a b s t r a c tDirect numerical simulation is used to assess the importance of compressibility in turbulent channel flow of a mixture of air and water vapor with dispersed water droplets. The dispersed phase is allowed to undergo phase transition, which leads to heat and mass transfer between the phases. We compare simulation results obtained with an incompressible formulation with those obtained for compressible flow at various low values of Mach number. We discuss differences in fluid flow, heat-and mass transfer and dispersed droplet properties. Results for flow properties such as mean velocity obtained with the compressible model converge quickly to the incompressible results in case the Mach number is reduced. In contrast, thermal properties such as the heat transfer, characterized by the Nusselt number, display a systematic difference between the two formulations on the order of 15%, even in the low-Mach limit. This shows the necessity of the use of a compressible formulation for accurate prediction of heat transfer, even in case of an initial relative humidity of 100%. Mass transfer properties display a difference between the models on the order of 5%, for example in the prediction of the droplet mean diameter near the walls.

We propose a point-particle model for two-way coupling of water droplets dispersed in the turbulent flow of a carrier gas consisting of air and water vapour. We adopt an Euler-Lagrangian formulation based on conservation laws for the mass, momentum and energy of the continuous phase and on empirical correlations describing momentum, heat and mass transfer between the droplet phase and the carrier gas phase. An incompressible flow formulation is applied for direct numerical simulation of differentially heated turbulent channel flow. The two-way coupling is investigated in terms of its effects on mass and heat transfer characteristics and the resulting droplet size distribution. Compared to simulations without droplets or those with solid particles with the same size and specific heat as the water droplets, a significant increase in Nusselt number is found, arising from the additional phase changes. The Nusselt number increases with increasing ambient temperature and is almost independent of the heat flux applied to the walls of the channel. The time-averaged droplet size distribution displays a characteristic dependence on position expressing the combined effect of turbophoresis and phase changes in turbulent wall-bounded flow. In the statistically steady state that is reached after a long time, the resulting flow exhibits a mean motion of water vapour from the warm wall to the cold wall, where it condenses on average, followed by a net mean mass transfer of droplets from the cold wall to the warm wall.

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a b s t r a c tDirect numerical simulation is used to assess the importance of compressibility in turbulent channel flow of a mixture of air and water vapor with dispersed water droplets. The dispersed phase is allowed to undergo phase transition, which leads to heat and mass transfer between the phases. We compare simulation results obtained with an incompressible formulation with those obtained for compressible flow at various low values of Mach number. We discuss differences in fluid flow, heat-and mass transfer and dispersed droplet properties. Results for flow properties such as mean velocity obtained with the compressible model converge quickly to the incompressible results in case the Mach number is reduced. In contrast, thermal properties such as the heat transfer, characterized by the Nusselt number, display a systematic difference between the two formulations on the order of 15%, even in the low-Mach limit. This shows the necessity of the use of a compressible formulation for accurate prediction of heat transfer, even in case of an initial relative humidity of 100%. Mass transfer properties display a difference between the models on the order of 5%, for example in the prediction of the droplet mean diameter near the walls.