Local heat transfer to impinging liquid jets in the initially laminar, transitional, and turbulent regimes

Abstract

Transport from small diameter, fully-developed liquid jets impinging normally on a constant heat flux surface has been investigated. This study focuses on jet Reynolds numbers spanning the laminar, transitional, and turbulent flow regimes at the nozzle exit, 300 ⩽ Re ⩽ 7000. Jet diameters studied include 0.584, 0.315, and 0.246 mm. Both free-surface and submerged jets were studied. Local heat transfer coefficient information was collected, and the 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 Re 0.5 and Re 0.8 for initially turbulent and laminar jets, respectively. This dependence was observed for both free-surface and submerged jet configurations. The Re 0.8 dependence of the Nusselt number in the laminar regime for free-surface jets is attributed to surface tension-induced jet broadening at the jet exit. For the submerged jet the Nu 0 ∼ Re 0.8 laminar regime functionality is explained by the dominating effects of jet destabilization. The free-surface configuration heat transfer data showed little dependence on nozzle-plate spacing. By contrast, the submerged jet data exhibit the usual potential core behavior for turbulent flow with its well-established dependence on nozzle-to-plate spacing. The Nusselt number was seen to be independent of nozzle-to-plate spacing in the initially laminar jet regime.