Study on fluid dynamic characteristics of a low specific speed centrifugal pump with emphasis on trimming operations

Abstract

In the present investigation, the fluid dynamic characteristics of a narrow channel centrifugal pump are studied using numerical simulations with special emphasis on pump trimming operations. The trimming operations at optimized rate, reduce the pressure fluctuations and hydraulic noise in pumps, but at the cost of pump performance characteristics. This has been extensively studied by the researchers in the past; however, the efficacy of trimming operations on industrial pumps with narrow channel passage impellers is to be explored. A low specific speed pump with narrow channel impeller passage and radial discharge volute (M1) with specific speed of 14.7 is considered for the present investigation. Detached Eddy Simulation (DES) is used in transient simulations and Fast Fourier Transform (FFT) is used to understand the pressure fluctuations and hydraulic noise at the Blade Pass Frequency (BPF) and its harmonics. The pressure fluctuation behavior along the impeller periphery is monitored at 30 equidistant probes and 7 probes covering the volute tongue. These results are compared with the similar study for impeller with oblique trimming of 30° (M2) and impeller oblique trimming along with volute trimming (M3). The flow behavior in M1 is dominated by a Jet-wake flow in the impeller passage which acts as a primary source of flow non-uniformity. The resulting non-uniform flow on the impeller periphery interacting with the volute tongue causes vortex shedding behind the volute tongue, subsequently leading to discharge recirculation in the impeller. The coefficient of pressure fluctuations near volute tongue at BPF is reduced by 10.3% in M2, whereas it is enhanced by 24% in M3 relative to M1. The enhanced vortex shedding behind the volute tongue in addition to the vortex train along the impeller periphery causes rise of the pressure fluctuations in M3.