Liquid Jet in Swirling Cross Flow

Abstract

Liquid jets in cross flows (JICF) have been an important part of spray research in the last few decades owing to the wide spectrum of applications ranging from agricultural sprays to aircraft gas turbine engines. However, the current study focuses on the liquid jet injected in swirling cross flows (JISCF), which is more specific to aircraft gas turbine engines. Since this configuration is relatively new in the research domain, various closely related problems are investigated for a better understanding of the spray dynamics. To begin with, the classical-JICF configuration is used to study the influence of liquid jet exit conditions on the resultant spray behaviour. The liquid jet exit velocity profiles are varied by altering the L/D ratio of the plain orifice nozzles between 10 and 100. While the smaller L/D nozzles produce laminar jets, the larger L/D nozzles allow the liquid to transition to fully turbulent conditions. Laser diagnostics are employed to measure the spray trajectory and drop sizes. It is observed that the laminar jets penetrate further into the cross flow as compared to the turbulent jet counterparts for similar flow conditions and produce larger droplets. This is attributed to the turbulent jet undergoing early breakup due to the inherent instabilities in the jet. This learning is then carried forward to experimental studies on liquid jet in swirling cross flow (JISCF) by employing longer L/D nozzles to promote better atomization. The swirl flow is generated with the aid of 3-D contoured axial swirl vanes with 30º and 45º exit angles and compared with baseline no-swirl case. The resultant spray shows interesting and counter-intuitive behaviour under the influence of swirl flows. While the centrifugal forces are expected to carry the droplets radially outward, the aerodynamics in the annular space has the opposing effect in the ABSTRACT ii wake regions of the jet. The drop size measurements also reveal the presence of coalescence among the droplets, which was not observed to be significant in classical JICF studies. Higher swirl numbers also caused the spray to bifurcate resulting in the larger droplets impinging on the outer wall much earlier as compared to the lower swirl conditions, whereas droplet impingement on the wall is not observed for no-swirl conditions. The JISCF problem is also studied computationally using an open source code, Gerris. As part of a validation study, secondary breakup is first simulated at a realistic density ratio of 1000:1. The Weber numbers were varied to cover the known regimes of breakup. Bag-and-stamen breakup, multi-bag breakup and shear breakup were captured with a close match to experimental results. The JISCF simulations are then carried out using similar annular geometry as that of the experiments for a density ratio of 180:1 owing to inherent limitations existing in the multi-phase numerical methodology. The spray characteristics such as trajectory and drop size distributions are analysed. The spray trajectory is observed to move radially outwards with increasing swirl numbers. The drop size measurements again indicate coalescence as the SMDs are observed to increase along the downstream direction. The numerical model is observed to effectively predict the qualitative behaviour of the spray in swirling cross flow. Overall, the present study helps further our understanding of the spray characteristics produced by liquid jets in swirling cross flows with combined efforts on experimental and numerical fronts.