Experimental Study On Pool Boiling Heat Transfer Enhancement On Composite Structure Surface

Electronic equipment is gradually developing towards integration,miniaturization and high efficiency,and the demand for heat dissipation is also increasing.Efficient boiling heat transfer is considered to be the most effective way to solve the high heat flux heat transfer problem of miniaturized electronic devices due to its advantages such as large latent heat of gas-liquid phase change and high heat transfer coefficient.There are many methods to enhance boiling heat transfer.Currently,enhancing heat transfer by changing surface structure is one of the main research directions in the field of phase change heat transfer.In this paper,based on the concept of surface temperature difference enhanced heat transfer,a new surface structure-composite structure surface(its appearance is like a "water cup",and a narrow cavity is formed between the extremely thin expansion wall and the heat transfer bottom surface)is designed independently.A pool boiling visualization experimental system was built.The heat transfer characteristics of composite structure surface(ETFS),micro cylinder structure surface(PFS)and ordinary light surface(PS)were analyzed and compared.The boiling heat transfer mechanism of ETFS structure is deeply explored.Firstly,saturated pool boiling heat transfer experiments were carried out on PS,PFS and ETFS surfaces.The results show that the ETFS structure can significantly enhance the boiling heat transfer,and the heat flux is increased by 107.5%and 34%respectively compared with the PS and PFS structures.The ETFS structure has ONB point delay at low heat flux and CHF point delay at high heat flux.During the boiling process,there is a temperature difference on the surface of the ETFS structure,resulting in a mixed boiling mode in the chamber,thereby achieving enhanced boiling heat transfer.According to the difference of bubble motion behavior in the boiling cavity of ETFS structure under different heat flux,the mixed boiling is divided into three modes.At low heat flux,it is nucleate/convective mixed boiling mode,at medium and high heat flux,it is partial film/nucleate boiling mode,and at critical heat flux,it is film/nucleate boiling mode.During the boiling process,the temperature fluctuation at the root of the extended wall of the ETFS structure is highly consistent with the bubble disturbance behavior.The greater the mixing boiling strengthening effect,the more severe the bubble disturbance and the greater the temperature fluctuation.The critical heat flux of ETFS structure increases first and then decreases with the increase of the extended wall height H,and there is an optimal value of H.Secondly,the subcooled pool boiling heat transfer experiments were carried out on PS,PFS and ETFS surfaces.The results show that subcooling has a significant impact on boiling heat transfer,and the heat flux density increases with the increase of subcooling,while the heat transfer coefficient continuously decreases.The existence of subcooling affects the mixed boiling mechanism inside the ETFS structure.Under low and medium heat flow,the increase of subcooling will hinder the nucleation of ETFS structure bubbles and weaken the heat transfer between the extended wall and the surrounding liquid film layer.Under high heat flux,the surface temperature difference increases with the increase of subcooling,the mixing boiling mechanism is strengthened,and the heat transfer effect of ETFS structure is enhanced.The subcooling degree has a great influence on the bubble evolution behavior in the boiling of ETFS structure.Under the medium and low heat flow,the temperature at the root of the extended wall increases first and then decreases.At high heat flux,the lowtemperature pool liquid weakens the effect of the extended wall height on bubble confinement,resulting in an increase in the optimal extended wall height H under supercooling conditions.Finally,the ETFS structure is arrayed to obtain the array composite structure element(A-ETFS),and the heat transfer performance of the A-ETFS structure is explored.The results show that the A-ETFS structure can achieve enhanced heat transfer,which is 51.4%and 10%higher than that of PS and PFS structures,respectively.The critical heat flux of A-ETFS structure increases first and then decreases with the increase of cavity diameter.Based on the thermal resistance theory,the variation of the critical heat flux of the A-ETFS structure with the cavity diameter is verified,and the theoretical optimal diameter value is consistent with the experimental results.