Browsing by Author "Ward, J."
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Item Open Access The effect of nozzle inclination on heat transfer in jet impingement systems(Cranfield University, 1981-09) Oladiran, M. T.; Ward, J.; Hammond, G. P.Jet impingement heating and cooling techniques are used extensively in industrial applications. in some of these installations, the axis of the jet can be inclined relative to the impingement surface. The impingement flow is then unsynLmetrical so that the heat transfer rates are modified. At present, there is a lack of information concerning the effect of inclination on jet impingement heat transfer. Thus, the experimental study reported in this thesis is primarily concerned with the measurement of local and average heat transfer coefficients associated with the impingement of inclined turbulent circular jets onto flat plates. A single free jet exiting into initially stagnant surroundings was considered and the nozzle inclination was varied from 300 to 900 to the surface. The tests covered the range: Z/d (nozzle-target separation) of 6 to 16 and Re (jet Reynolds number based on exit conditions) of 32500 to 65000. The effect of the exit nozzle shape was also determined. In multiple jet systems, the flow from the upstream jets can significantly affect those situated in the downstream section. Thus, the effect of nozzle inclination on the performance of an impinging jet exiting into a cross flow was also investigated. Thus, as well as the angle of inclination (a), the magnitude of the cross flow (Uc) and the width of the duct (H/d) were also altered in this confined situation. The ranges of these variables were 300Item Open Access Flow and heat transfers associated with impinging jets in crossflows(Cranfield University, 1977-11) Kabari, L.; Ward, J.This thesis reports the results of an experimental study into the flow and heat transfers associated with both inclined and orthogonally impinging axisymmetric air jets. The majority of previously reported studies have been mainly confined to orthogonally impinging jets in stagnant surroundings. In this investigation, free jets as well as the effects of crossflows are considered. This investigation is Primarily concerned with local heat transfer variations. The experimental tests were. conducted with a single 12.7 mm diameter jet impinging on a flat surface, and heat transfers were evaluated using a heat-mass transfer analogy (t e Chilton-Colburn analogy). The sublimation of naphthalene was employed as the mass transfer technique. The flowfield associated with impinging jets has a significant influence on their heat transfer characteristics., In view of the present limited level of understanding of this 'complex' flowfield, extensive flow visualisation techniques were employed in this present investigation. Those were primarily intended to aid interpretation of the experimental heat transfer results, and also to provide further physical understanding of the flowfields resulting from the interactions between impinging jets and crossflowing streams. The flow and heat transfer tests conducted in the programme of work reported in this thesis covered typical ranges of flow parameters of interest in many practical applications of jet impingement systems. Jet inclinations of 45°, 600, and 90°, nozzle to target spacings of 2,11, and 8 nozzle diameters were studied. The Reynolds nuinbers were 30,200,32,700 and 55,100 and mass velocity ratios In the range 4.0 to 3.8 were studied. The effects of these parameters on the flow and heal transfers associated with impinging jets are reported. Comparisons were drawn between the heat transfer results and those of previously reported studies where appropriate.Item Open Access Heat transfer from swirling impinging jets(Cranfield University, 1980-04) Mahmood, M.; Ward, J.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.Item Open Access Thermal design of a compact recuperative heat exchanger for a stirling engine(Cranfield University, 1977-12) Jewad, M. A.; Ward, J.This thesis is concerned with factors affecting the thermal design of a compact recuperative type crossflowing heat exchanger for the primary heater of a Stirling engine. The exchanger is constructed of small diameter metal tubes (in the range of 3.0 mm to 6.0 mm) and close spacings are maintained between the tubes (i. e. in the range of 0.30 mm to 1.80 mm). These small slender tubes are usually arranged in single or double rows and are mounted around the combustion chamber. The exchanger is used to heat hydrogen or helium which act as a working fluid for the Stirling cycle. A survey of the published literature indicated that the available data does not include results for the tube Geometries of interest in this study. Consequently the heat transfers and hydraulic resistances were measured experimentally for a single row of small diameter, closelyspaced tubes situated in a crossflowing fluid stream. The Reynolds numbers (bc,.s ed on the mainstream fluid velocity and the tube diameter) ranged between 300 and 6500. Where possible the accuracy of the experimental procedure was checked by comparing the present results where possible with those obtained by previous workers. Several arrangements of both bare tubes and tubes fitted with extended surfaces were studied, The results were analysed and discussed, and where appropriate compared with those published in the open technical literature. In most comparative cases excellent agreement was experienced and any departure, could be explained. For the bare tube arrangements the influences of flow blockage ratio, mainstream turbulence intensity and surface roughness on the average heat transfer performance were investigated. A comparison of the heat transfers and pressure drops characteristics of the different tube arrangements led to proposals for an optimal exchanger geometry. The validity of the empirical corrections suggested by previous workers to account for the influence of flow blockage on average heat transfers was examinod. An alternative modified empirical expression was then proposed for the particularly high flow blockage situations (D/1I>0.85). It was found that at those high flow blockages the overall average heat transfers were independent of the mainstream turbulence intensity. However preliminary tests suggested that an increase in the tube surface roughness increases the average tube heat transfers. Since the proposed heat exchanger operates at the higheet possible mean metal temperatures, it is likely that 'hot spots' occurring due to variations in local heat transfers can lead to premature failure. Consequently a detailed study of the local heat transfer distributions is presented for various Geometrical conditions. The influences of blockage ratio and mainstream Reynolds numbers are examined and the results are analysed, and discussed, and where possible compared with other published data. The accuracy of experimental procedure employed in these tests was checked by comparing the results for a single cylinder ease with those reported by other investigators. The influence of fitting a single longitudinal fin to the rear of tubes on both the heat transfer and pumping power was studied. The tube diameter- in these tests was kept constant, at 6.0 mm, but the angle of inclination of this longitudinal fin was varied incrementally so that an optimal angle for maximum performance is recommended. In a similar manner, transverse finned tubes with two different fin spacings were also investigated. The heat transfers and pressure losses obtained for the different finned tube arrangements were compared with each other and with those obtained for the bare tube geometries so that an optimal tube configuration was proposed . The data presented in this thesis were generalized, where possible, so that the results should be useful for future work. They should thus contribute to an understanding of the basic phenomena associated with modern compact heat exchangers. Recommendations for further work are also presented.