Thermodynamic Analysis And Optimization Design Of Heat Exchanger

Last decades witness the rapidly rising prices of coal, petroleum and other fossil fuels and severe environmental pollution. Energy shortage and environmental pollution have become two extremely serious problems facing the contemporary world. Reducing the energy dissipation in the energy transfer processes, and using energy sources efficiently are important approaches to reducing energy demand and lessening impact of the fossil fuels on the environment. Heat exchanger as a device of energy utilization is widely applied to high energy-consuming sectors, such as chemical process, petroleum, power, refrigeration, food processing, etc. Hence, it is of great importance to decrease the unnecessary energy dissipation and improve the performance of heat exchanger devices for energy conservation and environmental protection. In the present work based on the entropy generation minimization method, entransy dissipation theory and field synergy principle, we conduct the theoretical research on heat transfer enhancement and apply the results to optimization design of heat exchangers. Then the heat transfer enhancement mechanism of some heat transfer enhancement devices is investigated, and a waste heat power generation system is analyzed from the viewpoints of the first law and the second law of thermodynamics.Based on the genetic algorithm, a multiple-parameter heat exchanger optimization design method is developed by minimizing the dimensionless entropy generation. For liquid-liquid shell-and-tube heat exchanger with single tube pass and single shell pass, when heat duty and the heat capacity rate employed to non-dimensionalize the entropy generaton rate are given, both the entropy generation number defined by Bejan and the revised entropy generation number avoid the'entropy generation paradox'. When the heat duty is unfixed, the entropy generation number defined by Bejan induces the 'entropy generation paradox', while the revised entropy generation number avoids the 'entropy generation paradox'. Therefore, the revised entropy generation number has wider range of applications than the entropy generation number defined by Bejan in the optimization design of heat exchangers. Taking the revised entropy generation number as the objective function and ideal gases as the working fluids, we first show that there exists an optimal operation condition for plate-fin heat exchangers. With the revised entropy generation number taken as the objective function, the genetic algorithm which has powerful global optimization ability is adopted to optimize design of the plat-fin heat exchanger and a waste heat recovery ventilation system in which the plate-fin heat exchanger works as a component. The results show that the performance of both the heat exchanger and the waste heat recovery ventilation system has been improved significantly through the optimization. In order to improve the traditional entropy generation minimization method, a multi-objective optimization design method is developed by taking the dimensionless entropy generations induced by the heat conduction under finite temperature difference and fluid friction as two separate objective functions. The conventional entropy generation minimization method may improve the heat transfer performance, but in some situations it leads to large pumping power consumption. Through magnitude analysis, it is found that the irreversible loss caused by the fluid friction is far less than that induced by heat transfer for most liquid-liquid heat exchangers. Therefore the traditional entropy generation minimization with the total entropy generation number as the single objective function has the defect that the influence of the fluid friction has not been fully refected. In an attempt to resolve this problem, a multi-objective optimization design approach for heat exchanger is developed based on the multi-objective genetic algorithm, in which the dimensionless entropy generations induced by heat conduction and fluid friction are taken as two separate objective functions. The results of two heat exchanger optimization design problems show that in order to achieve the same exchanger effectiveness the multi-objective optimization of heat exchanger design requires less pumping power in comparison with the single-objective optimization design. In addition, the multi-objective optimization yields the Pareto optimal set including several optimal solutions. This gives the designer a great flexibility to choose an optimal design plan by fully considering the specific requirements and constraints of the heat exchanger design. Therefore, the multi-objective optimization of heat exchanger design demonstrates obvious advantages and great flexibility over the single-objective optimization.The heat transfer and fluid flow in the curved channels are investigated by the combination of numerical simulation and thermodynamic analysis. It's found that generally the revised entropy generation number decreases as Reynolds number increases, and the channel with the larger curvature ratio has the smaller revised total entropy generation number at the fixed Reynolds number. For the channels with the same radius of curvature, the revised total entropy generation number decreases as reducing the cross-sectional area. Although the Bejan number falls off rapidly with the increase of the Reynolds number, the best'trade-off' between the entropy generations induced by heat transfer and fluid friction does not appear in the range of the Reynolds number under consideration. If the best trade-off exits, it may occur in the turbulent flow regime or at least in the transitional region. The local heat transfer entropy generation concentrates in the narrow region adjacent to the outer walls, especially near the outer wall. The local heat transfer entropy generation near the inner wall is negligible in comparison with that near the outer wall. The local fluid friction entropy generation also mainly concentrates in the region closed to the walls, especially the outer wall, but the local fluid friction entropy generation near the inner wall is not so small to be neglected. When the working fluid is heated in the curved channel, the viscous dissipation decreases the Nusselt number relatively; for fluids with larger dynamic viscosity, the viscous dissipation may make the Nusselt number decrease as the mass flow rate increases; the revised heat transfer entropy generation number and revised fluid friction entropy generation number increase relatively under the viscous dissipation effect. The revised total entropy generation number may reach its extremum for aniline flow in the curved channel as the Reynolds number increases, and the extremum appears at a smaller Reynolds number when the aniline is heated than the case that the aniline is cooled. For the liquids having larger dynamic viscosity and smaller specific heat, such as ethylene glycol, the