Sudipto Debnath

The flow and heat transfer characteristics of multiple non-swirling and swirling impinging arrays of air jets have been numerically investigated. The air jets discharged from round orifices and perpendicularly impinge on a heated surface. On this array, total 25 nozzles are organized in inline arrangement, having nozzle to nozzle centerline spacing of 2D and nozzle orifice to heated impinging plate distance is fixed at H=2D (where D is the nozzle orifice diameter). Only a quarter portion of the model is constructed for computational analysis in order to save computational costing as the whole model is symmetric. That research is investigated for Reynolds number, Re = 11600 and swirl number, S = 0.74. Grid independence test is performed and a fine mesh consisting 890k nodes is found to be appropriate for this calculation. Calculations have been carried out by the applications of the SST k-ω turbulence models and performed on ANSYS v16.2. The jet flows are in downward direction and gravity is not considered during performing the simulation. Finally, comparisons are made between numerical results for different swirl number and conclusions drawn. This document also shows comparisons between numerical predictions with previously published literature about non-swirling jets. The results demonstrate that, the effects of higher swirl number ensure better industrial performances than non-swirl flows.

Impinging jets are generally utilized as an effective medium of heat and mass transfer. Various stream orientations and swirl effect may be a potential candidate to further improve the heat transfer characteristics. This paper numerically examines two different types of arrays of circular multiple jets and the effect of swirl on jets that impinges vertically onto a flat surface situated at a fixed vertical separation distance H=2D, D the nozzle diameter and at Reynolds number equal to 11,600. In this regard, three-dimensional simulation was performed using finite volume method whereas turbulence was described by SST k-ω model. The results demonstrate that staggered arrangement ensures better mixing and higher velocity as well as heat transfer. The strength of recirculation for swirl jet is larger than that of non-swirl jet. Maximum pressure coefficient occurs at individual stagnation point of each jet, with a secondary peak between two neighboring jets. Strong heat transfer is predicted to occur at around the stagnation region for each non-swirling jet and between the neighboring jets for swirling jets. Turbulences are strong around the periphery of each jet and they coalesce as it moves downward.

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Impinging jets are widely used for their effective heat and mass transfer for several decades. Arrays of jet impingement have also been studied before due to its practical relevance to electronics cooling. A number of jet variations and jet-to-jet orientations have previously been studied, mainly to further improve the magnitude and uniformity of heat transfer. In recent years, swirling jets has also gained interest in heat transfer application due to their inherent mixing and spreading characteristics, which is believed to be an improvement on overall heat and mass transfer. As such, this paper numerically investigates an array of circular jets with and without swirl that impinges vertically onto a flat surface located at a fixed vertical distance H = 2D and at Reynolds number equals to 11,600, where D is the nozzle diameter. As the entire was symmetric, only quarter of the model was constructed for numerical analysis to save computational cost. In this case, numerical calculations were done via commercial software package ANSYS Fluent using SST k-ω turbulence model. Inlet conditions were taken from experimental data. The jet flows were in downward direction and gravity was not considered. This paper also compares numerical predictions with previously published literature for non-swirling and swirling jets.