چكيده به لاتين
Abstract:
Employment of nano-coating method to enhance pool boiling performance has attracted too many researchers due to better control over surface properties and its significant effect on heat transfer improvement. In order to study pool boiling enhancement silver nanoparticles were deposited on copper substrate by boiling a new industrial nanofluid (DZ Nanocoolant) in two different concentrations to investigate the saturated pool boiling enhancement of distilled water under atmosphere pressure. Microstructure, surface topography, and contact angle of the polished surfaces were examined. As the nanoparticle concentration increased, the cluster deposition and hydrophobicity increased, however the deposition stability was decreased. The experimental results indicated that by increasing nanofluid concentration to reach nanocoated polished surfaces, the critical heat flux (CHF) and heat transfer coefficient (HTC) of pool boiling increased. Nano-fins are introduced as a new concept for heat transfer enhancement. In order to investigate nano-coating and structured surfaces blending effect inclined and reentrant microchannels were used. The optimum concentration with hydrophobic characteristics was deposited on the reentrant inclined microchannel. Effects of reentrancy and hydrophobicity on bubble dynamics were observed and pool boiling curves were compared. It was observed that inclination and reentrancy enhanced pool boiling performance in comparison with polished copper. Finally, as expected, the combined modification including coating the surface with deposited silver nanoparticles in internal side of reentrant inclined microchannel possesses the highest CHF and HTC of 196W/cm2 and 10W/cm2K, respectively which are 120% and 100% higher than those of the plain surface.
In the next stage of experiment, copper foams were welded to copper flat surfaces with two different properties, afterwards these two surfaces were coated using previous method. Pool boiling experiment were conducted using these surfaces. Enhancement of pool boiling heat transfer of these surfaces depend on increasing nucleation sites, increasing thermal contact area, decreasing bubble escape resistance and increasing capillary effect of foam to pump bulk liquid back. Coated surfaces exhibited better thermal enhancement and at the best performance they increased CHF and HTC about 53% and 65% respectively in comparison to copper polished surface. The enhancement was significant for the surface with less porosity. Nano-coating decreases bubbles escape resistance by decreasing bubbles departure diameter and improves heat transfer by increasing contact area through nano-fins.
Keywords: Pool boiling; Copper; Silver nanoparticle; Inclined and Reentrant Microchannel; Metal foam
Keywords: Pool boiling; Copper; Silver nanoparticle; Inclined and Reentrant Microchannel; Metal foam
