Heat Transfer Characteristics of Confined and Unconfined Submerged Air Bubble Jet Impingement

Heat transfer characteristics of two types of jet impingement, confined and unconfined is investigated using air and water as the working fluids. The effects of volumetric quality are evaluated on the lateral variation of the Nusselt number at low nozzle-to-plate spacing. The results show that the lateral Nusselt number of the unconfined submerged air bubble jet is similar to that of the confined submerged air bubble jet in Region I (bubble collision region) due to the effect of bubble collision. In Region II (bubble sliding region), however, the lateral Nusselt number of the unconfined jet was found to be 30%-40% lower than that of the confined jet flow due to the buoyancy effect that causes air bubbles to rise in Region I. In addition, from the flow visualization, high velocity bubble movement was observed from the midpoint of the heating surface to the periphery of the confinement plate in the confined jet set-up. This movement enhances the lateral Nusselt number in Region II. In the unconfined jet set-up, air bubbles rise past the heating surface in Region I and thus neither slide across the heating surface nor increase the lateral Nusselt number in Region II.