Numerical Validation of a Two-Phase Nozzle Design Tool Based on the Two-Fluid Model Applied to Wet-to-Dry Expansion of Organic Fluids

Abstract

Two-phase expansion could increase the power output of organic Rankine cycles by up to 30% in the temperature range of 150 to 250 $$^{\circ }$$ C compared to single-phase architectures. By employing molecularly complex fluids, it is possible to design a wet-to-dry cycle, in which the fluid transitions from a two-phase state to superheated gas during expansion. This opens the possibility of using turboexpanders, provided that the wet portion of the expansion is confined to the stator. Hence, the design of the stator becomes critical to ensure complete evaporation of the two-phase mixture. Given the rapid acceleration and large change in density across the expansion process, it may be essential to account for non-equilibrium effects when designing two-phase stators. This study aims to validate a previously developed two-phase nozzle design tool with non-equilibrium CFD simulations. The design tool assumes quasi-1D inviscid flow and employs a two-fluid model that solves separate mass, momentum and energy conservation equations for both phases. The design tool is evaluated by performing two-dimensional viscous simulations on the nozzle geometries generated from the nozzle design tool. To examine the importance of including non-equilibrium effects at the nozzle design stage, a separate nozzle was generated assuming homogeneous-equilibrium flow. The results reveal the predictions from the design tool were in relatively good agreement with the CFD simulations with the difference in streamwise variations of various flow properties typically not exceeding a few percent. However, this was not the case for expansions from low inlet pressure and low inlet vapour quality, which were found to be highly two-dimensional and could not be effectively treated by the design tool. Furthermore, a significant disparity between the CFD results and the homogeneous-equilibrium design was found, indicating that non-equilibrium effects should not be neglected when designing a nozzle for wet-to-dry cycles.