particles are now divided into several parts, for example, two parts. It can be intended that each part contains the same number of indicator particles. Of course, one part can also contain more or fewer indicator particles than the other.

[0034] The indicator particles of the different parts differ with respect to the indicator value and / or the time interval, particularly until a change in the indicator property as a unique function of the actual value of the state variable. For the first part of the indicator particles, the indicator value corresponds to the first indicator value or the first time interval, and for the second part, it corresponds to the second indicator value or the second time interval. In other words, the first part has one or more indicator particles with the first indicator value or the first time interval, and the second part has one or more indicator particles with the second indicator value or the second time interval. Naturally, however, any number of parts can be present, for example, an additional third part, a fourth part, and / or a fifth part.The use of indicator particles with different indicator values ​​or different time periods enables a particularly accurate determination of the actual value or actual value range of the state variable.

[0035] A further development of the invention provides that the first part of the indicator particles and the second part of the indicator particles are introduced into the fluid simultaneously or at different times and / or at the same or spaced-apart injection points, wherein the first part is provided with a fixed first marking independent of the respective indicator property, and the second part with a fixed second marking independent of the respective indicator property. The marking of the indicator particles, i.e., both the first and the second marking, serves for the simple identification of the respective indicator particle, despite any possible changes in its properties. For this purpose, the markings are fixed and, above all, independent of the respective indicator property.Therefore, changing the respective indicator property does not change the labeling; rather, it remains permanently the same.

[0036] The first part of the indicator particles has the first marking, and the second part has the second marking. The first and second markings differ from each other in such a way that, when the indicator particles are detected at the detection point, they can be assigned to the first and second parts. This means that for each indicator particle detected at the detection point, preferably for each detected indicator particle, the marking is determined, and the indicator particle is assigned to the part of the indicator particles corresponding to that marking.

[0037] For example, indicator particles are marked accordingly, depending on their original indicator properties and / or the respective point and / or time of introduction. The marking is selected based on at least one or more of the aforementioned parameters or indicator properties, ensuring that the indicator particles can be uniquely identified when collected at the collection point. Indicator particles with different parameters or properties will have different markings, while indicator particles with the same parameters or properties will have the same markings.

[0038] In principle, the indicator particles from the multiple components can be introduced simultaneously or sequentially. In the former case, all indicator particles from the multiple components are introduced into the fluid at the same time; in the latter case, this occurs sequentially, i.e., with a time interval. Additionally or alternatively, it is possible to introduce the indicator particles from the multiple components into the fluid at the same injection point or at injection points spaced apart. This allows for a high temporal and / or spatial resolution of the actual value or actual value range of the state variable in the fluid flow, thus achieving a higher level of detail with regard to the actual value and / or the actual value range or the fluid flow. Particularly preferred is the simultaneous introduction of the indicator particles at injection points spaced apart or at the same injection point with a time interval.However, staggered application at spaced-apart application points and simultaneous application at the same application point can also be useful.

[0039] A further development of the invention provides that the detection of the at least one indicator particle and the evaluation of its indicator properties are carried out contactlessly within the fluid or after the at least one indicator particle has been removed from the fluid. This has already been mentioned. The contactless detection and evaluation are performed, for example, optically, in particular by means of an optical sensor or an image-capturing camera. The optical sensor is based, for example, on light scattering (reflection, refraction, and / or diffraction) and / or light absorption by the indicator particle. Alternatively, the indicator particle can first be removed from the fluid, and its indicator properties evaluated only after removal. This is done, for example, by microscopic image acquisition (SEM / TEM) or contactless spectroscopic evaluation using an aerosol mass spectrometer.Additionally or alternatively, the evaluation is carried out tomographically.

[0040] A combination of the two aforementioned approaches is also fundamentally possible. In this case, for example, the indicator particle is first detected without contact, particularly optically and preferably by capturing the velocity field of the fluid or the velocity of the indicator particle at the detection point. After non-contact detection, the indicator particle is removed from the fluid, its properties are evaluated, and the actual value or actual value range of the state variable along the path or at the point of investigation is deduced from this. This results in a particularly high degree of flexibility for the described method.

