Direct numerical simulation of turbulent channel flow with condensation

Abstract

This thesis presents direct numerical simulations of turbulent humid air flow through a cooled vertical channel at atmospheric conditions. The combination of humidity, temperature and mixed convection is relevant to the ventilation of car interiors, where condensation on critical surfaces such as windows and windshield poses a problem to its immediate or long-term usability. The interaction between buoyancy, convective transport, and phase transitions creates a complex multi-physics problem at the intersection of thermodynamics and fluid dynamics. For the specific range of flow parameters relevant to automotive ventilation, the influence of the liquid phase dynamics is negligible compared to the flow of the gas phase. Leveraging this fact, the single-phase simulation approach uses direct numerical simulations for the solution of the flow fields for the gas phase, including the effect of the phase transition on the velocity, temperature, and vapor concentration of the humid air. It disregards the dynamics within the liquid phase, either neglecting the liquid completely or modeling condensate droplets as static, solid wall deformations. Simulations with and without phase transitions and with and without wall deformations serve to isolate the specific effect of the different influence factors. The combination of direct buoyancy from the cooling and drying of the humid air near the cooled channel wall and the opposing buoyancy contribution from the released latent heat damps the overall influence of the cooled surface on the flow fields compared to cooled channel flow without condensation. In simulations including wall deformations mimicking condensate droplets, the flow across these obstacles create a positive feed-back loop for condensation, increasing the rates at existing droplets.