Methods for determining temperature and / or pressure distributions

Abstract

Method for determining temperature and / or pressure distributions (18) of a gaseous fluid (11) in a measuring area (24), - wherein a temperature- and / or pressure-sensitive luminescent dye (2) is introduced into a liquid (6), - wherein gas-filled bubbles (9) are formed from the liquid (6) and suspended in the gaseous fluid (11), - wherein the bubbles (9) in the measurement area (24) are illuminated with excitation light (13), - wherein spatial intensity distributions (16) of light (14) emitted by the dye (2) in the measurement area (24) are recorded and - wherein the spatial intensity distributions (16) of the light (14) with respect to the temperature and / or pressure distributions (18) of the gaseous fluid (11) in the measurement area (24) are evaluated.

Description

TECHNICAL AREA OF INVENTION

[0001] The invention relates to a method for determining temperature and / or pressure distributions of a gaseous fluid in a measuring area using particles with a temperature- and / or pressure-sensitive luminescent dye.

[0002] Temperature- and / or pressure-sensitive luminescent dyes, i.e., especially fluorescent and phosphorescent dyes, exhibit luminescence behavior that depends on their temperature and / or the pressure in their environment, usually the oxygen partial pressure. Specifically, the half-life of the luminescent state of the respective dye, and thus the intensity of light emitted by the dye as luminescence, depends on the temperature and / or pressure. STATE OF THE ART

[0003] From DE 102015 111 162 B3, a method for measuring unsteady pressure fluctuations on a surface using pressure-sensitive paint is known. The pressure-sensitive paint is applied to the surface. Several light intensity distributions of the pressure-sensitive paint are recorded successively to determine signal intensity distributions across a large number of measurement points on the surface. A two-point correlation function of the signal intensity distributions is determined by a correlation analysis of the signal intensity distributions between individual measurement points. In this way, a wavenumber-frequency spectrum can be generated, and at a selected frequency within the wavenumber-frequency spectrum, a wavenumber can be determined. The velocity is then calculated from the quotient of the frequency and the wavenumber.If the measurement takes place in a medium with a flow velocity below the speed of sound (a subsonic medium), velocities equal to or greater than the speed of sound can only occur if they are caused by sound. Therefore, the presence of acoustic events at the surface can be inferred.

[0004] From DE 10 2015 104 585 B3, a ruthenium compound is known as a temperature end in a temperature-sensitive paint, in particular in a temperature-sensitive paint with a binder forming a matrix of synthetic resin.

[0005] From DE 20 2008 009 006 U1, a temperature-sensitive paint is known, comprising a binder for forming a polymer matrix and a transition metal complex embedded in the binder, which exhibits temperature-dependent fluorescence with a maximum relative temperature sensitivity at a first temperature time. Furthermore, a rare-earth complex is embedded in the binder, which exhibits temperature-dependent fluorescence with a maximum relative temperature sensitivity at a second temperature time, which is 50-150 Kelvin higher than the first temperature.

[0006] From EP 1 715 319 B1, a method for determining the pressure distribution over a surface is known, in which a layer of pressure-sensitive paint is applied to the surface and pressure-sensitive molecules in the paint are excited by excitation light to emit luminescent light, the intensity of which depends on the pressure over the surface and which is recorded from the outside when viewing the surface. A half-transmittance thickness of the paint, at which the paint transmits 50% of the intensity of the incident light, is adjusted at the wavelength of the excitation light and / or at the wavelength of the luminescent light such that only those proportions of the pressure-sensitive molecules located in an outer sublayer of the pressure-sensitive paint layer on the surface are recorded.When applying the layer of pressure-sensitive ink to the surface, a minimum layer thickness is maintained that is at least as thick as the outermost layer. This eliminates the need to apply a shielding layer to the surface before the pressure-sensitive ink to create a continuous background.

[0007] From Z. Deng et al., "A combined velocity and temperature measurement with an LED and a lowspeed camera," Meas. Sci. Technol. 33 (2022) 15301, a method for combined velocity and temperature measurement is known. To determine three-dimensional velocity and temperature fields in microfluidic devices, a fluid flowing through the respective microfluidic device is inoculated with particles colored with a temperature-sensitive luminescent dye. In a measurement region, the particles are illuminated in pulses with UV excitation light from an LED. The luminescence light subsequently emitted by the dye on the particles is imaged onto a camera. Two images are taken with the camera for each pulse of the excitation light, with the time regime for determining the velocity of the particles being different from that for determining the temperature of the particles.To determine the temperature, the first image is taken during an extended pulse of excitation light, and the second image is taken immediately afterward. The particle temperature is then calculated from the ratio of the luminescence intensity during the two successively acquired images.

