A Numerical Investigation of the Constant-Velocity Volute Design Approach as Applied to the Single Blade Impeller Pump

Abstract

This contribution addresses volute design as applied to single-blade-impeller pumps. Traditionally, volute design for multiblade impeller pumps has been carried out using either the constant-velocity or constant-swirl methodologies. Here, the constant velocity approach was investigated in order to determine whether or not it was appropriate for single-blade-impeller pumps, and whether the theoretical formulation would agree with numerically calculated data. In a numerical approach, three volutes were designed of the constant velocity type with design velocities of 0.8, 1.0, and 1.20 Cref. The performance of all three volutes was calculated using transient, three-dimensional, viscous numerical simulations, using the commercially available ANSYS CFX -11.0 code, over a range of flowrates 0.55<Qd<1.44. Analysis of the velocity distributions within the volutes was carried out by means of equispaced radially distributed planes on which the average circumferential velocity was calculated over full impeller rotations. The development of the initial constant velocity volute design (1.0 Cref) required the use of a somewhat arbitrary setting of the recirculation mass flowrate Qrc=0.35Qd. However in subsequent designs, a new iterative approach was developed, in which the velocity and mass flow distribution results from the numerical simulations were looped back into the design procedure, and an updated recirculation mass flowrate was obtained. These steps were then repeated until the desired constant velocity volute designs were obtained. The results of the investigation confirmed the strongly transient velocity pressure pulsation generated by the single blade impeller. When analyzed using average velocity measurements across an entire impeller revolution, clear agreement was seen between the velocity distributions predicted theoretically and calculated numerically for each of the constant velocity volutes. As expected, at flowrates above the dutypoint, the flow was seen to accelerate through the volute in all cases, while below the dutypoint, decelerating flow was observed. Examination of the hydraulic performance curves showed that an increase in the volute constant velocity design value led to a steeper head-flow curve. Further, increasing the design velocity provided for a higher overall hydraulic efficiency and a more peaked efficiency-flow curve.