LOCAL HEAT-TRANSFER TO IMPINGING LIQUID JETS IN THE INITIALLY LAMINAR, TRANSITIONAL, AND TURBULENT REGIMES

Transport from small diameter, fully-developed liquid jets impinging n ormally on a constant heat flux surface has been investigated. This st udy focuses on jet Reynolds numbers spanning the laminar, transitional , and turbulent flow regimes at the nozzle exit, 300 less-than-or-equa l-to Re less-than-or-equal-to 7000. Jet diameters studied include 0.58 4, 0.315, and 0.246 mm. Both free-surface and submerged jets were stud ied. Local heat transfer coefficient information was collected, and th e radial variation of the Nusselt number is explored. Correlations of stagnation Nusselt number as a function of Reynolds number for laminar and turbulent data are presented. The Nusselt number was observed to correlate approximately with Re0.5 and Re.0.8 for initially turbulent and laminar jets, respectively. This dependence was observed for both free-surface and submerged jet configurations. The Re0.8 dependence of the Nusselt number in the laminar regime for free-surface jets is att ributed to surface tension-induced jet broadening at the jet exit. For the submerged jet the Nu(o) is similar to Re0.8 laminar regime functi onality is explained by the dominating effects of jet destabilization. The free-surface configuration heat transfer data showed little depen dence on nozzle-plate spacing. By contrast, the submerged jet data exh ibit the usual potential core behavior for turbulent flow with its wel l-established dependence on nozzle-to-plate spacing. The Nusselt numbe r was seen to be independent of nozzle-to-plate spacing in the initial ly laminar jet regime.