Application of Metal Foams for Improving the Performance of Air-Cooled Heat Exchangers

Mostafa Odabaee (2011). Application of Metal Foams for Improving the Performance of Air-Cooled Heat Exchangers MPhil Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Mostafa Odabaee
Thesis Title Application of Metal Foams for Improving the Performance of Air-Cooled Heat Exchangers
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-04
Thesis type MPhil Thesis
Total pages 75
Total colour pages 11
Total black and white pages 64
Subjects 09 Engineering
Abstract/Summary This thesis investigates the application of metal foam heat exchangers to the air-cooled condensers of geothermal power plants in Australia where the resources are mostly located at the arid areas where there is no water for evaporative cooling of the power plant. One way to remove the heat from the thermodynamic cycles is to use (the most common) finned-tube heat exchangers, consisting of tubes with external fins to increase the air-side heat exchange surface. Another possible alternative is a class of designed porous materials called metal foams, containing such advantages as low-density, high area/volume ratio, and high strength structure. Therefore, they have been applied in a variety of industrial. The numerical study, in three main sections, has been conducted to explore that possibility. First, a comparison between the performance of a metal foam-wrapped solid cylinder in cross-flow and a commercially available finned-tune heat exchanger is investigated. Effects of the key parameters including the free-stream velocity and characteristics of metal foam such as porosity, permeability, and form drag coefficient on heat and fluid flow are examined. Being a determining factor in pressure drop and heat transfer increment, the porous layer thickness is changed systematically to observe that there is an optimum layer thickness beyond which the heat transfer does not improve while the pressure drop continues to increase. This has been verified by the application of Bejan’s Intersection of Asymptotes method. Results have been compared to those of a finned-tube heat exchanger to observe a higher area goodness factor for metal foam-wrapped cylinder. In the second part, an optimization study is carried out to discover an optimized design of metal foam heat exchangers as replacements for finned-tubes in air-cooled condensers of a geothermal power plant. Two different optimization techniques, based on first and second law (of thermodynamics) are reported. While the former leads to the highest heat transfer rate with as low pressure drop as possible, the latter minimizes the generated entropy in the thermodynamic system. Interestingly the two methods lead to the same optimal design. The new design has been compared to the conventional air-cooled condenser designed and optimized by using the commercially available software ASPEN. It is shown that while the heat transfer rate increases significantly (by an order of magnitude) compared to the finned tube for the same main flow obstruction height, the pressure drop increase is within an acceptable range. Further comparison between the two systems are carried out, making use of Mahjoob and Vafai's performance factor developed specifically for metal foam heat exchangers. Following that, the third part explores the heat transfer from a metal foam-wrapped tube bundle. Effects of key parameters, including the free stream velocity, longitudinal and transversal tube pitch, metal foam thickness and characteristics of the foam on heat and fluid flow are examined. It can be observed that the performance of the metal foam heat exchangers, measured in terms of area goodness factor, can be about four times better than that of the conventional design of finned-tube heat exchangers. It is also found that even a very thin layer of metal foam, when wrapped around a bare tube bundle, can significantly improve the area goodness factor. Finally, it is shown that while friction factor is more sensitive to the metal foam permeability than its porosity, the converse is true when it comes to the Colburn factor.
Keyword Metal foam, numerical, heat exchanger, finned tube, intersection of asymptotes
Additional Notes 17 24-27 36 38-39 61-62 73

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Created: Wed, 20 Jul 2011, 12:03:30 EST by Mr Mostafa Odabaee on behalf of Library - Information Access Service