Study On Bubble Dynamics And Heat Transfer During Pool Boiling Under Low Gravity

Boiling heat transfer is a complex process where thermo-dynamics, hydro-dynamics, mass transfer, heat transfer and interfacial phenomena are tightly interacted.It is of importance for energy storage and transportation processes such as heatexchange and electronic cooling. In earth gravity, natural convection and buoyancyare important mechanisms that affect boiling heat transfer through the rate at whichbubbles are removed from the surface. However, in low gravity environments, themagnitude of effects related to natural convection and buoyancy are relatively smalland other physical mechanisms, such as surface tension, viscosity, and inertial force，normally masked by natural convection in earth gravity becoming play an importantrole. Experiments to date have shown that commonly used correlations do notproperly account for the effect of gravity on boiling processes. Many of experimentalstudies regarding boiling heat transfer in microgravity environments have beenperformed during the past decades. The results of these experiments were somewhatcontradictory, with some experiments showing no effect of gravity on heat transferand others showing a strong dependence. Numerical simulation is also an importantmethod to study the mechanisms of boiling heat transfer. In this study, a mathematicmodel coupled flow field, temperature field and the time dependant gas-liquidinterface tracking has been established based on Phase Field Method. Boiling heattransfer and bubble behavior such as bubble formation, growth and departure on theheating surface has been simulated in microgravity environment.In the first section, a mathematical model of a single bubble based onNavier-Stokes equation and heat conduction equation is established to describe singlebubble dynamics during nucleate pool boiling. Phase Field Method is used to track thefree gas-liquid interface. Numerical calculation of single bubble behavior on theliquid-solid interface and in the liquid phase has been performed. The results ofnumerical simulation is qualitatively consistent compared with the results of physicalexperiments performed by NASA under reduced gravity and microgravityenvironment, thus testify that it is correct and practical for Phase Field Method to simulate heat transfer during pool boiling under low gravity.In the second section, single bubble dynamics and heat transfer process duringpool boiling have been simulated based on Phase Field Method. The growth anddeformation curve of single bubble, departure radius and time and the average heatflux density are acquired under different gravity, the degree of subcooling and thecontact angle. The results show that bubble base diameter expands and contractsduring growth of the bubble, and the top of the bubble goes across the liquidcontinuously. The departure radius and departure time increase with the decrease ofthe gravity gz, and the average heat flux decreases on the heating surface meanwhile.As the degree of subcooling increases, the departure time and average heat fluxincrease. However, the departure radius doesn’t change. With the increase of contactangle, the departure radius and departure time increase, and the average heat fluxdecreases.In the third section, numerical simulation of Marangoni convection duringnucleate pool boiling has been performed. Marangoni convection around a singlebubble and bubble dynamics on the heating surface has been simulated. The resultsshow that Marangoni convection caused a vortex near the gas-liquid interface, andbubble base velocity change. Marangoni convection enhances the heat transfer on theheating surface, and dominates the energy transfer near gas-liquid interface under0g.Small bubbles move on the heating surface and eventually merges with the primarybubble.In conclusion, bubble behavior during nucleate pool boilng is strongly effectedby gravity, the degree of subcooling and the contact angle on the heating surface.Bubble behavior is different under microgravity because the buoyancy effects arereplaced by surface tension effects, liquid momentum effects and Marangoniconvection. In this paper, numerical simulation of nucleate pool boilng has beenperformed and the results will provide valuable reference to experimental studies.