Method and device for detecting a laminar or turbulent boundary layer

Abstract

The invention relates to a method for determining the thickness d of a laminar or turbulent boundary layer on the surface (65) of a body (6) around which a medium is flowing and which has a different temperature than that of the flowing medium. At least one first temperature (TO), a second temperature (Tx), and a fourth temperature (TL) are measured at a respective specified distance (X1, X2, X4) to the surface (65) of the body (6), wherein at least one first device for determining a temperature (31), a second device for determining a temperature (32), and a fourth device for determining a temperature (34) are used to measure the temperatures (TO, Tx, TL), and additionally a third temperature (Ty) is measured using a third device for determining a temperature (33). The invention additionally relates to a device (1) for determining the thickness d of a laminar or turbulent boundary layer on the surface (65) of a body (6) around which a medium is flowing.

Description

[0001] Method and device for detecting a laminar or turbulent boundary layer

[0002] The invention relates to a method for determining a thickness d of a laminar or turbulent boundary layer on a surface of a body around which the flow is directed, which has a temperature difference to the flowing medium, at which at least a first temperature (T o ), a second temperature (T x ) and a fourth temperature (T L ) is measured at a predeterminable distance (X1, X2, X4) from the surface of the body being flowed around, whereby the temperatures (T o , T x , T L ) at least one first device for determining a temperature, one second device for determining a temperature, and one fourth device for determining a temperature are used. Furthermore, the invention relates to a device for carrying out such a method.

[0003] From DE 102016 107 212 A1 it is known that the convective heat transfer coefficient h c a convection surface using a device that measures a first temperature difference between a surface temperature and an ambient temperature, and a second temperature difference between a temperature within the boundary layer and the ambient temperature. For this purpose, three temperature sensors are used at different distances from the surface. This known sensor is based on the knowledge that the temperature profile within the boundary layer has an exponential profile, with the boundary layer thickness d as a constant. By determining three support points, the exponential profile and, from this, the convective heat transfer coefficient can be determined. This known device is also referred to below as a CHM sensor.

[0004] Alternatively, the exponential drop in air temperature over a convection surface can be determined optically using a laser differential interferometer. This measurement setup enables precise recording of the temperature profile and thus the convective heat transfer coefficient h c However, the equipment required is very complex, so that such a measurement is not suitable for routine measurements.

[0005] It is known from practice that the heat dissipation of a convection surface in laminar flow differs from the heat dissipation of a convection surface in turbulent flow. However, the device known from DE 102016 107 212 A1 has the disadvantage that turbulent and laminar flows cannot be distinguished and therefore the convective heat transfer coefficient h c a convection surface and / or the thickness d of the boundary layer is incorrectly recorded in turbulent flow.

[0006] Based on the prior art, the invention is therefore based on the object of specifying a method and a device which enable an accurate detection of the convective heat transfer coefficient h c and / or the thickness d of a laminar or turbulent boundary layer under changing flow conditions and with little equipment effort.

[0007] The object is achieved according to the invention by a method according to claim 1 and a device according to claim 10. Advantageous developments of the invention can be found in the dependent claims. According to the invention, in order to determine a thickness d of a laminar or turbulent boundary layer on a surface of a body around which the flow is directed and which has a temperature difference to the flowing medium, it is proposed to measure not just three, but at least four temperatures. To record the temperatures, at least a first device for recording a temperature, a second device for recording a temperature, a third device for recording a temperature and a fourth device for recording a temperature are available, each of which is arranged at a predeterminable and respectively different distance from the surface of the body around which the flow is directed.In some embodiments of the invention, the at least three temperature-detecting devices can be resistance thermometers and / or thermocouples. For the purposes of this description, the distance to the surface of the body around which the flow is directed refers to the length of the normal vector between the respective temperature-determining device and the surface.

[0008] In laminar flow, the temperature curve within the boundary layer between the surface temperature and the ambient temperature follows an exponential curve with the thickness d of a laminar or turbulent boundary layer as a constant. If the measured values ​​from the temperature measurement devices lie on this exponential curve within predefined error limits, laminar flow above the surface can be concluded. If the measured values ​​deviate from this exponential curve within predefined error limits, the flow beyond a laminar sublayer close to the surface is turbulent.

