Heat Transfer Performance of an Air-Assisted Impinging Water Jet Under a Fixed Pumping Power Constraint

Air injection into a liquid impinging jet has been shown to be a method of improving non-phase change heat transfer rates by up to twice the normal amount. Previous work has shown that there exists an optimal operating point in terms of the volumetric fraction of air injection when the pumping power is held constant because of an optimal two-phase flow pattern. However, previous work focused on heat transfer from the impingement point only, and neglected performance at other points. The present work studies the local heat transfer performance of an air-assisted water jet, at the impingement point and at positions moving radially outward, under constant pumping power conditions. The area-averaged heat transfer is also considered. Heat transfer at the stagnation point is shown to be optimized between β = 0.1∼0.2, where a bubbly flow pattern exists. Nu_avg(r/D ≤ 1) is optimized when the flow pattern was plug-flow and off-center peaks in Nu_r exist. Nu_avg(r/D > 1) is optimized when the water is accelerated by the injected air, but splattering is avoided. Flow patterns have no direct effect outside the impingement region.