[0008] In Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV), a flow whose flow field is to be determined is seeded with particles that follow the flow. A well-known form of these particles are helium-filled soap bubbles, which, due to the low density of their filling, float in air despite the higher density of their outer shell (see J. Bosbach et al., Large scale particle image velocimetry with helium-filled soap bubbles, Exp. fluids 46: 539-547 (2009)).

[0009] From M. Machacek and T. Rösgen, "A Quantitative Visualization Method for Wind Tunnel Experiments Based on 3D Particle Tracking Velocimetry (3D-PTV)," PAMM, Proc. Appl. Math. Mech. 1, 2002, it is known to add a fluorescent dye to a soap solution used to form helium-filled soap bubbles in order to separate the light emitted by the soap bubbles from illumination light reflected from a background and model surfaces. However, only a few fluorescent dyes proved soluble in the soap solution, and none proved suitable for wind tunnel measurements because the available UV-A lamps could not provide sufficient excitation light intensity.

[0010] From Schmeling et al., Simultaneous measurement of temperature and velocity fields in convective airflows, Measurement Science and Technology, 2014, Vol. 25, No. 3, p. 035302, a method for the simultaneous measurement of temperature and flow fields in convective airflows is known. The airflows are seeded with thermochromic liquid crystals (TLCs) as tracer particles. Using these tracer particles, the flow fields are determined by PIV (particle image velocimetry) and the temperature fields by PIT (particle image thermography).

[0011] From Trigona, Carlo, et al., Bubble Sensors for Temperature Measurements through a Colorimetric Approach, Sensors, 2024, Vol. 24, No. 4, p. 1278, a method for temperature measurement using bubbles coated with a thermochromic paint is known. The thermochromic paint exhibits reversible color modifications in response to temperature changes. The thermochromic paint is applied to bubbles formed from synthetic resin. The approximately spherical bubbles have a mean diameter of 10 cm. TASK OF INVENTION

[0012] The invention is based on the objective of demonstrating a method with which temperature and / or pressure distributions of a gaseous fluid can be determined with high spatial and temporal resolution. SOLUTION

[0013] According to the invention, the problem is solved by a method with the features of independent claim 1. The dependent claims relate to preferred embodiments of the method according to the invention. DESCRIPTION OF THE INVENTION

[0014] According to the invention, to determine the temperature and / or pressure distributions of a gaseous fluid in a measurement area, a temperature- and / or pressure-sensitive luminescent dye is introduced into a liquid. Gas-filled bubbles are formed from the liquid. The gas-filled bubbles are suspended in the fluid. In the measurement area, the bubbles are illuminated with excitation light, and the spatial intensity distribution of light emitted by the dye in the measurement area is recorded. The spatial intensity distributions of the light are evaluated with respect to the temperature and / or pressure distributions of the fluid in the measurement area.

[0015] Surprisingly, it is possible to add a temperature- and / or pressure-sensitive luminescent dye to a liquid that can form gas-filled bubbles, which can then be suspended in a gaseous fluid where they remain suspended for extended periods and whose currents they follow. This dye transforms the gas-filled bubbles into temperature- and / or pressure-sensitive probes within the gaseous fluid. These probes can be detected by illuminating the bubbles with excitation light and recording the intensity distributions of the light emitted by the dye. It is surprising both that the dye can be introduced into the liquid in such a way that gas-filled bubbles can still be formed, and that these gas-filled bubbles containing the dye can actually be used as temperature- and / or pressure-sensitive probes after suspension in the gaseous fluid.Temperature- and / or pressure-sensitive luminescent dyes are very specialized compounds, just as liquids used to form gas-filled bubbles have very specific compositions to enable the controlled formation of small, gas-filled bubbles with typical diameters in the range of 0.1 to 1 mm and sufficient long-term stability. It is equally important to consider that the temperature and / or pressure sensitivity of luminescent or fluorescent dyes is not solely a material property of the dyes themselves, but also strongly depends on their environment, particularly on the accessibility of the dyes to oxygen contained in the gaseous fluid. This is because the lifetime of the luminescent or fluorescent states of the dyes often depends critically on the oxygen concentration in their surroundings.

