1. Thermal anemometer
A speed measuring instrument that converts a flow rate signal into an electrical signal that also measures fluid temperature or density. The principle is that a thin wire (heating wire) that is heated by electricity is placed in the airflow, and the heat dissipation amount of the hot wire in the airflow is related to the flow velocity, and the heat dissipation causes the temperature of the hot wire to change, causing the resistance change, and the flow rate signal is converted into electric signal. It has two working modes: 1 constant current. The current through the hot wire remains unchanged, and when the temperature changes, the heat line resistance changes, and thus the voltage across the two changes, thereby measuring the flow rate. 2 thermostatic type. The temperature of the hot wire remains the same, such as maintaining 150 ° C, the flow rate can be measured according to the current applied. The thermostatic type is more widely used than the constant current type.
The length of the hot wire is generally in the range of 0.5 to 2 mm, and the diameter is in the range of 1 to 10 μm. The material is platinum, tungsten or platinum-rhodium alloy. If a thin metal film (thickness less than 0.1 micron) is used instead of the metal wire, it is a hot film anemometer, which functions similarly to the hot wire, but is mostly used to measure the liquid flow rate. In addition to the ordinary single line type, the hot line can also be a combined two-wire or three-wire type to measure the velocity component in all directions. The electric signal output from the hot line is amplified, compensated and digitized and input into the computer, which can improve the measurement accuracy, automatically complete the data post-processing process, and expand the speed measurement function, such as simultaneously completing the instantaneous value and the time average value, the combined speed and the sub-speed, and the turbulence. And measurements of other turbulence parameters. Compared with the pitot tube, the hot wire anemometer [1] has a small probe volume and small interference to the flow field; the response is fast, and the unsteady flow rate can be measured; and the low speed (for example, as low as 0.3 m/s) can be measured.
When a thermal probe is used in turbulent flow, airflow from all directions simultaneously impacts the thermal element, which can affect the accuracy of the measurement. When measured in turbulent flow, the temperature indicator of the thermal anemometer is often higher than that of the rotary probe. The above phenomenon can be observed during the pipeline measurement process. Depending on the design of the management pipe turbulence, it can occur even at low speeds. Therefore, the anemometer measurement process should be performed in the straight section of the pipe. The starting point of the straight line should be at least 10 × D (D = pipe diameter, in CM) before the measuring point; the end point is at least 4 × D behind the measuring point. The fluid section must not have any obstructions (angles, rehangs, 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 passing through a proximity induction, "counting" the rotation of the impeller and generating a pulse series, and then converting the processing to obtain the rotation speed. value. The anemometer's large bore probe (60mm, 100mm) is suitable for measuring turbulence at medium and small flow rates (eg at the pipe outlet). The anemometer's small-caliber probe is more suitable for measuring airflow with a pipe cross-sectional area greater than 100 times the probe cross-sectional area.
3, pitot tube anemometer
The 18th century was invented by the French physicist H. Pitto. The simplest pitot tube has a metal capillary tube with a small hole at the end as the pressure guiding tube, and the total pressure of the fluid is measured in the direction of the flow beam; and a guide is drawn on the main pipe wall near the front side of the metal thin tube. Pressurize the tube and measure the static pressure. The differential pressure gauge is connected to the two pressure guiding tubes, 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 by the pitot tube. After the structural improvement, it becomes a combined pitot tube, that is, a pitot-static tube. It is a double tube bent at right angles. The outer sleeve is sealed with the inner sleeve, and there are a plurality of small holes around the outer sleeve. When measuring, insert this sleeve into the middle of the pipe being tested. The nozzle of the inner sleeve is facing the direction of the flow beam, and the orifice of the small hole around the outer sleeve is perpendicular to the direction of the flow beam. At this time, the pressure difference between the inner and outer sleeves is measured to calculate the flow rate of the fluid at the point. Pitot tubes are commonly used to measure the velocity of fluids in pipes and wind tunnels, as well as river speeds. If the flow rate of each section is measured as specified, it can be integrated to measure the flow rate of the fluid in the pipeline. 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 rate of the particle-free fluid. Therefore, the pitot tube can also be used to measure the wind speed and wind flow, which is the principle of the pitot tube anemometer.
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