Analysis of Indicator Diagrams of a Water Injected Twin-shaft Screw-type Expander

Twin-shaft screw-type expanders offer a high potential for energy conversion in the lower and medium power range, for instance as expansion engines in Rankine cycles for exhaust heat recovery. With regard to minimizing internal leakages and lubricating moving machine parts, an auxiliary liquid or liquid working fluid can be fed to the screw expander. In this paper, indicator diagrams of a twin-shaft screw-type expander prototype designed at the Chair of Fluidics at TU Dortmund University are presented and analyzed in detail. The experimental investigations are carried out at a hot-air test rig with expander inlet manifold water injection. The time-dependant working chamber pressure is recorded by means of high-resolution absolute pressure transmittters. Hereby, specific aspects of working chamber pressure measurements are mentioned. Based on the indicator diagrams, relevant influence mechanisms on the expander's operational behavior resulting from water injection are determined. Additionally, the impact of the injected water on the volumetric and isentropic efficiency is presented.