Numerical Investigation of the Pressure-Time Method, Head loss in Developed and Developing Flows

抄録

Hydraulic efficiency is a crucial parameter in estimating the performance of hydraulic turbines. However, the flow rate makes such estimation challenging. Several methods have been developed over the years to measure the flow rate. The pressure-time method is an accurate and inexpensive alternative for flow rate estimation, based on transforming momentum into pressure during the deceleration of a liquid mass. The flow rate is obtained by integrating the differential pressure and the pressure loss history between two cross-sections. In the present work, three-dimensional (3D) computational fluid dynamics (CFD) analyses are performed to investigate in detail the influence of the head loss due to friction over the method accuracy when applied in developing flows. One important novelty of the CFD analyses is the use of the immersed solid method for the valve movement modeling for studying the pressure-time method, which is less expensive and more stable than the dynamic mesh method applied in previous CFD studies. The losses are investigated with the assumptions of constant, quasi-steady and unsteady friction factors and compared with detailed data obtained from CFD simulation. The calculated flow rate is not found to be precisely related to the initial pressure drop based on quasi-steady and unsteady friction factors in developing flows. Therefore, a friction factor correction coefficient is proposed and implemented, decreasing the error. The numerical results are validated with experimental data and compared with the dynamic mesh method.