Heat transfer from swirling impinging jets

Abstract

This thesis is concerned with the determination of the local and average mass/heat transfer--from swirling jets impinging orthogonally onto flat surfaces. ApplicatioCn> of swirl alters the flow field of the' jet considerably and eventually the maximum velocity in the jet is displaced from the axis resulting in a typical 'double-peak' profile. Further increase of swirl , can even result in a recirculation in the inner core of the flow. The turbulence characteristics of the jet are also affected. it is expected that these changes will, in addition, modify the heat transfer behaviour. Consequently, a study of both single free jets and a square array (of 3x 3) of jets was undertaken. The range of swirls examined in this study was from zero through to weak and then medium swirl, i. e. the swirl number S was varied from 0 to 0.48. The other parameters varied in the experiments were the nozzle-to-target spacing z/D from 2 to 12, the nozzle pitches x/D (for arrays of jets) which were 3.2,4.8 and 6.4, and the jGt Reynolds number. In the single frce jet tests, two flow rates corresponding to ReD-":2 32pOOO and 60,000 were studied whilst, for the 'multiple' jets, the measurements were confined. to the lower of these Reynolds numbers. (ReD is based on the mean exit velocity in the non-swirling case and the diameter of the nozzle. ) Limited velocity and turbulence measurements were also undertaken on the single jet to characterise the flow and also to compare the behaviour of the present jets with those in previous studies. The flows were similar to those observed previously for swirling jets so that the heat transfer results should be generally applicable. A 'thin-film' naphthalene sublimation technique was used to measure mass transfers over the target surface. The heat transfer coefficients were then derived using the Chilton-Colburn analogy. A rig was developed to spray a uniform coating of naphthalene on the target surface. This 'thin-film' technique was found to provide repeatable results and the validity of the experiments was further assessed by comparing the results with previously published data for the no swirl case. These were in reasonable agreement. For the single free jets, the application of swirl was found to continuously reduce the heat transfers. The heat transfers, however, became more uniform. An empirical correlation has also been suggested for the average Nusselt number associated with these single swirling ' jets and is valid for S= 0.12-to 0.48. In some circumstances in the 'multiple' jet tests (e. g. at close nozzle-to- target spacings) the average heat transfers increased to a maximum of IS =0.24 (approximately). Further increases in the degree of swirl brought about a subsequent reduction in average heat transfer coefficients until eventually the performance of the swirling jets was poorer than that of the non-swirling flows. 'This discrepancy in behaviour can be explained in terms of changes in the local heat transfer distributions