Research On Design And Property Of The Boundary Layer Type Thermal Gas Mass Flowmeter

Thermal Gas Mass Flowmeter,which is based on the principle of convective heat transfer,can directly measure the mass flow of gas.In order to overcome the disadvantages of traditional insertion thermal mass flowmeter such as flow interference and poor sensitivity,A thermal mass flowmeter with high precision,wide range,low cost and high reliability was designed.The main research contents and conclusions are as follows:Firstly,the physical principles and mathematical descriptions of thermal mass flowmeters were analyzed in detail,including boundary layer flow,the flow distribution of parallel pipes and integral calculation of cumulative flow.Secondly,a type of thermal gas mass flow meter with constant power,branch pipe and restrictor was designed.The small temperature rise of the gas in the branch pipe was used to reflect the mass flow of the fluid in the pipe.The range of the flowmeter is increased by 100 times by using the branch structure.Thirdly,the most important factors which affects the performance of the flowmeter was analyzed by CFD.The flow field in the designed flowmeter presents a stable laminar flow pattern;the temperature field in the fluid boundary layer is sensitive to the flow velocity;the flow ratio between the branch pipe and the main pipe is stable;The inflection point of the temperature difference measured by the sensor was found.On this basis,the optimum design scheme of restrictor,the optimal position of the sensor in the pipeline,the optimal length and diameter of the branch pipe is determined and a reasonable structural design scheme of thermal mass flowmeter is formed.Fourthly,the STM32 L series MCU module and the MEMS temperature sensor with three platinum electrodes were used to design the circuit with constant power and the prototype was manufactured.Finally,the flow conversion formula for different gases was derived with using N2 as the calibration gas.The performance of the flowmeter,including its reliability,repeatability and indication error was tested.The inflection point of the test coincides with the calculated inflection point.The final test results show that the thermal gas mass flowmeter designed in this paper meets the relevant national standards.