Hot-wire anemometer (HWA) was invented in the 1920s. Its basic principle is that a thin metal wire is placed in a fluid, and current is applied to heat the metal wire to make its temperature higher than that of the fluid, so it is called a "hot wire". When the fluid flows through the wire in the vertical direction, it will take away part of the heat of the wire and make the temperature of the wire drop. According to the theory of forced convection heat exchange, the relation (D3.4.4a) can be derived between the heat loss q of the hot wire and the fluid velocity v.
The above formula is called the gold (L.V.King, 19 14) formula, where R and I are the resistance of the hot filament and the current intensity flowing through it, δ T is the temperature difference between the hot filament and the fluid, and A and B are the physical constants related to the fluid and the hot filament. Considering the resistance-temperature characteristics of hot wire materials, equation (D3.4.4a) can be changed to (D3.4.4b).
In the above formula, u is the output voltage of the hot wire, and A' and B' are physical constants related to the temperature coefficient of the hot wire resistance, which are determined by experiments. In this way, the flow can be determined by measuring the voltage across the hot wire.
As shown in Figure 2. 1, the standard hot-wire probe consists of two brackets and a short thin wire. Metal wires are usually made of metals with high melting point and good ductility, such as platinum, rhodium and tungsten. The commonly used wire diameter is 5μm and 2 mm; In length; The smallest probe diameter is only 1μm and the length is only 0.2 mm According to different uses, hot-wire probes are also made into double-wire, three-wire, diagonal, V-shaped and X-shaped. In order to increase strength, metal films are sometimes used instead of metal wires. Usually, a thin metal film is sprayed on the insulating substrate, which is called a hot film probe, as shown in Figure 2.2. Hot-wire probes must be calibrated before use. Static calibration is carried out in a special standard wind tunnel, measuring the relationship between flow velocity and output voltage, and drawing a standard curve; Dynamic calibration is carried out in the known pulsating flow field, or pulsating electrical signals are added to the heating circuit of the anemometer to check the frequency response of the hot-wire anemometer. If the frequency response is not good, the corresponding compensation circuit can be used to improve it.
The advantages of hot-wire anemometer are (1) small volume and little interference to flow field; (2) Wide application range. It can be used for both gas and liquid, and can be used for subsonic, transonic and supersonic flow of gas; In addition to measuring the average velocity, it can also measure the pulsation value and turbulence amount; In addition to measuring unidirectional motion, velocity components in multiple directions can also be measured at the same time. (3) High frequency response, up to 1 MH z(4) High measurement accuracy and good repeatability. The disadvantage of hot wire anemometer is that the probe interferes with the flow field to some extent, and the hot wire is easy to break.
The main purpose of hot-wire anemometer is (1) to measure the average velocity and direction. (2) Measure the pulse speed and frequency spectrum of the incoming flow. (3) Measuring Reynolds stress in turbulent flow, and the velocity correlation and time correlation of two points. (4) Measurement of wall shear stress (usually carried out with a hot film probe placed flush with the wall, and the principle is similar to hot wire speed measurement). (5) Measure the temperature of the fluid (measure the change curve of the probe resistance with the temperature of the fluid in advance, and then determine the temperature according to the measured probe resistance. In addition, many professional applications have been developed.