Effect of droplet interaction on droplet-laden turbulent channel flow

We present results of direct numerical simulation of heat transfer and droplet concentration in turbulent flow of a mixture of dry air, water vapor, and water droplets in a differentially heated channel. In particular, we study the effects of droplet collisions by comparing results of simulations with and without droplet collision model for several overall droplet volume fractions. The results show that droplet collisions have a large influence on droplet concentration. Maximum local concentrations, which occur close to the walls of the channel, are reduced by almost an order of magnitude for the case with the highest overall volume fraction. In addition, the positive skewness of the local volume fraction is reduced by a factor of two near the walls. These findings show the importance of including four-way coupling, even in cases where the overall droplet volume fraction is only on the order of 10^-4 and the Stokes number in wall units is only about 10. In spite of this large effect of droplet collisions on droplet concentration, the effect on the overall heat transfer between the walls of the channel is not more than approximately 17%. That the effect on the overall heat transfer is relatively small can be explained by the lower heat exchange area between droplets and gas in the near-wall areas, which results in a higher temperature difference between droplets and surrounding gas.