[0041] A further development of the invention provides that if the property change does not occur, the indicator value is changed until the property change occurs, or if the property change occurs, the indicator value is changed until the property change does not occur, whereby the actual value or actual value range of the state variable is deduced from the indicator value I 1, at which the property change first failed to occur, and from the indicator value I 2, at which the property change first occurred.In other words, it is intended that if the property change does not occur, the indicator value is changed until the property change occurs, or if the property change occurs, the indicator value is changed until the property change does not occur, whereby the actual value or actual value range of the state variable is deduced from a first indicator value of the indicator value where the property change did not occur, and from a second indicator value of the indicator value where the property change did occur.

[0042] In other words, several indicator particles with different indicator values ​​are used; for example, indicator particles that partly exhibit a first indicator value and partly a second indicator value. The plan is to first introduce the first group of indicator particles into the fluid and evaluate their properties at the sampling point. If the indicator property matches the original indicator property—that is, if no change in property has occurred—then the second group of indicator particles, with a second indicator value that differs from the first, is introduced into the fluid, for example, at the same sampling point. If no change in property occurs again, indicator particles with yet another indicator value, different from both the first and second indicator values, are introduced into the fluid.This process continues, and the indicator value is changed accordingly, until the property change occurs for the first time (at I 2) and is detected during the evaluation.

[0043] If a change in properties is detected for the first group of indicator particles, the indicator value is adjusted and the second group of indicator particles is introduced. The indicator value is adjusted until the change in properties ceases for the first time (at I 1). If a change in properties also occurs for the second group of indicator particles, the third group of indicator particles, exhibiting an indicator value further than the first and second group, is introduced into the fluid. This process is only interrupted once the change in properties of the indicator particles ceases.

[0044] From the different indicator values ​​of the indicator particles introduced into the fluid, the first indicator value I1 and the second indicator value I2 are determined. The first indicator value is the value of an indicator particle for which the property change was first absent, and the second indicator value is the value of an indicator particle for which the property change was first observed. Preferably, the first indicator value I1 and the second indicator value I2 are chosen such that their values ​​are as far apart as possible. Thus, the first and second indicator values ​​define the actual value or actual value range of the state variable in the fluid flow.From the two indicator values, it is therefore possible to draw particularly precise conclusions about the actual values ​​of the maximum and minimum values ​​of the state variable, or the actual value range of the state variable, with respect to the investigated locations. If the investigated locations are spatially in close proximity, the determined indicator values ​​I1 and I2 approach each other and correspond to the actual value at the investigated location.

[0045] The change in the indicator value of an indicator particle from the first to the second indicator value can occur in different ways. For example, the actual value of the minimum value of the state variable is set to the average of the first and second indicator values ​​if there is no change in the property of the first indicator value and a change in the property of the second indicator value occurs. However, it is also possible to use more complex methods, particularly those not based on averaging, to determine the actual values ​​of the maximum and minimum values ​​of the state variable or the actual value range.For example, the actual values ​​of the maximum and minimum values ​​of the state variable, or the actual value range of the state variable, are determined from the indicator properties of the indicator particle or the indicator values ​​of multiple indicator particles using machine learning and / or a neural network. This applies regardless of the described method for using the different indicator values. This enables a particularly accurate, time-efficient, and cost-effective determination of the actual values ​​of the maximum and minimum values ​​of the state variable, or the actual value range of the state variable, with respect to the investigated points.

[0046] A further development of the invention provides that, using a model of the fluid flow and / or the movement of the indicator particle in the fluid flow, at least one prediction value or a prediction value range for the state variable, in particular along the path of the at least one modeled indicator particle, is calculated and verified using the at least one indicator particle. If the prediction value or the prediction value range deviates from the actual value or actual value range determined using the at least one indicator particle, the model is adjusted to the actual value or actual value range. The model is in particular in the form of a digital twin of the fluid flow or the fluid-carrying device.The model is designed and configured to determine the behavior of the fluid flow and thus the movement of the indicator particle, for example in the form of its trajectory, particularly with at least spatial resolution. Additionally, the model can incorporate a temporal resolution of the fluid flow behavior. The model is based, for example, on finite difference, finite volume, finite element, or a mesh-free method.

