Effect of droplet interaction on droplet-laden turbulent channel flow

Abstract

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.

Clustering and entrainment effects on the evaporation of dilute droplets in a turbulent jet

“…This aspect has been found to be of critical importance in turbulent channel flows where, besides small-scale clustering, turbophoresis increases the mean liquid volume fraction at the wall even by orders of magnitude. In these flow configurations, even with bulk volume fractions of the order of 10 −4 , the effect of droplet collisions is significant (see, e.g., Kuerten and Vreman [35]).…”

Direct Numerical Simulation of Turbulent Flows Laden with Droplets or Bubbles

“…It is noted that nucleation of droplets and droplet breakup were not accounted for in this study. Kuerten & Vreman (2015) extended the DNS study of Russo et al (2014) to include the effects of droplet collisions. The prescribed Weber number of the droplets was small such that coalescence between colliding droplets was negligible.…”

Predicting the impact of particle-particle collisions on turbophoresis with a reduced number of computational particles

“…Oblique collisions redistribute particle streamwise velocity fluctuations into the wall-normal direction (Caraman et al, 2003). Thus, the wall-normal mixing of particles near the wall is greatly enhanced, flattening the concentration profiles even at relatively low bulk volume fractions ∼ 10 −4 (Yamamoto et al, 2001;Kuerten and Vreman, 2015). The quadratic scaling of collision rate with particle number density creates the scenario in which the particle concentration profile, as a function of wall-normal distance, is very sensitive to the bulk volume fraction of the particle-laden flow (Johnson, 2018;Johnson et al, 2019).…”