Influence Of The Upstream Pipe Bends On The Measurement Errors Of A Single-Point Speed Flow Meter

Speed type flow meter is a flow meter based on direct measurement of the flow velocity of closed pipeline full of fluid,thermal gas flow meter is a typical single point speed type flow meter.At present,the study of thermal gas flow meter mainly focus on the flow sensor and hardware circuit,while less study on the flowmeter flow characteristics.This paper focus on the influence of the pipeline condition on the measurement error of the thermal gas flow meter.Numerical simulations have been carried out to analyze the influence of measurement accuracy of upstream planar single pipe bend and spatial double pipe bends on the thermal gasflow meter,structure and parameter optimization are studied to improve the measurement accuracy,and the pressure loss of the flow meter before and after optimization is evaluated.the main research and results are shown as follows;(1)Study on the flow in the flow meter and effect of the straightener.Firstly,the velocity distribution by numerical calculation in the measure channel section is compared with that measured by the Hot wire anemometer.The results agree well with the numerical simulation,which proves the rationality of the numerical simulation.Numerical simulations have been carried out with Fluent on DN32?DN50?DN100 three kinds flow meter,the results show that installation of the straightener can reduce the influence of secondary flow and vortex which caused by upstream pipe bend,compared with without straightener,the measurement error can be reduced by about 2%at the flow of more than Q0.4max and different degrees of reduction below Q0.4max.The greater the distance between the pipe bend and the flow meter,the smaller the flow measurement error.(2)Study on the calculation model of screen.Screen as common flow straightener in pipe,which used for cutting medium vortex and below,for better reducing the turbulence,thereby effectively improve the flow velocity distribution,so it can reduce the measurement arror of the thermal gas flow meter,screens are used in the optimization model of the straightener.In the numerical simulation the screens are managed as porous media model,using Forchheimer equation combined with the experimental data to obtaine the damping viscous resistance coefficient and inertia resistance coefficient calculation formula,resistance coefficient can be obtained by the porosity.(3)Straightener type and parameter optimization.Three optimization schemes are studied.The results show that the optimize model which keep the original square honeycomb as the first straightener,and screens as the second straightener can achieve good effect,the measurement errors decrease for different diameters,especially for spatial double pipe bends as upstream baffles.The pressure loss values of the three kinds of optimization schemes have different degrees of reduction compared with the original model,and the structure optimization has a good effect on reducing the pressure loss.