[0016] Against this background, a person skilled in the art would not have expected the method according to the invention to be successfully suitable for determining temperature and / or pressure distributions of a gaseous fluid. In fact, however, very rapid temperature and / or pressure fluctuations up to the kHz range, for example over a range from 1 Hz to 1 kHz or even up to 10 kHz, can be detected with the aid of the gas-filled bubbles containing the temperature- and / or pressure-sensitive luminescent dye, such as those associated with sound waves in the gaseous fluid. The method according to the invention is therefore particularly suitable for detecting sound pressure distributions in the gaseous fluid, i.e., not only over a surface, but over a three-dimensionally extended volume in which sound can propagate, with three-dimensional resolution. Each bubble acts as its own microphone.In other words, the method according to the invention provides a three-dimensional microphone distribution with a high spatial density relative to the wavelength of the sound.

[0017] In the method according to the invention, very rapid pressure fluctuations can be detected both directly with a pressure-sensitive dye and indirectly with a temperature-sensitive dye by exploiting the temperature changes during adiabatic changes of state as a result of the pressure fluctuations.

[0018] In the process according to the invention, it can be advantageous to dry the bubbles before using them as temperature- and / or pressure-sensitive probes in the gaseous fluid. Drying can be carried out before or after suspending the bubbles in the fluid. During drying, components of the liquid or of an excipient introduced into the liquid along with the dye evaporate from the liquid, thereby altering, and in particular improving, the accessibility of the dye in the liquid to oxygen in the gaseous fluid.

[0019] In the process according to the invention, a surface-active agent, in particular a surfactant, can be added to the liquid from which the gas-filled bubbles are formed as an aid to bubble formation. Typically, a soap solution with a reduced surface tension compared to air is used for bubble formation. This solution can contain polymers as functional components in addition to surfactants. In the process according to the invention, the surface-active agents and polymers are selected and concentrated such that small gas-filled bubbles with a diameter in the range of 0.1 to 1 mm can be formed, with diameters that are as uniform as possible and with good long-term stability.

[0020] In the process according to the invention, a solvent can be added to the dye and / or the liquid as an aid to dissolving the dye in the liquid. It has proven advantageous not to attempt to dissolve the dye directly in the liquid to form the bubbles, but rather to first dissolve the dye in another solvent and then introduce it in dissolved form into the liquid to form the bubbles. Specifically, an alcoholic solution can be prepared with the dye and the solvent, from which an aqueous solution is then formed. The alcohol can subsequently be removed, either directly from the liquid or later by drying the resulting bubbles, depending on whether the alcohol hinders bubble formation or not.

[0021] More specifically, the solvent methanol can be used to ultimately dissolve the dye EuTTA (CAS 21392-96-1) in a soap bubble fluid (e.g. SAI 1035 BFS (Bubble Film Solution), Sage Action, Inc., AZ, USA).

[0022] In the inventive method, the bubbles can be filled with helium and shaped in such a way that they float in the gaseous fluid, in a manner known in principle. It is assumed that the fluid consists of molecules with a higher molecular mass than helium, so that the mass of the bubble shell can be compensated for by the lighter helium.

[0023] The emission of luminescent light from dyes depends not only on the influence of temperature and / or pressure on the half-life of their luminescent or fluorescent state, but also on the intensity of the excitation light, the amount of dye in the respective bubble, and other parameters that can vary from bubble to bubble. To account for the influence of the excitation light intensity, a spatial excitation light intensity distribution can be determined in the measurement area and used as a correction factor for the spatial intensity distributions of the light emitted by the dye in the measurement area. The spatial excitation light intensity distribution in the measurement area can be determined, for example, using non-luminescent particles added to the gaseous fluid that reflect the excitation light from the measurement area depending on its local intensity.If this reflected light is registered with the same instruments as the light later emitted by the dye in the measurement area, different sensitivities of these instruments for different parts of the measurement area are also recorded in such a way that they are automatically corrected when correcting the spatial intensity distribution of the light emitted by the dye in the measurement area.

