Studies on Mixing Characteristics in the Critical Flow Regime of Supersonic Ejectors

Abstract

Ejectors are passive devices having ubiquitous applications in several engineering domains, including modern-day applications in supercritical CO2-based power and refrigeration cycles. The ejector compresses a low-enthalpy fluid (secondary flow) through gasdynamic interactions with a co-flowing high-enthalpy fluid (primary flow). The ejector operates in the critical mode when the secondary flow is choked aerodynamically, and the entrainment ratio (ER, ratio of secondary to primary mass flow rate) becomes independent of the compression ratio (CR, ratio of the ejector exit pressure to the secondary flow stagnation pressure). In the mixed mode of operation, the ER decreases with CR. Applications of ejectors in energy conversion systems prefer the critical mode of operation. Ejectors have been studied using experimental, analytical, and computational tools with an emphasis on evaluating performance parameters. The gasdynamic mixing which drives the performance of the ejector is not well understood. An optical diagnostic evaluation of mixing in the ejector has been performed for the mixed mode of operation. The mixing characteristics and flow structures inside an ejector are not reported for the critical mode operation, which is the prime motive of this work. With an aim to achieve the critical flow regime, eight mixing duct geometries of different lengths and heights and three supersonic nozzles are designed and fabricated. Experiments are conducted in the supersonic ejector facility at Laboratory for Hypersonic and Shockwave Research, Indian Institute of Science, Bengaluru. Mass flow rate measurements yield the ER, and the static pressure profile indicates the mixing progress and compression. High-speed schlieren images are captured to observe unsteady flow features. Mixing characteristics in terms of non-mixed length (Lnm) is quantified using the planar laser Mie-scattering technique...