A method for visually measuring wind speed in airflow fields

Abstract

This invention discloses a method for visually measuring the wind speed of an airflow field. The measurement method includes six steps: placing a constant-power electric heating plate with a temperature higher than that of the airflow in the path of the airflow to be measured; using a thermal imager to photograph the airflow in front of the constant-power electric heating plate to obtain a thermal image of the temperature distribution of the constant-power electric heating plate during heat exchange; exporting the temperature values ​​from the image; using a hot-wire anemometer to detect the air speed at a specific point; fitting a correlation equation using the temperature and velocity values ​​at the specific point; exporting the temperature and coordinate values ​​of N or all pixels in the temperature distribution image of the electric heating plate, and substituting them into the correlation equation to obtain the velocity values ​​at each point in the airflow field. This allows for a direct visualization of the velocity distribution and range of the measured airflow field. This invention can accurately acquire the velocity field of the airflow, and the method is applicable to the velocity field detection of various complex airflows, and can accurately determine the diffusion range of a gas jet.

Description

Technical Field

[0001] This invention belongs to the field of thermal measurement technology, specifically relating to a method for visually measuring jet wind speed using infrared technology. Technical Background

[0002] The need to measure each type of air jet individually with a point anemometer is not conducive to quickly and conveniently obtaining the velocity field of the air jet. The most effective method is to use a visual wind speed measurement method based on infrared thermal imaging technology.

[0003] Currently, there are various technologies for measuring airflow, generally categorized into point velocimetry and area velocimetry. Point velocimetry technologies include bituminous anemometers based on differential pressure; hot-wire and hot-wire anemometers based on heat transfer principles; and laser Doppler velocimetry (LDV). Area velocimetry technologies mainly include particle tracking velocimetry (PTV), particle stripe velocimetry (PSV), and particle image velocimetry (PIV), all of which are based on optical principles. For three-dimensional airflow, point velocimetry struggles to accurately and comprehensively represent the velocity field within the airflow. Furthermore, typical point velocimeters have a range limited to less than 50 m / s, making them unsuitable for measuring high-speed airflow. While optical-based surface measurement techniques can obtain the velocity field of airflow, their implementation is extremely difficult. It requires pre-scattering tracer particles with good tracking and reflectivity in the fluid, illuminating a cross-sectional area of ​​the flow field with a laser sheet, and continuously capturing two or more exposures of particle images using an imaging recording system. The captured PIV images (a single PIV instrument can cost hundreds of thousands of yuan) are then analyzed using image cross-correlation methods to obtain the average displacement of the particles in each small region, thus determining the velocity distribution. Furthermore, it has certain requirements regarding operating conditions; for example, surface measurement instruments are relatively complex, and in daylight, bright light cannot capture particle images on the laser sheet.

[0004] Infrared thermal imagers can quickly and intuitively detect the temperature distribution on an object's surface, also known as the temperature field. While any image is composed of countless pixels, each pixel in an infrared thermal image represents a temperature point. Currently, a method for measuring airflow velocity on the hot surface of a building radiator using infrared thermal imaging is relatively close to the technical solution of this invention (CN).

[0005] (106124799A) This method uses an infrared thermal imager to capture images and videos, obtains frame images, filters the frame images to obtain the correlation between two adjacent frame images, and finally calculates the velocity vector. The drawback of this method is that it calculates the airflow velocity vector by measuring the temperature of the heat sink surface, but it cannot obtain the magnitude of the velocity field of the airflow itself.

[0006] Therefore, this invention proposes a method for visually measuring the wind speed of an airflow field using infrared thermal imaging technology. The difference lies in using a constant-power heating plate placed behind the airflow, and obtaining a two-dimensional temperature field showing how the heating plate temperature changes with the airflow velocity using an infrared thermal imager. Then, a simple experiment is conducted to obtain the relationship between airflow temperature and velocity, thus yielding a visualized velocity field of the airflow. Summary of the Invention

[0007] To address the problems existing in current airflow temperature detection methods, the purpose of this invention is to provide a method for visually measuring wind speed in an airflow field. Infrared thermal imaging technology is used to obtain a temperature cloud map of a heating plate, which is then converted into a fluid velocity cloud map using a formula. This visualizes the airflow velocity, facilitating accurate determination of the wind speed range.