[0024] If the bubbles are suspended in the gaseous fluid at a concentration of such a high concentration and the spatial intensity distributions of the light emitted by the dye in the measurement area are recorded in such a way that the light can be attributed to individual bubbles, then, firstly, flow velocity fields of the fluid in the measurement area can be determined using a PIV or PTV method that evaluates the spatial intensity distributions of the light emitted by the dye in the measurement area. Secondly, the individual bubbles can be tracked across the spatial intensity distributions of the light emitted by the dye in the measurement area based on the light emitted by their dye. Tracking methods such as those known to those skilled in the art from DE 10 2013 105 648 B3 can be used for this purpose.While tracking individual bubbles can also be used to determine the fluid flow velocity fields in the measurement area, alternatively or additionally, averaged intensities of the light emitted by the dye in the measurement area, based on several temporally successive spatial intensity distributions, can be used as a reference value for the light intensity of each tracked bubble. This averaged intensities preferably take into account the local intensity of the excitation light and the local sensitivity of the light recording. It is assumed that the averaged intensities of the light from each tracked bubble accurately reflect all bubble-specific influences on the light intensity of that particular bubble.

[0025] In many cases, it is sufficient to use a spatial intensity distribution of light averaged over several successive spatial intensity distributions of the light emitted by the dye in the measurement area as a reference value for the individual spatial intensity distributions of light. This approach is based on the consideration that temperature and / or pressure changes of interest, for example as a result of sound, occur much faster than the distribution of bubbles in the respective measurement area changes significantly.

[0026] In the procedures described so far within the framework of the inventive method, the bubbles in the measurement area can be continuously illuminated with the excitation light. However, the intensity distributions of the light emitted by the dye in the measurement area are always snapshots. These snapshots can also be taken at equal time intervals corresponding to pulses of the excitation light.

[0027] In another embodiment of the method according to the invention, the dye in the measurement area is illuminated with the excitation light in pulses, and after each pulse of the excitation light, at least two spatial intensity distributions of the light emitted by the dye in the measurement area are recorded successively. In this way, the lifetime of the luminescent state of the dye can be determined independently of the local intensity of the excitation light, the sensitivity of the light recording, and the individual properties of the bubbles, and can be recorded as a measure of the temperature or pressure of interest.

[0028] Devices suitable for carrying out the method according to the invention are generally known to those skilled in the art in the field of PIV and PTV and to those skilled in the art in the field of the use of pressure- and / or temperature-sensitive dyes.

[0029] Advantageous further developments of the invention result from the patent claims, the description and the drawings.

[0030] The advantages of features and combinations of features mentioned in the description are merely exemplary and can have an effect alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention.

[0031] Regarding the disclosure content—not the scope of protection—of the original application documents and the patent, the following applies: Further features can be derived from the drawings—in particular, the geometries depicted and the relative dimensions of several components to one another, as well as their relative arrangement and functional connection. The combination of features from different embodiments of the invention or from features of different claims is also possible, deviating from the chosen cross-references of the claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features from different claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

[0032] The features mentioned in the claims and the description are to be understood, with regard to their number, as meaning that exactly that number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least". Thus, for example, if a solvent is mentioned, this is to be understood as meaning that exactly one solvent, two solvents, or more solvents are present. The features listed in the claims may be supplemented by further features or may be the only features that the subject matter of the respective claim possesses.

[0033] The reference numerals contained in the patent claims do not constitute a limitation of the scope of the subject matter protected by the patent claims. They merely serve the purpose of making the patent claims easier to understand. BRIEF DESCRIPTION OF THE FIGURES

[0034] The invention will now be further explained and described with reference to preferred embodiments shown in the figures. Fig. Figure 1 is a flowchart for the method according to the invention. Fig. Figure 2 shows an arrangement for carrying out a first embodiment of the method according to the invention. Fig. 3 shows parts of a opposite Fig. 1 simplified arrangement for carrying out the first embodiment of the method according to the invention and Fig. 4A and billustriching a further embodiment of the inventive method with a similar device as in Fig. 3. FIGURE DESCRIPTION

[0035] The in Fig. The process according to the invention, illustrated in Figure 1 as a flowchart, begins with a first step 1 in which a temperature- and / or pressure-sensitive luminescent or fluorescent dye 2 is dissolved in a solvent 3 to produce a dye solution 4. In a second step 5, the dye solution 4 is introduced into a liquid 6 such that the dye 2 is dissolved in the liquid 6. In a third step 7, helium-filled bubbles 9 are then formed from the liquid 6 containing the dissolved dye 2 by adding helium 8. Directly during their formation or in a separate fourth step 10, the bubbles are introduced into a gaseous fluid 11, the pressure and / or temperature distributions of which are of interest in a measurement area. In a fifth step 12, the bubbles in the measurement area are illuminated with excitation light 13.Subsequently, the light 14 emitted by the bubbles 9 or the dye 2 contained therein is recorded in a sixth step 15 in the form of spatial intensity distributions 16 in the measurement area. In a seventh step 17, the spatial intensity distributions 16 are evaluated in order to determine the temperature and / or pressure distributions 18 of interest.

