Hydrodynamic Behavior of a Pump as Turbine under Transient Flow Conditions

Abstract

Centrifugal pumps as turbines (PATs) are widely used in chemical engineering for recycling the abundant energy from high-pressure fluid. The operation of PATs is significantly affected by their upstream conditions, which are not steady (i.e., with a constant flow rate) in reality, thus, research on the flow mechanism of PATs under transient conditions should be considered of higher importance. In this study, a numerical model of a PAT was developed by employing the sliding mesh method to describe turbine rotation, and a user-defined function (UDF) for characterizing transient flow conditions. Corresponding experiments were also conducted to provide validation results for the simulation, and the simulation results agreed well with the experimental outcomes. The instantaneous characteristic curves under the current working conditions were obtained for different transient flow rates. The results show that the turbine’s efficiency is significantly affected by transient flow conditions. In particular, a rapid increase (large time derivative) of flow rate results in a large energy dissipation at the turbine outlet, and therefore, the turbine efficiency decreases. In addition, as the flow rate increases, the hydrodynamic force on the impeller, and the pressure fluctuation amplitude in the volute first decrease and then increase, reaching the minimum near the design flow rate. The current study provides a reliable and precise approach for the estimation of hydrodynamic performance of fluid machinery under transient flow conditions.

Selection and Performance Prediction of a Pump as a Turbine for Power Generation Applications

“…In addition to mesh resolution, the value of y + is dependent on factors such as the distance from the wall to the cell center (y), fluid density (ρ), molecular viscosity (µ), and the wall shear stress (τ w ) (Equation ( 9)). For the k-ε and SST k-ω turbulence models, the near boundary wall should have a y + value of around 200 [16] and ≤100 [16,31], respectively.…”

“…The physical model used in the solver employed the finite volume method with a second-order spatial discretization scheme for the convection terms in the governing equations. In the study conducted by J. Hu [31], the contribution of heat transfer was deemed negligible; thus, the energy conservation equation was not included. Thus, the continuity, Equation (10), and momentum, Equation (11), were used to govern the model, as described in [2,11].…”

“…In order to ensure the accurate convergence of the numerical simulations, a relative error criterion of less than 0.000001 was set. In addition to the standard k-ε, which is based on model transport equations for the turbulence kinetic energy (k) and its dissipation rate (ε) [2], the SST k-ω turbulence model was employed due to its ability to account for the transport of the principal shear stress within boundary layers under adverse pressure gradients [31]. The physical model used in the solver employed the finite volume method with a second-order spatial discretization scheme for the convection terms in the governing equations.…”

The acceleration effect of pump as turbine system during starting period

“…From the overall diagram, the pump as turbine starts to operate at 0.3 s from the calculation moment, and its dimensionless head also starts to decrease from the maximum value at 0.3 s from the calculation moment. From the local graph, the dimensionless head (15) www.nature.com/scientificreports/ coefficient reaches its stable value at 0.486 s, 0.900 s and 1.400 s, and its stable values are 2.3834, 2.3496 and 2.3824, respectively. it can be seen that the time to reach the stable value of the dimensionless head coefficient during the start-up process is also related to the acceleration time of the pump as turbine, and there is a slight delay in the rapid start-up.…”

“…The results show that the cavitation performance of the vortex pump as turbine is poor, and its performance conversion coefficients of flow and head are much higher than those of centrifugal pumps with the same speed, and the efficiency of the vortex becomes low as the viscosity increases or the impeller Reynolds number decreases. Hu et al 15 studied the hydraulic characteristics of pump as turbine under instantaneous flow conditions. The results show that the efficiency of the pump as a turbine is greatly influenced by the instantaneous flow conditions.…”