Optimization of mass and energy transfer process is critical to improve energy efficiency. In this contribution we introduce the field synergy principle as a unified principle for analyzing and improving the performance of the transfer process. Three field synergy numbers are introduced for heat, mass, and momentum transfer, respectively, and three cases are demonstrated for validation. The results indicate that the field synergy numbers will increase when reducing the angle between the velocity vector and the temperature gradient or the species concentration gradient fields in the convective heat or mass transfer, and the overall heat or mass transfer capability is therefore enhanced. In fluid flows, it will reduce the fluid flow drag to decrease the synergy number between the velocity and the velocity gradient fields over the entire domain and to decrease the product between the fluid viscosity and the velocity gradient at the boundary simultaneously. 1. Introduction Energy conservation is not only a scientific or engineering problem but also a social one that most people on this planet are facing [1–3]. Solutions of this problem include both developments of new substitutable energy products [4–6] and optimizations of current energy utilizations [7–9]. The energy efficiency is always expected maximized by enhancing the energy output for a given cost, or by minimizing the input energy loss for a given output [10]. In recent years, the multiphysical transport process in energy systems, involving mass and energy transfer, has been a hot but challenging topic [6]. The difficulties come mainly from the coupling of the multiphysical transport processes. In this work, we focus on the fluid flow coupled with species or heat transfer, which is one of the most popular transport phenomena in energy systems. In such a multiphysical process, three kinds of transports are involved: the fluid flow (momentum transport), heat transfer (energy transport), and species transfer (mass transport) [11–13]. During the past several decades, a great number of heat, mass transfer enhancement, and fluid flow drag reduction technologies have been developed including using extended surfaces, spoiler elements, and external electric or magnetic field [14–17] for heat and mass transfer, riblet surfaces, guide plates, and drag-reduction additives of low viscosity for fluid flow [18–20]. All these methods have successfully cut down not only the energy consumption but also the cost of equipment itself. In the meanwhile, in the interest of revealing the essence of these methods from the
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