Study On Heat And Mass Transfer With Cross Diffusion Effects In Multi-Physical Fields

To control and remain the healthy environment, the characteristics of heat and mass (e.g. Water vapor and volatile organic compounds) transfer in multi-physical fields should to be well understood. The investigation of simultaneous heat and mass transfer may help to know about the distribution of temperature, humidity and the concentration of contaminant indoor environment. Further, the knowledge of transfer process in multi-physical fields is widely used in some engineering fields and hence has attracted considerably attention in the recent years. The objective of the present dissertation is to investigate the modeling and characteristics of the buoyancy driven convection in multi-physical fields.According to the principle of thermodynamics of irreversible processes, it is well known that when heat and mass transfer occur simultaneously in a closed system, the relations between the fluxes and the driving potentials are more intricate while considering the thermal-diffusion and diffusion-thermo effects. Thermal-diffusion implies that mass diffusion is induced by thermal gradient, which is so-called the Soret effect. Diffusion-thermo implies that the heat transfer is induced by concentration gradient, namely the so-called the Dufour effect. The influence of the cross diffusive effects on heat and mass transfer are taken into account and a novel mathematic model has been developed for laminar natural convection. Further this model has been transformed into the non-dimensional governing equations to study the general characteristics.The non-dimensional mathematical model is solved numerically applying a numerical technique based on the common finite element method. The validation of mathematical model and the numerical code has been performed for pure thermal natural convection and double diffusion natural convection. The heat and mass transfer in two close cavities connected by a capillary pipe was numerically studied and the influence of cross diffusion effects was investigated specially.Further more, the natural convection, driven by temperature gradient and concentration gradients of two components, was numerically studied. The influence of the non-dimensional parameters in the model, such as Rayleigh number, buoyancy ratio number, Lewis number, and cross diffusive effect coefficients, on heat and mass transfer have been studied numerically in detail. The numerical results show that the natural convection flow velocity is controlled by both Rayleigh number and buoyancy ratio number. Generally, the natural convection become intense with the increase of the Rayleigh number, however the influence of buoyancy ratio number on the flow involves various forms. For given component, the average Sherwood number augments with the increase of its Lewis number. If the direction of temperature gradient and the concentration gradient is the same, the positive cross diffusion effects cause the heat and mass transfer enhance mutually.In present dissertation, a modified two-equation model, k-ε, has been developed to investigate the natural convective heat and mass transfer in a multi-component system for big Rayleigh number. Taking into account of cross diffusion effects, the turbulence model has been studied numerically by FEM method. The influence of Rayleigh number and buoyancy ratio number are the same to the laminar one. Compared with laminar one, the cross diffusion effects have slightly effect on heat and mass transfer.The heat and mass transfer between two close containers connected by a superfine pipe was experimentally studied. The experimental results show that the contaminative gas under temperature gradient move from the warm container to the cold container with positive thermal-diffusion coefficient and the heat flux move to the lower concentration container with positive diffusion-thermo coefficient. And a multi-component convection-diffusion experimental room has been built to investigate the flow, heat and mass transfer experimentally. Along the horizontal direction, temperature gradient and the concentration of water vapor and volatile organic compound gradients were kept unchangeable. The values of temperature and the concentration of two components for various conditions were presented.The characteristic of heat and mass transfer in multi-physical fields is intricate, and all the factors may have a coupled and synergy effect. In this dissertation, for the convenient of study, the author presents the effects of parameters one by one. According to the importance of each parameter, one may decide to conduct some measures to control the heat and mass transfer. The present results may give an introduction to some thermal engineering fields.