Scaling of Gas Turbine From Air to Refrigerants for Organic Rankine Cycle Using Similarity Concept

Similitude, or similarity concept, is an essential concept in turbomachinery to allow the designer to scale a turbine design to different sizes or different working fluids without repeating the whole design and development process. Similarity concept allows the testing of a turbomachine in a simple air test bench instead of a full-scale organic Rankine cycle (ORC) test bench. The concept can be further applied to adapt an existing gas turbine as an ORC turbine using different working fluids. This paper aims to scale an industrial gas turbine to different working fluids, other than the fluid the turbine was originally designed for. The turbine performance map for air was generated using the 3D computational fluid dynamics (CFD) analysis tools. Three different approaches using the similarity concept were applied to scale the turbine performance map using air and generate the performance map for two refrigerants: R134a and R245fa. The scaled performance curves derived from the air performance data were compared to the performance map generated using CFD analysis tools for R134a and R245fa. The three approaches were compared in terms of the accuracy of the performance estimation, and the most feasible approach was selected. The result shows that complete similarity cannot be achieved for the same turbomachine with two different working fluids, even at the best efficiency point for particular expansion ratio. If the constant pressure ratio is imposed, the location of the optimal velocity ratio and optimal specific speed would be underestimated with calculation error over 20%. Constant Δh_0s/a₀₁² was found to provide the highest accuracy in the performance estimation, but the expansion ratio (or pressure ratio) is varying using different working fluids due to the variation of sound speed. The differences in the fluid properties and the expansion ratio lead to the deviation in turbine performance parameters, velocity diagram, turbine's exit swirl angle, and entropy generation. The use of Δh_0s/a₀₁² further limits the application of the gas turbine for refrigerants with heavier molecular weight to a pressure ratio less than the designed pressure ratio using air. The specific speed at the best efficiency point was shifted to a higher value if higher expansion ratio was imposed. A correction chart for R245fa was attempted to estimate the turbine's performance at higher expansion ratio as a function of volumetric flow ratio.