Secondary flows and losses in radial turbine volutes

Abstract

Secondary flows had been found in radial turbine volutes in the past, but the flow mechanisms governing their existence are far from clear. This paper describes an analytic and numerical work on the mechanism of the secondary flows, and the effects of volute shape, A/R distribution, and turbine speed on the development and associated losses of the secondary flows in the turbine volutes. The results show that the centrifugal and viscous forces acting on the wall boundary-layer, as well as the diffusion of radial flow in volute’s cross-sections, are the main generating mechanism of secondary flows, but they are worked against by volute discharging. Compared with the rectangular and trapezoidal shape volutes, the circular volute generates stronger secondary flows, resulting in no hydrodynamic benefit over the other two volutes. By using the dissipation function to separate the secondary flow loss from the total loss in the three volutes, it shows for the first time that although the loss from secondary flows alone is small, the flows can push the main flows toward volute side walls thereby increase wall friction loss. Three twin-entry volutes with different aspect ratios of their cross-sections are also studied. The offset of the cross-section center to volute exit affects volute discharging and the aspect ratio impacts on the radial flow in sidewall boundary layers, they therefore influence the secondary flows in the volutes. The slope of A/R curves is found to correlate well to the strength of the secondary flows in the volutes, and the reason behind is explained. Turbine speed’s influence on the secondary flows is largely through flow compressibility, and if the speed change reduces turbine inlet Mach number, it will increase discharge flow angle at the volute exit, and so too the strength of the secondary flows, but the changes will be small.