[0047] The model is used to calculate at least one predictive value or predictive value range for the state variable and / or the movement of the indicator particle or its trajectory. The predictive value is understood to be a value of the state variable that, according to the model, the state variable exhibits at a specific location, in particular the point of investigation. The predictive value range is understood to be a range of values ​​of the state variable within which, according to the model, the state variable lies within a specific time period or within a specific distance along its trajectory. To check or verify the predictive value or predictive value range, the at least one indicator particle is introduced into the fluid. Upon reaching the detection point, the indicator particle is detected and its indicator properties are evaluated.The indicator property is then used to infer the actual value or actual value range of the state variable. If the actual value corresponds to the predicted value, or if the actual value range corresponds to the predicted value range, the model can be considered confirmed, at least with regard to the actual value at the point of investigation or with regard to the actual value range along the investigated section of the railway line. Naturally, several predicted values ​​or predicted value ranges can be determined and verified using one or more indicator particles in the manner described.

[0048] If, however, the actual value or the actual value range deviates from the predicted value or the predicted value range, or if they are not consistent, the current model is insufficient to describe the fluid flows completely and accurately. Therefore, the model is corrected so that at least one (recalculated) predicted value or predicted value range subsequently corresponds to the actual value or actual value range, at least within a predefined tolerance. It may be necessary to repeat the described procedure multiple times, particularly for a large number of different test points or for a large number of different sections along a railway line, so that a thorough review and adaptation of the model to the actual fluid flow can be carried out.In each case, a new prediction value or a new prediction value range is calculated and verified in the manner described, so that ultimately a plurality of prediction values ​​or prediction value ranges are available.

[0049] The predictive value is preferably calculated for the test site, particularly for the second embodiment of the indicator particle. However, it can also be provided that the model is used to model the path of the indicator particle from the point of introduction to the point of detection, so that the course of the actual value or the actual value range along the path traveled by the indicator particle is subsequently known. This procedure is primarily carried out for the first and third embodiments of the indicator particle. From the course of the actual value or the actual value range, it is subsequently concluded whether a change in the properties of the indicator particle due to the fluid is to be expected or not. This finding is then used to verify and adjust the model.In the third embodiment, the course of the actual value along at least a part of the path or the entire path can be determined from the indicator properties of the individual sub-areas. Overall, the described procedure enables the creation of a model or digital twin of the fluid flow or the current flow conditions in the fluid-carrying device with extremely high accuracy.

[0050] A further development of the invention provides that a base body is used on which several sensor elements and / or sensor areas are formed, wherein a first part of the sensor elements or sensor areas consists of the sensor material and a second part of the sensor elements or sensor areas consists of a sensor material different from the sensor material. The indicator particle thus has different sensor materials, which are assigned to the several sensor elements or sensor areas. For example, the sensor materials are selected for the same state variable, but with different indicator values ​​and / or time intervals. However, it can also be provided that the sensor materials serve to determine different state variables.

[0051] In principle, any number of sensor elements and / or sensor areas and any number of different sensor materials can be used. If only two sensor materials are provided—namely, the sensor material itself and a different sensor material—the sensor elements or sensor areas are divided into the first part and the second part. Of course, more different sensor materials are also possible, in which case the number of parts into which the sensor elements or sensor areas are divided corresponds to the number of different sensor materials. The use of different sensor materials enables higher accuracy in determining the actual value or actual value range and / or a reduction in the time required for determination.

[0052] A further development of the invention provides that an indicator particle is used in which the sensor elements and / or sensor areas are covered by a protective shell of varying thickness. This ultimately means that the sensor elements or sensor areas are exposed to the fluid after different time intervals following the introduction of the indicator particle. If the sensor elements or sensor areas serve to determine actual values ​​or actual value ranges of the same state variable, and if the changes in their indicator properties are sensitive to the state variable value, either via different indicator values ​​or indicator properties that exist as a unique function of the actual values, then the actual value or actual value range of the state variable can be detected at different times or at different time intervals.This allows for a particularly precise determination of the actual value or actual value range along the path traveled by the indicator particle in the fluid. For example, the third embodiment of the indicator particle mentioned at the beginning is designed with a multitude of sensor elements or sensor areas covered by different thicknesses of coating. The coating is preferably dimensioned such that a first sensor element or sensor area is exposed to the fluid immediately upon introduction of the indicator particle, and a last sensor element or sensor area is exposed immediately before the detection point. This allows for a conclusion to be drawn about the actual value or actual value range of the state variable along the entire path.The invention further relates to a method for operating a fluid-carrying device with the features of claim 9, wherein an actual value and / or an actual value range of at least one state variable of the fluid in a fluid flow present in the device is determined by means of at least one indicator particle introduced into the fluid using the method according to the description in this document.

