Mixing and Entrainment Characteristics in Circular Short Ejectors

Abstract

Analytical and experimental investigations have been conducted to characterize the performance of "short" ejectors. In short ejectors, the core of primary (motive) flow still exists at the mixing duct exit, and nonuniform mixed flow is discharged from the mixing duct. Due to incomplete mixing, short ejector pumping performance is degraded and cannot be predicted by the existing "long" ejector models. The new analytical short ejector model presented in this paper is based on the control volume analysis and jet expansion model. The secondary (entrained) flow velocity and the corresponding shear layer (between the primary and the secondary flows) growth rate variations along the mixing duct are taken into account. In addition, measurements have been made in ejectors with length ratios (LRs) of two and three for an area ratio (AR) of 1.95; and a LR of two for an AR of 3.08. Velocity profiles at the mixing duct inlet and exit, and static pressure distribution along the mixing duct have been measured with pitot probes and pressure taps. All of the tests have been carried out at a Reynolds number of 420,000. Comparison shows that the new ejector model can accurately predict flow characteristics and performance of short ejectors. For all of the test cases, the velocity profiles at the mixing duct inlet and exit are well predicted. Also, both predictions and measurements show pumping enhancement with increasing mixing duct length. The pumping enhancement is due to the increase in the static pressure difference between the mixing duct inlet and atmosphere as the mixing duct is lengthened. Furthermore, both measured and predicted static pressure distributions along the mixing duct show a kink. According to the analysis, the kink occurs when the outer shear layer reaches the mixing duct wall, and the secondary flow velocity decreases along the mixing duct upstream of the kink and increases downstream of the kink. Thus, the new ejector model can accurately predict not only the integral performance but also different mixing regimes in short ejectors.