Screw Type steam expander, screw expander design, application oriented design, chamber model simulation
Screw expanders offer a high potential for energy conversion in the lower and medium power range, for instance as expansion engines in closed loop power cycles. A common field of application is waste heat recovery where a certain heat flow is available to evaporate a working fluid in an organic Rankine cycle (ORC). In order to reach maximum cycle efficiency, the expander has to be designed with regards to the specific case of application. Design parameters, such as rotor geometry, size and inner volume ratio, have to be chosen in accordance to system parameters, for instance pressure levels and mass flow. Given that the available heat flow for the application is restricted, it is necessary to variably determine inlet and outlet pressure as well as the expansion mass flow in order to maximize the power output of the case specifically designed screw expander. The paper contains results of an exemplary design and dimensioning process of a screw expander for an organic Rankine cycle for exhaust heat recovery of internal combustion engines. Both geometric parameters and system parameters are varied in a wide range to maximize the overall power output. It is shown that especially for small scale applications a combination of an uncommonly large inner volume ratio with high inlet pressures can be advantageous despite the fact that only relatively small isentropic expander efficiency is reached. Further proceeding includes consideration of several part load Rankine cycle operation points where a method of averaging the final expander size is presented. During the dimensioning process chamber model simulation is used to predict the operation behavior of the screw expander where the chamber model of the machine is scaled under consideration of geometrical similarity (except of gap heights) during the simulation process at constant circumferential speed. Thus, it is assured that the actual expander mass flow matches the calculated ORC mass flow. Moreover, to estimate overall performance of the heat recovery system, a characteristic map of the selected screw expander is calculated to find pressure and expander speed depending operating points for part and full load. With the aim of validating the theoretical approach, experimental investigations of the designed machine are carried out on a water steam test rig at TU Dortmund University. To increase system simplicity, the oil-free prototype screw expander operates without timing gear which is enabled by a hard coated rotor surface. The experimental results are finally compared to corresponding chamber model simulations (will be presented but not be included in the paper).