Research Of Bubble Growth Characteristics Basing On Gas Kinetic Theory Of Interface Mass Transport And Microlayer Evaporation Under Subcooled Conditions

Bubble dynamic is an attractive topic in the study of flow boiling heat transfer.Nucleate boiling phenomenon under subcooling conditions is generally existed inindustrial equipment of condensation enhancement, electronic heat control and compactnuclear reactor etc. Bubble growth, evolution, motion mechanisms in such conditionsare crucial to enhance heat exchange rate and to ensure the safety. On the other side,large scale span and instant changes in flow and temperature field challenge bothexperimental and numerical researches. In this paper, water vapor bubble growth modelbasing on gas kinetic theory of interface mass transport and microlayer evaporationunder subcooled conditions in regular narrow channel are established. Influences ofinlet subcooling degree, system pressure and inlet velocity are analyzed in this paper.Moreover, forces in bubble growth, shape evolution and microlayer changes are furtherdiscussed.The bubble growth model, basing on gas kinetic theory of interface mass transportand microlayer evaporation, established in this paper finely describes the growingprocess under subcooling conditions and coincides well with experimental results.Proper grid generation and calculation region setup solve the scale span between bubblesurrounding and channel in dimension. The modification of mass and energy source inFluentTMactualizes the bubble growth model in numerical program.Influences of inlet subcooling degree, system pressure and inlet velocity arediscussed in this paper. Under the condition of higher inlet subcooling, as theoverheated liquid layer is thinner and condensation effect at the cap is stronger, thebubble tends to lift off from heating wall earlier with smaller diameter; Under highersystem pressure, the force balance vertical to the heating surface is harder to break upand the bubble lifts off later with smaller diameter; For higher inlet velocity, the bubblelifts off earlier with no significant diameter changes owing to higher heat exchange rateat the bubble cap.In further research, shape evolution, force effects and microlayer thickness areanalyzed. According to simulation results, there are four stages in bubble growth:fast-grow-up stage, contact angle changing stage, lift-off stage and condensation stage.The alternation of stages are mainly caused by changes of forces on the bubble. Anumerical model of microlayer under the bubble is established to analyze the changes of thickness and microlayer existing time. In general thickness decreases as the time goeson. It drops down slowly in the beginning and sharply at the end with a littlefluctuations. And with higher inlet velocity and inlet subcooling, microlayer exist for ashorter time and sharper decrease.