Performance analysis of heat transfer processes from wet and dry surfaces : cooling towers and heat exchangers
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Doctoral thesis (article-based)
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Report / Helsinki University of Technology, Department of Mechanical Engineering, Laboratory of Heating, Ventilating and Air Conditioning. A, 10
AbstractThe objective of this work is to study the thermal and hydraulic performance of evaporatively cooled heat exchangers, including closed wet cooling towers, and dry tube heat exchangers with various geometries. Applications utilising such equipment exist in almost every thermal process. The investigation includes theoretical analysis, computational approaches, and experimental measurements. In this work, a computational model is presented for the thermal performance of closed wet cooling towers intended for use in conjunction with chilled ceilings in cooling of buildings. A variable spray water temperature inside the tower is assumed. A prototype tower was subjected to experimental measurements to find its characteristics. Optimisation of the tower geometry and flow rates for specified design conditions is carried out in order to achieve a high value of the coefficient of performance (COP). Results from a global simulation program (including the tower model, a transient building model, a chilled ceiling model, system control etc.) show that closed wet cooling towers can be used with chilled ceilings to achieve acceptable indoor air temperatures in locations having suitable climatic conditions. This is supported by published results from a performance test of an office building using this method of cooling. Simplification of the model is obtained by assuming a constant temperature for the spray water. The tower performance predicted by the simplified model and the computational model shows comparable results. The results of the simplified model are then incorporated with Computational Fluid Dynamics (CFD) to assess the temperature distribution inside the tower. It is shown that CFD can be implemented to study the effect of air distribution inside the tower on its performance. The effect of introducing plate fins in evaporatively cooled plain circular tubes is experimentally studied. The measurement results show a 92% to 140% increase in the amount of heat transfer for the finned tubes. This is accompanied by an increase in the pressure drop, so that an indication of the combined thermal hydraulic performance is found to be close for the two geometries. However, it shows higher heat transfer rates per volume for the finned tubes. The performance of oval tubes in the evaporatively cooled heat exchanger is then experimentally investigated. The measurement results for the oval tubes show good heat and mass transfer characteristics; its average mass transfer Colburn factor is 89% of that for the circular tubes. Furthermore, it shows low friction factor for the air flow, which is 46% of that for the circular tubes. It is concluded that the tested oval tube is better than the circular tubes in combined thermal hydraulic performance. The features of oval tubes appear clearer in a dry heat transfer process. Five shapes of dry oval tubes are experimentally investigated in a cross-flow of air. The measurement results for the oval tubes are compared with those for an equivalent circular tube. It is found that the Nusselt numbers NuD for the studied tubes are close for Reynolds numbers ReD < 4000. While for higher ReD, the NuD decreases with the increase of the oval tube axis ratio. The drag measurements indicate lower drag coefficients Cd avg for the oval tubes. It is revealed that the investigated oval tubes have favourable combined thermal-hydraulic performance, which is expressed in terms of (NuD / Cd avg). The ratio of (NuD / Cd avg) for the oval tubes to that for the circular tube is from 1.3 to 2.5.
cooling tower, heat exchanger, heat transfer, mass transfer, fin, oval
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- Hasan A., 2004. Thermal-hydraulic performance of oval tubes in a cross-flow of air. Heat and Mass Transfer, accepted for publication. [article5.pdf] © 2004 by author and © 2004 Springer-Verlag. By permission.