Mechanism Study On Microscale Droplet Phase Change Behaviors And Jet Impingement Cooling At Near-critical Pressures

The power densities of the electronic devices and the space vehicles become higher and higher,which requires efficient cooling methods to be developed in urgent.On the other hand,nuclear fusion energy is a new type of renewable energy,which is a promising solution for the energy crisis.Here,efficient heat transfer between the working fluids and the high heat flux surfaces of the reactor is one of the key issues that must be realized for the utilization of the nuclear fusion energy.Therefore,efficient heat transfer and thermal management are the fundamental issues both for the development of the electronic divices and the space vehicles and the utilization of new energies,and reliable and efficient cooling method has to be developed.With the advantages of high heat transfer coefficient and cooling uniformity,spray cooling has been wildly used in many industry applications.To reveal the heat transfer mechanisms of spray cooling,studies on flow and heat transfer of microscale droplet impacting on solid surfaces was conducted,where higher surface tension,stronger evaporation effect and much smaller timescale of momentum and heat transfer were involved for droplets at microscale.The results showed that the captured air bubbles during droplet impacting acted as the nucleus of bubble growth inside the microscale droplets on the polished surface.A theoretical model for characterizing the relation between the droplet impact parameters and the bubble growth superheat was established based on the lubrication theory and the boiling nucleation theory.Then the effects of the micro and nano surface structures,such as Zinc oxide nanowires and copper inverse opals,on the impacting flow and heat transfer of microscale droplet were studied.The results showed that more remarkable precursor film occurred on the nanowire surface,which enhanced the droplet evaporation rate dramatically.With the increase of the surface temperature,the contribution of the precursor film to droplet evaporation decreased.Both of the two structured surfaces enhanced the heat transfer between the microsacale droplet and the surface,and raised the Leidenfrost temperature of the microscale droplet.Phase change cooling methods at subcritical pressures,such as spray cooling,has the critical heat flux(CHF),which is a shortcoming for cooling of high heat flux surfaces.There is no CHF for jet impingement cooling with fluids at supercritical pressures,which means it is more reliable for cooling of high heat flux surfaces.Therefore,the heat transfer characteristics of jet impingement cooling with CO₂ at subcritical and supercritical pressures were studied.Flow visualization near the cooled surface was also conducted.The effects of heat flux,mass flow rate,pressure,jet inlet temperature and surface micro structures on the heat transfer were studied,where the effects of the dramatic variations of thermo-physical properties of CO₂ near the pseudocritical point were discussed.Results showed that single-phase convection and boiling heat transfer coexisted on the cooled surface and there was no temperature overshoot after onset of boiling at subcritical pressures.At supercritical pressure,when the jet inlet temperature was lower than the pseudocritical temperature,the heat transfer coefficient increased with the increase of the heat flux and then decreased with further heat flux increase.The results of flow visualization and numerical simulations showed that there was a fluid layer of large specific heat near the cooled surface,which enhanced the heat transfer rate between CO₂ and the cooled surface.In addition,the local and average Nusselt number of jet impingement cooling with CO₂ at supercritical pressures were correlated with property modifications based on the existing correlations.There is no CHF for jet impingement cooling with CO₂ at supercritical pressures.Therefore,when the surface heat flux is higher than the CHF of jet impingement cooling with CO₂ at subcritical pressures,the heat transfer coefficient of jet impingement cooling with CO₂ at supercritical pressures is still pretty high.Surfaces with micro pin fins were developed and the heat transfer rate of jet impingement cooling with CO₂ at supercritical pressure was enhanced with the micro surface structures.