Browsing by Author "Oresta, Paolo"
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Item Effect of vapor bubbles on velocity fluctuations and dissipation rates in bubbly Rayleigh-B ?enard convection(Physical Review E, 9/19/2011) Lakkaraju, Rajaram; Schmidt, Laura E.; Oresta, Paolo; Toschi, Federico; Verzicco, Roberto; Lohse, Detlef; Prosperetti, AndreaNumerical 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.Item Effects of particle settling on Rayleigh-Bénard convection(Physical Review E, 6/20/2013) Oresta, Paolo; Prosperetti, AndreaThe effect of particles falling under gravity in a weakly turbulent Rayleigh-Bénard gas flow is studied numerically. The particle Stokes number is varied between 0.01 and 1 and their temperature is held fixed at the temperature of the cold plate, of the hot plate, or the mean between these values. Mechanical, thermal, and combined mechanical and thermal couplings between the particles and the fluid are studied separately. It is shown that the mechanical coupling plays a greater and greater role in the increase of the Nusselt number with increasing particle size. A rather unexpected result is an unusual kind of reverse one-way coupling, in the sense that the fluid is found to be strongly influenced by the particles, while the particles themselves appear to be little affected by the fluid, despite the relative smallness of the Stokes numbers. It is shown that this result derives from the very strong constraint on the fluid behavior imposed by the vanishing of the mean fluid vertical velocity over the cross sections of the cell demanded by continuity.Item Heat transfer mechanisms in bubbly Rayleigh-Benard convection(Physical Review E, 8/17/2009) Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, AndreaThe 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 pointlike 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 Jakob 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.Item Heat transport in bubbling turbulent convection(Proceedings of the National Academy of Sciences, 6/4/2013) Lakkaraju, Rajaram; Stevens, Richard J.A.M.; Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, AndreaBoiling 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 much-enhanced 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 × 106 and 5 × 109. 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.Item Modification of turbulence in Rayleigh-B ?enard convection by phase change(New Journal of Physics, 2/3/2011) Schmidt, Laura E.; Oresta, Paolo; Toschi, Federico; Verzicco, Roberto; Lohse, Detlef; Prosperetti, AndreaHeavy or light particles introduced into a liquid trigger motion due to their buoyancy, with the potential to drive flow to a turbulent state. In the case of vapor bubbles present in a liquid near its boiling point, thermal coupling between the liquid and vapor can moderate this additional motion by reducing temperature gradients in the liquid. Whether the destabilizing mechanical feedback or stabilizing thermal feedback will dominate the system response depends on the number of bubbles present and the properties of the phase change. Here we study thermal convection with phase change in a cylindrical Rayleigh–Bénard cell to examine this competition. Using the Reynolds number of the flow as a signature of turbulence and the intensity of the flow, we show that in general the rising vapor bubbles destabilize the system and lead to higher velocities. The exception is a limited regime corresponding to phase change with a high latent heat of vaporization (corresponding to low Jakob number), where the vapor bubbles can eliminate the convective flow by smoothing temperature differences of the fluid.