[0036] In a specific embodiment, the dye 2 is EuTTA (CAS 21392-96-1), the solvent 3 is methanol, and the liquid 6 is SAI 1035 BFS (Bubble Film Solution), Sage Action, Inc., AZ, USA. 30 mg of EuTTA are dissolved in 10 ml of methanol by stirring while heated to 50°C. The resulting solution is mixed with 100 ml of SAI 1035 by stirring while heated.

[0037] Fig. Figure 2 shows a device 19 for carrying out an embodiment of the method according to Fig. 1. The bubbles 9 are generated by a bubble generator 20 and introduced into the gaseous fluid 11 via a nozzle 21. The gaseous fluid 11 is, for example, air 22 in a plenum 23. Due to their helium filling, the bubbles 9 float in the air 22. In the measurement area 24, the bubbles 9 are illuminated with the excitation light 13 from excitation light sources 25 in the form of LEDs 26, which are followed by wavelength-selective filters 27. The light 14 emitted by the bubbles 9 or the dye 2 contained therein as a result of excitation by the excitation light 13 is recorded by two cameras 28 in a stereo arrangement in order to register the spatial intensity distributions 16 of the light 14. The cameras are fitted with 28 additional wavelength-selective filters 29 that only allow the light 14 to pass through, but block the excitation light 13 and other background light.Specifically, the excitation light 13 can have a wavelength of approximately 335 nm, while the light 14 falls within a wavelength range of 615 to 650 nm. These values ​​are suitable for the dye EuTTA. The spatial intensity distributions of the light 14, recorded by the cameras 28, are evaluated to determine a temperature distribution 18' and / or a pressure distribution 18" in the measurement area 24 with time resolution. The temperature and / or pressure distributions 18 can each indicate the locations of individual bubbles 9 and the associated temperature and / or pressure at the respective location. Fig. Figure 2 further illustrates a control system 30 with a longer trigger pulse 31 for controlling the light sources 25 and several shorter trigger pulses 32 for controlling the cameras 28 during the longer trigger pulse 31.

[0038] When using device 19 according to Fig. 2. First, a spatial excitation light intensity distribution in the measurement area 24, provided by the light sources 25, can be determined. This can be done by first introducing non-luminescent and non-fluorescent bubbles into the plenum 23. Alternatively or additionally, the individual bubbles 9 within the plenum 23 can be tracked to determine reference intensities for the individual bubbles 9. When evaluating the spatial intensity distributions of the light 14, the excitation light intensity distribution, the reference intensity of the individual bubbles 9, and the spatial sensitivity distribution of each of the cameras 28 must be taken into account.In acoustic applications where the movements of the bubbles 9 are several orders of magnitude slower than the pressure or temperature changes to be detected, an intensity of light 14 averaged in a spatially, temporarily limited area of ​​the measurement area 24 can also be used as a reference intensity.

[0039] Fig. Figure 3 illustrates, by displaying only one of the light sources 25, how the cameras 28, controlled by the trigger pulses 32, make visible the movement 33 of a bubble 9 in the measurement area 24. Determining the movement 33 depends on the locations from which the light 14 is emitted from the measurement area 24. The intensity of the light 14 is crucial for determining the temperature and / or pressure at each location.

[0040] Fig. 4A and Fig. Figure 4B illustrates an alternative control of the light sources 25 by short trigger pulses 34 and a synchronized control of the cameras 28 such that these each according to Fig. 4A a first light intensity distribution during the respective pulse of the excitation light 13 and according to Fig. 4B A second intensity distribution of the light 14 is recorded after each pulse of the excitation light 13. From the ratio of the intensities of the light 14 of the respective bubble 9, the lifetime of the luminescent or fluorescent state of its dye 2 can be directly determined, which is a measure of the temperature and / or pressure at the respective location in the measurement area 24. Following the procedure according to Fig. 4. No calibration of the device 19 is required with regard to the excitation light intensity distribution, the reference intensity of the individual bubbles 9 or the spatial sensitivity distribution of the cameras 28. REFERENCE MARK LIST 1. First step 2 Dye 3 solvents 4 dye solution 5 second step 6 Liquid 7 third step 8 Helium 9 bubbles 10 fourth step 11 gaseous fluid 12 fifth step 13 Excitation light 14 emitted light 15 sixth step 16 spatial intensity distribution of the emitted light 14 17 seventh step 18 Temperature and / or pressure distribution 18' Temperature distribution 18" pressure distribution 19 Device 20 Bubble Generator 21 nozzle 22 air 23rd Plenary 24 Measurement area 25 light sources 26 LED 27 filters 28 Camera 29 more filters 30 Control 31 Trigger pulse 32 trigger pulses 33 Movement 34 trigger pulses