[0009] In some embodiments of the invention, the first device for detecting the temperature is arranged directly on the surface of the body around which the flow is carried, so that it detects the temperature T othe surface. The fourth temperature measurement device is located at a greater distance from the surface, so that it records the undisturbed ambient temperature. The second and third temperature measurement devices are located within the boundary layer forming above the surface. This allows for particularly simple evaluation of the measured values ​​to determine whether the flow at the surface is laminar or turbulent.

[0010] In some embodiments of the invention, the first device for determining a temperature and the second

[0011] Device for determining a temperature and the third

[0012] Device for determining a temperature and the fourth

[0013] A device for determining a temperature using thermocouples can be formed. This can reduce the equipment required for measuring the measured values.

[0014] In some embodiments of the invention, the thickness d of the boundary layer can be determined from the temperatures (T o , T x , T y , T L ), the distance X2 of the second device for determining a temperature above the surface, the distance X3 of the third device for determining a temperature and the thermal conductivity coefficient λ L of the flowing medium and λ M the sensor arrangement can be determined twice as follows: The flow is recognized as turbulent if d1=d2, and the flow is recognized as laminar if d1=d2. Due to measurement uncertainties, a deviation of the values ​​within specified tolerances can also be considered laminar. This can be used, for example, to detect a flow stall on the wing of an aircraft or the blade of a wind turbine, allowing appropriate countermeasures to be initiated.

[0015] In some embodiments of the invention, the distance X2 at which the second temperature (T x ) from the surface, is about 0.2 mm to about 2 mm. In other embodiments of the invention, the distance X2 at which the second temperature (T x ) from the surface, is about 0.3 mm to about 1 mm. In still other embodiments of the invention, the distance X2 at which the second temperature (T x ) from the surface, is approximately 0.7 mm. This ensures that even a laminar sublayer of an otherwise turbulent flow is reliably detected.

[0016] In some embodiments of the invention, the distance X3 at which the third temperature (T y) from the surface, is about 4 mm to about 7 mm. In other embodiments of the invention, the distance X3 at which the third temperature (T y ) from the surface, should be approximately 5 mm. This ensures that turbulent flow can be reliably detected even if it still forms a laminar sublayer near the surface.

[0017] In some embodiments of the invention, the distance X1 may be approximately 0 mm, so that the first temperature (T o ) corresponds to the temperature of the surface.

[0018] In some embodiments of the invention, the distance X4 can be chosen so large that the fourth temperature (T L) corresponds to the temperature of the environment of the body around which the flow occurs. For this purpose, in some embodiments of the invention, the distance X4 can be between approximately 9 mm and approximately 20 mm. In other embodiments of the invention, the distance X4 can be between approximately 10 mm and approximately 14 mm. In yet other embodiments of the invention, the distance X4 can be between approximately 11 mm and approximately 16 mm.

[0019] In some embodiments of the invention, the first device for determining a temperature and the second

[0020] Device for determining a temperature and the third

[0021] Device for determining a temperature and the fourth

[0022] A device for determining a temperature is formed by thermocouples, wherein a first thermoelectric voltage is measured between the first and the fourth device for determining a temperature, a second thermoelectric voltage U2 is measured between the second and the fourth device for determining a temperature, and a third thermoelectric voltage U3 is measured between the third and the fourth device for determining a temperature. This facilitates the evaluation of the measurement, so that with little equipment outlay, e.g., an analog computing circuit, an electrical signal representing the convective heat transfer coefficient can be directly generated. Such an analog computing circuit can be implemented, for example, with operational amplifiers.

[0023] In some embodiments of the invention, the parameter be determined by a calibration measurement. A calibration measurement takes into account that the temperature T measured by the second or third device for determining a temperature x and T y is influenced by both the flowing medium and the sensor's lead wires and optional mechanical mounting devices. This influence can be determined either mathematically through a computer simulation of the heat flows or, particularly simply, through a calibration measurement for each sensor or sensor type.

