Effects of design conditions and flow layout on the design of ultra-low aspect ratio radial outflow turbines

Abstract

Small-scale radial outflow turbines offer a promising alternative to conventional axial turbines, for example in waste heat recovery applications. One of the design challenges with these machines is that the common single flow design approach leads to the need for axial thrust compensation. A convenient solution to this challenge is to use a double flow layout, which, however, reduces the turbine efficiency due to increased aerodynamic losses. The public literature lacks information on the design of these turbines under different design conditions and power levels. Therefore, this study aims to fill this research gap by examining three design cases as both single and double flow turbines. It hereby shows that the aspect ratio and tip-clearance losses mostly explain the observed differences between the flow layouts. The role of high flow turning in the rotor losses is also highlighted under different design conditions.