Beyond the Reynolds analogy: bubble-induced modulation of turbulent heat transfer

Boiling and bubble injection are effective strategies for enhancing heat transfer between solid boundaries and a working fluid in numerous industrial applications, including nuclear reactors and molten metal processing. Motivated by this, we conduct direct numerical simulations of a vertical, turbulent, differentially heated, bubble-laden channel flow. The Prandtl number Pr, kept identical in both phases, is varied across three representative values-0.07 (liquid metals), 0.7 (vapour) and 7 (water)-to span thermal transport regimes across three orders of magnitude. The simulations are conducted at a friction Reynolds number Reτ =150, void fraction α =5.4 % and a density ratio ρr=0.1 (defined as the bubble-to-carrier density). The bubbles substantially alter the hydrodynamic structure of the flow, amplifying turbulent fluctuations and mixing. Their interaction with the thermal boundary layers disrupts the characteristic streaky structures near the heated walls, fragmenting them into smaller and more chaotic patterns. To elucidate this mechanism, we examine the bubble-induced modifications to the temperature field and show that temperature becomes decorrelated from velocity. Consequently, the heat-transfer enhancement arises primarily from an increase in convective heat flux driven by intensified wall-normal velocity fluctuations. The thermal boundary layer is markedly thinned, and the Nusselt number nearly doubles across all examined cases.