Jet Impingement Heat Transfer from a Circular Cylinder Located Between Confining Walls

Abstract

*The heat transfer characteristics of a circular cylinder (diameter d) located between confining walls (at varying separation distance H apart) exposed to slot jet of air (of width w) from a contoured nozzle, and at a range of spacing z from the jet exit, has been studied experimentally and computationally. The study focussed on Reynolds number Re in the range 1000 – 12000. The results reveal that while the slot jet impinging on a cylinder in confined space generally yields higher average heat transfer rates relative to the uniform cross flow case; these are however almost always found to yield lower heat transfer rates in comparison with corresponding slot jet impingement without confining walls. Furthermore, in the Reynolds number range studied, it was found that a dimensionless confinement spacing H/d = 10 exists which consistently exhibits a minimum in cylinder heat transfer rate over all the non-dimensional jet exit to cylinder spacing z/w and ratio of cylinder diameter to slot width d/w that were investigated. It was established that reduction in heat transfer for confinements H/d between 10 and 16 is due to the jet becoming unstable and thereby periodically flapping across the cylinder, switching between the confining walls, in tandem with vortex shedding from the cylinder. This reduces the effectiveness of impingement and hence reduces the heat transfer rates. It is therefore concluded that jet impingement does not always enhance heat transfer rate, but may in fact reduce it under conditions in which the impingement target is located in the midst of confining walls. It was observed that the average of all the Nusselt numbers in a confined space reduces by about 17% when compared with a non confined cylinder. In summary therefore, the influence of confining walls around the cylinder has an unfavourable impact on the flow and therefore the heat transfer rates with jet impingement. The reduction in heat transfer rates with confining walls implies that a jet impingement heat transfer system designed using data obtained for jet impingement on a non-confined cylinder could compromise the effectiveness of the system, and may have a large bearing on the quality, efficiency and productivity as well as on the health and safety aspects of products and processes.