The mathematical equations describing transient heat transfer between the fluid flowing through a fixed bed and a moving bed of packing were formulated. The resistance to heat transfer within the packing due to its finite thermal conductivity was taken into account. An approximate integral method was applied to obtain an analytical solution to transient response of the bed packing. Results for two cases of fixed and moving bed were obtained. The validity of the approximate method was checked against the more exact method employed by Handley and Heggs who obtained the results for a fixed bed of packing with a step change in fluid inlet temperature. It was concluded that the approximate method gives results that agree well with the more exact methods. The method considered here provides a quick determination of the packing mean temperature in order to obtain the effectiveness. The other peculiarity of this method is that the effect of packing thermal conductivity can be examined very quickly since the solution is in analytical form. The analysis of the results revealed that as the thermal conductivity of the packing decreases the difference between its surface and mean temperature increases. A series of charts showing the comparison between the packing surface and mean temperatures for different thermal conductivities are presented. The approximate method was a moving bed of packing. It was packing thermal conductivity is series of charts representing versus dimensionless length conductivities are presented. then applied to the case of concluded that the effect of more severe than expected. A the moving bed effectiveness for different thermal / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Thesis (MEng)--Stellenbosch University, 2000. / ENGLISH ABSTRACT: The main objective of this study to determine the air-side heat transfer and pressure drop performance characteristics of commercially available round and elliptical fined tubes. A . computer code to compare the performance of industrial forced and induced draught aircooled heat exchangers (henceforth referred to as f\CHE's) commonly found in the petrochemical industry is also developed. The comparison is extended to include both round and elliptical finned tubes. From the tests conducted, it is found that there is generally an increase in the heat transfer parameter with a decrease in the fin pitch. The decrease in the fin pitch however also results in an increase in the pressure drop across the tube bundle. The performance of the tubes is compared with round reference tubes having circular or plate fins for which performance correlations are available in the literature. The fan power required by an induced draught aircooled heat exchanger is found to be higher than that required by a forced draught heat exchanger rejecting the same amount of heat. / AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie studie is om die werksverrigting van industriële geforseerde trek -- en geinduseerde trek lugverkoelde warmteruilers (LVWR) soos algemeen aangetref in die petrochemiese nywerheid te vergelyk. Warmteruiier bundels word gewoonlik opgebou uit ronde vinbuise. Die werksverrigting van laasgenoemde word vergelyk met die werksverrigting van warmteruiier bundels wat gebruik maak van elliptiese vinbuise. Die werksverrigting van 'n reeks kommersieël verkrygbare ronde en elliptiese vinbuise word deur middel van toetsing bepaal. In die algemeen word 'n toename in die warmteoordrag-parameter waargeneem met In afname in die vinsteek. 'n Toename in die vinsteek gaan egter gepaard met" 'n toename in die drukverlies oor die bundel. Die werksverrigting van hierdie buise word vergelyk met bestaande korrelasies vir die werksverrigting van ronde verwysingsbuise wat gebruik maak van ronde of plaatvinne. Daar is bevind dat die drywing wat benodig word deur 'n waaier vir 'n geinduseerde trek lugverkoelde warmteruiler, heelwat hoër is as in die geval van 'n geforseerde trek lugverkoelde warmteruiler, vir dieselfde hoeveelheid hitte verwydering

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PhD, Faculty of Engineering and the Built Environment, University of the Witwatersrand / In this study the heat transfer and frictional pressure drop performance characteristics of plate heat exchangers (PHE’s) used as refrigerant liquid overfeed evaporators were investigated. PHE’s have been gaining new applications in the refrigeration industry, especially as evaporators, during the last few decades, but the available information in the open literature for operation in this mode is rather limited. This study aims to extend the knowledge of PHE evaporator performance and to develop a model for use in evaluating heat transfer and pressure drop over as wide a range of operating conditions as possible. A laboratory experimental facility was constructed and the thermal-hydraulic characteristics of three middle-size industrial PHE’s were measured. The exchangers all had 24 plates of the same size but with different chevron angle combinations of 28°/28°, 28°/60°, and 60°/60°. Two sets of tests were carried out with the three units: single-phase performance tests with water, and evaporator performance tests with R134a and R507A, for which the exchangers operated as refrigerant liquid over-feed evaporators. The tests with water served to provide accurate water-side heat transfer information for the evaporator performance analysis which is the primary purpose of this study. In the evaporator performance tests, refrigerant flow boiling heat transfer and two-phase pressure drop data were obtained under steady conditions, over a range of heat flux from 1.9 to 6.9 kW/m2, refrigerant mass flux from 5.6 to 31.4 kg/(m2s), outlet vapour quality from 0.2 to 0/95, and saturation temperatures from 5.9 to 13.0 °C. Additional field data of thermal performance were collected on an ammonia and a R12 PHE water chiller, operating as thermo-siphon evaporators at their design working conditions. All experimental data were reduced and analyzed to obtain the refrigerant-side heat transfer coefficients and frictional pressure drops in the PHE evaporators. The heat transfer results showed a strong dependence on heat flux and a weak dependence on mass flux, vapour fraction and the chevron angle. Along with the ii observations from the ammonia and R12 evaporators, it is concluded that the dominating heat transfer mechanism in this type of evaporator is nucleate boiling rather than forced convection. In contrast to the heat transfer characteristics, the refrigerant two-phase frictional pressure drop was found to be strongly influenced by mass flow rate, vapour fraction and also the chevron angle. An almost linear increase of the frictional pressure drop with the homogeneous two-phase kinetic energy per unit volume was observed for both refrigerants. Based on the experimental data, correlations were developed for predicting the refrigerant boiling heat transfer coefficient and two-phase frictional pressure drop in PHE liquid over-feed evaporators. Two correlations were developed for boiling heat transfer, one of these reflecting the h-q relationship in pool boiling, the other with all constants and exponents determined by regression analysis. The mean absolute errors are respectively 7.3% and 6.8% for these correlations. For two-phase frictional pressure drop, data were correlated using two established methods, namely the homogeneous and the Lockhart-Martinelli methods, with means absolute errors of 6.7% and 4.2%, respectively. The homogeneous model showed a slightly higher discrepancy with the experimental data but is likely to be more physically sound for PHE evaporators, and is much simpler to apply. Validation of these correlations with other data has been difficult due to the shortage of published information. For other refrigerants operating at comparable conditions, these correlations should serve as a guide, while more accurate design or evaluation may need to be based on further testing. The performance analysis carried out in this study was based on systematic experimental investigations and field tests on industrial PHE units. Correlations were developed covering a rather extensive range of flow parameters, plate geometry and various refrigerants. Such correlations have not been reported previously for PHE liquid over-feed evaporators. The results simplify the performance analysis of PHE evaporators and provide a reliable thermalhydraulic model of PHE liquid over-feed evaporators, which can be used for system modeling of water-chilling machines employing this type of evaporator.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2003. / Thesis advisor(s): Ashok Gopinath. Includes bibliographical references (p. 101-102). Also available online.

Thesis (M. Phil.)--University of Hong Kong, 1989.

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