Entransy Analysis Of Thermal Performace For Heat Exchangers And Cooling Channel Networks

Energy is a substantial basis of modern society. Improving the performance of heat-exchange equipments plays an important role in efficient energy utilization. The performance of heat-exchange equipments for given heat duty can be enhanced by reducing its irreversibility. In this dissertation, equivalent resistances are used to measure the irreversibility due to heat transfer and fluid flow in heat-exchange equipments, and the equivalent resistances are minimized for the optimization of heat exchangers, conductive and convective cooling channel networks and steam condensers, to improve energy utilization efficiency.Based on the analogy between thermal system and electric system, entransy of an object has been defined as half of product of its internal energy and temperature. The entransy describes heat transfer ability of the object, and the entransy is irreversibly dissipated during heat transfer process while heat conserves. Based on the derived expression of entransy dissipation rate of heat exchanger, the equivalent thermal resistance of heat exchanger is defined as ratio of the entransy dissipation rate to the heat transfer rate squared, and the equivalent thermal resistance is considered as a measure of the heat exchange irreversibility of heat exchangers. An analytical formula is derived between the non-dimensionalized equivalent thermal resistance and the effectiveness for heat exchangers. The formula is independent of heat exchanger flow arrangements, which is more convenient for comparison of thermal performance among different heat exchanger flow arrangements. Further analysis of the formula shows that the thermal effectiveness increases monotonically with decreasing of the equivalent thermal resistance, so the heat exchanger irreversibility analysis based on equivalent thermal resistance avoids the entropy paradox in heat exchanger analysis.The thermal performance optimization of heat exchangers is summarized as minimizing the (nondimensionalized) equivalent thermal resistance with some specified constraints, which is the thermal resistance minimization principle for heat exchanger optimization. The decrease of the nondimensionalized equivalent thermal resistance corresponds to the increase of the uniformity of heat exchanger temperature difference field, which reveals the physical essence of the uniformity principle of temperature difference field as minimizing the heat exchange irreversibility measured by equivalent thermal resistance. Because the equivalent thermal resistance and the entropy generation rate describe the irreversibility of heat-exchange process and heat-work conversion process respectively, the thermal resistance minimization principle is suitable for optimization of heat exchangers involved in heating/cooling, while the entropy generation minimization principle is suitable for optimization of heat exchangers within a heat-work conversion cycle.For the conductive and convective cooling channel networks of electronic devices, equivalent thermal resistance and equivalent flow resistance are defined to measure the irreversibility during heat conduction and fluid flow. Structural parameters of cooling channel networks with minimum equivalent thermal or flow resistance are optimized. For general transport processes in channel networks, the transfer ability of that transport process, or the entransy, must be equally distributed in a channel network when the parameters of the channel network is optimal.The tube bundle design problem of steam condensers is considered as the problem of optimally distributing porous material with high permeability in a low-permeability porous zone, and the optimal distribution of porous materials is obtained based on minimizing the equivalent seepage resistance of the porous zone. A new tube bundle design, the quasi-diamond tube bundle with sparse steam passages, is extracted based on the optimal high-permeability material distribution. Numerical simulations show that the pressure drop of the purposed tube bundle design is reduced approximately 15% comparing to the original design.