Properties of some pressure-ratio line of radial inflow turbines and application to off-design considerations

Abstract

Radial turbines are commonly used in applications involving operation of the stage through severe off-design conditions. The emergence of variable-geometry systems leads to the distinction between two off-design concepts: operational or geometric off-design. Both of these operating constraints must be integrated in the design procedure for efficient results on a wide range of operating conditions. Recent developments in prediction and optimization methods allowed such an integration but these methods are based on complex algorithms coupled with semi-empiric loss models. This paper provides a basis to obtain simple information, from an existing or predesigned machine, on both operational and geometric offdesign conditions. An alternative turbine map is defined using loading and flow coefficients, where best efficiency regions are identified. A one-dimensional analysis shows that the constant-pressure-ratio operating line is a straight line whose slope is remarkably correlated with the pressure-ratio and geometrical characteristics. This theoretical approach is validated against experiments performed with two machines: an air-conditioning-pack turbine stage and a turbocharger turbine stage. The linearity is observed in both cases, the influence of the stator configuration on the gradient of the pressure-ratio line confirms the applicability of this work to variable-geometry stages through the definition of a dimensionless cross-section of the stator. However, the unexpected displacement of the intercept of the pressure-ratio lines limits the application field of this method. Such a phenomenon needs detailed studies and leads to discuss the hypothesis of the one-dimension analysis. Nevertheless a simple performance prediction analysis is proposed for blocked mass-flow operation.