[0024] In some embodiments of the invention, at least the second, third, and fourth devices for determining a temperature can each consist of a thermocouple formed from a common first wire made of a first material and at least one further wire made of a second material, which is connected to the first wire at a respective contact point, wherein the wires each extend from the surface into the half-space adjacent to the surface and have a diameter of approximately 300 μm or less. The contact points can be formed by a clamping and / or soldering and / or welding connection.

[0025] In some embodiments of the invention, the first, second, third, and fourth wires may each have a diameter of about 50 μm to about 250 μm. In other embodiments of the invention, the first, second, third, and fourth wires may each have a diameter of about 50 μm to about 150 μm. In still other embodiments of the invention, the first, second, third, and fourth wires may each have a diameter of about 75 μm to about 125 μm. In other embodiments of the invention, the first, second, third, and fourth wires may each have a diameter of about 90 μm to about 110 μm. In other embodiments of the invention, the first, second, third, and fourth wires may each have a diameter of about 100 μm.

[0026] In some embodiments of the invention, the first, second, third and fourth wires may extend freely into the half-space surrounding the surface.

[0027] In some embodiments of the invention, the first, second, third, and fourth wires may not be connected to a support structure, at least in the longitudinal section not in contact with the surface. These features can reduce the thermal load and the response speed and / or measurement error. Furthermore, the support structure can prevent interference with the flow, which could lead to flow disturbance and thus falsification of the measurement results.

[0028] The invention will be explained in more detail below with reference to figures and an exemplary embodiment. In the following:

[0029] Figure 1 shows, as an example, the known flow conditions for laminar and turbulent flow on a surface

[0030] Figure 2 shows a schematic representation of a CHM sensor according to the invention.

[0031] Figure 3 shows the measured values ​​of the second and third devices for recording a temperature against their distance X2 or X3 from the surface in laminar flow.

[0032] Figure 4 shows the measured values ​​of the second and third devices for recording a temperature against their distance X2 or X3 from the surface in turbulent flow. Figure 1 shows a cross-section through a body 6 with a surface 65 around which air flows. The body 6 can, for example, be part of a vehicle, aircraft, or ship. In other embodiments of the invention, the body 6 can be part of a wind turbine. In yet other embodiments of the invention, the body 6 can be part of a room climate measuring device which determines the convective heat transfer coefficient and / or the exchange of radiant heat with the environment. For this purpose, the surface 65 can be heated, for example with an electric heating device. In other embodiments of the invention, the surface 65 can be the outside of a heat exchanger.

[0033] During operation of the boundary layer sensor or device 1 for determining a thickness d of a laminar or turbulent boundary layer, the body 6 is surrounded by a forced flow or a convection flow, so that a flow develops in the half-space of the body 6 adjacent to the surface 65. Figure 1 shows a laminar and a turbulent flow as well as the transition region using a flat plate.

[0034] In section I, laminar flow 5 prevails. The fluid flows in layers that do not mix with each other. In laminar flow, the streamlines, i.e., the paths of the fluid particles, run virtually parallel to each other. The temperature within the boundary layer decreases exponentially from the surface 65 to the environment outside the boundary layer: where T x the temperature at a distance x from the surface, T odenotes the temperature of the surface and T L is the temperature of the flowing medium, and d is the thickness of the boundary layer. Section II shows a turbulent flow 55. In turbulent flow, the fluid particles move chaotically in all directions, but macroscopically still in the direction of flow. Due to the turbulence, the fluid is thoroughly mixed, so that the heat transfer coefficient h c in the turbulent boundary layer and the thickness d of the boundary layer is greater than in the laminar boundary layer. The heat flow from surface 65 into the environment is therefore greater in the turbulent boundary layer than in the laminar boundary layer.

[0035] Figure 1 further shows in Section II that a laminar sublayer 50 forms adjacent to the surface 65. Its thickness corresponds approximately to 3% - 5% of the turbulent boundary layer.

[0036] Between the first section I and the second section II there is a transition zone within which the thickness of the laminar sublayer 50 decreases and the thickness of the turbulent boundary layer increases.

[0037] Figure 2 again shows a cross section through a body 6 with a surface 65. During operation of the boundary layer sensor or the device 1 for determining a thickness d of a laminar or turbulent boundary layer, the body 6 is flowed around by a forced flow or a convection flow, as explained above in connection with Figure 1. To detect the thickness d of the boundary layer and / or the convective heat transfer coefficient h c According to the invention, a boundary layer sensor or CHM sensor 1 is used above the surface 65.

