Theoretical Investigation On Bubble Growth And Leidenfrost Temperature Of Single And Bi-component Liquid Boiling

As a high-efficiency cooling scheme,boiling phase change possesses the advantages such as low heat transfer temperature difference and high heat dissipation capacity.Therefore,it is now widely used in various fields including the thermal management of high-heat-flux devices and fast cooling of extremely high-temperature surfaces.For the former case,the microscopic mechanism of heat transfer and mass diffusion during bubble growth in the nucleate boiling regime is currently unclear.For the latter,the triggering mechanism and predictive models for the Leidenfrost point temperature have yet to be developed.Considering the abovementioned inadequate research,the present study focuses on the thermodynamic characteristics of singleand bi-component bubble growth in uniformly superheated liquid,and also the Leidenfrost temperature of different liquid patterns,aiming at revealing the underlying physics via a combination of theoretical modeling and experimental observation.(1)A theoretical model is developed to study the single-component bubble growth in uniformly superheated liquid.The model assumes that there exists a thin temperature boundary layer adjacent to the bubble surface,in which the liquid temperature profile is quadratic.This assumption is coupled with the energy conservation equations for the bubble and liquid,with the effect of pressure difference,viscosity and surface tension taken into consideration.The model calculation provides the bubble growth characteristics and also stress conditions at the interface.The model predictions agree very well with bubble radius data under different working conditions.The model shows a higher accuracy,better university and wider scope of application than the existing simplified analytical solutions in the literature.(2)The above single-component bubble growth model is further extended to study the bicomponent bubble growth in uniformly superheated liquid.It is assumed that the mass fraction profile of the more volatile component is also quadratic within the concentration boundary layer.Then the bubble growth characteristic parameters can be calculated by coupling the liquid energy equation and species diffusion equation.The non-random two-liquid(NRTL)equation is adopted to calculate the vapor-liquid equilibrium and activity coefficient,and the variation of thermophysical properties with species concentration is also considered.The controlled rates by heat transfer and mass diffusion are quantitatively proposed to investigate their competitive mechanism and the effect of component concentration and liquid temperature on bubble growth characteristics.This new model agrees much better with data than the existing simplified analytical solution,and also can provide details in the early stage of bubble growth.(3)Taking a unit cell of vapor valley underneath the saturated liquid pool on a horizontal surface as the research object,the triggering mechanism for the Leidenfrost temperature is proposed based on the force balance criterion.With the decrease of wall heat flux,the gravity and pressure difference,which point to the heated surface,gradually exceed the surface tension force and stagnation pressure in the opposite direction.In this case,the vapor-liquid interface moves towards the heated surface,and once the vapor film thickness becomes thinner enough to contact the wall,the Leidenfrost point will be triggered and film boiling terminates.With the combination of film boiling heat transfer coefficient expression,a theoretical predictive model for liquid pool Leidenfrot temperature is developed.The new model is shown to provide good agreement with data for various liquids with a broad range of pressures.It also shows better university and higher accuracy than the traditional hydrodynamic models,and simultaneously can reveal the mechanism of the Leidenfrost point compared to the thermodynamic models.(4)A thermodynamic model is developed to study the sessile droplet evaporation on a high-temperature surface in the film boiling regime.The model considers the evaporation from both the upper and lower surfaces of the droplet,and can provide the droplet shape evolution and the vapor flow dynamics underneath the droplet.Based on the calculated results,the triggering mechanism for the sessile droplet Leidenfrost temperature is proposed by considering the surface roughness effect.The vapor layer thickness gradually decreases with decreased surface temperature,and a thin enough vapor film is extremely vulnerable to be penetrated by the surface irregularities.As the vapor film stability is destroyed,direct liquid-solid contact occurs,which terminates the film boiling and then transition boiling occurs.A new theoretical model is developed based on this mechanism,and the predicted Leidenfrost temperatures using correlated surface roughness relations compare satisfactorily with experimental data,which shows the rationality and effectiveness of the triggering mechanism and predictive model.(5)The triggering mechanism for the Leidenfrost temperature of an impacting droplet is proposed based on the force balance criterion with the combination of surface roughness effect.When the water hammer pressure exceeds the vapor pressure,the vapor-liquid interface moves towards the heated surface,inducing a thinner vapor layer.When the vapor layer thickness reaches a critical value,it will be penetrated by the surface roughness protrusions to induce droplet-solid contact,signaling the attaining of Leidenfrost temperature.An approximate predictive model is developed which is applicable to both single-and bi-component impacting droplets.Experiments are also conducted using water,lower alcohols and various bi-component mixtures as the working liquid.It is found that the alcohol additives can counteract the negative influence of smaller impact energy on the Leidenfrost temperature.The model effectiveness is verified by comparing the predicted trends with experimental results.Therefore,the model can be used to reveal the underlying mechanism for the remarkable improvement in the Leidenfrost temperature by a small amount of alcohol additive in a semiquantitative manner.