Energy is an essential and critical commodity and our reliance on it has fuelled much of the debate and interest in society and academia alike. Environmental concerns, depleted energy resources and higher energy prices are the main factors that drive this interest. Energy efficiency is one of the main avenues to preserve and better utilize this valuable commodity. The energy exchange by employment of heat exchangers is extensive and tube-fin heat exchangers are widely used in industrial and commercial applications. Smarter designs could not only improve energy efficiency but may also save on material costs. Although mass production and improved manufacturing techniques have reduced manufacturing costs, tube fin heat exchangers have not evolved greatly to take advantage of these improvements. There has been a large range of fin surface enhancements proposed, such as waffled fins or louvres and while limited improvements in capacity have been achieved, this is generally accomplished at a much larger pressure drop penalty. Numerous studies have been performed in order to examine the potential of various surface enhancement geometries on an ad hoc basis. These presumably operate on the basis of enhanced convection due to increased turbulence levels. However very few of these studies examine the actual nature of turbulence that is responsible for convection enhancement. A series of experiments and numerical studies have been conducted to quantify the effect of the turbulence vortex characteristics on the air side heat convection of a tube-fin heat exchanger. Homogeneous, transverse and streamwise vortical structures were investigated. The thermal transfer performance resulting from these flows was compared to that of standard louver fin geometries by considering sensible heat transfer only, applicable to radiator applications. Several novel coils designed to achieve these vortex structures, were developed and their heat transfer characteristics were quantified. These coil designs can be described as the Tube Mesh, Tube Strut and a Delta-Winglet fin surface.The Tube Mesh heat exchanger consisted entirely of horizontal and vertical tubes arranged in an approximate homogeneous turbulence generating grid. While they had a lower heat transfer of between 53% to 63% of that of the louvre fin surface, they had an extremely low pressure drop of 25% to 33%. This has the potential to make them suitable for certain low pressure drop applications, especially if energy saving is a prerequisite. The range of Tube Strut coils consisted of a tube bundle with interconnecting heat conducting struts to form a parallel plate array were also investigated. Three different strut thicknesses and strut spacing were trialled. In general these had similar performance to the tube mesh at 45% to 65% the heat transfer capacity of the louver fin surface. The resulting pressure drop was 38% to 42% of that of the louver fin surface. A delta-winglet design which positioned the deltas in a flow up configuration just in front of the tubes was examined. It was found that this configuration had an almost comparable capacity of 87% to a louver surface having the same fin pitch. On the other hand it had approximately half the pressure drop of 54% of the similar louver fin surface. This particularly low pressure drop makes this design preferable from an energy utilisation perspective. While a slight increase in coil area is required, this is offset by an almost 50% reduction in operating costs by reducing the parasitic energy requirements of the convection fans. The experimental data gathered for this Delta-Winglet design served to validate a succession of numerical simulations which were performed to estimate the performance of other configurations of multiple vortex generators. In addition the performance of combining a delta-wing with a louvred surface was investigated. It was found that increasing the number of delta-winglets or combining deltas with a louvred surface provided little improvement in heat transfer but increased pressure drop substantially. The louvre design itself was examined, and simulations were undertaken to estimate the effect of louvre angle, as well as louvre pitch. A hitherto unexamined concept was to investigate the effect of having louvres with serrated edges. It was found that an increase in louver angle by 5 degrees had negligible effect on heat transfer but increased the pressure drop by 17%. A variation in louver pitch showed a minimal variation in both heat transfer and pressure drop. Surprisingly a serrated louver showed a slight reduction in both heat transfer and pressure drop but this was miniscule. It was established throughout the course of the investigations that the bulk of the coil heat transfer is performed by the first tube row. Therefore the potential for increasing heat transfer by shifting some heat exchange to the down stream rows was examined. This was attempted by having progressively increasing louvre angles from the front of the coil to the rear. While a slight increase in heat transfer performance was achieved, this accomplished at the expense of a 13%-14% increase in pressure drop. The outcomes have shown that substantial net improvement of heat exchanger energy efficiency can be achieved through optimization of the turbulence generation along the fins of a tube fin heat exchanger. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1253254 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2006
| Identifer | oai:union.ndltd.org:ADTP/285580 |
| Date | January 2006 |
| Creators | Allison, Colin Bidden |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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