[0053] The advantages of such a procedure or such a design of the fluid-carrying device have already been mentioned. Both the fluid-carrying device and the method for operating it can be further developed as described in this document, and reference is made to these details. The reference regarding the actual value and the actual value range remains valid.

[0054] A further development of the invention provides that a malfunction of the fluid-carrying device is detected when the actual value deviates from a previously determined actual value and / or from a predicted value determined using a fluid flow model. For example, it is intended that the actual value or actual value range is determined using the described method at a first point in time, particularly immediately after commissioning the fluid-carrying device. Subsequently, the actual value or actual value range is repeatedly determined for the same test point or section of track at time intervals.If a change occurs in the actual value or actual value range, i.e., if the most recently determined actual value or actual value range deviates from the previously determined actual value or actual value range, particularly by more than a permissible tolerance, a malfunction of the fluid-carrying device is detected and this is displayed, for example, to a user of the device. Conversely, if the actual value or actual value range corresponds to the previously determined actual value or actual value range, preferably again within the desired tolerance, then proper functioning of the fluid-carrying device is recognized.

[0055] Additionally or alternatively, it may be possible to use the model to determine the predictive value, particularly for the test site or section of the railway line for which the actual value or actual value range is also determined using at least one indicator particle. Preferably, the model is first calibrated and adjusted using the actual value or actual value range such that the predictive value corresponds to the actual value or the predictive value range corresponds to the actual value range. Subsequently, the actual value or actual value range is determined again, for example periodically, preferably using at least one further indicator particle, which is, for example, identical in design to the indicator particle. At the same time, the predictive value or actual value range is determined.A predictive value range is determined, specifically for the same inspection point or section of track for which the actual value or actual value range is also determined. If the actual value or actual value range deviates from the predictive value, a malfunction is detected. If the actual value corresponds to the predictive value or the actual value range corresponds to the predictive value range, proper functioning is detected. In both cases, the aforementioned tolerance range is preferably observed. This allows for a diagnosis of the fluid-carrying equipment during operation with minimal effort and without significant or continuous disruption of the fluid flow.

[0056] Finally, the invention relates to a device for determining an actual value and / or an actual value range of at least one state variable of a fluid in a fluid flow by means of at least one indicator particle introduced into the fluid, using the method as described in this description. The device has the features specified in claim 11.

[0057] The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without thereby limiting the invention. The drawing shows: Figure 1 is a schematic representation of a device for determining an actual value or actual value range of at least one state variable of a fluid, in particular a liquid, in a fluid flow, as well as a fluid-carrying device; Figure 2 is a schematic representation of a first embodiment of the indicator particle; Figure 3 is a schematic representation of a second embodiment of the indicator particle; Figure 4 is a schematic representation of a third embodiment of the indicator particle; and Figure 5 shows an indicator particle and a diagram in which the pressure of the fluid is plotted along the pathlines traveled by several indicator particles in the fluid.

[0058] The Figure 1Figure 1 shows a fluid-carrying device 1, namely a jet pump, which has a suction medium inlet 2, a motive medium inlet 3, and an outlet 4. The motive medium inlet 3 is nozzle-shaped and opens into a mixing chamber 5. A diffuser 6 is fluidically connected to the mixing chamber 5 as an extension of the motive medium inlet 3. The mixing chamber 5 is fluidically connected to the outlet 4 via the diffuser 6. The suction medium inlet 2 is also fluidly connected to the mixing chamber 5.

[0059] A first fluid, the so-called motive fluid, is supplied to the fluid-carrying device 1 at high pressure and – optionally – high temperature via the motive fluid inlet 3. Acceleration of this first fluid through the nozzle 3 results in a lower pressure of the first fluid in the mixing chamber 5 relative to the pressure at the suction medium inlet 2. Consequently, a second fluid, the so-called suction medium, which is present at the suction medium inlet 2 – optionally at a lower temperature – is conveyed from the suction medium inlet 2 into the mixing chamber 5 under further pressure reduction. The first and second fluids mix in the mixing chamber 5 and the subsequent diffuser 6, with temperature equalization and a further pressure increase, before a fluid mixture consisting of the two fluids leaves the device through the outlet 4. The first and second fluids can be of the same material.

