Investigation Of The Effect Of Surface Characteristics On The Pool Cryogenic Quenching Heat Transfer In Liquid Nitrogen

Quenching heat transfer is of fundamental interest in cryogenic chilldown applications.Surface configuration can improve quenching heat transfer and clear understanding of effect of different surface characteristics is necessary.In this paper,the effect of different surface characteristics on pool quenching in liquid nitrogen was studied.Quenching experiments on seven kinds of surface were carried out consisting of electropolishing surface,rough?HCl etched?surface,thermal resistance coating?Teflon coated-thin,Teflon coated-thick?surface,row anodic aluminum oxidation?RAAO?surface,anodic aluminum oxidation?AAO?surface and multi-layer nanoporous?MNP?surface.The influence of surface thermal resistance was further studied by a theoretical calculation model.On the basis of experimental analysis and numerical calculation,the mechanisms of effect of surface roughness,multi-layer nanoporous structure and other surface characteristics on cryogenic quenching heat transfer were deeply studied and demonstrated in this paper.The focus was put on the analysis of mechanisms of surface thermal resistance and surface nanopores structure,and theoretical illustration models for those two factors were proposed.In addition,in order to improve the application of anodic oxidation in the practical process of cryogenic quenching,two kinds of anodic oxide film were prepared on the stainless steel surface,and were quenched in liquid nitrogen.The work of this paper is mainly focused on the following aspects:1.The preparation of different surface characteristics on 6061aluminium alloy and the analysis of surface characteristics:6061 aluminium alloy rodlets were used as the substrate,and the AAO surface was prepared with uniform and ordered nanopores structure and the layer thickness of 15?m,RAAO surface was prepared without obvious surface structure and with the same layer thickness as the AAO surface.MNP surface was prepared with multi-layer interconnected nanopores,HCl etched surface was prepared with roughness of about 5?m and two kinds of Teflon coated surfaces were prepared with different thicknesses.Through analysis of wettability,the HCl etched surface was proved to be the most hydrophilic.And the thermal properities of the AAO and RAAO surfaces in the temperature range of the experiments were further analysed,which indicated that those two kinds of surfaces have the same surface thermal resistance.2.Experimental study on cryogenic pool quenching heat transfer:In this paper,cryogenic quenching experiments were carried out on seven kinds of surfaces in liquid nitrogen.The results showed that all susface characteristics have improved the performance of the cryogenic quenching.And the total time for MNP surface to cool from room temperature to liquid nitrogen saturation temperature was the shortest,which was about 25.1s,and the transition boiling took place on it at the beginning of the quenching process.The Leidenfrost Point?LFP?temperature was increased from 112.3 K on electropolishing surface to 211.6 K on the AAO surface and a second heat flux peak appeared in the nucleate boiling regime on the AAO surface,which was different from the traditional boiling curve.3.Theoretical study and analysis of mechanism of the effect of different surface characteristics on cryogenic quenching heat transfer:In this paper,numerical simulation of the quenching on the RAAO surface based on the theoretical model of Kikuchi^[1]was carried out.The LFP temperature and the time when the LFP occurred on the RAAO suraface were calculated,and the results were consistent with the experiment.it is indicated that the end of film boiling on the surface with the thermal resistance layer is resulted from the decrease of local surface temperature that is not hot enough to sustain the local vapor film,while the bulk temperature is still high,and the temperature of the LFP was improved.Compared with the experimental results of RAAO surface,the independent effect of nanopores on the AAO surface was obtained.For the nanopore structure,it is theoretically estimated that the nanopores will be kept filled with vapor at the surface temperature higher than the critical temperature?94.4 K?.This phenomenon reduces the liquid-solid contact area on the surface and decreased the local vapor generation rate,resulting in a higher temperature of the LFP and the lower heat flux in the partial nucleate boiling regime.When the surface temperature drops below the critical temperature,liquid flows into the nanopores and a second heat flux jump appears besides the critical heat flux?CHF?.Micron-scale rough structures on the rough?HCl etched?surface disturbed the vapor film and slightly increased the temperature of the LFP.Comparisons of the experimental results of thin and thick Teflon coating surfaces indicated that the increased surface thermal resistance would introduce improved performance of cryogenic quenching.Because of the multi-layer nanoporous structure of thickness of 15?m on the MNP surface,the generated vapor flowed within the nanoporous structure,and resulted in the direct contact between the surface of MNP and the liquid nitrogen.This phenomenon eliminated the film boiling regime and made the transition boiling take place on the surface at the beginning of the quenching process.4.Preparation of anodic oxide films and experimental study of cryogenic quenching on 316L stainless steel surfaces:Two kinds of stable anodic oxide films with properties of water insolubility and tolerance of large range of temperature were prepared on the 316L stainless steel surface,and pool quenching experiments were carried out on them.The anodic oxide film with thickness of about 1?m achieved a reduction of about 10%of quenching time duration,which verified the feasibility of applying anodic oxidation technology to improve the performance of quenching process in practice.