Numerical Study on the effect of port geometry of intake manifold in a Steam Wankel Expander

Abstract

A volumetric Wankel steam expander has numerous advantages over other positive displacement machines as an expansion device 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. Pressure drop of steam during admission through rotary valves, is inevitable across the intake manifold of the expander during admission duration. These pressure losses during intake, changes the design pressure ratio across the actual expander, which leads to a reduced power output by a reasonable margin of 20 to 30%. Therefore, it is crucial to reduce it to improve the net power output. The goal of the present research is twofold. In the first part, the pressure losses across the intake manifold of the expander is estimated for an existing rectangular port geometry. In the second part, a trapezoidal port profile of same hydraulic diameter is designed for the intake manifold with an aim to reduce the intake losses, thereby delivering a higher power output. The thermodynamic analysis is carried out for the theoretical pressure-volume cycle of the expander using Python 3.8 and the obtained data are fed into the developed CFD model on the intake manifold in ANSYS Fluent 19.2. The boundary conditions are obtained from the aforementioned thermodynamic model, and the state point values are obtained from the REFPROP database. It is observed that the trapezoidal port significantly reduces the pressure losses by a margin of around 50%, thereby delivering around 7 to 21% higher net power output and a increment of isentropic efficiency by a margin of 14% over a range of rotational speed varying from 1200 to 3000 RPM. Further investigations are conducted to study the effect of different fluid flow and turbulent parameters on the pressure loss, power output and isentropic efficiency of the expander.