CFD simulations of compressed air two stage rotary Wankel expander – Parametric analysis

Abstract

A small scale volumetric Wankel expander is a powerful device for small-scale power generation in compressed air energy storage (CAES) systems and Organic Rankine cycles powered by different heat sources such as, biomass, low temperature geothermal, solar and waste heat leading to significant reduction in CO2 emissions. Wankel expanders outperform other types of expander due to their ability to produce two power pulses per revolution per chamber additional to higher compactness, lower noise and vibration and lower cost. In this paper, a computational fluid dynamics (CFD) model was developed using ANSYS 16.2 to simulate the flow dynamics for a single and two stage Wankel expanders and to investigate the effect of port configurations, including size and spacing, on the expander’s power output and isentropic efficiency. Also, single-stage and two-stage expanders were analysed with different operating conditions. Single-stage 3D CFD results were compared to published work showing close agreement.The CFD modelling was used to investigate the performance of the rotary device using air as an ideal gas with various port diameters ranging from 15mm to 50mm; port spacing varying from 28mm to 66mm; different Wankel expander sizes (r=48, e=6.6, b=32)mm and (r=58, e=8, b=40)mm both as single-stage and as two-stage expanders with different configurations and various operating conditions. Results showed that the best Wankel expander design for a single-stage was (r=48, e=6.6, b=32)mm, with the port diameters 20mm and port spacing equal to 50mm. Moreover, combining two Wankel expanders horizontally, with a larger one at front, produced 8.52kW compared with single-stage which gave 4.75kW power output at the same operating conditions. Also, a maximum isentropic efficiency of 91% was calculated with inlet pressure of 6bar and inlet temperature of 400K at 7500rpm for the two-stage compared to the 87.25% for the single-stage.