F ree-surface jet heat transfer reexamined: from indifferent regimes to the re-emergence of off-center peaks
School of Mechanical Engineering Seminar
Wednesday, December 2, 2020 at 14:00
Free-surface jet heat transfer reexamined: from indifferent regimes to the re-emergence of off-center peaks
Msc of Dr. Herman Haustein
Laminar jet impingement is an efficient method for heat transfer processes, though much of its hydrodynamics and the resulting convection are still not fully understood. In this work, it is shown that, stagnation-point heat transfer depends directly on the near-axis radial acceleration, varying strongly with nozzle diameter, normalized nozzle length, normalized nozzle-to-plate spacing, and flow rate. Studying the hydrodynamics of all stages in the jet’s history up-to plate impingement, therefore, led to the development of a universal heat transfer model.
In this study experiments and two-phase flow simulations were conducted to determine the flow regime of a jet inside a nozzle, during free flight, upon impingement and subsequent wall flow. From the results it was seen that existing theory deals only with two extremes, and therefore a general description is required. Relying on previous pipe-flow analysis by our group, the crucial dynamics of the jet’s centerline velocity decay and evolution of the jet width were dealt with under varying levels of flow rate, issuing velocity-profile development and relaxation, surface tension and gravity. In other words, the dependence of the arriving jet’s key characteristics on all relevant crucial dimensionless numbers (Re, L/d, H/d, We & Fr, accordingly) was addressed. For the jet’s impingement, a previous streamline-bending analysis was adapted to the present case of a free-surface jet, thereby tying the characteristics of the velocity profile arriving at the wall and transitioning from free flight to stagnation flow. The present analysis recovers the radial acceleration’s dictation of stagnation heat transfer, as well as the downstream radial distribution. The model is seen to be valid up to the point of boundary layer emergence from the wall-jet film, beyond which Watson’s classical solution is seen to be valid if appropriately scaled.
Finally, it is seen that the present analysis is physically sound and is able to continuously capture the occurrence or re-emergence of an off-center peak in the heat transfer which occurs for under-developed or over-relaxed arrival profiles – a phenomena previously dealt with only in a case-specific manner. The study has also led to the clarification and definition of a criterion for the occurrence of the “indifferent jet regime” – wherein gravity’s acceleration is countered by the profile relaxation to generate a constant-centerline velocity jet. Such a jet’s stagnation heat transfer is indifferent to the nozzle-to-plate spacing, and has multiple useful applications.
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