[0008] To achieve the objective of this invention, the technical solution adopted is to place a constant power electric heating plate on the back of the airflow field to be measured. The specific steps of the measurement method are as follows:

[0009] (1) Arrange the electric heating plate that generates heat with constant power and uniform heat distribution in the airflow path.

[0010] (2) When the air is flowing, use an infrared thermal imager to take pictures of the constant power electric heating plate to obtain an infrared thermal image of the temperature distribution when the air flow exchanges heat with the constant power electric heating plate.

[0011] (3) Each pixel in the infrared thermal image is a temperature point. The temperature value of each pixel in the infrared thermal image is obtained and exported to an Excel spreadsheet. At the same time, the temperature values ​​of 3-5 specific points are selected in the infrared thermal image along the direction of decreasing airflow velocity, and the spacing and location of the temperatures of the 3-5 specific points are recorded.

[0012] (4) A temperature distribution image is formed based on the temperature value of each pixel.

[0013] (5) Use a multi-point hot-wire anemometer to detect the air speed at the 3-5 specific points mentioned in step 3.

[0014] (6) Using the temperature value of the heating plate and the airflow velocity value at a specific point, the formula v = At ​​for velocity and temperature is fitted. 3 -Bt 2 +Ct-D, where: A, B, C, and D are coefficients in the polynomial; t represents temperature; and v represents the velocity to be determined.

[0015] (7) Based on the infrared thermal image of the temperature distribution of the electric heating plate obtained in step 2, export the temperature and coordinate values ​​of N or all of the pixels and substitute them into the correlation formula between temperature and velocity to obtain the velocity value of each point in the air flow field.

[0016] (8) The velocity distribution image of the airflow is finally obtained from the velocity values ​​of each point in the airflow field, which can intuitively show the velocity distribution and range of the measured airflow field.

[0017] Compared with current flow field measurement technologies, this invention, taking reference (CN 106124799 A) as an example, has the following technical solution:

[0018] (1) The airflow field on the hot surface of a building radiator is tested using an infrared thermal imager, including velocity measurement. This is not the airflow field itself.

[0019] (2) This method obtains frame images by taking thermal video with an infrared thermal imager; then obtains the image after filtering the frame frequency; obtains the correlation between "two adjacent frame images" through a calculation formula; and finally obtains the velocity vector and draws the velocity vector diagram, etc.

[0020] (3) The displacement l of the surface brightness temperature is obtained by calculating the correlation between two adjacent frames, and the velocity vector V is calculated based on the displacement difference of the surface brightness temperature. This method is not only cumbersome, but also introduces a considerable random error due to filtering, conversion and formula calculation.

[0021] The innovation of this invention lies in placing a constant-power heating plate with a temperature higher than that of the airflow behind the airflow to be measured. This is because the temperature difference between the airflow and the ambient temperature is relatively small. Using a background plate increases the temperature difference between the airflow in front of the background plate and the surrounding environment, thus allowing the thermal imager to capture a very clear thermal image of the airflow after it passes through the background plate. Thermal imagers measure the surface temperature of objects; without a background plate, it is difficult to distinguish the temperature difference between the airflow and the environment in the thermal image (the temperature field of the airflow and the ambient environment).

[0022] Regarding the correlation between temperature and velocity points, all temperature values ​​in the thermal image are exported. Simultaneously, several specific temperature points are selected along the direction of decreasing airflow velocity in the thermal image, and the distances between these points and their geometric positions are recorded. Based on this, a multi-point hot-wire anemometer is used to actually detect the airflow velocity values ​​at these specific points. Then, a correlation is fitted based on the measured temperature and velocity values ​​to finally obtain an airflow velocity distribution image.

[0023] The features and beneficial effects of this invention are as follows:

[0024] (1) It can accurately detect the temperature field of the heating plate when the airflow exchanges heat with the constant power heating plate, and then obtain the airflow velocity through conversion. This method is applicable to various complex airflows. (2) The operation procedure is simple and the measurement accuracy is high. Using a hot-wire anemometer to measure the actual wind speed ensures that the measurement results are closer to the actual results. (3) Compared with the current testing technology, the cost of the instruments and equipment used is lower, which makes up for the shortcomings of the point velocity measurement technology LDV and the cumbersome steps and expensive equipment investment of PIV measurement. (4) This invention is not affected by the environment and any airflow, and can accurately determine the diffusion range of the gas jet. Attached Figure Description

[0025] Figure 1 This is a flowchart of the measurement method steps of the present invention.

