Numerical Investigation of Droplets Condensation on a Windshield: Prediction of Fogging Behavior

An accurate model to predict the formation of fogging and defogging which occurs for low windshield temperatures is helpful for designing the air-conditioning system in a car. Using a multiphase flow approach and additional user-defined functions within the commercial CFD-software STAR-CCM+, a model which is able to calculate the amount of water droplets on the windshield from condensation and which causes the fogging is set up. Different parameters like relative humidity, air temperature, mass flow rate and droplet distributions are considered. Because of the condition of the windshield's surface, the condensation occurs as tiny droplets with different sizes. The distribution of these very small droplets must be obtained to estimate numerically the heat transfer coefficient during the condensation process to predict the defogging time. This distribution is obtained in a preliminary experiment by using a camera mounted with a microscope on which the picture of the condensation on a windshield's sample is taken. The results show how the relative humidity, the air temperature, the velocity of the car and the droplet distribution influence the fogging pattern and the defogging time. Because of the uncertainties of the boundary conditions in this simple preliminary experiment with the microscope, a complete test bench is currently under investigation and real vehicle tests are planned.