Research On Heat Transfer Enhancement Of Flow Boiling Based On Micro-scale Slotted Copper Foam Structuret

The heat transfer enhancement with porous structure can increase the specific surface area and the gasification core of boiling process more effectively,which is one of the most widely used and prominent heat transfer enhancement technologies.Open-cell metal foam materials show considerable advantages due to their own particularity,such as uniform pore size distribution,large specific surface area,high permeability,strong capillary force and large thermal conductivity.In this paper,an experimental system for boiling heat transfer of copper foam was designed and built.The effects of structural parameters on boiling heat transfer and pressure drop characteristics of copper foam were investigated by using deionized water as working fluid.Combined with visual observation,the bubble dynamics and boiling flow pattern of copper foam were studied.First of all,the properties of copper foams with different specifications were explored.The study found that for 45 PPI samples,the lower the porosity was,the better the boiling heat transfer performance was.For the sample with pore density of 90 PPI,the boiling heat transfer performance of 80%and 87%porosity is better.Overall,the boiling heat transfer performance of 90 PPI sample was better than that of 45 PPI.When the mass flow rate was18.5 kg/m~2·s,the maximum critical heat flux(CHF)was about 70 W/cm~2.Then,the performance of copper foam with channel was systematically explored.It was found that the heat transfer performance of copper foam with channel was lower than that of copper foam without channel in the low heat flux range.With the increase of heat flux,the boiling heat transfer coefficient of copper foam with six channel was generally higher than that of copper foam without channel and nine channel.The boiling heat transfer performance of nine-cell copper foam is not as good as that of non-cell copper foam.For the six-cell copper foam,when the porosity is 70%and the pore density is 110 PPI,the boiling heat transfer performance is the best.The heat transfer coefficient can reach 21 k W/m~2·K at medium and high heat flux,and the critical heat flux reaches 113 W/cm~2.In terms of the pressure drop characteristics,as a whole,the more the number of slots is,the smaller the average pressure drop is.However,at high heat flux,the pressure drop of copper foam with nine slots is higher than that of copper foam with six slots.Through visual observation,it is found that for the grooveless copper foam,the air mass is formed at the outlet end,and the heat flux increases and the air mass area increases.For copper foams with grooves,when the heat flux is low,it is mainly the nucleate boiling mechanism.The flow pattern is bubble flow.With the increase of heat flux,annular flow is easily formed in the channel,which is the dominant mechanism of convective boiling.When entering the region with medium and high heat flux,the boiling is very intense,and it is easy to form an atmospheric plug covering multiple channels,indicating that the boiling mechanism of copper foam is dominated by convective boiling and thin film evaporation.Visual observation also found that there are a large number of bubbles in the copper foam mesh at the bottom of the channel,which is also the main obstacle for copper foam to further improve CHF,and copper foam oxidation will aggravate this phenomenon.