[0026] Figure 2 This is a schematic diagram of the experimental apparatus used in the measurement method of this invention. In the diagram: 1-support and base, 2-constant power heating plate, 3-gas supply pipe.

[0027] Figure 3 This invention uses an infrared thermal imager to capture an infrared thermal image of the airflow field. Different colors on the infrared image represent different temperatures: high-temperature areas are displayed as white, followed by red, yellow, green, and blue, with the lowest-temperature areas displayed as purplish-black.

[0028] Figure 4 This invention is based on Figure 3 The acquired temperature region cloud map shows that the airflow temperature exhibits an elliptical diffusion trend, with a larger temperature distribution in the middle and smaller temperatures at both ends.

[0029] Figure 5 This is a diagram of the experimental setup for the method of this invention.

[0030] Figure 6 It is an airflow velocity distribution cloud map obtained using the measurement method of this invention. Detailed Implementation

[0031] The principles and steps of the present invention will be described in detail below with reference to the accompanying drawings and embodiments. However, the scope of the present invention is not limited to the following embodiments.

[0032] A method for visually measuring wind speed in an airflow field, equipped with an infrared thermal imager and a multi-point hot-wire anemometer, involves placing a background plate with a temperature lower than the air temperature behind the airflow field to be measured. The specific steps of the measurement method are as follows:

[0033] (1) Arrange the electric heating plate that generates heat with constant power and uniform heat distribution in the airflow path.

[0034] (2) When the air is flowing, use an infrared thermal imager to take pictures of the constant power electric heating plate to obtain an infrared thermal image of the temperature distribution when the air flow exchanges heat with the constant power electric heating plate.

[0035] (3) Each pixel in the infrared thermal image is a temperature point. The temperature value of each pixel in the infrared thermal image is obtained and exported to an Excel spreadsheet. At the same time, the temperature values ​​of 3-5 specific points are selected in the infrared thermal image along the direction of decreasing airflow velocity, and the spacing and location of the temperatures of the 3-5 specific points are recorded.

[0036] (4) A temperature distribution image is formed based on the temperature value of each pixel.

[0037] (5) Use a multi-point hot-wire anemometer to detect the air speed at the 3-5 specific points mentioned in step 3.

[0038] (6) Using the temperature value of the heating plate and the airflow velocity value at a specific point, the formula v = At ​​for velocity and temperature is fitted. 3 -Bt 2 +Ct-D, where: A, B, C, and D are coefficients in the polynomial; t represents temperature; and v represents the velocity to be determined.

[0039] (7) Based on the infrared thermal image of the temperature distribution of the electric heating plate obtained in step 2, export the temperature and coordinate values ​​of N or all of the pixels and substitute them into the correlation formula between temperature and velocity to obtain the velocity value of each point in the air flow field.

[0040] (8) The velocity distribution image of the airflow is finally obtained from the velocity values ​​of each point in the airflow field, which can intuitively show the velocity distribution and range of the measured airflow field.

[0041] Example

[0042] There are no special requirements for the technical specifications of the infrared thermal imager and the multi-point hot-wire anemometer; ordinary instruments used in testing experiments are sufficient. The testing process is as follows: Figure 1 As shown.

[0043] The experimental apparatus used in the measurement method of this invention is as follows: Figure 2 As shown. The heating plate 2 is fixed to the base of the bracket 1, and the air supply pipe 3 is fixed to the upper end of the bracket. The heating plate is heated with a constant power to serve as a background plate, and the uniformity of the heating plate temperature can be monitored using a thermal imager; air is passed through the air supply pipe and flows over the heating plate.

[0044] Infrared images of the airflow on the surface of the heating plate are captured using a fixed or handheld infrared thermal imager. Figure 3This is an infrared thermal image of the airflow field acquired using a thermal imager. Different colors in the image represent different temperatures, with the order of high to low temperature regions being white, red, yellow, green, and blue, and the lowest temperature region appearing as purplish-black. It can be seen that the highest temperature in the image is 25℃, and the lowest temperature is 11℃. Furthermore, the higher the jet velocity, the lower the temperature of the constant-power heating plate. As the jet diffuses, the temperature of the heating plate gradually increases, and the jet temperature gradually decreases along the direction of jet diffusion.

