Surface Enhancement Of Nucleate Boiling Heat Transfer Performance With Dispersed Free Particles In Porous Media

With the increasingly prominent resource problems,researchers are increasingly demanding heat utilization.Boiling heat transfer is used to solve the problem of high heat dissipation of high-power devices due to more latent heat of vaporization.Improving the surface structure of boiling heat transfer is an important method to enhance heat transfer performance.The excellent heat transfer surface has more nucleation points,larger heat transfer area and faster bubble separation frequency.In order to further reduce the difficulty of boiling on the heat transfer surface,increase the number of nucleation points,and accelerate the frequency of bubble separation,a free particle/porous medium composite surface is designed in this paper.The design idea of this structure is to use the high thermal conductivity and freedom of free particles to disturb the thermal boundary layer,reduce thermal resistance,and reduce the difficulty of boiling in the natural convection stage;in the nucleate boiling stage,free particles interact with each other.The angle formed by the porous medium forms a cavity structure,which increases the number of nuclei.In addition,the free particles disturb the micro-liquid layer to promote the reflux of the liquid phase and shorten the bubble growth cycle.In this paper,sintered wire mesh and sintered copper particles are used as porous media,and free particles are dispersed into it to form a composite surface of free particles/porous media.Boiling experiments were carried out using water and absolute ethanol with different surface tensions as working fluids,and the mechanism of free particles in the porous media on the enhancement of boiling heat transfer performance was studied.The volume fractions of free particles(10%,15%,20%,25%)were investigated.the pore size of the metal wire mesh(150 mesh,180 mesh,200 mesh,220 mesh)and thickness(2 layers,3 layers,4 layers,5 layers)on the boiling heat transfer performance,and use visualization Methods The boiling phenomenon of water and anhydrous ethanol on the porous surface of the bubble nucleation,growth and detachment was experimentally verified,and the boiling enhancement mechanism of the free particle/porous medium composite surface was studied from the perspective of bubble dynamics.theoretical analysis.The boiling heat transfer performance of the composite surface of free particles/wire mesh with sintered wire mesh as the porous medium has been significantly enhanced.Using water as the working medium,the free particle(20%volume fraction)/porous medium(200 mesh 3-layer wire mesh)composite surface has the best heat transfer performance.Compared with a smooth surface,the initial superheat of the surface is reduced by 5.9~oC.The critical heat flux is increased by 2.3 times,and the heat transfer coefficient is increased by more than 3.5 times;using ethanol as the working medium,free particles(20%volume fraction)/porous medium(200 mesh 3-layer wire mesh)composite surface has the best heat transfer performance Compared with a smooth surface,the initial superheat of the surface is reduced by 14.8~oC,and the heat transfer coefficient is increased by more than 4 times.The boiling heat transfer performance of the free particle/sintered particle composite surface with sintered metal particles as the porous medium has been significantly enhanced.For the composite surface of free particles/sintered particles with different volume fractions:using water as the working medium,the free particle(20%volume fraction)/porous medium composite surface has the best heat transfer performance.Compared with a smooth surface,the initial superheat of the surface is reduced At 6~oC,the critical heat flux is increased by 3 times,and the heat transfer coefficient is increased by more than 3 times.With ethanol as the working medium,the free particle(20%volume fraction)/porous medium composite surface has the best heat transfer performance.Compared with a smooth surface,The initial superheat of the surface is reduced by 14.6~oC,and the heat transfer coefficient is increased by more than 2.5 times.Based on the boiling heat transfer model of porous media,the mechanism of enhanced boiling heat transfer of dispersed free particles in porous media is analyzed,and the enhancement effect of free particles is verified according to the relationship as follows:in the stage of natural convection,under the action of the traction force produced by the natural convection of hot and cold fluid,the free particles in porous media disturb the thermal boundary layer and reduce the thermal resistance.In the nucleate boiling stage,the microlayer evaporation and liquid phase reflux promote the free particles to move to the microliquid layer,disturb the microliquid layer,further promote the liquid phase reflux and prevent the microliquid layer from drying up,thus shortening the bubble growth cycle.In addition,the heat conduction rate of metal free particles is higher than that of liquid phase,and the angle between metal free particles and porous media forms a cavity structure,which increases the number of nucleation and enhances the heat transfer efficiency,thus enhancing boiling heat transfer.By comparing the nucleate boiling empirical fitting with the boiling experimental results,it is found that there is little difference between the boiling experimental results and the empirical simulation results,which proves the reliability of the experimental results.