WO2011026445A1 - Combined measurement instrument for flowfield pressure and velocity and method thereof - Google Patents

Abstract

A combined measurement instrument for flowfield pressure and velocity is composed of a five-freedom bracket (1) with three translational freedoms and two rotational freedoms, a multi-hole pressure probe and a one-dimension hot-wire anemometer probe (4) fixed in the rotational freedom. And a combined measurement method for flowfield pressure and velocity is provided.

Description

 Combined measurement tool and method for flow field pressure and velocity

Technical field

 The invention relates to the field of parameter measurement of fluids. Specifically, a device and method for measuring the pressure and three-dimensional transient velocity of a flow field using a porous pressure probe and a one-dimensional hot wire anemometer in a flow field, including design, calibration methods, and operating procedures of a pressure probe and a hot wire anemometer .

2. Background technology

 With the continuous development of electronic and optical technologies, the measurement methods of fluid parameters have been greatly improved. For example, hot wire anemometers, laser Doppler velocimeters, particle tracers and other technologies have been widely used in the measurement of fluid velocity. Applications. However, the fluid measuring instruments described above often require large manufacturing costs, and both Doppler velocity and tracer particle velocity require tracer particles. In addition to the measurement range limited to small areas, the followability of the tracer particles is also considered.

 In many cases, especially for the measurement of the average flow velocity of fluids, porous pressure probes are still widely used due to their simple manufacturing and low cost of use. Porous Pressure Probes (including three, five, and seven holes) The basic principle for measuring three-dimensional velocity vectors is that the pressure difference measured from different orifices determines the magnitude and direction of fluid flow. The pressure probe needs to transmit the pressure wave at the measuring end point to the pressure sensor through the air duct, and the damping of the air will cause the high-frequency signal during the transmission to disappear, so that the porous pressure probe can only obtain the low-frequency average speed signal. In the case where the angle between the pressure probe and the flow direction is too large, the fluid causes flow separation on the probe, causing the measurement to fail. This is often the case with three- and five-hole probes. In contrast, the seven-hole pressure probe has a high application value. The six dynamic pressure holes can be evenly distributed around the central static pressure hole, and a partitioning strategy can be adopted to prevent flow separation and make the measurement of a larger flow angle more reliable. In addition, the measurement circuit required to use the pressure probe has fewer heat generating devices and the data processing method is simpler.

Hot wire anemometers, including hot wire probes and measuring circuits, are a common tool for measuring fluid transient velocity. It is a method for determining the flow velocity by using a fluid to flow through a hot wire to cause a heat loss to the hot wire, thereby causing a change in the resistance of the measuring circuit. Especially with fast response and high accuracy, it is often used for transient speed measurement. Usually, the measurement of the three-dimensional flow field requires the use of a three-dimensional hot wire anemometer, and the three-dimensional hot wire probe is expensive to manufacture, and the hot wire is easily damaged in practical applications. Especially the calibration of 3D hotlines is very complicated and time consuming. If applied to large flow angle measurements, it is necessary to calibrate more measurement points and speed conditions, resulting in too long calibration time. The overheating and drift of the electronic instrument used for measurement makes the instrument unusable. Nevertheless, the hot wire anemometer is still the most effective transient flow measurement tool, especially the one-dimensional hot wire anemometer for measuring one-dimensional flow velocity. The structure is simple and the production cost is low. The general cost is only 1% of the three-dimensional hot wire anemometer. Thus it is widely used.

3. Summary of the invention

 The present invention utilizes the respective advantages of a porous pressure probe and a hot wire anemometer to design a combined measurement tool and method for pressure and velocity of a fluid, which combines a porous pressure probe with a one-dimensional hot wire probe to measure fluid flow (including large flow). Angle) The pressure, instantaneous velocity involved in the device, the calibration process, and the operating process.

 To complete the measurement, you first need a five-degree-of-freedom bracket driven by a stepper motor. In addition to moving in Cartesian coordinates, there should be two directions of rotation at the end where the measuring probe is mounted, which can produce rotational freedom.

