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Numerical Investigation of Pressure Losses and its Effect During Intake in a Steam Wankel Expander

A Wankel steam expander has numerous advantages over other positive displacement machines as an expansion device. This is due to its high power to weight ratio, compactness, lower noise, vibration, and potentially lower specific cost making them a favourable choice over reciprocating expanders. Admission in the expander chamber occurs through rotary valves fed with steam supply from a boiler. The present study aims to investigate the magnitude of pressure losses of the steam during intake and its effect on the net power output of the expander over a range of rotational speed varying from 1200 to 3000 RPM. The thermodynamic analysis is carried out for the theoretical pressure-volume cycle of the expander using Python, which is then used for CFD analysis in Ansys Fluent 19.2. Three dimensional models are developed for the flow domain stretching from the exit of the intake valve port to the port in the rotor housing at different rotor angles ranging from admission to cutoff. Computations are performed to investigate the associated pressure drop of steam during admission. The boundary conditions at different rotor angles are obtained from the thermodynamic model of the expander's theoretical pressure-volume plot, and the state point values were obtained from the REFPROP database. Validation of the CFD models are carried out by comparing pressure drop values obtained through an analytical approach using correlations and expressions reported in literature. A thorough performance analysis of this expansion device is made and the loss in power output due to the intake pressure loss is calculated. These losses during admission change the pressure ratio across the actual expander designed for a given pressure ratio, leading to a reduced power output by a reasonable margin of 20 to 30%. It is observed that the percentage loss in power output increases with an increase in shaft speed.