[0038] The boundary layer sensor 1 is configured to detect four temperatures or two temperature differences. For this purpose, the boundary layer sensor comprises a first temperature detection device 31, which is arranged at a first distance X1 from the surface 65. In the illustrated embodiment, the first temperature detection device 31 is located directly on the surface 65. The distance X1 is therefore 0 mm.

[0039] Furthermore, the boundary layer sensor 1 has a second device 32 for detecting a temperature, which is arranged at a distance X2 above the surface 65. The distance X2 can, for example, be between about 0.2 mm to about

[0040] 1 mm. The distance X2 is selected such that the second device 32 for detecting a temperature in laminar flow is located within the boundary layer forming above the surface 65. In turbulent flow, the second device 32 for detecting a temperature is located within the laminar sublayer explained above, over a wide velocity range.

[0041] Furthermore, the boundary layer sensor 1 has a third device 33 for detecting a temperature, which is arranged at a distance X3 above the surface 65. The distance X3 can be, for example, between 4 mm and 7 mm. For example, the third distance X3 can be approximately 5 mm. The distance X3 is selected such that the third device 33 for detecting a temperature is located within the boundary layer forming above the surface 65 in the case of laminar flow. In the case of turbulent flow, the third device 33 for detecting a temperature is located within the turbulent boundary layer explained above.

[0042] Finally, the boundary layer sensor 1 has a fourth device 34 for determining a temperature, which is arranged at a distance X4 above the surface 65. The distance X4 can, for example, be between about 9 mm and about 20 mm or between about 10 mm and about 14 mm or between

[0043] The distance X4 is selected such that the fourth device for determining a temperature detects the ambient temperature of the medium flowing over the surface 65 outside the boundary layer. Thus, the distance X4 above the surface 65 can be selected depending on the expected flow velocity, so that at lower flow velocities, a sensor with a shorter distance is selected, and at higher flow velocities, a sensor with a longer distance is selected.

[0044] In the illustrated embodiment, the first, second, third, and fourth devices 31, 32, 33, and 34 for determining a temperature are designed as thermocouples. For this purpose, the boundary layer sensor 1 has a first wire 20 made of a first material. One end of the first wire 20 is connected to one end of a second wire 21. The second wire 21 is made of a second material, so that a thermoelectric voltage can develop at the contact point, which is a measure of the temperature T o the surface 65.

[0045] The first wire 20 further has a second end, which is arranged at a distance X4 from the surface 65. At this end, a fourth contact point is formed with a fifth wire 24. This contact point forms the fourth device 34 for determining the temperature T L .

[0046] Similarly, along the longitudinal extent of the first wire 20, there is another contact point with a third wire 22 at a distance X2. This contact point forms the second device 32 for determining the temperature T x .

[0047] Similarly, along the longitudinal extent of the first wire 20, there is another contact point with a fourth wire 23 at a distance X3. This contact point forms the third device 33 for determining the temperature T Y. In some embodiments of the invention, three thermoelectric voltages can thus be determined. A first thermoelectric voltage is determined with a first measuring device 41 between the second wire 21 and the fifth wire 24. A second thermoelectric voltage is determined with a second measuring device 42 between the fifth wire 24 and the third wire 22. A third thermoelectric voltage is determined with a third measuring device 43 between the fifth wire 24 and the fourth wire 24. The first thermoelectric voltage is thus a measure of the temperature difference T o - T L The second thermoelectric voltage is a measure of the temperature difference T x - T L The third thermoelectric voltage is a measure of the temperature difference T Y - T L .

[0048] The boundary layer sensor 1 can be easily attached to the surface 65 using an adhesive tape 7. This makes the boundary layer sensor 1 suitable for temporary or mobile applications, for example, for experiments in a flow channel. Furthermore, the adhesive tape allows for installation without disturbing the surface geometry.

