An anemometer is an instrument that measures the speed of air. There are many types of it. The most commonly used in meteorological stations is the wind cup anemometer. It consists of three parabolic cone empty cups fixed on the bracket at 120° to each other to form the sensing part. The concave surfaces of the empty cups are all in one direction. The entire induction part is installed on a vertical rotating shaft. Under the action of the wind, the wind cup rotates around the shaft at a speed proportional to the wind speed. Today, let's introduce three anemometers:
1. Thermal anemometer
A tachometer that converts a flow rate signal into an electrical signal, and can also measure fluid temperature or density. The principle is that a thin metal wire (called a hot wire) that is heated by electricity is placed in the air flow, and the heat dissipation of the hot wire in the air flow is related to the flow rate, and the heat dissipation causes the temperature change of the hot wire to cause the resistance change, and the flow rate signal is converted into electric signal. It has two working modes: ①Constant flow. The current through the hot wire remains unchanged, and when the temperature changes, the resistance of the hot wire changes, and thus the voltage across the two ends changes, thereby measuring the flow rate. ② Constant temperature type. The temperature of the hot wire is kept constant, such as 150°C, and the flow rate can be measured according to the current required to be applied. The constant temperature type is more widely used than the constant flow type.
The length of the hot wire is generally in the range of 0.5 to 2 mm, the diameter is in the range of 1 to 10 microns, and the material is platinum, tungsten or platinum-rhodium alloy. If a very thin (thickness less than 0.1 micron) metal film is used to replace the metal wire, it is a hot film anemometer. In addition to the ordinary single-wire type, the hot wire can also be a combined two-wire type or three-wire type to measure the velocity components in all directions. The electrical signal output from the hot wire is amplified, compensated and digitized and then input into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement functions, such as simultaneous completion of instantaneous value and time average value, combined speed and sub-speed, turbulence degree and other turbulence parameters. Compared with the pitot tube, the hot-wire anemometer [1] has the advantages of small probe size, small interference to the flow field, fast response, and can measure unsteady flow velocity;
When using thermal probes in turbulent flow, airflow from all directions hits the thermal element simultaneously, affecting the accuracy of the measurement results. When measuring in turbulent flow, thermal anemometer flow sensors tend to have higher indications than rotor probes. The above phenomena can be observed during pipeline measurement. Depending on the design that manages the turbulence in the pipe, it can occur even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight section of the pipeline. The starting point of the straight line part should be at least 10×D before the measurement point (D=pipe diameter, in CM); the end point should be at least 4×D behind the measurement point. The fluid section must not have any obstructions (edges, re-suspensions, objects, etc.).
2. Impeller anemometer
The working principle of the impeller probe of the anemometer is based on converting the rotation into an electrical signal, first through a proximity induction head, "counting" the rotation of the impeller and generating a pulse series, and then converted by the detector to obtain the speed. value. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flow with medium and small flow velocity (such as at the pipe outlet). The small-diameter probe of the anemometer is more suitable for measuring the airflow whose cross-sectional area of the pipe is more than 100 times larger than the cross-sectional area of the probe.
3. Pitot tube anemometer
It was invented by French physicist H. Pitot in the 18th century. The simplest pitot tube has a thin metal tube with a small hole at the end as a pressure guide tube, which measures the total pressure of the fluid in the direction of the flow beam; another guide tube is drawn from the wall of the main pipe near the front of the thin metal tube. Press the tube and measure the static pressure. The differential pressure gauge is connected with the two pressure guiding pipes, and the measured pressure is the dynamic pressure. According to Bernoulli's theorem, the dynamic pressure is proportional to the square of the flow velocity. Therefore, the flow rate of the fluid can be measured with a pitot tube. After the structural improvement, it becomes a combined pitot tube, that is, a pitot-static pressure tube. It is a double-layered tube bent at a right angle. The outer sleeve and the inner sleeve are sealed, and there are several small holes around the outer sleeve. When measuring, insert this sleeve into the middle of the pipe under test. The nozzle of the inner casing is facing the direction of the flow beam, and the orifice of the small hole around the outer casing is just perpendicular to the direction of the flow beam. At this time, the flow velocity of the fluid at this point can be calculated by measuring the pressure difference between the inner and outer casings. Pitot tubes are often used to measure the velocity of fluids in pipes and wind tunnels, as well as river velocity. If the flow velocity of each section is measured according to the regulations, it can be used to measure the flow rate of the fluid in the pipeline after integration. However, when the fluid contains a small amount of particles, it may block the measuring hole, so it is only suitable for measuring the flow of particle-free fluids. Therefore, the pitot tube can also be used to measure wind speed and wind flow, which is the principle of the pitot tube anemometer.