[0045] Export the temperature value of each pixel in the acquired thermal image to an Excel spreadsheet. Figure 4 It is based on Figure 3 The temperature values ​​of the pixels obtained from the heat map are used to form a temperature distribution cloud map. It can be seen that the airflow temperature exhibits a diffusion trend similar to an ellipse, with the temperature being higher in the middle and lower at both ends.

[0046] This embodiment is in Figure 3 Five specific points were selected along the direction in which the airflow velocity decreased. Figure 5 As shown), export and record the temperature values ​​and locations of these 5 specific points. The distance between the background board and the air supply pipe outlet is approximately 2 mm. Using a multi-point hot-wire anemometer, detect the airflow velocity at the above-mentioned specific points, and fit the temperature and velocity values ​​at these specific points to derive a correlation between temperature and velocity:

[0047] v = 0.178t 3 -8.6366t 2 +140.66t-764.2

[0048] according to Figure 3 The temperature distribution of the airflow during heat exchange with the heating plate (at constant power) is displayed. The temperature and coordinate values ​​of all pixels are derived and substituted into the correlation between velocity and temperature to obtain the velocity value of each point in the airflow field. Figure 6 It is an airflow velocity distribution cloud map obtained based on the correlation between temperature and velocity. The airflow velocity ranges from 0 to 13 m / s, and the diffusion range of the airflow velocity is consistent with the diffusion range of the temperature cloud map, showing an elliptical diffusion trend. The wind speed value at any location can be read from the map.

[0049] Infrared thermal imaging technology is used to obtain the temperature distribution of the heating plate when the airflow exchanges heat with the constant power heating plate. Then, through experimental testing and correlation, it is converted into a velocity distribution map of the fluid, thereby realizing the visualization of the airflow velocity and facilitating the accurate determination of the wind speed range.

Claims

1. A method for visually measuring wind speed in an airflow field, equipped with an infrared thermal imager and a multi-point hot-wire anemometer, characterized in that: A constant-power electric heating plate is placed behind the airflow field to be measured, so that the airflow just passes over the heating plate. The specific steps of the measurement method are as follows: (1) Arrange the electric heating plate that generates heat with constant power and uniform heat distribution in the airflow path; (2) When the air is flowing, use an infrared thermal imager to take pictures of the constant power electric heating plate to obtain an infrared thermal image of the temperature distribution when the air flow exchanges heat with the constant power electric heating plate. (3) Each pixel in the infrared thermal image is a temperature point. The temperature value of each pixel in the infrared thermal image is obtained and exported to an Excel spreadsheet. At the same time, the temperature values ​​of 3-5 specific points are selected in the infrared thermal image along the direction of decreasing air flow speed, and the spacing and location of the temperatures of the 3-5 specific points are recorded. (4) Based on the temperature value of each pixel, a temperature distribution image is generated; (5) Use a multi-point hot-wire anemometer to detect the air velocity at the 3-5 specific points mentioned in step 3; (6) Using the temperature value of the heating plate and the airflow velocity value at a specific point, the formula v = At ​​for velocity and temperature is fitted. 3 -Bt 2 +Ct-D, where: A, B, C, and D are coefficients in the polynomial; t represents temperature; v represents the velocity to be determined; (7) Based on the infrared thermal image of the temperature distribution of the electric heating plate obtained in step 2, export the temperature and coordinate values ​​of N or all of the pixels and substitute them into the correlation between temperature and velocity to obtain the velocity values ​​of each point in the air flow field. (8) The velocity distribution image of the airflow is finally obtained from the velocity values ​​of each point in the airflow field, which can intuitively show the velocity distribution and range of the measured airflow field.

2. A method for visually measuring airflow field wind speed according to claim 1, characterized in that: The two-dimensional temperature distribution infrared image acquired by the infrared thermal imager is the temperature field of the constant power electric heating plate under the condition of heat exchange between the airflow and the electric heating plate. The change in its temperature distribution is related to the speed of the airflow.

3. A method for visually measuring airflow field wind speed according to claim 1, characterized in that: The heating plate is a copper plate, with a resistance wire embedded in it as a heating element. The copper plate is then encapsulated with a material having an emissivity ε≥90%.

4. A method for visually measuring wind speed in an airflow field according to claim 1, characterized in that: The temperature of the constant power heating plate must be higher than the temperature of the fluid being tested.

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