孔板流量计瞬时孔流系数的数值预测

Numerical prediction of transient discharge coefficient in orifice flow meter

为了研究孔板流量计在测量流量快速变化时的特性,以孔板流量计瞬时孔流系数C为研究对象,采用计算流体动力学(CFD)方法,基于Realizable分离涡模拟(DES)描述瞬时湍流流动,模拟研究了流量直线加速过程瞬时C和内流场随时间的演变结果.为了对比分析,将加速过程离散为不同流量下的稳态点,采用Realizable k-ε模拟各个稳态点的孔流系数和流场结构.稳态孔流系数C0的模拟结果与ISO试验回归曲线相比,误差在3%以内.将加速过程和稳态假设下模拟的孔流系数结果进行对比,结果表明:加速过程瞬时C从0逐渐增加至稳定值,而稳态C0基本保持在0.6附近.进一步将孔流系数与内流场和压力场分布的演化结合起来分析,得出以下结论:加速流动的漩涡滞后于稳定状态,加速前期压能没有在短距离内全部转换为动能,是导致C与C0产生偏差的内流原因.研究内容可为瞬时流量的测量提供参考基础.

To study the characteristics of the orifice flow meter when measuring linear accelerating flow rate,taking the curves of the transient discharge coefficient（C） as the study object,the computational fluid dynamics（CFD） method was used to simulate the C and the evolutions of the corresponding flow field.The transient turbulent flow was described by the Realizable detached eddy simulation（DES）.For comparative analysis,the accelerating process was discretized to separate steady-state conditions at different points of flow rate.The Realizable k-ε model was chosen as the steady-state turbulent closure.The error between curves of present steady discharge coefficient（C0） and the ISO data is less than 3%,so the current calculation results is validated.Comparison of discharge coefficients between flow acceleration process and steady-state condition was performed.Comparison results show that the transient C curve increases from 0 to a constant value gradually and the steady C0 remained in the vicinity of 0.6.The discharge coefficient together with the evolutions of velocity and pressure field were combined to further analysis the internal flow mechanisms.Reasons of deviation between transient and steady discharge coefficients are that the vortex of the acceleration state lags behind the steady-state conditions,so the pressure energy was not able to transform into the kinetic energy at the early process within a short distance.Contents of current study can serve as the reference of the instantaneous flow measurements.