Mechanism Of Heat Transfer In Different-mode-interacting Boiling At Microscopic Scale Applying Laser Interferometry

Nucleate boiling is widely used in industrial equipment and technologies such as heat exchangers and aerospace electronics due to its excellent heat transfer capability.However,when the heat flux exceeds the critical heat flux(CHF)of nucleation boiling,the boiling mode will change to membrane boiling,while the superheat increases dramatically to burn out the heat transfer equipment.Different-mode-interacting boiling is a method to improve CHF by arranging low thermal conductivity materials inside the heat transfer plate and changing the internal structure of the heat transfer plate to make the heat transfer surface produce different boiling modes.There have been a lot of studies on the influencing factors of different-mode-interacting boiling,but the explanation of the interaction between different boiling modes of abnormal interference boiling has not been given.The purpose of this study is to investigate the heat transfer mechanism of different-mode-interacting boiling at the microscopic scale using laser interferometry.By etching conductive indium tin oxide(ITO)films at intervals on the glass surface to form a spaced distribution of high and low temperatures on the heat transfer surface,the boiling phenomena on the non-uniform heat transfer surfaces of quartz and sapphire glasses were observed by laser interferometry,and the visualization of different-mode-interacting boiling at the microscopic scale was achieved.On the heat transfer surface of quartz glass,the nucleation phenomena are scattered in the heated and unheated regions,and the nucleation points have no obvious dependence on the heated region.In the sapphire glass heat transfer surface,the nucleation point density in the heated region is higher than that in the unheated region,and the nucleation point density increases with the increase of heat flux in different regions until the critical heat flux is reached.In addition,during the boiling process,dryout is formed at the bottom of the bubbles generated on the heat transfer surface,with the dryout in the non-heated area accounting for 15% and the dryout in the heated area accounting for more than40%.Under the medium-high heat flux condition,when a large dryout occurs in the heating area,the non-heating area is still in the wetting state,and the liquid in the nonheating area moves laterally to the heating area on both sides under the influence of bubbles,which expands the wetting area.Two different boiling modes existed simultaneously on the heat transfer surface,and the dryout was repeatedly wetted to delay the occurrence of large dryout spread.The non-uniform copper block model and the spaced ITO model with the material of sapphire glass were established by Fluent,and the temperature and heat flux distributions on the heat transfer upper surface of the two models were simulated and calculated with the actual experimental conditions imposed,and the relationship between the average superheat degree and the average heat flux was analyzed,with the increase of the heat flux,the difference of the wall superheat degree between the high and low heat conduction regions also increased and showed an expanding trend.For the sapphire heat transfer surfaces with heating area widths of 1.6 mm,2.0 mm,and 2.4mm,the wall superheat differences between the heated and unheated areas near the critical heat flux reached 35 K,16 K,and 19 K.The increase in temperature difference between the different areas led to the rupture of the glass with stresses exceeding the limits it could withstand.In addition,the analysis of the dryout infiltrating frequency and velocity of the heat transfer surface of the sapphire glass with grooves compared with that of the uniform sapphire glass.The dryout infiltrating frequency of sapphire glass with grooves reached a maximum of 175 Hz,which is about 2.2 times the maximum infiltrating frequency of the uniform heat transfer surface,and the dryout infiltrating velocity of the former reached a maximum of 1.18 m/s,which is higher than the maximum infiltrating velocity of the uniform heat transfer surface of 0.88 m/s.Therefore,there is a stronger transverse interference between bubbles in differentmode-interacting boiling compared to uniform heat transfer.