«° and . As shown in Figure 1, the two degrees of freedom of rotation of one end of the measuring probe mounted on the five-degree-of-freedom bracket are shown in the schematic. Among them, the measuring probe 1 refers to a porous rod-shaped porous pressure probe or a one-dimensional hot-wire wind probe, which is respectively mounted on the rotational freedom of the five-degree-of-freedom bracket at different stages of measurement. The figure also shows the measurement relationship of the angle of rotation degrees generated in the Cartesian coordinate system (X, y, z). Reference numeral 2 in Fig. 1 refers to the measuring end of the probe, that is, the measuring point of the flow field. Figure 2 shows the layout of the measurement tool when it is in use. The five-degree-of-freedom bracket 1 is placed downstream of the incoming flow direction 3. The incoming flow direction is generally set to the horizontal direction. The translational degrees of freedom 6, 7, and 8 are driven by the three stepping motors in the x, z, and y directions of the Cartesian coordinate system. At the end of the bracket in the X direction near the direction of the flow, the bracket produces rotational degrees of freedom 2, 5, which are driven by two other stepper motors.

 The object of the present invention is to utilize the advantages of a multi-hole pressure probe and a one-dimensional hot wire probe in combination, instead of a three-dimensional hot wire anemometer commonly used for measuring three-dimensional transient flow rates. The core technology is the process of measuring the flow field using a measuring probe. Figure 3 is a flow chart of the measurement method. The figure shows that the invention first uses a porous pressure probe to obtain the flow direction of all measurement points, and then uses a one-dimensional hot wire probe to perform a second measurement at these measurement points, this time measuring the transient velocity along the flow direction. Calibration is done in a one-dimensional wind tunnel. The measurement process is divided into four steps.

 The first step of the measurement begins with the calibration of the pressure probe. For each calibration angle, the steps are:

 1. Position the probe with the freedom of rotation of the five-degree-of-freedom bracket according to the given calibration angle;

 2. Measure the pressure of multiple holes to obtain the pressure coefficient;

 3. Check if the flow at the measuring end is separated.

The second step of the measurement is the calibration of the one-dimensional hot wire anemometer. The hot wire probe is placed along the flow direction of the fluid (the direction of the flow in the wind tunnel, generally horizontal). The third step in the measurement is to obtain the flow angle of the measuring point with a pressure probe. The steps are:

 1. Adjust the pressure probe to the flow direction (usually horizontal);

 2. Measure the pressure value at the measuring point to obtain the pressure coefficient;

 3. Pressure probe partition selection;

 4. The flow angle is obtained by interpolation from the calibration value;

 5. Record the spatial coordinates and flow angle of the measurement points.

 The fourth step in the measurement is to replace the pressure probe with a hot wire probe for transient speed measurement. The steps are:

 1. Drive the five-degree-of-freedom bracket to position the hot wire probe along the local flow direction according to the spatial coordinates and flow angle of the measuring point;

 2. Local one-dimensional transient velocity measurement.

4. BRIEF DESCRIPTION OF THE DRAWINGS

 Figure 1 Schematic diagram of rotational freedom

 Figure 2 Measurement tool layout

 The number in the figure indicates:

1. Five-degree-of-freedom bracket 2. Rotational freedom "_; 3. Flow direction; 4. Porous pressure probe or hot wire probe;

5. Rotational freedom yT; 6. Translational freedom along the X direction; 7. Translational freedom along the z direction;

 8. Translational degrees of freedom along the y direction;

 Figure 3 Flow chart of measurement method

 Figure 4 Schematic diagram of the measuring end of the seven-hole pressure probe

 Figure 5 Measurement plan

6. Specific implementation

 The invention will be further described below in conjunction with the drawings and the examples of the description. This embodiment is a three-dimensional transient velocity of the flow field of an object in a horizontal wind tunnel using the combined pressure and velocity measuring tools and methods proposed by the present invention.

