Study On Boiling Heat Transfer Enhancement With Metal Foams

Metal foams have unique micro skeleton structure that results in high porosity, high thermal conductivity and high ratio of surface area to volume. These characteristics provide the opportunity for using metal foams in aerospace, efficient electronic cooling, building energy conservation and other fields related to heat transfer. The complex structures in metal foams make it challenging to completely understand the mechanism of heat transfer associated with them. Prior work in the literature is based on empirical formulas derived from macro level experiments. The limited number of experiments is inadequate for explaining the effects of pertinent physical parameters. For example, the effect of micro pore structure of metal foams on vapor-liquid two phase flow and heat transfer is not completely understood. Therefore, in order to improve upon our understanding about such phenomena, the present work aims at studying the boiling heat transfer enhancement with metal foams taking into consideration the characteristics of metal foam pore structure. The major contributions of the current research work may be summarized as follows:1. Numerical study on the bubble dynamics in porous medium by LBM method. In the simulations, the metal foam structure is approximated as a porous media. A physical model of bubble dynamics behavior in the porous medium is established. The Lattice Boltzmann method is applied to simulate the gas-liquid two phase flow in pore scale. By considering the interaction force between fluid-fluid and fluid-solid in porous media, the Shan-Chen model with multi-component single relaxation is used to track the motion of the bubbles in the porous structure. The effects of porous media on bubble dynamics behavior is studied, including the influence of bubble diameter, distance between porous media skeleton and porous media arrangements. The effects of flow field characteristics on bubble motion are explored. Based on the characteristics of a single bubble motion and its dynamic change process, the movement characteristics, such as motion trajectory, coalescence behavior etc., in an aggregate of bubbles is studied taking into account the interaction between bubbles, and between bubbles and fluid. The results show that the flow field of two-phase flow is affected by the existence of porous media, which alters the movement speed and motion morphology of the bubble. By taking advantage of this observation, proper allocation of structural parameters of porous media can increase the bubble movement speed, therefore enhancing the two-phase flow and heat transfer process.2. Experimental study on subcooled pool boiling phase change heat transfer on heated surface with metal foams. The simulation results provide the platform for further research into the heat transfer enhancements using metal foams. To this end, an experimental apparatus for measuring the heat transfer performance of pool boiling on surface with metal foam is designed and developed. The heat transfer characteristic of sub-cooled pool boiling on heating surface with porous metal foam is studied. The mechanisms and influence factors of the heat-removal from such surfaces, such as pores per inch (PPI), heat flux, and the heating surface inclination are explained through this work. Flow-visualization of the experimental process provides a better insight into the dynamic behavior of bubbles during pool boiling, such as bubble growth, dynamic changes of gas-liquid boundary and bubble fusion. The results show that with the increase of PPI the heat transfer area as well as the number of nucleation sites increase; however there is a penalty on the flow resistance and bubble escape resistance.3. Experimental study on flow boiling heat transfer enhancement in square tube filled with metal foams. The final part of this work is focused on experimental study of flow boiling heat transfer reinforcement in square tube filled with porous metal foam. The flow boiling pattern and heat transfer model in the square tube is established, which is verified with the experimental results. Characteristics of flow boiling pressure drop and heat transfer in the square tube filled with porous foam metal is studied. The influence factors of the heat transfer characteristics is explored, such as PPI value of metal foam, heat flux, mass flow rate, etc. The results show that, when PPI increases, the heat transfer coefficient as well as the pressure drop increase in the test section. It implies that the enhancement of flow boiling heat transfer by metal foam is at the expense of increased the pressure drop. Therefore, it is important to consider the overall performance based on both heat transfer and pressure drop.In this work, the boiling heat transfer characteristics in metal foam are studied using the combined method of theory and experiment. The influence factors of boiling heat transfer enhancement by metal foams are explored. The research provides an effective supplement for gas-liquid two phase flow and heat transfer theory in metal foam. At the same time, it provides theoretical and technical support for the application of metal foam in the field of cryogenic refrigeration.