Numerical Simulation Of Pressure Potential Differential Type Laminar Flow Sensing Element

Laminar flow meter is suitable for micro&small gas flow measurement,due to its features of wide turndown ratio,high accuracy and fast response.However,it is not fully developed laminar flow between the pressure taps for a traditional laminar flow sensing element,there are nonlinear pressure losses in the measured differential pressure.In this paper,a pressure potential differential type（PPD）laminar flow sensing technique is studied,with which the nonlinear pressure losses can be eliminated.The computational fluid dynamics is used to simulate and analyze the internal flow of the PPD sensing element.The main research contents and results are as follows:（1）Working principle and analysis of the PPD laminar flow sensing element.For the incompressible flow,the flow resistance characteristics of the two branches constituting the PPD sensing element are studied.The relative deviation of the pressure drops of the two branches is within±0.02%as mass flow being the same,indicating that the flow resistance characteristics of the two branches are consistent,which means that the assumption of the PPD principle is reasonable.There is an ideal linear relationship between pressure potential difference and volume flow in the whole calculated range,which proves the correctness of the PPD principle.Compared with the traditional laminar flow sensing element,the nonlinear pressure losses can be effectively offset and higher measurement accuracy and a larger turndown ratio can be obtained with the PPD sensing element.（2）Applicability of PPD laminar flow sensing element in measurement.The results show that the compressibility has an obvious influence on the gas flow and pressure loss with the compressible model used in numerical calculation.On the other hand,despite the compressibility effect,the relative deviation of the pressure drops of the two branches is within±0.05%,indicating that the flow resistance characteristics are consistent.There is little difference relative to the pressure potential difference,±0.01%,in the local pressure losses of the two upstream capillaries outlet flows,which can be ignored in a real measurement,further verifying that the PPD sensing technique is also applicable to the gas flow measurement.By comparing the calculation data of traditional laminar flow element and the PPD sensing element,the measurement error of the latter is reduced by at least one order of magnitude,which proves that the latter can greatly eliminate the nonlinear effect.The influence of gas expansion in laminar flow measurement is theoretically analyzed,and the value of correction coefficient is simulated.It is found that the deviation between the measured flowrate and the real flowrate is the smallest when the expansion correction coefficient K_exp=1,verifying the results of previous works of researches.（3）Study on the structure and influencing factors of the PPD laminar flow sensing element.The effects of structural factors such as short capillary length,pressure chamber length,pressure tap position and capillary arrangement and distribution on differential pressure measurement are studied.The results show that the longer short capillary length,the higher measurement precision of the PPD sensing element can be achieved.If the measurement error is required to be within±1%,the dimensionless length of short capillary L^*should not be less than 0.035.When the length of pressure chamber L_c≥20d,the time average pressure value is stable and the pulsation value is small.The pressure distribution inside the pressure chamber is not completely uniform,which means that selecting the pressure tap point in the middle of the pressure chamber is reasonable.The capillary arrangement and distribution consider two influencing factors:the distance between the capillary and the inner wall of the pressure chamber H and the capillary spacing s.When H≤d or s decreases,the pressure pulsation will increase.In the application of the PPD sensing element in gas flow measurement and the influence of capillary arrangement,the experimental test method is used for comparison and verification.The experimental results are consistent with the simulation results,which further verifies the reliability of the numerical simulation.The research results of this paper further improve the theory of the PPD laminar flow sensing technique and the design method of sensing elements,which is conducive to the application of this technique in practice.