Research Of Measurement Characteristics Of Thermal Gas Mass Flow Sensor

The thermal gas mass flow sensor is a flow measurement sensor based on the principle of heat transfer.It has the advantages of simple structure,small pressure loss,wide measuring range and no overload damage,and has been widely used in various industrial scenes.However,it is still difficult to measure the small flow velocity accurately and study the influence of temperature,pressure and other factors on the output signal.Based on the King's law,the measurement characteristics of the thermal gas mass flow sensor under different working conditions are thoroughly studied in this thesis.Firstly,three commonly used forced convection heat transfer models are compared with the experimental data: King's law,Churchill & Bernstein model,and heat transfer model of the plate.Based the King's law,the measurement model of the thermal gas mass flow sensor is given.Secondly,the heat transfer model of free convection is studied at a low velocity range(0.03m/s~1.00m/s).By changing the heating current and the direction of flow,the influence of free convection on the measurement model of thermal gas mass flow sensor is discussed.Next,based on the King's law,a modified heat transfer model of the mixed convection for thermal gas mass flow sensor was proposed and discussed from the perspective of free convection at a low velocity range(0.03m/s~1.00m/s).According to the prototype of thermal gas mass flow sensor with constant current,the relationship of free convection and forced convection over a wide velocity range was discussed.In the light of the influence of free convection,the relationship between the exponent n of King's model and the convection intensity ratio was observed.Based on the experimental data,the mathematical model of exponent n was established.Compared with the results of the King's model,the model for mixed convection improves the measurement accuracy.Finally,the change of the pressure,temperature and other factors of the measured gas will lead to the change of the heat transfer of the thermal gas mass flow sensor,and then affect its output signal,which leads to the decrease of the measurement accuracy.In this study,the influence of temperature and pressure on the measurement accuracy of thermal gas mass flow sensor is analyzed and calculated from the perspective of physical parameters and empirical formula,and verified by experiments.