Experimental and numerical investigation of a shaped microchannel evaporator for a micro Rankine cycle application

Abstract

In the present study, numerical analysis and experimental observations have been conducted to investigate the heat and mass transfer behavior in a shaped microchannel evaporator and to determine its ability to provide high quality steam for micro Rankine cycle application. The micro evaporator is designed to prevent the presence of liquid droplets at the exit by incorporating re-entrant wall angles for capillary flow stabilization and a stream wise temperature gradient to allow a range of wall temperatures along its length. Meniscus evaporation, periodic bubbly evaporation and boiling regimes were observed based on experiments at different mass flow rates and working temperatures. Experimental results show that the meniscus evaporation and periodic bubbly evaporation regimes provide fully evaporated steam without droplets at the exit. Thus they are successful operating regimes for the micro evaporator in micro Rankine cycle application. One dimensional conjugate heat transfer was considered along the microchannel evaporator with thin film theory used to model the evaporation rate. The numerical results are in good agreement with experimental observation for the meniscus evaporation regime. This kind of micro evaporator achieves the objective of providing a constant flow of fully evaporated steam without the presence of mixed flow at the exit, but only up to a limit flow rate.