Brinkman number may become very large, and the revised total entropy generation number extremum does not appear due to the predomination of fluid friction entropy generation in the total entropy generation. When the aniline with temperature-dependent viscosity is heated in the curved microchannel, the Nusselt number is larger than the case that the aniline's viscosity is constant. While the revised heat transfer entropy generation number and fluid friction entropy generation number for the aniline with temperature-dependent viscosity are smaller than the case that the aniline's viscosity is constant. When the aniline is cooled, one may have the opposite conclusion.The entropy generation minimization approach demonstrates some inconsistencies and paradoxes in the application of heat exchanger designs, therefore, the entransy dissipation theory is also studied in this thesis. Based on the entransy dissipation theory a non-dimensionalizing method for entransy dissipation rate in the two-fluid heat exchanger is proposed and an entransy dissipation number is thus defined which can be employed to evaluate the performance of heat exchangers. The entransy dissipation number represents the ratio of actual entransy dissipation to possible maximum entransy dissipation in heat exchangers. Compared with the exchanger effectiveness, the entransy dissipation number not only reflects the overall performance of heat exchangers, but also quantifies the irreversibility caused by the flow imbalance in heat exchangers. In comparison with the entropy generation number defined by Bejan, the entransy dissipation number avoids the'entropy generation paradox'. Compared with the revised entropy generation number, the entransy dissipation number does not directly rely on the inlet and outlet temperatures of the working fluids. Therefore, it is more suitable for evaluating the performance of different types of heat exchangers.Under the condition that the heat load and heat transfer area are given the principle of equipartition of entransy dissipation (EoED) is established, namely the total entransy dissipation rate reaches the minimum when the local entransy dissipation rate is uniformly distributed along the heat exchanger. In comparison with the principles of equipartition of temperature difference principle (EoTD) and equipartition of heat flux (EoHF), the EoED principle is more advantageous when the heat transfer coefficient is not fixed. The difference between the results obtained by the EoED and EoTD principles is very small, far less than that between results obtained by the EoED and EoHF principles. In practice, the EoTD principle can be regarded as the approximate expression to the EoED principle.In a practical two-fluid heat exchanger, the optimization design is often conducted by changing the parameters of one fluid and fixing ones of other fluid. Under the condition that the heat duty and heat transfer area are given, the applications of the EoTD principle to two-fluid heat exchanger show that the improper choice of optimization parameters between the two alternative fluids may lead to worse performance in terms of entransy dissipation. Therefore, choosing the appropriate fluid parameters to be optimized plays an important role in the successful applications of the entransy dissipation minimization principles. The criterion for choosing the appropriate fluid is to increase the total heat transfer coefficient, or to make the temperature profiles of the hot and cold fluids more parallel and lessen the temperature difference between the hot and cold fluids simultaneously.The investigation of heat transfer in the curved channels shows that the field synergy principle may explain the phenomenon that the higher convective heat transfer occurs when the heat transfer surface is specified on the outer wall of the curved channel, rather than the inner wall. The field synergy principle can also explain that the curved channel can improve the convective heat transfer significantly at the expense of slight increase of the flow resistance in comparison with the straight mircochannel. The field synergy number comprehensively reflects the field synergy principle, so it is more appropriate to be taken as the criterion to evaluate the degree of the synergy between the temperature gradient and the velocity vector.The numerical investigations of heat transfer and fluid flow in the tube fitted with helical screw-tape inserts show that the SSTκ-omega turbulence model is more advantageous than the RNGκ-epsilon model for describing the swirling flow in the low Reynolds number regime, and it is also in better agreement with experimental results. It is found that the helical inserts with alternative right and left twists have better heat transfer performance than the helical inserts with uniformly right twists. The field synergy principle could give a good explanation to the heat transfer enhancement mechanism of the local and overall heat transfer performance of tubes fitted with right and left twists alternatively.In practice the distributions of temperature gradient and velocity vector in heat exchangers are normally unknown or hard to get. Under this consideration, the field synergy number maximization is employed as the objective function to optimize the design of shell-and-tube heat exchangers based on the empirical heat transfer correlations. The results show that the exchanger effectiveness increases and the pumping power decreases through the optimization. The field synergy number maximization approach is more attractive in the sense that the reduction of water consumption or the heat exchanger effectiveness improvement can lead to much more profit than the total cost cut achieved by the traditional heat exchanger optimization design approach with the total cost as the objective function.The investigation of a waste heat power generation system based on the first- and second-law of thermodynamics shows that there are two approaches to improving the system performance:one is to improve the heat/exergy input; the other is to enhance the heat-work conversion ability of the system. The former would deteriorate the environment if the heat-work conversion ability of the system remains unchanged; the latter could reduce the environmental impact but it's restricted by the heat/exergy input. Therefore, the optimal operation condition should be achieved at the'trade-off between the heat/exergy input and the heat-work conversion ability of the system. For the same criterion, different expressions would lead to different conclusions. Thus, choosing the appropriate expressions for the performance criteria is crucial for the optimization design of the system. The heat/exergy loss to the environment should be taken into account in the exergy analysis of the similar systems, since the available energy in exhaust gas depends on the gas inlet condition and the environmental condition, rather than on the energy absorbed by the system.