Investigation On Flow And Heat Transfer Of Fluid In Porous Spherical Beads

Porous media is the material with high porosity that has been widely used and has therefore gained considerable attentions in not only industrial section but also numerical simulation. However, it is hardly to be visualized because of its transparency. Therefore the flow mechanism in the porous media is yet to be fully understood. The porous spherical beads with uniform porosity can provide clear visualization and are widely employed in researches.Boiling heat transfer is the convection with phase change. In this thesis, self-rewetting fluid, a solution composed of deionized water and alcohol with high carbon numbers, is chosen as the working fluid for boiling heat transfer. The self-rewetting fluid could be widely used in, for example, heat pipe, because its surface tension could increase with temperature beyond a critical transition point. With the porous spherical beads being treated as the heat transfer channel, it focuses in this thesis on the boiling phenomena of some self-rewetting fluids, comparing with that of deionized water. The experimental results show that the bubble numbers of rewetting fluids are less than that of deionized water at low heat fluxes, and the bubble accumulation phenomenon in self-rewetting fluids is not as obvious as in deionized water for the same superheat. With the increase of heat flux, the surface tension of alcohol becomes an important factor in heat transfer enhancement. The bubble detachment diameter in self-rewetting fluids is much smaller than that of deionized water, and smaller bubbles accelerate the fluid flow onto the heating wall. Hence the self-rewetting fluid could significantly enhance the boiling heat transfer, comparing with deionized water. It also indicates that the self-rewetting fluid with higher carbon number can further enhance the boiling heat transferin porous spherical beads.Many studies have shown that the convection in porous medium mainly occurs in the contact zone, or the meniscus area, in between the heating surface and the bead, so we focus in this thesis on the meniscus, in an aim to investigate the heat transfer enhancement in porous media. For convective heat transfer, the velocity and temperature are the most important physical parameters, so the measurement of velocity and temperature fields in the meniscus becomes the key for the heat transfer enhancement mechanism in porous media. In the second section, the velocity and temperature fields at the meniscus are obtained by the method of total internal reflection (TIR). It is found that the velocity distribution is of the "V" shape while increasing the distance between the wall and the porous spherical beads. The temperature in the meniscus is also obtained with the TIR method.The experimental researches have provided the experimental basis and reference for further researches on the flow and heat transfer of fluid in porous spherical beads.