Research about Heat Transfer Performance and Flow Characteristics of Ultra-light Porous Metal Foam

Ultra-light Porous metallic foams (the metal foam) are novel multi—functinal porous materials, it has the remarkable features of little density, high porosity, large specific surface area and others. It has become a research hot spot due to strong features in the field of heat transfer. Study of its heat transfer performance and flow chatacteristics of fliud in it is suit for the pursuit of high-performance materials in line with the people of the demand for light. First，using deionized water as working fluid, pool boiling on the copper foam covers was studied. Through the study found that the metal foam covers will significantly enhance the pool boiling heat transfer, reduce the surface superheat at the boiling incipience. In some cases the heat transfer coefficients were increased by 2~3 times than the smooth surface. Pore density and thickness of metal foam has an important effect on boiling heat transfer, each showing a strengthening and weakening of the two sides of heat transfer. The effects of porosity on the heat transfer are complex. The effect of Subcooled on the pool boiling is relation to the thickness of the metal foam covers, and has different characteristics compare to smooth surface. Second，High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, using acetone as the working fluid. The surface superheats are from -20 K to 190 K, and the heat fluxes reach 140 W/cm2. The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. The pattern of heat transfer is different at region III and boiling curves are crossed between the high and low ppi foam covers. Third，a series of pool boiling experiment were conducted with different inclination of copper foam covers. The result shows that the orientation of metal faom surface has effects on the pool boiling heat transfer, but the degree of effect is relation to liquid subcooled. The heat transfer is decreased with the angle increases, at low heat flux, but is just opposite at high heat flux. Parts of boiling curves are crossed between the big and small angle for the 90ppi foam covers. In succession, using deionized water as working fluid, experiments were conducted to study the one-phase forced convective flow and heat transfer characteristics in metal foam embedded in a copper channel. The effects of heat flux, pore density of the metal foam and mass flux on the pressure drops, wall temperatures and heat transfer coefficients were obtained during the laminar flow range. It is found that the metal foam could enhance the heat transfer and lower the wall temperature dramatically. The heat transfer capability is increased with the increase of the Reynolds number. Compared with the common copper channel without the embedment of metal foam, the maximum Nusselt number during convective flow through the copper channel embedded with the metal foam is thirteen times larger, but the pressure drops are obviously increased either. The increases of the Reynolds number and the pore density of metal foam also lead to lager pressure drop. Last, experiments were conducted to study the pressure drop of flow boiling in high porosity metal foam embedded in a copper channel. Cyclical fluctuations phenomena were found in the course of flow boiling, including fluctuations of temperature at the outlet, pressure drop and mass flux which were affected by the heat flux. The experimental results show that the pressure drop of fluid and friction pressure drop multiplier were affected by porous density, mass flux and vapor mass quality. On the basis of analyzing of pressure drop, the two-phase pressure drop correlation was acquired which is suitable for channel filled with high porosity metal foam