Comparison of mean-line design methods on matching characteristics of three coaxial-impellers in an aircraft environmental control system

High-speed turbo-cooler of coaxial expander-compressor is a promising solution for reverse Brayton cycle refrigeration systems to improve compactness, power recovery and efficiency. The performance curves of coaxial impellers are crucial for their matching characteristics as well as the system efficiency under off-design conditions. In this paper, three mean-line methods for the design of high-speed turbo-coolers are evaluated by comparing the matching characteristics of three coaxial-impellers, including the specific speed and characteristic ratio (SS-CR) approach, the power ratio (PR) approach and velocity coefficient (VC) approach. A coupling model is established to predict the off-design performance of turbo-cooler and the system size. The results show that the rotating speed and mass flow rate by all three methods lie within 2% of each other. At the design point, SS-CR approach gives 84.0% expander efficiency compared to 83.0% by PR approach and 80.9% by VC approach, but the PR approach has better performance at speeds below 35 krpm. The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger designed with all the three methods. However, SS-CR gives 6.0% advantage in system volume 5.6% over VC approach and 3.7% over PR approach. When the cooling capacity is controlled at 2000 W, SS-CR approach gives 79.3% expander efficiency compared to 78.9% by PR approach and 76.4% by VC approach.