Numerical Simulation And Experimental Study On The Transient Flow In Centrifugal Pump During Starting Period

The startup process of a centrifugal pump is a special type of transient process. The centrifugal pump during starting period can be used to supply instantaneous hydraulic power in some special application areas, while instantaneous performances of high-power pumps during starting period may lead to excessive loads or impact failures of facitilies and piping systems. Due to the lack of systematic study on the transient flow in a centrifugal pump during starting period at present, how to achieve methods of performance prediction, design optimization and flow control of the centrifugal pump during starting period is an important issue.Taking the centrifugal pump during starting period as the research object of this dissertation, in view of the special transient hydraulic performance casued by the pump's startup process, the methods of numerical simulations and diagnostics on the unsteady flow in the centrifugal pump during startup process were established. A centrifugal pump performance test rig was developed. Numerical simulations and experimental studies were combined to explore the internal transient fluid flow mechanism which is the direct reason for the external transient effects. The major contents of the current dissertation consist of the following five aspects:First, in order to validate the effectiveness and accuracy of the dynamic mesh based finite volume method in solving the unsteady flow caused by the moving boundary, numerical simulations on the two-dimensional unsteady incompressible viscous flow caused by a circular impulsively started into translation and rotary translation were performed. Spring analogy combined with remeshing method was employed to realize the deformation of the computational domain. Numerical results are compared well with the experimental and numerical results in literature both qualitatively and quantitatively. The dynamic mesh based finite volume method is proved to be feasible in solving the unsteady flow caused by the boundary motion.Second, according to the problem that using dynamic mesh method alone to deal with the changing of fluid domain due to the accelerated impeller may lead to the deterioration of the updating mesh quality, the dynamic slip region (DSR) method was put forward to guarantee the updating mesh quality. The DSR method was successively applied to solve the two-dimensional and three-dimensional transient flows in the centrifugal pump during starting period. Furthermore, to eliminate the errors introduced by externally prescribed unsteady inlet and outlet boundary conditions during starting period, the three-dimensional cycling pipe system along with the whole pump model was set up, and then the transient flow was solved exactly via numerical self-coupling computation process without specifying any boundary conditions.Third, a centrifugal pump performance test rig was originally designed and built up. The rig was composed of power storage device, pump, piping system, data acquisition and electric control system. Instantaneous explicit characteristics such as flow rate, head, rotational speed and shaft torque of the pump during starting period were tested and collected experimentally. Particle image velocimetry (PIV) technique was employed to record the two-dimensional instantaneous flow field. Evolutions of the velocity vector field corresponding with the explicit performance under same condition were given. The experimental results can served as the realistic references in exploring the transient flow mechanism during starting period.Fourth, three-dimensional numerical model of cycling pipe and pump system equivalent with the real test rig was established. The transient flow in the numerical system model was solved under the experimental rotational speed history. Comparisons of the explicit and internal results between numerical simulation and experiment were satisfactory.Fifth, based on the numerical calculation results of the transient flow, unsteady vortex dynamics diagnostics on the pump were performed by analyzing the total pressure flux distribution in the flow cross section and the axial component of boundary vortex flux on the blade surface.Research results indicate that, during starting period, the explicit factor of unsteady non-dimensional head deviation from the steady-state level is the form of start acceleration, while the fluid inertial and evolution of flow structure are the internal factor. Transient characteristics in the early stage of the starting period show rapid rises of rotational speed, head and power with low flow rate, non-dimensional head suddenly falls below the steady-state level from infinity; in the mid-stage flow rate rises rapidly, lead to the increase rate of power exceeds that of head; in the later stage, start acceleration abruptly decreases to zero, rotational speed turns to stable, the increase of flow rate slowdowns, head and power come to the steady-state level after fluctuation and impact. During the whole process of the starting period, the flow rate-time curve shows like cubic trait. The motor power shows clear instantaneous impact when the valve was completely closed.The evolutions of the internal flow field present strong transient effects. That applying the quasi-steady assumption to the design of a centrifugal pump used in the startup process is not appropriate. The content and conclusions of the current dissertation can provide references for the performance prediction, design optimization and fluid control of the hydraulic machinery used in a variety of transient process.