Numerical Simulation And Analysis Of Windshield Defogging Performance

Passenger safety has been considered as the key issue during vehicle design since many years. One of the important factors that strongly affects driving safety is visibility through the windshield. The presence of fog on the windshield glass reduces or blocks the field of vision of the driver and it severely deteriorates the driving safety. Impairing driver visibility due to inadequate window defogging has been shown to be a common concern. So, how to defog the windshield efficiently and quickly is still a central issue in vehicle HAVC system design.In this thesis, CFD simulation combined with the theory of heat transfer and mass transfer has been applied to do analysis of the windshield defogging performance.Water on a fogged windshield glass surface is distributed in the form of water-drops and does not form a film. Because fogging is related with many factors and the distribution of water-drops is changeful, several assumptions are given:①Once the windshield temperature drops below the dew point temperature of the surrounding air, the glass will be fogged.②The water-drops distributed on a fogged windshield homogeneously.③Here, the shape of each water-drop is assumed to have a hemisphere of radius r, and the contact angle between the glass surface and water surface is assumed to be 90-degree.④Assume that each water-drop have the same radius r, and the value of radius equals to 10μm ,it comes from experiment of someone else.In this thesis, a new fog droplet evaporation model was established and applied based on the mechanism of fog droplet condensation. Then, research has been done about the evaporation principle and heat transfer and mass transfer of that course, and a fog droplet evaporation formula has been educed as follows: The fog droplet evaporation formula provides a good method for calculating the defogging time and analyzing automobile windshield defogging performance.Based on the analysis of water-drop evaporation principle near the glass and heat transfer and mass transfer in the process of evaporation, following conclusions are obtained:①The evaporation of fog on windshield is a complex process including mass transfer, heat transfer and momentum transfer. The fog droplet shrinks when mass-transfer happens between the surrounding air and the fog droplet.②When the evaporation of fog droplet is progressing, there is a boundary-layer of saturated vapor on the fog droplet, and the air far from the boundary-layer is called main-airflow. The evaporation of fog droplet comes from the vapor pressure difference between saturated boundary-layer and the main-airflow.③The evaporation rate on windshield glass is influenced by fog droplet temperature- Ts , main airflow temperature- T∞, main airflow relative humidity-Φand main airflow velocity.Then, computational simulation has been carried out by applying CFD software combined with the theory of heat transfer and mass transfer to analyze the windshield defogging performance and the influence factors of defogging. It provides some references for the optimization design and performance estimation of the HAVC defogging system by comparing different parameters.①The first step of computational simulation is to make up a initial surrounding condition under which windshield is fogged. In this thesis, an assumption of initial surrounding condition of fogging was set, and simulation result of it validates that the assumption of initial surrounding condition is reasonable.②In this thesis, a reference operation condition was chosen, then analysis of the defogging process and the vapor characteristics near the wall has been done accounting for different evaporation rate on windshield glass. A further study has been done about the effects of velocity on the temperature and relative humidity distribution near the wall.The following conclusions are obtained:The difference of evaporation rate on the glass surface comes from the difference impingement degree of inlet airflow to the windshield. Evaporation rate of fog droplet depends on velocity but not directly, however, velocity distribution provides a precondition to evaporation rate. Velocity distribution affects heat transfer coefficient and mass transfer coefficient firstly. Then, the heat transfer coefficient and mass transfer coefficient effect on fog droplet temperature, main airflow temperature and main airflow relative humidity. The evaporation rate of fog depends on the synthetic action of all the factors.③Then, the effects on the defogging performance of the automobile air conditioning system of different inlet-airflow parameters, such as temperature, velocity, and relative humidity has been compared and analyzed. The following conclusions are obtained:The larger the inlet-airflow flux is, the better the defogging performance is. The smaller the inlet-airflow relative humidity is, the better the defogging performance is. In a certain scale, the lower the inlet-airflow temperature is, the better the defogging performance is. However, considering comfort of the passengers and performance of air conditioning system, customer should choose a suitable condition for defogging.