The principle of the thermal probe of anemometer
The basic principle of an anemometer is to place a thin metal wire in a fluid, apply current to heat the wire, and make its temperature higher than the temperature of the fluid. Therefore, the metal wire anemometer is called a "hot wire". When the fluid flows through the metal wire in a vertical direction, it will take away some of the heat from the wire, causing the temperature of the wire to decrease. According to the theory of forced convective heat exchange, a relationship can be derived between the heat dissipated by the hot wire Q and the velocity v of the fluid. A standard hot wire probe consists of two brackets tensioned with a short and thin metal wire. Metal wires are usually made of metals with high melting points and good ductility, such as platinum, rhodium, tungsten, etc. The commonly used wire diameter is 5 μ m and the length is 2mm; The small probe has a diameter of only 1 μ m and a length of 0.2mm.
According to different purposes, the hot wire probe can also be made into double wire, triple wire, diagonal wire, V-shaped, X-shaped, etc. In order to increase strength, metal film is sometimes used instead of metal wire. Usually, a thin metal film is sprayed on a thermally insulated substrate, called a hot film probe, as shown in Figure 2.2. The hotline probe must be calibrated before use. Static calibration is carried out in a specialized standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawing it into a standard curve; Dynamic calibration is performed in a known pulsating flow field or by adding a pulsating electrical signal to the heating circuit of an anemometer to verify the frequency response of the hot wire anemometer. If the frequency response is poor, corresponding compensation circuits can be used to improve it.
The measurement range of flow velocity from 0 to 100m/s can be divided into three sections: low speed: 0 to 5m/s; Medium speed: 5 to 40m/s; High speed: 40 to 100m/s. The thermal sensitive probe of the anemometer is used for measurements ranging from 0 to 5m/s; The rotary probe of the anemometer is ideal for measuring flow velocities ranging from 5 to 40m/s; By using a pitot tube, results can be obtained within a high-speed range. An additional criterion for correctly selecting the flow velocity probe of an anemometer is temperature, and typically the temperature range for the thermal sensor of an anemometer is around+-70C. The rotary probe of the specially designed anemometer can reach up to 350C. Pitot tubes are used for temperatures above+350C.