[0049] The boundary layer sensor 1 may in some embodiments contain further elements, in particular mechanical fastening devices, which the device 31, 32, 33 and 34 for determining the temperatures T o , T L , T y and T x at their intended locations. This can prevent deformation or changes in the distances X2, X3, and X4 and / or reduce the risk of mechanical damage to the boundary layer sensor 1.

[0050] The temperature profile within the boundary layer above the surface 65 follows an exponential function in laminar flow, which is why known methods for evaluating the measured values ​​are essentially based on deriving the measured values ​​T o , T x and T L to derive the coefficients of an exponential function, which represent the boundary layer thickness and / or the convective heat transfer coefficient. However, according to the invention, it was recognized that the temperature profile within the boundary layer above the surface 65 in turbulent flow can be described by two exponential functions, namely one for the laminar sublayer and another for the turbulent boundary layer. Turbulent and laminar flow can thus be distinguished if, in addition to the temperature T x at a distance X2 another temperature T Y measured at a distance of X4. This is explained below.

[0051] Figures 3 and 4 illustrate the influence of the flow on the measured values ​​of sensor 1. The measured values ​​T x - T L or T Y - T L in Kelvin on the ordinate in a logarithmic scale and the distance X2 or X3 of the second and third device 32 and 33 for determining a temperature T x on the abscissa. Figure 3 shows the temperature curve for laminar flow, and Figure 4 shows the temperature curve for turbulent flow.

[0052] As Figure 3 shows, the measured values ​​T x and T Y In the illustrated embodiment, the flow is on a straight line in laminar flow. This means that the thickness d1 of the boundary layer determined by the second device for determining a temperature at a distance X2 is just as large as the thickness d2 of the boundary layer determined by the third device for determining a temperature at a distance X3, ie

[0053] Since the thickness d of the boundary layer does not change over the entire measuring range, the case shown in section I of Figure 1 exists, namely a laminar flow with exponential temperature progression.

[0054] As Figure 4 shows, the measured values ​​T x and T Y In the illustrated embodiment, in turbulent flow, the boundary layer thickness d1 of the laminar sublayer, determined by the second device for determining a temperature at a distance X2, assumes a different value than the boundary layer thickness d2 of the turbulent boundary layer, determined by the third device for determining a temperature at a distance X3, ie

[0055] Since the convective heat transfer coefficient varies across the thickness of the boundary layer, the case shown in Section II of Figure 1 exists, namely turbulent flow. As Fig. 4 shows, this case can be modeled in a logarithmic representation with two straight lines or with two exponential curves, with the intersection of the two straight lines of the logarithmic representation indicating the thickness of the laminar sublayer.

[0056] A digital or analog arithmetic circuit can be used to evaluate the measurement signals. This circuit indicates the transition from turbulent to laminar flow with an acoustic and / or optical signal when the boundary layer thicknesses d1 and d2 deviate by a predefined value. This signal can also be used for the automatic control of a wind turbine or the attitude of an aircraft.

[0057] Of course, the invention is not limited to the illustrated embodiments. The above description is therefore not to be considered restrictive, but rather explanatory. The following claims are to be understood as meaning that a stated feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. Where the claims and the above description define "first" and "second" embodiments, this designation serves to distinguish between two similar embodiments without establishing a priority.

Claims

Claim 1. Method for determining a thickness d of a laminar or turbulent boundary layer on a surface (65) of a body (6) around which a flow occurs, which has a temperature difference to the flowing medium, wherein at least a first temperature (T o ), a second temperature (T x ) and a fourth temperature (T L ) each at a predefinable distance (X1, X2, X4) to the surface (65) of the body (6) around which the flow is measured, whereby the temperatures (T) are measured o , T x , T L ) at least a first device for determining a temperature (31) and a second device for determining a temperature (32) and a fourth device for determining a temperature (34) are used, characterized in that a third device for determining a temperature (33) is further used to determine a third temperature (T y) is measured.

2. Method according to claim 1, characterized in that the boundary layer thickness d is determined from the temperatures (T o , T x , T y , T L ), the distance X2 of the second device for determining a temperature (32) above the surface (65), the distance X3 of the third device for determining a temperature (33) and the thermal conductivity coefficient λ L of the flowing medium and λ M The sensor arrangement is determined in two ways as follows: where the flow is recognized as turbulent when d1 ≠ d2 and the flow is recognized as laminar when d1 = d2.

