Visual Measurement Of Microlayer Structure By Laser Interferometry During Pool Boiling Process

As a widely accepted model of boiling heat transfer,the so-called microlayer evaporation model has been the focus of recent research work.The research about microlayer formation and evaporation characteristic will give a deep insight of boiling heat transfer mechanism,provide theoretical guidance for the development of boiling surface with excellent heat transfer performance and be of significant importance both to foundational research and industry applications.The formation of thin liquid film with a thickness on the order of several micrometers(i.e.,a microlayer)beneath boiling bubbles during nucleate pool boiling has been confirmed by numerous researchers.However,the characteristic of microlayer formation and evaporation,dynamic variation of bubble growth,expansion of dry spot beneath single bubble as well as the quantitative contribution of microlayer to bubble growth has been quite limited owing to a lack of experimental conditions.Moreover,the boiling crisis,called the critical heat flux(CHF),which limits the nucleate boiling heat transfer,still remains controversial since the decisive evidences have not yet been sufficiently collected for the mechanism by which CHF occur.Hence,aiming at the characteristic of microlayer formation and evaporation together with the visualization of CHF phenomenon,the following three parts were incorporated in the present study:Firstly,the laser interferometry method was adopted to investigate the detailed structure of microlayers and their variation during nucleate pool boiling for water and ethanol in the present study,with a transparent glass with ITO film coated at the backside as the heating element.In the experiments,the interference fringes in the microlayer region and the profile of the boiling bubble were simultaneously observed and recorded by a high-speed camera through a microscope.The 2-dimensional distribution of the initial microlayer thickness during the whole range of pool boiling process was obtained for the first time,ranging from the Onset of Nucleate Boiling to the Critical Heat Flux.It was confirmed that the linear distribution of initial microlayer thickness in the relatively smaller radius of microlayer,and the crest-like structure of initial microlayer when getting close to the maximum of microlayer radius.As a consequence,the initial microlayer thickness and its relation to the dynamic variation of bubble growth could also be obtained.Secondly,on the basis of the two-dimensional distribution of microlayer thickness and its variation obtained in one bubble cycle,the characteristic of microlayer evaporation was acquired.Hence,the contribution of microlayer evaporation was analyzed by comparing the variation in the microlayer volume(the accumulated value from bubble inception time)to the bubble volume,by considering the density difference between vapor and liquid.The significant effect of microlayer evaporation on boiling heat transfer was thus elucidated quantitatively.It was confirmed that significant contribution of microlayer evaporation was achieved to the total evaporation.The ratio of microlayer evaporation to the total bubble volume increased with the increase of surface superheat at bubble inception time.It was approximately 39 % for ethanol and 14-44 % for water,for the experimental conditions in this study.Lastly,for the visual observation of critical heat flux mechanism in horizontal pool boiling process,the boiling phenomenon at CHF was recorded with high resolution and sampling frequency,by increasing the heat flux continuously.The dynamic variation of microlayer and dry spot at the beneath of small individual bubble under the mushroom vapor was analyzed in this study.Moreover,the appearance and merger of separate dry spot into large dry patch was visualized microscopically,and its relationship with CHF mechanism was also discussed in detail.The existence of microlayer beneath individual bubble was confirmed experimentally and the microlayer model seems to be more powerful for the elucidation of CHF phenomenon of nucleate pool boiling.