Design Optimization of A Small Scale High Expansion Ratio Organic Vapour Turbo Expander for Automotive Application

Abstract

Environmental concern on a global scale and the concept of energy conservation in heavy duty trucks nowadays have attracted the increasing attention among researchers on the automotive waste heat recovery applications based on Organic Rankine Cycle (ORC) system. However, the system efficiency is theoretically low because of comparatively small temperature difference between exhaust gas and atmosphere, which brings about increasing expansion ratio as a feasible method to improve system performance. However, turbo expander, the most significant component in the system, might generate huge losses due to supersonic flow caused by high expansion ratio, therefore study on supersonic organic vapor turbo expander design emerged as required.In this paper, a single-stage radial-inflow organic vapor turbo expander with pressure-ratio up to 8 is preliminarily designed, and its performance and internal flow are numerically studied. The performance is simulated with three dimensional computational fluid dynamic (CFD) method at operating conditions. Several geometry modifications are conducted in order to learn their influence on turbine performance. Shock waves and trailing edge losses are observed as main losses in the nozzles. In addition, flow separations are the main losses in the rotors. Turbine blade is accordingly optimized after the analysis, and the nozzle total pressure loss coefficient decrease clearly, meanwhile a rise of 3.4% on turbine efficiency can be obtained. Shock waves and reflected waves are suppressed, and a more uniform stream at nozzle outlet can be observed.