3. Method according to claim 1 or 2, characterized in that the distance X2, at which the second temperature (T) x) is measured from the surface (65), and is approximately 0.2 mm to approximately 2 mm, or approximately 0.3 mm to approximately 1 mm, or approximately 0.7 mm.

4. Method according to any one of claims 1 to 3, characterized in that the distance X3, at which the third temperature (T) is measured y ) is measured from the surface (65), approximately 3 mm to approximately 7 mm or approximately 5 mm.

5. Method according to any one of claims 1 to 4, characterized in that the distance X1 is approximately 0 mm, so that the first temperature (T o ) corresponds to the temperature of the surface (65).

6. Method according to one of claims 1 to 5, characterized in that the distance X4 is selected to be so large that the fourth temperature (T L) corresponds to the temperature of the environment around the body (6) around which the fluid flows.

7. Method according to any one of claims 1 to 6, characterized in that the distance X4 is between about 9 mm and about 20 mm or between about 10 mm and about 14 mm or between about 11 mm and about 16 mm.

8. Method according to any one of claims 1 to 7, characterized in that the first device for determining a temperature (31), the second device for determining a temperature (32), the third device for determining a temperature (33), and the fourth device for determining a temperature (34) are formed by thermocouples, wherein a first thermoelectric voltage U1 is measured between the first and the fourth device for determining a temperature (31, 34). a second thermoelectric voltage U2 is measured between the second and fourth devices for determining a temperature (32, 34) and a third thermoelectric voltage U3 is measured between the third and fourth devices for determining a temperature (33, 34).

9. Method according to one of claims 2 to 8, characterized in that the parameter is determined by a calibration measurement.

10. Device (1) for determining a thickness d of a laminar or turbulent boundary layer on a surface (65) of a body (6) around which a flow occurs and which has a temperature difference to the flowing medium, comprising at least a first device for determining a temperature (31) and a second device for determining a temperature (32) and a fourth device for determining a temperature (34), each of which is arranged at a different predefinable distance (X1, X2, X4) to the surface (65) of the body (6) around which a flow occurs, characterized in that the device (1) further comprises a third device for determining a temperature (33), which is arranged at a third distance (X3) to the surface (65) of the body (6) around which a flow occurs.

11. Device according to claim 10, characterized in that the distance X2, at which the second temperature (T) is determined x) measured from the surface (65), is approximately 0.2 mm to approximately 2 mm or approximately 0.3 mm to approximately 1 mm or approximately 0.7 mm and / or that the distance X3, at which the third temperature (T y) is measured from the surface (65), is approximately 4 mm to approximately 7 mm or approximately 5 mm, and / or that the distance X4 is between approximately 9 mm and approximately 20 mm, or between approximately 10 mm and approximately 14 mm, or between approximately 11 mm and approximately 16 mm.

12. Device according to one of claims 10 or 11, characterized in that the first device for determining a temperature (31), the second device for determining a temperature (32), the third device for determining a temperature (33), and the fourth device for determining a temperature (34) are formed by thermocouples. 13.Device according to one of claims 10 to 12, characterized in that at least the second, third and fourth devices for determining a temperature (32, 33, 34) each consist of a thermocouple formed from a common first wire (20) made of a first material and at least one further wire (22, 23, 24) made of a second material, which is connected to the first wire (20) at a contact point, wherein the wires (20, 22, 23, 24) each extend from the surface (65) into the half-space adjacent to the surface and have a diameter of about 300 μm or less.

14. Device according to one of claims 10 to 13, characterized in that the first, second, third and fourth wire (20, 22, 23, 24) each have a diameter of about 50 μm to about 250 μm or of about 50 μm to. approximately 150 μm or from approximately 75 μm to approximately 125 μm or from approximately 90 μm to approximately 110 μm or approximately 100 μm.

15. Device according to one of claims 10 to 14, characterized in that the first, second, third and fourth wires (20, 21, 23, 24) project freely into the hemisphere surrounding the surface and / or that the first, second, third and fourth wires (20, 21, 23, 24) are not connected to a support structure at least with the longitudinal section which is not in contact with the surface (65).

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