Dynamic simulation of a test rig for organic vapours

A blow-down facility for experimental analysis of real gases is under construction at Politecnico di Milano (Italy), in collaboration with Turboden s.r.l. and in the frame of the research project named Solar. Experiments are meant to characterize flow fields representative of expansions taking place in Organic Rankine Cycle (ORC) turbine passages. Indeed, ORC power plants represent a viable technology to exploit clean energy sources, but ORC turbines design tools still require accurate experimental data for validation. A significant improvement of turbine efficiency is expected from detailed investigations on vapour streams; in fact, ORC turbines design tools still require accurate experimental data for validation. The facility is equipped with a straight axis supersonic nozzle as a test section and a batch-closed loop plant has been designed in order to reduce investment and operational costs. Due to the batch operation, the evaluation of the time evolution of main processes involved in the cycle is of great importance. To this purpose a dynamic simulation of the test rig has been carried out using a dynamic simulator based on an object-oriented modeling language, Modelica, allowing an easy development of component models structured with a hierarchical approach. Models include control loop devices, strongly influencing processes duration. This paper presents how the test rig has been modelled, with particular emphasis on the models framework and on simulation procedure; the calculation results are finally discussed. With a lumped parameter approach, a first scheme of the facility has been built by modelling each of the three main plant section (heating, test, condensation) using components included in a self-made library. Several models, not embedded in the Modelica standard libraries, have been created using Modelica code; among them the most important has been the supersonic nozzle. In order to better describe the facility behaviour and the thermal losses, a plant calculation refinement has been carried out by the development of finite volume based one-dimensional models of ducts and reservoirs, either in radial or axial direction; in particular, a novel distributed-parameters model has been built for the heating section. All simulations have been performed using Siloxane MDM and Hydrofluorocarbon R245fa as reference fluids and FluidProp® to calculate thermodynamic properties. A quasi 1-D steady nozzle flow calculation has also been carried out by implementing FluidProp® routines in a dedicated Fortran software. Since the unsteady nozzle expansion is well approximated by a sequence of steady states, the computation provides all thermodynamic properties and velocity along the nozzle axis as a function of time. Simulation results have given a fundamental support to both plant and experiments design.