Two-phase chamber modeling of a twin-screw expander for Trilateral Flash Cycle applications

Abstract

Abstract Low temperature (<100°C) streams have the largest share of waste heat recovery potential and may represent an attractive opportunity for a sustainable economy. Among the bottoming thermodynamic approaches that have been proposed to convert this waste heat into electricity, the Trilateral Flash Cycle (TFC) proved to be theoretically capable to recover more heat from a low-temperature single-phase heat source than any simple Rankine cycle. However, the commercialization of TFC recovery units has been so far prevented by the lack of an expander technology that should efficiently operate with high mass flow rates of a two-phase flashing flow. A promising candidate to tackle these challenges is the twin-screw technology thanks to its positive displacement nature and the capability to run at high revolution speeds without remarkable efficiency drops. In the current research work, a twin-screw expander has been modeled in the commercial software GT-SUITE™. The modeling activity resulted in a two-phase chamber model based on the coupling of the conservation equations and the REFPROP library to calculate the thermophysical properties of the liquid-vapor mixture of the R245fa working fluid at each time step. Using pre-processed geometrical data, the model includes a detailed breakdown of the leakage paths and allows to retrieve key information for a future optimization of the machine such as the indicator diagram and the quality-angle diagram. Parametric analyses were eventually carried out to assess the expander behavior at different operating conditions, namely manometric expansion ratio, revolution speed and inlet quality.