Irreversibility And Optimization Of Convective Transport Processes

Heat, mass and momentum transport processes are the three important parts in energy utilization. Transport performance improvement is of great significance to the energy conservation and emission reduction. In terms of the analogy among the heat, mass and momentum transport phenomena, the concepts of mass entransy and momentum entransy have been introduced, they represent mass and momentum transfer ability of a system. The mass entransy dissipation function and the momentum entransy dissipation function have also been defined, and it is the entransy dissipation rather than entropy generation that measures the irreversibility of a transfer process, that is unrelated with the heat-work conversion in a thermodynamic cycle. Further, the minimum entransy dissipation principle is pointed out to be the least action principle in transport processes. For constant transport coefficient, the Newton's law of viscosity, Fourier's law of heat conduction and Fick's law of diffusion have been deduced from this least action principle, suggesting that these laws themselves reflect the optimal transport processes.In order to illuminate the differences between the thermodynamic optimization and the transport optimization for convective heat transfer, two different Euler's equations have been deduced from the extremum principle of entransy dissipation and the entropy generation minimum principle, respectively, which lead to two different fluid velocity fields with the best heat transfer performance. The optimization results of the convective heat transfer of laminar flow in a tube show that the extremum principle of entransy dissipation is more suitable for the convective heat transfer optimization than the entropy generation minimum principle.By defining the concepts of heat entransy and the heat entransy dissipation function for the convective heat transfer of turbulent flow and utilizing a zero equation turbulence flow model, an Euler's equation, which leads to a velocity field with the best turbulent heat transfer performance, has been developed for a given pumping power. The extremum principle of entransy dissipation for laminar heat transfer optimization has then been extended to the turbulent heat transfer optimization. As an example, the field synergy analysis for turbulent heat transfer between parallel plates is presented. The results support that the tubes with micro fins effectively enhance heat transfer, and have clarified the best heights of the fins for different Reynolds numbers.The field synergy principle for convective heat transfer has been extended to the convective mass transfer. The concept of field synergy between the velocity vectors and concentration gradients is introduced, and then the convective mass transfer field synergy equation is deduced, it is just the Euler's equation for convective mass transfer optimization with the extremum value of the mass entransy dissipation. This principle has been validated experimentally through a convective mass transfer process between air and liquid water, and then been used to optimize the decontamination ventilation designs in space station cabins and the geometric structure designs of photocatalytic oxidation reactors. The results show that by utilizing the concentrated air supply system to substitute the uniform air supply system, the maximum contaminant concentration in the cabin is decreased from 0.47% to 0.22%, and the contaminant removal effectiveness for the discrete double-inclined ribs plate reactor is increased by 22 % compared to the smooth plate reactor.The field synergy principle for convective heat transfer has also been extended to the fluid flow. The concept of field synergy between the velocity vectors and velocity gradients is introduced, and then the fluid flow field synergy equation is deduced, it is just the Euler's equation for fluid flow optimization with the extremum value of the momentum entransy dissipation. As an example, the field synergy analyses for duct flow with two parallel branches and insulated transport process of thick oil are presented. The results show that by adding a flow divider nearby the fork may reduce the flow drag of duct flow by 5%, and the resulting multi-longitudinal vortex flow reduces the flow drag by 19% during the insulated transport process of the thick oil.