Thermodynamic Analysis And Application Of Convective Heat Transfer

Forced convective heat transfer presents widely in thermal equipments. Mostconvective heat transfer processes are characterized by the entropy generation due tofluid friction and heat transfer across a finite temperature difference. Efficient utilizationof exergy is one of the primary objectives in designing any thermodynamic system.Minimization of entropy generation in a thermodynamic system provides efficient useof exergy that is available. In the dissertation, first of all, the irreversibilities of entrancethrough a duct with constant wall temperature and constant heat flux are simulated bythe computer when the flow is laminar. The flow and temperature fields are obtainednumerically. Local entropy generation distributions are obtained based on the resultingvelocity and temperature fields by solving the entropy generation equation. The resultsshow that the irreversibilities with constant wall temperature are quite different fromthat with constant wall heat flux. More importantly, the irreversibilities of entrancethrough a duct is also quite different from that of a fully developed region. The resultsalso show the effect of Reynolds number on the local entropy generation distributionand total entropy generation for different thermal boundary conditions. Finally, it isfound that entropy generation by heat transfer is relatively dominant over viscousentropy generation by viscous flow. In engineering, the effect of entrance is pursued, butmoreover, the effect of irreversibilities should be paid attention. On the other hand, in the past years, second law analysis was carried out onconvective heat transfer to minimize the entropy generation or the loss of availableenergy. Unfortunately, the factor of fouling which strongly affecting the performance ofheat transfer and flow process in heat transfer equipment has not yet been considered insecond law analysis in literatures, but the fouling always appears in many thermalequipments. Especially, the existing study on the fouling effect is only performed fromthe view point of the first law of thermodynamics, but neglecting the irreversible loss ofit. The fouling affects not only the pressure drop, but also the heat transfer. This impliesthat the irreversibilities associated with the heat transfer and viscous friction are alsoaffected. Thus, as an extension to the second law analysis on the convective heattransfer process through a duct, it is required to investigate the effect of fouling duringthe heat transfer and flow processes. Based on the first and second laws ofthermodynamics simultaneously, the effect of fouling on the thermodynamic III重庆大学博士学位论文performance of convective heat transfer process through a duct with constant walltemperature and constant heat flux is investigated analytically when the flow isturbulent or laminar. The results with constant wall temperature are compared with thatwith constant wall heat flux. In the meantime, the effect of fouling on theirreversibilities of heat transfer process in heat exchangers are also investigated. A newcriterion evaluating the effect of fouling is defined as the entropy generation increaserate per unit heat transfer rate. The effect of Reynolds number (not considering fouling),dimensionless inlet temperature difference, dimensionless wall heat flux, heat transferunit number and the ratio of heat capacity rate of cold and heat fluids on the entropygeneration increase rate per unit heat transfer rate is discussed. In addition, a newformula for calculating the excess area of heat exchanger is given. A new method isprovided to analyze quantitatively the effect of fouling on the heat transfer and flowperformance. Finally, as an important application of thermodynamic analysis on single phaseconvective heat transfer through a duct, the exergy-economic evaluation of enhancedheat transfer performance and exergy-economic optimization design for thermodynamicpipeline and its...