Investigations On Microbubble Emission Boiling And Its Formation Mechanism

Microbubble Emission Boiling(MEB)is a special boiling phenomenon characterized by an extremely high heat transfer capacity,and has a great potential application in cooling the equipments and systems like high speed chip,high power semiconductor,nuclear reactors etc.However,research regarding MEB is still in its beginning currently.Influencing factors involved and mechanism triggering MEB are not fully understood.Therefore,a detailed study was conducted in this paper on bubble dynamic behavior,acoustic characteristics,heat transfer characteristics of MEB,as well as the role of condensation effect and Marangoni effect in MEB formation process.In first,a visualized experimental system was designed and fabricated to achieve MEB stably and repeatedly on the basis of solving the key issues such as sealing,insulation under high heat flux condition etc.The heating surface is circular and has a diameter 10 mm.Experiments were carried out under atmospheric pressure and subcooling of 0-60 K,and degassed water was used as the working fluid,and typical MEB phenomenon was observed successfully.When liquid subcooling is higher than 20 K and heat flux is over CHF,MEB occurred remarkably with a maximum heat flux over 7 MW/m2 after a briefly step increase in the wall temperature(lasting about 2-5 s).In MEB,large irregular bubbles commonly collapse rapidly in the bubble edge or on the bubble top accompanied by the emission of microbubbles and loud boiling sound,but do not departure from the heating surface,which is different from that in the other boiling modes.The average bubble collapse frequency increases with the increase in heat flux,and is always over 100 Hz,much higher than that in nucleate boiling.Moreover,liquid subcooling and heat flux have limited effect on average bubble diameter in MEB,whereas the maximum variation in bubble radius increases with the increase in wall superheat and even reaches to 2.5 m/s.In the region of MEB,the acoustic signal fluctuates violently and the frequency bands of its energy peaks approach to those of bubble collapse frequency.These energy peaks alternately appear in the time-frequency spectrum,indicating that more than one bubble collapse during MEB.In addition,it is also found that natural convection,nucleate boiling,film boiling and MEB can be distinguished by the deviation of the signal and method of discrete wavelet transform.Considering the relationship between bubble collapse and heat transfer,as well as condensation and Marangoni effect,a dimensionless heat transfer correlation for MEB is proposed.In order to illustrate the influence of condensation at the vapor-liquid interface on the formation of MEB,a visualized experimental system was set up to investigate the condensation and collapse process of a vapor bubble.Once liquid subcooling and the vapor injection rate exceed 20 K and 0.74 m3/h,respectively,scenario similar to MEB could be observed in the experiments.Moreover,it is found that capillary wave with relatively high wave number would form at the interface before the collapse,and the amount and size of the generated microbubbles are determined by the wave number.Linear instability analysis on a spherical surface further indicates that at relatively high liquid subcooling,substantial condensation mass flux across the interface may trigger Landau-Darrieus instability,resulting in the formation of capillary wave on the surface.The bubble condensation process is therefore enhanced and Birkhoff instability is excited afterwards,leading to the bubble breaking up into a great amount of microbubbles.In addition,at low liquid subcooling,Landau-Darrieus instability could not be triggered with any surface wave number,therefore no capillary wave forms at interface and no bubble collapse process occurs,which reveals that why MEB can be only observed at a relatively high liquid subcooling.Finally,in order to investigate the effect of Marangoni convection on the formation of MEB,micro-convection around a bubble before collapse was analyzed with the software of FLUENT.For water,due to the relatively violent Marangoni convection near a vapor bubble edge at relatively high liquid subcooling,the instability of vapor-liquid interface is thus intensified,as a result,partially collapse occurs more easily than the other regions,showing a good agreement to the experimental results.For ethanol,at high liquid subcooling,although the condensation effect is strong,the Marangoni convection adjacent an ethanol bubble is quite weak and the maximum velocity of fluid is just a third of that for water,resulting in that no MEB occurs under this condition.Therefore,Marangoni convection is very likely another important factor to trigger bubble collapse in MEB except the condensation at vapor-liquid interface.