The measuring device is arranged as shown in Figure 2, with the device as a whole placed downstream of the flow. The five-degree-of-freedom bracket has three translational degrees of freedom and two degrees of freedom of rotation. The freedom of motion is done by a computer-controlled five stepper motor mounted on a five-degree-of-freedom bracket. The pressure probe used is a seven-hole pressure probe in the form of a thin rod. Probe tip 5毫米的球体。 The cone is 30 degrees, the cone volume is smaller than the diameter of 3. 5mm. The partitioning strategy can be illustrated from the schematic of the measuring end of the seven-hole pressure probe (Figure 4). The middle hole is numbered 7 and the surrounding holes are from 1 to 6. The seven wells are divided into six zones, each zone having four holes, for example, 7-4-3-5; 7-3-2-4; 7-2-1-3; 7-1-2-6 7-6-1-5; 7-5-4-6. The measurement is effective as long as there is fluid adhesion in a certain area. Compare the three- and five-hole probes, each of which must be in the active area with fluid attachment. Thus, the seven-hole pressure probe further increases the range of measurement angles. There are seven thin tubes connected to the tail of the seven-hole pressure probe, which are connected from the wall of the wind tunnel to the pressure sensor with the same number of holes, and then connected to the signal amplifier and data acquisition system. Hot wire anemometers are one-dimensional hot wire probes that are connected by wires to measurement circuits, including bridges, analog to digital converters, signal amplifiers, and data acquisition systems. Among them, the bridge and the analog-to-digital converter are easy to generate heat, but compared with the measurement circuit of the three-dimensional hot wire anemometer, the measurement circuit of the one-dimensional hot wire anemometer is simple, the heat generation is small, and the instrument does not generate drift phenomenon. Figure 5 shows a diagram of the measurement scheme for this example. The figure shows that the porous pressure probe and the one-dimensional hot wire probe are respectively connected to the pressure sensor and the analog-to-digital converter, and the amplifying circuit, and then through the data acquisition system, that is, through the data acquisition board, the data is input into the computer microprocessor and analyzed. The processing unit, then the microprocessor drives the stepper motor on the five-degree-of-freedom bracket, positions the probe position and angle, and performs the combined measurement. When measuring, the static pressure of the wind tunnel can be obtained from the static pressure hole on the wall surface, and the total pressure ptot is determined by the state of the wind tunnel entrance. The wind tunnel produces a horizontal one-dimensional flow, and the flow velocity can be adjusted from subsonic to supersonic speed. Under different wind speeds, the total pressure of the wind tunnel does not change, and the dynamic pressure changes.

The first step in the measurement process is the calibration of a seven-hole pressure probe. Adjust the rotational freedom of the seven-hole pressure probe at a fixed point in the wind tunnel with a five-degree-of-freedom bracket. The angle of the calibration can be from -50° to 50° with an interval of 5°. A total of about 400 The angle of the calibration. For each angle represented by a different (", ), which needs to be calibrated, corresponding to the fourteen coefficients, which are calculated from the seven pressure values obtained from the seven-hole pressure probe. They are, the radial pressure coefficient C and the tangential pressure coefficient are expressed as

The subscript of the variable in the formula indicates the label of the hole, which is also the area code, i=l, 2, . . . 6 (seven holes are divided into six areas). If i=l, then jl=6; j2=2, if i=6, then jl=5; j2=l, when equal to others, then jl=i+l; j2=i~h for small angles of flow direction (When the flow angle is less than 30°), the fluid is completely attached to the probe, and the radial pressure coefficient and the tangential pressure coefficient can be obtained by the following formula. Cpr ₇ = Cpta + Cptb),

 among them

Cpta =

 It can be seen from this process that there is no total pressure and static pressure value of the wind tunnel related to the incoming flow velocity in the formula for calculating the pressure coefficient. Therefore, the calibration of the flow angle is independent of the incoming flow velocity, and no other speed conditions are considered. . If it takes 20 seconds at each calibration angle, the full range of 400 angle calibrations takes a total of 2. 2 hours. After the calibration is completed, calibration data is formed, each of which has a one-to-one correspondence with its pressure coefficient in each partition. If a new pressure coefficient is measured at an unknown angle, the angle value can be obtained by two-dimensional interpolation in the calibrated pressure coefficient data.

 The second step of the measurement is to calibrate the one-dimensional hot wire probe in the wind tunnel. This process is a speed calibration and the calibration method is well known. At the time of calibration, the probe with a thin rod shape is placed in the direction of flow. This process is quick and easy. According to the calibration of 20 speed conditions, each working condition takes 20 seconds, and it takes about 7 minutes in total. The calibration process, which measures the first and second steps, does not exceed 3 hours in total.

 The third step of the measurement is to measure the flow direction of the point. Use a five-degree-of-freedom bracket to adjust the seven-hole pressure probe to the same orientation along the flow direction of the wind tunnel, ie ° = 0, ° = 0. Seven pressure values were obtained at each measurement point. Comparing the seven pressures, the highest pressure hole determines the partition selection. There are four pressure factors in each zone. Comparing A with the other three pressures, if the difference is less than the set threshold, it means that the flow is separated and the measured angle is outside the calibration range.

 Under normal circumstances, the angle is interpolated at the measurement point using the measured pressure coefficient and the calibrated pressure coefficient data to obtain the flow direction of the measurement point, that is, the measurement point obtained by interpolation (", yT). The spatial position, flow angle and pressure coefficient of the measuring point are recorded as a file using the data analysis and processing unit of Figure 5.

