Heat transfer with impinging gaseous jet systems
Abstract
Heat transfer during materials processing and manufacturing is enhanced through jet impingement for many different applications including, the tempering and shaping of glass, the annealing of metal and plastic sheets, and the drying of textiles, veneer, paper, and film materials. Literature studies have examined average convective coefficients for jet arrays, but the understanding of the observed trends in the average transport coefficients and local effects are limited. Therefore, the physical mechanisms of impinging gaseous jets that affect the heat transfer characteristics were investigated. A liquid crystal technique was developed to measure local and average convective coefficients for impinging air jets, and the effects of separation distance, jet-to-jet spacing in an array, Reynolds number, and spent air exits were examined. Small separation distances, H/D $\le$ 1, were found to significantly enhance the local and average convective coefficients for both single and multiple jet systems. Besides the presence of local secondary maxima situated in rings around the stagnation point of an individual jet, the dominant physical mechanism which influences the heat transfer coefficient and separation distance relationship for jet arrays is adjacent jet interference before impingement. Thus interference before impingement is minimized by using small separation distances. The array jet-to-jet spacing was found to significantly affect the uniformity and magnitude of the average convective coefficient by varying the percentage of impingement surface area covered by the stagnation and wall jet regions. The larger percent area covered by the stagnation region with high local convective coefficients (smaller jet-to-jet spacing) resulted in higher average and more uniform local values. However, on an equivalent mass flux basis the larger jet-to-jet spacing was most efficient in cooling the impingement surface. For small separation distances the Reynolds number was found to have a strong influence on the convective coefficient by increasing the size and magnitude of the secondary maxima in the local coefficient distributions. Spent air exits resulted in reduced wall jet interactions between adjacent jets and a more uniform coverage of the impingement surface. The elimination of crossflow is the main benefit of spent air exits for all but small separation distances.
Degree
Ph.D.
Advisors
Viskanta, Purdue University.
Subject Area
Mechanical engineering
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