In small Rankine cycle power plants, it is advantageous to use organic media as the working fluid. A low-cost single-stage turbine design together with the high molecular weight of the fluid leads to high Mach numbers in the turbine. Turbine efficiency can be improved significantly by using an iterative design procedure based on an accurate CFD simulation of the flow. For this purpose, an existing Navier-Stokes solver is tailored for real gas, because the expansion of an organic fluid cannot be described with ideal gas equations. The proposed simulation method is applied for the calculation of supersonic flow in a turbine stator. The main contribution of the paper is to demonstrate how a typical ideal-gas CFD code can be adapted for real gases in a very general, fast, and robust manner.

The aim of this study was to make a computer program that simulates a standard refrigeration process, and a process provided with a bubble intercooler. A further object of the study was to establish the suitability of a turbocompressor for small refrigeration plants. Firstly the fundamentals of refrigeration machines and industrial refrigeration systems are discussed. An iteration procedure of steady state refrigeration process calculation is introduced. Fluid properties are calculated with the program units created for the modelling of an ORC power plant. Specific input files were made for 6 process fluids R134a, R123, isopentane, isobutane, toluene and ammonia. The compressor program is linked to the refrigeration process simulation program in order to model single stage and two stage radial compressors. The Turbo Pascal program made for microcomputers is modular, which makes it possible to develop and test the program unit by unit. The maximum deviations of fluid properties from those in tables was found to be less than 1 per cent. To simulate tailor-made refrigeration plants, a simple model is required. On nominal loads the program estimates an optimum intermediate pressure for the bubble process and optimum rotational speed for the radial compressor(s). The lowering of the rotational speed by an inverter gives high COP-values on partial loads of the plant. Based on the example calculation, a two stage turbocompressor calculated with isopentane as the working fluid, a cooling capacity of 1200 W seems to be feasible.