[0060] A device 7 is associated with the fluid-carrying device 1, which serves to determine an actual value or an actual value range of at least one state variable of a fluid present in the fluid-carrying device 1. The device 7 can also be referred to as a measuring device. To determine the actual value or the actual value range, it is provided that one (or more) indicator particles 9 are introduced into the fluid at one or more introduction points 8. In the embodiment shown here, the introduction point 8 is located at the suction medium inlet 2. It can also be located between the suction medium inlet 2 and the mixing chamber 5, in any case upstream of the mixing chamber, the diffuser 6, and the outlet 4. The indicator particles 9 are introduced by means of an introduction device 10, which is only indicated here.After the indicator particles 9 are introduced into the fluid, they pass through the fluid-carrying device 1, starting from the introduction point 8, along a respective path 11a, 11b, or 11c. Downstream of the introduction point 8, there is a detection device 12, which serves to detect the indicator particles 9. The detection of the indicator particles 9 is carried out, for example, by means of an optical detection device. This is only indicated here in a very schematic way.

[0061] The indicator particles 9 are designed and configured such that, upon the presence of a specific indicator value of the state variable to be determined, they undergo an irreversible change in an indicator property of the respective indicator particle 9. In the embodiment shown here, the shape of the indicator particles 9 is used as the indicator property. It can be seen that in the area of ​​the detection device 12, one of the indicator particles 9 has a modified shape, whereas the other indicator particles 9 have the same shape as when they were introduced into the fluid at the introduction point 8.From this, it can be concluded that the indicator particle 9, which has changed its shape, has entered a region of the fluid flow along the respective path 11a, 11b, or 11c where the state variable has an actual value that corresponds to, exceeds, or falls below an indicator value of the indicator particle 9. Upon such correspondence, exceedance, or falling below of the indicator value by the actual value, an irreversible change in property occurs, in this case, a change in shape. For this purpose, the indicator particles 9 are provided in a first embodiment in which the state variable is the fluid pressure and the indicator value of the indicator particles is selected such that the resulting change in shape indicates the occurrence of cavitation in the fluid flow. Figure 2Figure 1 shows a schematic representation of a second embodiment of the indicator particle 9, or of one of the indicator particles 9. In the illustrated embodiment, the indicator particle 9 has a base body 13 on which at least one sensor element 14 made of a sensor material (in the embodiment shown here, several sensor elements 14) is formed. Only a few of the sensor elements 14 are characterized by way of example. Preferably, several identical sensor elements are evenly distributed over the surface of the indicator particle. The sensor elements 14 are covered by a protective covering 15, wherein the protective covering 15 has different covering thicknesses for the different sensor elements 14, which are constant over the respective sensor element 14. Preferably, identical sensor elements have the same protective covering, in particular with the same covering thickness.The protective casing 15 is designed such that it is decomposed by the fluid after the indicator particle 9 is introduced into it, so that the sensor elements 14 are only directly exposed to the fluid after a certain period of time. The sensor material from which the sensor elements 14 are made is state-variable sensitive or state-variable value sensitive. This means that the sensor material undergoes an irreversible change in its indicator property as soon as it comes into contact with the surrounding fluid, whereby the indicator property changes according to the actual value or actual value range of the state variable immediately after the respective sensor element 14 is exposed to the fluid. It can be provided that the property of the sensor material of the different sensor elements 14 changes irreversibly when the same or different indicator values ​​are present.It can also be provided that the property of the sensor material of the different sensor elements 14 changes as a unique function of the actual value of the state variable immediately after the sensor elements 14 come into contact with the surrounding fluid. The sensor material can additionally or alternatively be designed for different state variables of the fluid.

[0062] The use of multiple identical sensor elements distributed across the particle surface enables a very simple optical detection of the indicator properties of an indicator particle. A unique correlation between the thickness and surface area of ​​a sensor element allows for a very simple correlation between the detected indicator property and the time (after the indicator particle was introduced into the fluid) at which the actual value or actual value range was present.

