Impact of different clearance heights on the operation of a water-flooded twin-screw expander—experimental investigations based on indicator diagrams

Twin-screw expanders offer a high potential for energy conversion in the lower and medium power range, for instance in Rankine cycle systems for exhaust heat recovery. With the aim of minimising internal leakages within the expander and lubricating moving machine parts, an auxiliary liquid can be carried with the main flow or liquid working fluid can be fed to the twin-screw expander. Moreover, the operation of twin-screw expanders in Rankine cycle systems at high liquid mass fractions and in trilateral flash cycles is predicted to be a very promising application for expanders in lower power ranges. Thus, a fundamental understanding of the operation of liquid-flooded twin-screw expanders is mandatory. This paper presents results of an experimental investigation into a water-flooded twin-screw expander prototype SE 51.2 designed at the Chair of Fluidics at TU Dortmund University. On the one hand, the aspect of a two-phase working fluid is discussed considering integral characteristic numbers such as mass flow, delivery rate, and effective isentropic efficiency. On the other hand, in order to explain the influence mechanisms of a two-phase working fluid on the operating behaviour of the twin-screw expander, indicator diagrams are recorded by means of high-resolution pressure transmitters to determine indicated power. Hence, mechanical and hydraulic losses, indicated isentropic efficiency, and mechanical efficiency of the twin-screw expander can be calculated. In order to determine the influence of the narrow clearance on the hydraulic losses and the clearance sealing effect in terms of a two-phase working fluid, a systematic variation of the rotor-tip clearance height is carried out. As a result of the investigations, a water surge at the rotor tip is proved to be a significant mechanism affecting hydraulic losses in a water-flooded twin-screw expander. (paper)