Structured Surface With Metal Frost Layer For Enhancement Of Boiling Heat Transfer

Nucleate boiling can transmit a large heat flux density at a small wall superheat,so it is often used for cooling industrial equipment,such as nuclear reactors,electronic components,heat exchangers,etc.However,there is a critical heat flux(CHF)in nucleate boiling.For the equipment using the boiling phenomenon,when the heat flux density exceeds this value,the boiling state will transition from nucleate boiling to film boiling.At this time,the surface temperature of the equipment rises rapidly and is likely to be burned.In order to prevent the occurrence of this phenomenon and transmit a higher heat flux density as much as possible,many scholars have explored various ways to enhance the nucleate boiling CHF,such as porous coating,chemical deposition,machining,etc.The frost layer is a kind of porous structure,which plays a great role in promoting the boiling heat transfer of objects in the cryogenic liquid nitrogen.However,in general industrial applications,boiling heat transfer temperatures range from 100°C to 300°C,so water-formed frost structures are not suitable.In this paper,a kind of low melting point metal zinc is used to simulate the frosting process of water,thus forming a similar porous structure,and the effect of this porous structure on the nucleated pool boiling property of water is investigated.In this paper,the zinc vapor is obtained by using this system,and the condensation of zinc vapor on the surface of heat transfer plate is realized,so the metal “frost” is successfully obtained.In order to better improve the performance of nuclear boiling heat transfer of water,the structure of metal frost was investigated under different experimental conditions.The effect of surface temperature and cooling time of heat transfer plate on metal frost structure was investigated under the condition of the same amount of zinc evaporation.The structure of metal frost was observed by optical Microscope and Scanning Electron Microscope.For a more complete analysis of the discharge,the weight and height of the metal frost obtained under each condition were measured,and the density and porosity of the metal frost were calculated.For the obtained metal frost structure,this paper selects one of the structures under one condition to carry out the nuclear pool boiling test of water,and compares with the boiling characteristics of smooth surface,and further explores the enhancement effect of metal frost porous structure on surface boiling heat transfer characteristics.It was found that boiling CHF increased by 16.5% compared with smooth surface.And the quenching test of the structure shows that the time required for metal frost is half less than that for smooth surface when cooling to the same temperature.