On The Microscale Mechanisms Of The Effects Of Surface Properties On The Cooling Rate And Boiling Heat Transfer During Quenching

Quenching,as an efficient way of cooling with high heat flux,is a widely-used technique for rapid cooling in a variety of industrial applications,such as heat treatment of metallic materials,and cooling of nuclear fuels after a loss of coolant accident.Boiling heat transfer performance during quenching will determinte the cooling efficiency.In this work,quenching experiments were performed with stainless steel spheres and rodlets to investigate the pool boiling behaviors and vapor film and bubble evalution during quenching.Some surface modifaction methods,such as nanofluids,spraying,vapor deposition,plasma cleaning and chemical etching,were used to fabricate mico/nanostructures and thus to regulate surface properties,e.g.surface wettability,wickability,roughness,and porosity.The effects of these modified surfaces on pool boiling,expecially on film boiling and transition boiling regimes were further studied.Firstly,quenching experiments with stainless steel spheres were performed in aqueous nanofluids in the presence of carbon nanotubes(CNT)having various lengths and diameters.The effects of CNT dimension and quenching times on surface modification and pool boiling behaviors were investigated systematically.It was shown that the quenching efficiency were improved in these CNT nanofluids and the quenching time is progressively reduced upon consecutive runs in the nanofluids with any types of the four kinds of CNTs.In addition,the improvement of quenching efficiency is clearly related to the enhanced pool boiling heat transfer.The increases in CHF and LFP are exhibited for all cases,while the most remarkable CHF enhancement by nearly 56%is resulted from the use of the longer and thicker CNTs,which tend to form a highly-porous layer,in accordance with about 53%reduction in the quenching time.The modified quenching and boiling behaviors are elucidated by the accumulative changes in surface properties due to deposition of CNTs.In view of the nearly unvaried contact angles,the consistently increased surface roughness and the formation of porous structure seem to be responsible for quenching and boiling enhancement.The effects of surface wettability,from superhydrophilic to superhydrophobic,on pool boiling during quenching with stainless steel spheres were experimentally examined by preparing four kinds of classic surfaces,i.e.,superhydrophilic,hydrophilic,hydrophobic,and superhydrophobic surfaces.The CHF and cooling rate during quenching were shown to monotomously accelerate with increasing the surface hydrophilicity.As compared to the original hydrophilic case,the CHF for superhydrphilic case was found to increase by nearly 78%.Due to the surface superhydrophilicity,vapor film was significantly destabilized and there is no stable film boiling was observed even at high wall superheat.The transition boiling regime was divided into transitional film boiling subregime and transitional nucleate boiling subregime according to the liquid-solid contact models.To the other end,it was found impressed that surface hydrophobicity led to stabilized vapor film,and the LFP and CHF phenomena even disappeared for this case.Good agreement was exhibited for the film boiling heat transfer and vapor film thickness between the present experimental data and theoretical predictions.The boiling behaviors and evolutions of vapor film during quenching on stailess steel rodlets and spheres with different wettabilities in water with various degrees of subcooling were further investigated.The quench front propagates much faster with increasing the degree of subcooling.When the water is subcooled by 50°C,the average quench front velocity on the superhydrophilic surface is 32.6 mm/s,which is increased by about 800%from 3.6 mm/s for original hydrophilic case at saturated condition,as a result the CHF was enhanced.At a subcooling of 50°C,the CHF is enhanced from 400 kW/m~2(original case)to 2475 kW/m~2,nearly 519%.As for spheres,the CHF was enhanced by 508%at a subcooling of 70°C.It was shown that film boiling is the overwhelming mode of heat transfer during the entire course of quenching as a result of the retention of stable vapor film surrounding the supherhydrophobic spheres,even at very low wall superheat that normally corresponds to nucleate boiling.The heat flux enhancement was found to be up to fivefold for the subcooling degree of 70°C in comparison to the saturated case,at the wall superheat of 200°C.A modified correlation in the ratio form was proposed to predict pool film boiling heat transfer from spheres as a function of the subcooling degree.At last,both hemi-wicking(hydrophilic)and wicking(superhydrophilic)surfaces were fabricated using nanoparticle deposition and chemical etching methods,respectively,on stainless steel spheres.Quenching experiments were carried out on these microporous surfaces in saturated water to reveal the influence of surface wickability on the collapse of vapor film during transition boiling.It was shown that the the presence of wickability changes the liquid-solid contact mode during transition boiling by extending the transitional film boiling sub-regime through capillary action.In addition,the CHF and transitional heat flux(THF)at the critical transitional point(CTP),which separates the transitional film boiling sub-regime and transitional nucleate boiling sub-regime,was significantly enhanced with improving the surface wickability,and that the most wicking surface leads to a maximum of 656%THF increase as compared to the bare non-porous surface.The Weber number was modified to characterize the instantaneous imbibition of water through the microporous structures upon liquid-solid contact.Based on the hydrodynamic stability model,a linear correlation was proposed between the enhancement ratio of THF and the modified Weber number.