A Visualization Experimental Research On Micro-bubble Emission Boiling

Boiling heat exchange is widely used in various heat exchangers due to its high heat transfer coefficient and low required wall superheating.Boiling experiences nucleate boiling,transition boiling and film boiling consecutively.Transition boiling is instable and often brings on the deterioration of heat transfer,heating surface temperature increasing quickly and heat transfer coefficient decreasing rapidly,sometime even burnout the heating surface.As a result,the heat transfer ability of water-cooled heat exchangers has to be limited.Micro-bubble Emission Boiling(MEB)is a special boiling phenomenon,in which the heat transfer coefficient increases quickly with the increase of heating temperature after CHF.Therefore,heat flux higher than CHF can be acquired without increasing the heating temperature too much,which can break through the bottleneck problem of heat transfer limitation of heat exchangers.An experimental setup was built to study the boiling phenomenon and characteristics of MEB.Stable MEB phenomena was realized on the setup,temperature and boiling sound data were acquired from the experiments.Another experimental setup for vapor injection was also built to study the condensation of steam vapor.Main results for this study are as follows:1.The experimental results showed that a remarkable degree of MEB will occur when subcooling exceeds 25 K.The heat flux increased rapidly with the increasing of subcooling.The mechanism of MEB heat transfer is similar with steam-water convection,therefore,a heat transfer correlation of MEB is derived from Rohsenow correlation.2.The bubble behaviors from nucleate boiling to MEB region are recorded,In addition,the influences of noncondensable gas as well as the installation of heating surface on MEB were also discussed.3.The data of boiling sound shows that the characteristics on the amplitude spectrum is different between different boiling modes.When fully MEB occurs,a characteristic peak came into being around the frequency of 300 Hz.Along with the increase of heat flux,the bubble behaviors become violent,thus,the peak value rises accordingly;The chaotic analysis shows,MEB has chaos in characteristics with higher complexity as well as chaotic extent than nucleate boiling.4.Steam injection experiments were carried out.The behaviors of vapor-liquid interface as well as the effect of injection rate and the intensity of ultrasound were investigated.The results shows that vapor bubbles become fragile with moderate injection rate(about 0.34m/s)and enough intensity of ultrasound.