[0063] The Figure 3Figure 1 shows a schematic representation of a third embodiment of the indicator particle 9. Reference is made to the preceding explanations, and only the differences are discussed below. These differences lie in the fact that several sensor element assemblies 16 extend from the base body 13, of which only a few are shown as examples. Each of these sensor element assemblies 16 has several sensor elements 14. This is also only indicated by way of example. The sensor element assemblies 16 extend outwards from the base body 13 and are spaced apart from one another. The protective shell 15 is located in the spaces 17 between the sensor element assemblies 16 (again, only shown as examples). After the indicator particle 9 is introduced into the fluid, the protective shell 15 is worn away from the outside inwards.This means that the sensor elements 14 are exposed to the fluid one after the other in terms of timing.

[0064] Each of the sensor elements 14 is configured such that the property of the sensor material changes irreversibly when exposed to the fluid, provided the specified indicator value of the state variable is present. Alternatively, each of the sensor elements 14 is configured such that the property of the sensor material changes irreversibly as a unique function of the actual value of the state variable immediately after the sensor material comes into contact with the surrounding fluid. Particularly preferably, the sensor elements 14 are configured such that the irreversible change in property is only possible immediately at the beginning of the exposure of the respective sensor element 14 to the fluid.Accordingly, when evaluating the indicator property of the indicator particle 9 or the indicator properties of the sensor elements 14, a temporal sequence of the actual value or actual value range of the state variable can be evaluated along one of the pathlines 11a, 11b and 11c.

[0065] The Figure 4Figure 1 shows a schematic representation of a fourth embodiment of the indicator particle 9. Reference is again made to the preceding descriptions. Again, several sensor element assemblies 16 are provided, each comprising several sensor elements 14. The sensor elements 14 are each arranged or formed directly on the base body 13. For example, a central sensor element 14 is provided, which is surrounded by the other sensor elements 14, for example, in a ring-like fashion. Each of the sensor element assemblies 16 is covered by the protective shell 15, which is formed, for example, in a segment-shaped form on the base body 13. The protective shell 15 is arranged such that, before the indicator particle 9 is introduced into the fluid, it covers at least a portion of the sensor elements 14, preferably all of them.Since the protective shell 15 dissolves over time due to the fluid, and the shell thickness varies for the sensor elements 14, the sensor elements 14 are sequentially exposed to the fluid. This configuration of the indicator particle 9 also enables a fine temporal resolution of the current value or current value range of the state variable, particularly along the respective path 11a, 11b, or 11c of the indicator particles 9 (Langrange description) or of the local current values ​​of the state variable in the fluid flow (Euler description), preferably after the path 11a, 11b, and 11c of the indicator particles 9 have been determined within the framework of a calibrated model of the fluid flow and particle motion.

[0066] The Figure 5 shows an indicator particle 9 as well as a diagram in which a pressure p over a distance s of the railway lines 11a, 11b and 11c for the based on the Figure 1The fluid-carrying device 1 described is applied. The indicator particle 9 shown represents several identical indicator particles 9. These each have the base body 13, which is hollow and made of a defined material. The base body 13 is defined by an outer diameter D, a cavity 18 with an inner diameter d, a reference pressure in the cavity 18 of the base body 13, and a specific wall thickness w = D - d. A curve 19a shows the pressure profile along path 11a, a curve 19b along path 11b, and a curve 19c along path 11c. All path lines begin at pressure p0 at the injection point 8 and end at pressure p4 at the outlet 4.For the indicator particle 9 assigned to track 11a, the local fluid pressure initially falls below the vapor pressure pD of the fluid, starting from pressure p0. This leads to the formation of cavitation bubbles, which, as a result of the pressure increase after exceeding the vapor pressure pD, implode and result in very high pressure peaks in the fluid (exceeding the defined indicator value pI). These peaks then cause a permanent change in the shape of the indicator particle. This is not the case for the indicator particles 9 assigned to tracks 11b and 11c, so they retain their shape, as indicated.

