Dynamics of Liquid-Vapor Phase Change as Applied to Bubbles and Droplets (original) (raw)
Related papers
Nanoscale and Microscale Thermophysical Engineering, 2020
Heat transfer via phase change is a major contributor to heat removal in numerous engineering applications. Thin films of liquid result in increased heat transfer due to a reduction of conduction resistance, in addition the pressure jump at the liquid-vapor interface also affects the rate and direction of rate of phase change. Because of these effects the morphology of the substrate surface is expected to affect the film shape, hence heat transfer, especially in thin films. In this study, the influence of surface characteristics on the rate of phase change from micron and sub-micron sized 2-D droplets-i.e. films extending to infinity-forming on a substrate are modeled. Surface film profiles are generated on both flat and non-flat surfaces, triangular or wavy in nature, and a kinetic model for quasi-equilibrium phase change is applied. In case of wavy surfaces, the surface is assumed to be a harmonic wave with an amplitude equal to the surface roughness and a wavelength corresponding to values commonly encountered in applications. Due to the presence of intermolecular forces at the contact line, which render the solution of the augmented Young-Laplace equation stiff, an implicit scheme is employed for the numerical integration. To verify the method, the predictions of a molecular dynamics (MD) simulation of a nano sized droplet present on a V-grooved surface is compared to the continuum model. The augmented Young-Laplace equation is solved numerically along with a phase change model originating from kinetic theory to calculate the shape of the two-phase interface forming the droplet and study the effect of various parameters on the rate of phase change. Results are obtained for droplets with liquid pressures higher and lower than that of vapor, resulting in opposite contribution to phase change due to the pressure jump at the interface. The results show that the heat transfer rate can be substantially altered due primarily to the combined effects of surface morphology and disjoining pressure.
Dynamics of vapor bubbles and associated heat transfer in various regimes of boiling
2018
The dynamics of bubble formation during boiling is highly significant considering its influence on the heat transfer rate associated with various applications. Depending on the heat flux, the mode of boiling transforms from the nucleate boiling regime to the film boiling regime. The present thesis is focused on the study of the varying characteristics of boiling regimes through direct numerical simulations. The liquidvapor interface-capturing is performed using the CLSVOF (Coupled Level-Set and Volume of Fluid) approach. In the film boiling regime, the phenomenon of bubble formation is governed by the instabilities at the liquid-vapor interface instigated by the combined influence of surface tension, buoyancy, heat flux, vapor thrust or any other applied external field (electric field in the present study). The dynamical disturbances destabilize the interface which results in bubble formation with the passage of time. The bubble release during film boiling is found to be more of a d...
Physical Chemistry Chemical Physics, 2014
Heat and mass transfer through interfaces is central in nucleation theory, nanotechnology and many other fields of research. Heat transfer in nanoparticle suspensions and nanoporous materials display significant and opposite correlations with particle and pore size. We investigate these effects further, for transfer of heat and mass across interfaces of bubbles and droplets with radii down to 2nm. We use square gradient theory at and beyond equilibrium to calculate interfacial resistances, in single-component and two-component systems. Interface resistances as defined by non-equilibrium thermodynamics, vary continuously with the interface curvature, from negative (bubbles) to zero (planar interface) to positive (droplet) values. The interface resistances of 2 nm radii bubbles/droplets are in some cases one order of magnitude different from those of the planar interface. The square gradient model predicts that the thermal interface resistances of droplets decrease with particle size, in accordance with results from the literature, only if the peak in the local resistivity is shifted toward the vapor phase. Curvature will then have the opposite effect on the resistance of bubbles and droplets. The model predicts that the coupling between heat and mass fluxes, when quantified as the heat of transfer of the interface, is of the same order of magnitude as the enthalpy change across the interface, and depends much less on curvature than the interface resistances.
Experimental analysis of a single vapor bubble condensing in subcooled liquid
Chemical Engineering Journal, 2002
An experimental investigation of the dynamics of, and the heat transfer to, the vapor bubbles condensing in a miscible or an immiscible liquid is presented in this paper. Unpublished experiments of Freon-113, pentane and hexane bubbles condensing in water and Freon-113 bubbles condensing in subcooled Freon-113 are analyzed and compared to previously published experiments of pentane/water, isopentane/water and pentane/glycerol systems. The experimental results of both the mechanical and thermal behaviors are compared to existing models. Throughout the comparison, we examine the effect of the shape and rigidity of condensing bubbles as well as the effects of the contaminants and noncondensibles on the velocity of, and the heat transfer to, the bubbles. Empirical correlations for the drag coefficient and the Nusselt number for a wide range of experimental parameters are developed. These correlations are simple to use (especially in contrast to existing complicated models requiring numerical solutions) and agree well with the experimental results.
An analytical model for bubble growth and convective heat transfer coefficient of a single liquid/vapour two-phase bubble evaporating in direct a contact with another immiscible liquid media has been developed. The model was based on the solution of the energy equation in a spherical coordinate with a potential flow assumption and a cellular model configuration. A new expression of the convective heat transfer coefficient in terms of the vaporization ratio and the Pe was derived and used to develop the bubble growth rate expression. Different parameters such as the vaporization ratio, Ja, Pe and Fro were tested. The solution results compared well with available experiment data and other theories.
Vapor bubble growth in heterogeneous boiling—I. Formulation
International Journal of Heat and Mass Transfer, 1995
Abstraet--A numerical analysis is carried out to study bubble growth in saturated heterogeneous boiling. The bubble growth is determined by considering the simultaneous energy transfer among the vapor bubble, liquid microlayer, and heater. Finite difference solutions for the temperature fields in the microlayer and heater are obtained on expanding coordinates as the bubble grows. The parameters characterizing the bubble shape and microlayer wedge angle are determined by matching the existing experimental data. The predicted bubble growth rate compares very well with the reported experimental data over a wide range of conditions.
Forced convection heat transfer around large two-phase bubbles condensing in an immiscible liquid
Heat Recovery Systems and CHP, 1993
Al~tract-In this investigation, an equation is theoretically developed to predict the Nusselt number and hence the collapse rate of large spherical two-phase bubbles condensing in quiescent immiscible liquid. Heat transfer from the thin film of condensate and in the wake is determined. Theoretical prediction and experimental data show satisfactory agreement. A parameteric study is also carried out for different parameters affecting the collapse rate. NOMENCLATURE A heat transfer area (m 2) a defined by equation 29 B dimensionless bubble radius (R/Ro) b defined by equation 30 Cp specific heat (J kg-I K-I) Fo Fourier number (¢tt/R~o) Fr Froude's number (u2/2Rg) Fro initial Froude number (U2o/2Rog) g acceleration due to gravity (m s-z) h heat transfer coefficient (W m-2 K-~) Ja Jakob number (Pc C~ AT/pdv2d) k thermal conductivity (W m-~ K-t) Nu Nusselt number, (2hR /kc) Pe Peclet number (2uR/ct~) Peo initial Peclet number, (2uoRo/rtc