Research On Heat Transfer Characteristics Of Ultra-thin Vapor Chamber With Screen Mesh Wick

Smart electronic devices are developing towards high performance,integration and thin-thickness,and the problem of high heat flux in limited space has become a key challenge for current smart electronic devices.As a high-efficient and compact vapor-liquid phase change heat transfer device,the ultra-thin vapor chamber（UTVC）with a total thickness of≤2.0 mm is an effective solution to the heat dissipation problem of smart electronic devices.The wick provides capillary pressure to return the condensate and acts as the interface for vapor-liquid phase change,which is the key to the heat transfer performance of the UTVC.The screen mesh wick has the advantages of low cost,thin thickness,moderate capillary pressure and permeability,which was used in industry and made them become the subject of this paper.However,the thickness of the screen mesh decreases with the thinning of the thickness of UTVC,which not only results in different transport and heat transfer characteristics compared to conventional wick,but also results in difficulties in wick structure design and fabrication.Therefore,this paper focuses on the mesh-type UTVC,the capillary transport characteristics and boiling heat transfer characteristics of ultra-thin screen mesh are investigated,and the heat transfer characteristics of UTVC with different mesh structures are investigated by optimizing the wick structure（mesh layer and mesh number）and wick surface structure（surface modification）.The main research contents of this paper are as follows:（1）The effects of the mesh layer,mesh number and composite screen mesh on the capillary transport performance of ultra-thin multilayer screen mesh were investigated by capillary rise experiments.The results show that increasing the mesh layer increases the capillary micro pores along the thickness direction,and the wicking coefficient,maximum capillary rise height and volume flow rate of the ultra-thin screen mesh increase.Increasing the mesh number,the smaller pore size causes a larger capillary pressure,and the maximum capillary rise height of the ultra-thin screen mesh increases.When the mesh thickness is 0.17 mm,increasing the mesh size increases the wicking coefficient and volume flow rate of the ultra-thin screen mesh;when the thickness is 0.33 mm,the capillary pressure of 2#100 mesh is small and its wicking coefficient and volume flow rate are the smallest,while the capillary pressure of 4#180 and6#350 mesh is large,the inverse changes in capillary pressure and permeability caused by pore size offset,and the composite screen mesh has large capillary pressure and permeability,so these three meshes have similar wicking coefficients and volume flow rates.（2）A pool boiling visualisation experiment platform was established to investigate the effects of the mesh layer,mesh number and degree of subcooling on the boiling heat transfer characteristics of the working fluid on the ultra-thin screen mesh heating surface.The results show that the capillary micro pores of the mesh have capillary pressure,and increase nucleation sites and heat transfer area compared to plate surfaces,thus enhancing the critical heat flux and boiling heat transfer coefficient.At saturated boiling,the critical heat flux and maximum boiling heat transfer coefficient become larger by increasing the mesh layer.Compared to 6#350 mesh with similar thickness,2#100 mesh has larger wire diameter and lower pore density,which reduces the frequency of bubble merging at high heat flux and increases isolated bubbles,thus increasing the critical heat flux.2#100 mesh has less layer and lower contact thermal resistance,and when Q≥60 W/cm²,boiling inversion accurs,thus increasing the boiling heat transfer coefficient.Compare to the saturated boiling,when degree of subcooling is 20 K,the maximum heat flux on the mesh surface rises,but the boiling heat transfer coefficient decreases,the bubble size becomes smaller and a number of microbubbles are generated.（3）An experiment platform was built to investigate the effect of the mesh layer,mesh number and composite screen mesh on the thermal performance of UTVC.The results show that the thermal performance of UTVC is dependent on the effective capillary radius and mesh thickness.For larger pore radius mesh wick(100 in^-1),UTVC with 0.33 mm thick mesh wick shows better performance,while for smaller pore radius mesh wick(180 in^-1 and 350 in^-1),UTVCs with 0.17 mm thick mesh wick have better thermal performance.For 0.17 mm thick mesh wick,UTVC with smaller pore radius has better thermal performance,while for 0.33 mm thick mesh wick,UTVC with larger pore radius shows better thermal performance.Among the mesh wick whose thickness is below 0.12 mm,the mesh wick with larger thickness and smaller pore radius can promote the thermal performance of the UTVCs.The composite screen mesh wick can improve the thermal performance of UTVCs with 0.33 mm thick mesh wick.（4）The laser etching method was used to form superhydrophilic microstructures on the screen mesh surface,and the capillary rise experiment reveals that the maximum capillary rise height and the wicking coefficient of the laser etched mesh increase by 85.60%and 29.81%,respectively,compared to the original mesh.When the laser etching area ratio（LEAR）of the central evaporator mesh of the UTVC is increased from 0%to 83.3%,the minimum thermal resistance decreases from 0.431 K/W to 0.328 K/W,i.e.decreases 23.9%,the maximum heat transfer capacity increases from 100 W to 120 W,i.e.increases 20%,and the partial dry-out limit increased from 60 W to 70 W,i.e.increases 16.7%.