Mediating heat transport by microbubble dispersions: The role of dissolved gases and phase change dynamics

Recently a theory for additional heat convection by microbubbles was posited. The mechanism proposed is that the latent heat of vapor of the liquid is carried by microbubbles from hot zones that vaporize more liquid to cold zones where condensation releases the latent heat. In this paper, the proposition that the additional heat flux is controlled by the phase fraction of microbubbles, is tested by steady state solutions of the canonical hot wall / cold wall buoyant convection problem. The simulations show that the range of additional heat transfer varies monotonically with length scale of the cavity, between 5 and 45% with phase fractions varying from 0.02 and 0.2. The larger characteristic lengths introduce insufficient heat flux from the hot wall to maintain a “driven cavity” flow structure, so that the steady state structure that emerges is a stable stratification with thin boundary layers near the hot and cold walls, with weak shear flow convection. The stable stratification resultant at higher characteristic lengths suppresses the additional heat flux due to microbubble mediation, yet only moderately deviating from proportionality. These conclusions hold qualitatively for a variant of the model which simply treats the gas exchange between the microbubble phase and dissolved gases in the liquid, resulting in variation of the microbubble phase fraction with temperature, hence with position in the domain. Quantitatively, the percentage increases are 3–30% in the parameter regime above, with the effects of dissolved gases captured dynamically.