振荡流态下孔板流量计的瞬时压力-流量特性分析

为了研究孔板流量计在动态非稳定流态或振荡流态下的瞬时压力-流量特性,理论分析了孔板前后的旋涡域大小随流速变化是引起孔板进出口瞬时流量差的主要原因.借助CFD数值解析方法,建立孔板模型,并在模型入口加载某一频率下的正弦流速,对孔板流量计在振荡流态下的瞬时压力-流量特性进行分析.结果表明:当孔板流量计处于低频振荡流动状态时,孔板两端差压也处于周期振荡状态,差压与节流孔瞬时流量同频不同相,差压幅值随入口流速振幅增大而线性增大,且线性增长系数与振荡频率相关;孔板的入口与出口存在周期波动的瞬时流量差,振荡频率越大或入口流速峰值越小,瞬时流量差的波动越小.由于相位滞后和瞬时流量差的存在,使孔板流量计的测量流量与实际出口流量之间存在偏差.振荡频率越大,偏差也越大.

Analysis of transient pressure-flow characteristics of an orifice meter in oscillating state

To investigate pressure-flow characteristics of an orifice meter under unsteady or oscillating flow conditions, the size variation of vortices up- and downstream an orifice plate was theoretically analysed when the flow rate was changing. This variation is the main reason for the instantaneous difference in flow rate between the inlet and the outlet of the orifice. An orifice meter model was built, and the pressure-flow characteristics were analysed based on computational fluid dynamic(CFD)method when the inlet flow velocity was imposed by a sine function at a certain frequency. Results indicated that when the inlet flow was in an oscillating state of low frequency, the pressure drop across the orifice was also in an oscillating state with the same frequency but a different phase. The amplitude of pressure drop increased linearly with increasing amplitude of the inlet velocity, and the linear growth coefficient was correlated with the oscillation frequency. There was a transient difference in flow rate between the inlet and the outlet. The greater the oscillation frequency, or the lower the inlet velocity, the smaller the fluctuation of the transient flow rate. Since a phase delay and an instantaneous flow rate difference exists, there is a deviation between the measured flow rate and the actual flow at the outlet, and the higher the oscillation frequency, the larger the deviation.