Claims

[1] Method for determining temperature and / or pressure distributions (18) of a gaseous fluid (11) in a measuring area (24), - wherein a temperature- and / or pressure-sensitive luminescent dye (2) is introduced into a liquid (6), - wherein gas-filled bubbles (9) are formed from the liquid (6) and suspended in the gaseous fluid (11), - wherein the bubbles (9) in the measurement area (24) are illuminated with excitation light (13), - wherein spatial intensity distributions (16) of light (14) emitted by the dye (2) in the measurement area (24) are recorded and - wherein the spatial intensity distributions (16) of the light (14) with respect to the temperature and / or pressure distributions (18) of the gaseous fluid (11) in the measurement area (24) are evaluated. [2] Method according to claim 1, wherein the bubbles (9) are dried before or after they are suspended in the gaseous fluid (11). [3] Method according to claim 1 or 2, wherein a surfactant is added to the liquid (6) as an aid to the formation of the bubbles (9). [4] Method according to any of the preceding claims, wherein a solvent (3) is added to the dye (2) and / or the liquid (6) as an aid to dissolving the dye (2) in the liquid (6). [5] Method according to claim 4, wherein an alcoholic solution is formed with the dye (2) and the solvent (3) and wherein an aqueous solution is formed from the alcoholic solution and the liquid (6). [6] Method according to one of the preceding claims, wherein the bubbles (9) are filled with helium (8) and are designed to float in the gaseous fluid (11). [7] Method according to one of the preceding claims, wherein a spatial excitation light intensity distribution in the measurement area (24) is determined and used as a correction parameter for the spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measurement area (24). [8] Method according to one of the preceding claims, wherein the bubbles (9) are suspended in the gaseous fluid (11) in such a concentration and the spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measuring area (24) are recorded in such a way that the light (14) can be attributed to individual bubbles (9). [9] Method according to claim 8, wherein flow velocity fields of the gaseous fluid (11) in the measuring area (24) are determined by a PIV or PTV method evaluating the spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measuring area (24). [10] Method according to claim 8 or 9, wherein the individual bubbles (9) are tracked over the spatial intensity distributions (16) of the light (14) emitted by their dye (2) in the measurement area (24) using the light (14) emitted by their dye (2). [11] Method according to claim 10, wherein intensities of the light (14) from the individual tracked bubbles (9) averaged over several of the temporally successive spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measurement area (24) are used as a reference for the intensity of the light (14) from the respective tracked bubble (9). [12] Method according to any one of claims 1 to 10, wherein a spatial intensity distribution (16) of the light (14) averaged over several of the temporally successive spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measurement area (24) is used as a reference value for the individual spatial intensity distributions (16) of the light (14). [13] Method according to one of the preceding claims, wherein the dye (2) in the measuring area (24) is illuminated with the excitation light (13) in pulses. [14] Method according to claim 13, wherein after each of the pulses of the excitation light (13) two spatial intensity distributions (16) of the light (14) emitted by the dye (2) in the measurement area (24) are recorded successively. [15] Method according to one of the preceding claims, wherein the spatial intensity distributions (16) of the light (14) with respect to temporal pressure and / or temperature fluctuations of the gaseous fluid (11) in the measuring area (24) are evaluated in a range from 1 Hz to 5 kHz or preferably up to 10 kHz. [16] Method according to one of the preceding claims, wherein the spatial intensity distributions (16) of the light (14) are evaluated with respect to three-dimensional sound pressure distributions of the gaseous fluid (11) in the measurement area (24). [17] Method according to one of the preceding claims, wherein pressure fluctuations of the gaseous fluid (11) in the measuring area (24) - directly with a pressure-sensitive luminescent dye (2) and / or - by utilizing temperature changes of the gaseous fluid (11) in the measuring area (24) which occur during adiabatic changes of state as a result of pressure fluctuations, indirectly detected with a temperature-sensitive luminescent dye (2).

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