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Thermal Mass Flow Meter Design - Focus on Microfluidic Control Technology

In modern industrial production processes, the physical parameters of various media in the production process need to be measured in order to effectively perform production operations and control. Among them, the flow rate of the fluid is one of the important parameters that are often measured and controlled. Due to the complexity of the flow measurement, the flow measurement instrument has become a type of instrument with the most variety of specifications in the process detection control instrument. Thermal flowmeters are meters that directly measure fluid mass flow using heat transfer principles. At present, thermal distributed thermal flowmeters and heat dissipating thermal flowmeters are more commonly used. Thermal flowmeters can measure very small flow rates at very low flow rates with low pressure losses and high reliability due to the absence of moving parts.

In this paper, the invading heating resistance element is used to perform constant power heating on the flowing gas, and the temperature difference is used to measure the gas mass flow in the circular pipeline. In order to use the thermal flow meter to measure the flow of the easily-vaporizable liquid, a cooling type thermal liquid mass flowmeter was studied at the same time. The main research work is summarized as follows:

1. Based on the detailed reading of the relevant literature, the development process of the flowmeter is summarized, and the types and measurement principle of the thermal mass flowmeter are emphatically discussed.

2. The basic concepts of fluid mechanics, convection heat transfer and thermoelectric refrigeration involved in the measurement principle of thermal flowmeters are described, as well as the mathematical description and analysis methods of these physical phenomena; a constant power thermal dissipation type thermal gas flowmeter is deduced. The flow equation of the cold and hot type liquid flowmeters demonstrates the feasibility of the two flow measurement methods.

3. A fluid simulation experiment was conducted for the case where the air in a circular pipe flows through a cylindrical constant-power heating resistor element. The simulation results showed the velocity and temperature distribution of the fluid around the heating element under different air mass flow rates, which directly reflected the influence of the heating element on the flow; the velocity distribution of the fluid on the surface of the heating element was investigated, and the naturalness caused by buoyancy was revealed. The effect of convection on the flow measurement was given and the simulation results of the lower limit value were given. The flow equation of the constant power heat dissipation type thermal flowmeter was verified by simulation data.

4. A temperature signal measuring circuit was designed for the constant power dissipating thermal gas flow meter and the refrigerating thermal liquid flow meter respectively, and an experimental platform based on a microcontroller was set up. Combining the statistical method with the mean filtering method, a software filtering of the sampling data is proposed. By dynamically changing the power saving management mode and system clock frequency of the microcontroller, the system operation power consumption can be effectively reduced.

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