Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a muchenhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh-Bénard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 10 6 and 5 × 10 9. We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh-Bénard convection.two-phase convection | latent heat | boundary layers | point bubble model | simulations T he greatly enhanced heat transfer brought about by the boiling process is believed to be due to several interacting components (1-3). With their growth the bubbles cause a microconvective motion on the heating surface, and as they detach by buoyancy, the volume they vacate tends to be replaced by cooler liquid. Especially in subcooled conditions, the liquid in the relatively stagnant microlayer under the bubbles can evaporate and condense on the cooler bubble top. This process provides for the direct transport of latent heat, which is thus able to bypass the low-velocity liquid region adjacent to the heated surface due to the no-slip condition. The bubble growth process itself requires latent heat and, therefore, also removes heat from the heated surface and the neighboring hot liquid. Finally, with their buoyancy, the bubbles enhance the convective motion in the liquid beyond the level caused by the well-known single-phase Rayleigh-Bénard (RB) convection mechanisms (4, 5). This last process is the aspect on which we focus in the present article.In classical single-phase RB convection, the dimensionless heat transport, Nu, the Nusselt number, is defined as the ratio of the total heat transported through the cell to the heat that would be transported by pure conduction with a quiescent fluid. This ratio increases well above 1 as the Rayleigh number Ra = gβΔL 3 νκ is increased due to the onset of convective motion in the cell. Here g is the acceleration of gravity, β the isobaric thermal expansion coefficient, Δ = T h − T c the dif...

The heat transfer mechanism in Rayleigh-Bénard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jacob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number.

No abstract

. (2011). Effect of vapor bubbles on velocity fluctuations and dissipation rates in bubbly Rayleigh-Bénard convection. Physical Review E, 84(3), 036312-1/7. [036312]. DOI: 10.1103/PhysRevE.84.036312 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Numerical results for kinetic and thermal energy dissipation rates in bubbly Rayleigh-Bénard convection are reported. Bubbles have a twofold effect on the flow: on the one hand, they absorb or release heat to the surrounding liquid phase, thus tending to decrease the temperature differences responsible for the convective motion; but on the other hand, the absorbed heat causes the bubbles to grow, thus increasing their buoyancy and enhancing turbulence (or, more properly, pseudoturbulence) by generating velocity fluctuations. This enhancement depends on the ratio of the sensible heat to the latent heat of the phase change, given by the Jakob number, which determines the dynamics of the bubble growth.