Aerogel coated metal foams for dehumidification applications

Metal foams;Heat transfer coefficient;Pressure drop;Aerogels;Dehumidification;Adsorption;Desorption

Nawaz, Kashif

关键词: Metal foams;    Heat transfer coefficient;    Pressure drop;    Aerogels;    Dehumidification;    Adsorption;    Desorption;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46827/Kashif_Nawaz.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Separate sensible and latent cooling systems offer significant increases in the overall performance of cooling/dehumidification systems compared to conventional vapor-compression air-conditioning systems. Key to the energy efficiency of such systems is the performance of the heat and mass exchangers that provide sensible cooling and dehumidification. Metal foams have emerged as a potential material for advanced heat exchangers in air-cooling systems. Metal foams have a large surface-area-to-volume ratio and a tortuous structure, which promotes flow mixing in heat exchanger applications. The subject of this thesis is the use of metal foams for air-side heat and mass transfer in air-conditioning heat exchangers. In this work, the thermal-hydraulic performance of metal foams is studied. Experimental data are obtained, leading to new correlations for the friction factor and the Colburn j factor, valid over a wide range of foam geometry and flow rate. Geometrical parameters (pore size, ligament size, etc.), the base metal of the metal foam, and the geometry of the heat exchanger govern its performance. Metal foams are shown to provide very high air-side heat transfer coefficients, but they also induce high pressure drops.Notwithstanding potential increases in the fan power, it is shown that the overall thermal-hydraulic performance of metal foams can surpass the performance of louvered-fin heat exchangers. Hence, metal foams can compete with state-of-the-art heat exchangers in managing the sensible load. In order to manage the latent load, metal foams are studied as substrates for aerogel desiccants. Silica aerogels are excellent desiccants, with much higher moisture adsorption rates and capacities than other solid desiccants, such as carbon sieves or salts. In this work, it is shown that silica aerogel can be deployed over the large surface area of metal foams in the form of a thin film. In this way, the effect of the low thermal conductivity of the desiccant can be mitigated, allowing the heat of adsorption to be removed and regeneration heat to be added via the metal foam substrate. The dehumidification performance of silica aerogels is affected by their micro-structure, which depends on the catalyst used in the sol-gel process to manufacture the desiccants. Dynamic vapor sorption experiments are used to determine mass diffusivity, and the data show that silica aerogel coated on metal foam has the same mass diffusivity in adsorption/desorption as bulk silica aerogel; however, the catalyst used in the sol-gel process significantly affects the mass diffusivity. A silica aerogel coating prepared using hydrofluoric acid as a catalyst (with tetra methyl orthosilicate as a precipitator and methanol as a solvent) results in a mass diffusivity that can be an order of magnitude higher than using other catalysts, such as potassium hydroxide, steric acid etc. Analysis of the simultaneous heat and mass transfer processes in the silica aerogel coating shows that the moisture adsorption rate and the moisture saturation time depend on the type of foam and the thickness of coating, as well as the thermophysical properties of the desiccant coating. Silica aerogel coated on the metal foams provides better moisture removal rate and adsorption capacity per unit volume than does a coated flat plate or louvered-fin substrate.Metal foam heat and mass exchangers have excellent thermal-hydraulic performance and may find application in separate sensible and latent cooling systems for air conditioning. However, questions regarding fouling, manufacturing cost, and heat exchanger geometry constraints remain to be addressed.