Thermal design of a compact recuperative heat exchanger for a stirling engine

Abstract

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.