 The fourth step in the measurement is to measure the transient velocity with a one-dimensional hot wire probe. It is controlled by the microprocessor of the computer, and the point is found according to the recorded spatial position at each measuring point with a five-degree-of-freedom bracket, and the hot-wire probe is repositioned for the second measurement. At the measuring point, the five-degree-of-freedom bracket is driven by the stepping motor to adjust the angle of the hot wire probe until it is in the direction of fluid flow, ie known (", ), processed into a local one-dimensional flow, and then subjected to transient measurement. , to get the local transient flow velocity, it is easy to find the three-dimensional component of the transient velocity

u - Γ cos a cos β; ν = F sin a; w = -Γ cos a sin β ₀ Through such a measurement process, the pressure of the flow field and the three-dimensional transient velocity field are obtained by a combination of a porous pressure probe and a one-dimensional hot wire probe. Specifically, two measurements were made for each measurement point of the flow field. The flow direction of the flow field was obtained with a porous pressure probe for the first time, and the transient velocity of the flow field was obtained for the second time with a one-dimensional hot wire probe. The total measurement time did not increase significantly, as the measurement time of the pressure probe and the one-dimensional hot wire was much lower than that of the three-dimensional hot wire. Although it is necessary to position the probe twice, the process is done by a computer-controlled five-degree-of-freedom bracket. Positioning is also required when using a three-dimensional hot wire anemometer, so the spatial accuracy of the two-position method is not affected. If this embodiment uses a three-dimensional hot wire anemometer, the calibration range of the same angle, the calibration time of each angle, and the calibration range of the speed conditions, that is, according to 400 calibration angles, each calibration angle takes 20 seconds, 20 speed conditions. To calculate, only the calibration time is required: 20 seconds x 20 x 400 = 44 hours. The use of the present invention makes the measurement process simple and fast. As mentioned above, the calibration time does not exceed 3 hours, saving 90% of the time. Especially for the measurement of large flow angle, the hot line calibration time and process are greatly shortened and simplified, and the error caused by long-time overheating of the measuring instrument is reduced, and the measurement result is more reliable. The manufacturing cost of a porous pressure probe is about 10% of that of a one-dimensional hot wire probe, while the cost of a one-dimensional hot wire probe is about 1% of that of a three-dimensional hot wire probe. The five-degree-of-freedom support proposed by the present invention is also necessary for the calibration and measurement of the three-dimensional hot wire, so that the three-dimensional transient measurement using a porous pressure probe and a one-dimensional hot wire anemometer is generally used to reduce the three-dimensional hot wire anemometer method. The manufacturing cost is over 95%. Since the one-dimensional hot wire probe is downstream in the flow direction during calibration and measurement, the interference of the probe on the flow field is minimized. If it is a supersonic flow field, there is no interference. This invention also mentions

Claims

A combined measuring tool for flow field pressure and velocity, characterized by a five-degree-of-freedom support having three translational degrees of freedom, two rotational degrees of freedom, and a rotation mounted on a five-degree-of-freedom bracket A porous pressure probe on the degree of freedom and a probe of a one-dimensional hot wire anemometer. The porous pressure probe of the combined measuring tool for flow field pressure and velocity according to claim 1 is three-hole, five-hole, or seven-hole, and the probe has a thin rod shape as a whole and a thirty-degree end. Conical, pressure sensor with the same number of holes in the tail. A hot wire anemometer in a combined measurement tool for flow field pressure and velocity according to claim 1 comprising a one-dimensional hot wire probe in the shape of a thin rod, and a matching bridge, analog to digital converter, Signal amplifier, data acquisition system. A combined measurement method of flow field pressure and velocity, characterized in that the measurement comprises the following steps: first, calibration of the porous pressure probe, followed by calibration of the one-dimensional hot wire probe, and measurement of each measurement point in the flow field by using a porous pressure probe The pressure and three-dimensional flow direction, and finally the transient velocity in the flow field is measured by the one-dimensional hot wire probe along the flow direction of each measurement point. A combined measurement method of flow field pressure and velocity according to claim 4, wherein when the transient flow velocity is measured by the one-dimensional hot wire probe, the one-dimensional hot wire probe is completed by the five-degree-of-freedom support along the flow direction of the measuring point. Positioning on. A combined measurement method of flow field pressure and velocity according to claim 4, wherein the one-dimensional hot wire probe is placed in the flow direction when the one-dimensional hot wire probe is calibrated.