The Influence of Working Fluid Characteristic Parameters on Turbine Performance for the Small Scale ORC System

Abstract

The small scale Organic Rankine Cycle (ORC) using a turbine as expanders is considered as one of the most efficient ways to convert the waste heat energies of automobile engine into electrical energy, in the power range from several kW up to dozens of kW. In general, two important factors must be taken into account when designing the ORC turbine: the real gas effects of the organic fluid and the high expansion ratio presented in the machinery due to thermodynamic and efficiency factors. The characteristic parameters of R245fa have great difference compared with air. When designing an ORC turbine, the initial problem is the influence of the working fluid characteristic parameters on the turbine performance. Similarity analysis method is used to analyze this problem. When using the similarity theory to design two similar operating conditions, gas constant R and dynamic viscosity μ are easy to be taken into account as they have a dimension; specific heat ratio κ is hard to be taken into consideration. In this paper, a high expansion ratio ORC turbine is generated for a small scale ORC system working with the organic fluid R245fa, and the influence of κ on the turbine performance parameters analyzed. Firstly, the turbine geometry is designed in an iterative process using the commercial design tool Concepts-NREC Rital program, and the designed operating condition is determined. Secondly, the similarity criterion numbers are deduced with no consideration of the specific heat ratio κ, and the similar operating condition is calculated by the deduced similarity criterion numbers. The results achieved by the one-dimensional design software Rital shows that the deviation of similarity criterion numbers is small, which indicates that the two operating conditions are similar. Then, to verify the conclusion above, the flow field simulation using one channel model is conducted based on the R245fa operating condition and air operating condition using the computational fluid dynamics (CFD) code NUMECA-FineTurbo. However, deviations of the similarity criterion numbers from the CFD results are more than twice to those from the one-dimensional results. The larger deviation may indicate that the influence of the specific heat ratio κ cannot be ignored. After that, the distributions of the total pressure, total temperature, the relative Ma number and entropy in the nozzle and rotor channel are compared. These results show that except the total temperature the distributions of the other three parameters are very similar. The similar distribution may indicate that the specific heat ratio κ has little influence on the efficiency and expansion ratio, but κ has greater influence on the temperature and output power. All of these conclusions are made according to the simulation results, the accuracy should be verified through the test results.