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

表面活性剂对池沸腾换热的影响

本文以SDS（十二烷基硫酸钠）为表面活性剂，研究了在不同过冷度下SDS浓度对池沸腾换热的影响。结果表明在低SDS浓度下，沸腾换热在一定热流密度范围内得到显著强化。在不同过冷度下，均存在相应的最佳SDS浓度值，最大换热系数可达纯水的2—4倍。在饱和沸腾状态下，临界热流密度（CHF）随SDS浓度的增加而减小，沸腾曲线呈现三类分歧，且存在“S”型沸腾曲线及较明显的沸腾滞后现象

流体在超轻多孔金属泡沫中的流动和换热特性

以去离子水为冷却液，对其在超轻多孔铜泡沫中的流动和换热特性进行了实验研究。在测定和分析流量、压力降和温度等实验参数的基础上，获取了热流密度、金属泡沫孔密度、液体流量等参数对层流流体流过金属泡沫时的压力降、通道壁面温度、对流换热等特性的影响。结果表明金属泡沫会显著强化对流换热，大大降低通道的壁面温度，其对流换热能力会随Reynolds数的增大而逐渐增强，最大Nusselt数可达空矩形通道的13倍，但与空通道相比，金属泡沫通道的压力降显著增大，并随Reynolds数及金属泡沫孔密度的增大而增大