Application-Oriented Design and Theoretical Investigation of a Screw-Type Steam Expander

摘要

This paper contains results of an exemplary design and dimensioning process of a screw expander for an organicrnRankine cycle for the exhaust heat recovery of internal combustion engines. Both geometric parameters and systemrnparameters are varied in a wide range to maximize overall power output. It is shown that, for small scalernapplications in particular, a combination of an uncommonly large inner volume ratio with high inlet pressures can bernadvantageous, despite the fact that only relatively small isentropic expander efficiency is achieved. The remainder ofrnthe study includes a consideration of several part load Rankine cycle operation points and presents a method forrnaveraging the final expander size. During the dimensioning process, multi chamber model simulation is used tornpredict the operation behavior of the screw expander. The multi chamber model of the machine is scaled takingrngeometrical similarity into consideration (excluding clearance heights) during the simulation process at constantrncircumferential speed. In this way it is ensured that the actual expander mass flow always matches the calculatedrnORC mass flow. Moreover, to estimate the overall performance of the heat recovery system and to find commonrnpressure- and speed-dependent operating points for part and full load, a characteristic map of the selected screwrnexpander is calculated.