Claims

1. Method for determining an actual value and / or an actual value range of at least one state variable of a fluid in a fluid flow by means of at least one indicator particle (9) introduced into the fluid, wherein the at least one indicator particle (9) is provided and designed for an irreversible property change of an indicator property of the indicator particle (9) - in the presence of a specific indicator value of the at least one state variable in the fluid flow, and / or - as an unambiguous function of the actual value at the end of a certain period of time after the indicator particle (9) is introduced into the fluid, wherein the indicator particle (9) is detected at a detection point, the indicator property of the indicator particle (9) is evaluated and the actual value and / or the actual value range of the state variable upstream of the detection point is inferred from the indicator property, characterized in that - the indicator particle (9) is provided and designed for an irreversible change in properties of an indicator property of the indicator particle (9) in the presence of a specific indicator value of the at least one state variable in the fluid flow, wherein the state variable is a normal stress and / or a shear stress of the fluid and the indicator particle (9) has a base body (13) having a particle shell enclosing a cavity, wherein a reference pressure is present in the cavity and the particle shell is provided and designed to irreversibly change and / or break the indicator property in the form of its shape when the normal stress deviates from the reference pressure by a specific pressure difference, and / or wherein the particle shell is provided and designed to irreversibly change and / or break the indicator property in the form of its shape for this purpose when the shear stress deviates from a reference tension, and / or that - the indicator particle (9) is provided and designed for an irreversible change in properties of an indicator property of the indicator particle (9) as an unambiguous function of the actual value at the end of a certain period of time after the indicator particle (9) has been introduced into the fluid, wherein the base body (13) is covered by a protective cover (15) so that the base body or the sensor material is only exposed to the fluid after the specific period of time has elapsed after being introduced into the fluid.

2. Method according to claim 1, characterized in that the at least one indicator particle (9) is part of a plurality of indicator particles (9), which are introduced into the fluid, wherein the indicator value of the state variable for a first part of the indicator particles (9) corresponds to a first indicator value and for a second part of the indicator particles (9) corresponds to a second indicator value and / or the period of time corresponds to a first period of time for the first part of the indicator particles and to the second period of time for the second part of the indicator particles.

3. Method according to the preceding claim, characterized in that the first part of the indicator particles (9) and the second part of the indicator particles (9) are introduced into the fluid simultaneously or in a time-delayed manner and / or at the same point of introduction (8) or at spaced-apart points of introduction (8), wherein the first part is provided with an unchangeable first identification independent of the respective indicator property and the second part is provided with an unchangeable second identification independent of the respective indicator property.

4. Method according to one of the preceding claims, characterized in that the at least one indicator particle (9) is detected and the indicator property is evaluated without direct contact in the fluid or after the at least one indicator particle (9) has been removed from the fluid.

5. Method according to one of the preceding claims, characterized in that if the property change does not occur, the indicator value (9) is changed until the property change occurs, or if the property change occurs, the indicator value is changed until the property change does not occur, wherein from a first indicator value of the indicator value, at which the property change did not occur, and from a second indicator value of the indicator value at which the property change occurred, the actual value and / or the actual value range of the state variable is inferred.

6. Method according to one of the preceding claims, characterized in that at least one prediction value for the state variable is calculated using a model of the fluid flow and the movement of a particle in the fluid flow, in particular along the trajectory of the at least one modeled indicator particle, and is verified using the at least one indicator particle (9), wherein the model is adjusted to the actual value and / or the actual value range if the prediction value deviates from the actual value and / or actual value range determined by means of the at least one indicator particle (9).

7. Method according to one of the preceding claims, characterized in that a base body (13) on which a plurality of sensor elements (14) and / or sensor regions are formed is used as base body (13), wherein a first part of the sensor elements (14) or sensor regions made of the sensor material and a second part of the sensor elements (14) or sensor regions consists of a sensor material that is different from the sensor material8. Method according to claim 7, characterized in that an indicator particle (9), in which the sensor elements (14) and / or sensor regions are covered by the protective cover (15) with different cover thicknesses, is used as indicator particle (9).

9. Method for operating a fluid-guiding device (1), wherein an actual value and / or an actual value range of at least one state variable of the fluid in a fluid flow present in the device (1) is determined by means of at least one indicator particle (9) introduced into the fluid, using the method according to one or more of claims 1 to 810. Method according to claim 9, characterized in that if the actual value and / or the actual value range deviates from a previously determined actual value and / or actual value range and / or from a predicted value and / or predicted value range determined using a model of the fluid flow, a malfunction of the fluid-guiding device (1) is inferred.

11. Device (7) for determining an actual value and / or an actual value range of at least one state variable of a fluid in a fluid flow by means of at least one indicator particle (9) introduced into the fluid, using the method according to one or more of claims 1 to 8.