Heat Transfer Of Frost Fog And Characteristics Of Defrosting/Defogging In Windshield

The phenomenon of frost and condensation is involved in the fields of aerospace, automobile, refrigeration and cryogenic engineering and heat pump systems. Frost and fog in the aircraft and windshield of automobile and airplane also strongly influence safety and comfort of driving. Frost and fog in the cold wall cause to decrease of heat transfer coefficient of refrigeration system, decrease of refrigerating capacity, increase of energy consumption, even serious blockage of system. It is very necessary to study the formation, dissolution and evaporation process of frost and fog, especially heat transfer of formation, characteristics of defrosting/defogging in windshield and the optimal defrosting/defogging method. The research is an important guarantee of security for automobile and aerospace, plays a significant engineering role and becomes the main research direction of VHVAC (Vehicle Heating Ventilating and Air Conditioning).Firstly, theoretical analysis the formation, dissolution and evaporation process of frost and fog, which use thermodynamics, crystallography and nucleation theory, is given. From the point of view of chemical potential, phase change driving force and critical radius, mechanism of frost and fog formation, dissolution and evaporation is analyzed. Furthermore, criterion of frost is defined, that is the cold wall temperature is lower than the solid-vapor or liquid-solid phase transition phase temperature corresponding to water vapor partial pressure. The criterion of fog is defined, that is water vapor partial pressure is higher than triple point pressure and cold wall temperature is lower than dew point temperature. During the analysis of defrosting, based on concentrating group analysis of single fog drop unit, single fog drop evaporating model is established to analyze heat and mass exchange for fog drop evaporating process and build up the mathematical relationships. Moreover, from the point of view of fog drop adhering area, the concept of defogging rate, which is used to evaluate the effect of defogging, is put forward to realize the quantify analysis of defogging process.According to the frequent appearance of frost and condensation in windshield, the characteristics of defrosting/defogging are studied. The research includes confirmatory experimental analysis and developing simulating analysis with CFD. By using the indoor air to defog, the experimental results show that fluid temperature increases quickly with exponential law when the defrosting process start, fluid temperature keeps constant at the later stage of defrosting when cooling system of engine is steady. The temperature change characteristics in the defrosting process has three stage, that is the temperature quick increasing stage before defrosting, temperature steady stage as defrosting and temperature increasing stage after defrosting. By using the outside air to defog, there are not apparent stage characteristics and increasing of fluid temperature is relative slow. In the process of defogging, the temperature of glass increase slowly with linear law, but there is great different velocity rate in the different location. Defogging process differs from the defrosting process, which has apparent stage characteristics and very long time. The results also shows the influence of some factors, such as structure and location of air outlet and so on.According to the phenomena, characteristics and heat transfer process of frost/fog in windshield, physical models of defrosting in the outside of windshield and defogging process in the inside of windshield are established to make numerical simulating calculation. At the same time, simulating analysis is confirmed by experimental research. By comparative analysis of simulating and experimental results of temperature changing characteristics of air outlet in the system of defrosting/defogging, where the simulating results is obtained by using of regression analysis of experimental data in the given conditions, the numerical simulating calculation of temperature changing characteristics, temperature field distribution, fluid phase rate and defrosting effect is confirmed to be feasibility. It lays the groundwork for future developing simulating research.Combined with numerical simulating calculation, the effect of the extent of bend of glass curved surface and glass incidence on the air cohesiveness and characteristics of defrosting is analyzed. In fact, windshield, air outlet structure (the type of nozzle, location, angle and so on) and parameters of air directly influence the characteristics and effect of defrosting. Many factors impact each other, so the effect characteristics of different factors are analyzed to put forward analytical method of defrosting characteristics. At the same time, the main factor is found and is analyzed to get the influencing characteristics.By studying of defrosting characteristics, it is found that the bent extent of glass influence little on the effect of defrosting though cohesiveness is produced in the air. Furthermore, the effect of glass inclination angle and air outlet structure is considerable, which can improve air characteristics, achieve the optimal defrosting characteristics and obtain the better effect of defrosting. The type of nozzle directly related to the air flow and diffusion, thereby affected the solution frost effect. The location of nozzle has a certain impact on defrosting, especially in the case of small glass inclination angle, is not the closer from the glass low edge the better, there exists an optimal location. Diversion angle of the nozzle directly affect the jet angle and sweep angle, is very important for the characteristics and efficiency of defrosting, there is an optimal range of angles. The optimal angle range is impacted by the bent extent of glass surface, glass inclination angle, nozzle type, location and other factors, and comprehensive analysis. The jet angle and sweep angle can be adjusted to improve efficiency of defrosting. Moreover, the velocity and temperature of air have an apparent and synthetical influence on the glass surface temperature, effect of defrosting, running time, running economy and thermal comfort and so on. There is the optimal velocity and temperature range.It is well known that there is considerable different for forming process of frost and fog in the windshield, so the characteristics of defogging is different from that of defrosting. The effect factor changing with time for defogging process is analyzed by using of cluster analytical method of fog drop. The results of defogging shows that when the fog drop evaporates, the radius of fog drop decreases linearly, defogging rate increases exponentially, the fog drop near impact air evaporates the best quickly. The influence of air parameters on the characteristics is very apparent. Under the condition of conventional climate and internal circulation air flowing, when it is use the cold air and air with the same temperature to defog, every fog drop radius is still linear, but by using of hot air to defog, every fog drop radius is nonlinear. The temperature of air has a considerable influence on the velocity of evaporation in the area which is far away from impacting location. Under the condition of the same defogging air, the effect of defogging is better by using the cold air than the same temperature air and hot air. The effect of defogging by using hot air is the worst, which even make more fog appear in the windshield. When defog is done by the air of the same humidity, the increase of air temperature can make the effect of defogging is better than that of defogging by using the same temperature air and cold air. At this time, the effect of defogging by using of cold air is the worst. The relative humidity of air directly influences on the responsive extent of defogging. The smaller the relative humidity is, the quicker the velocity of defogging is. With the increasing of air flow rate, the velocity of evaporation for fog increases. But when the flow rate is very large, the influence extent is smaller. The optimal flow rate exists. By analysis of economy and comfort, it is satisfactory to use the lower flow rate (100m3/h-200m3/h) to defog, which is beneficial to decrease the work, noise and the influence of the flow field in the car.The research of defogging also shows that the parameters in the car also influence on the characteristics of defogging. It has a considerable influence on the effect of defogging in the area away from the air impact. The air parameters outside the car directly influence on the heat transfer ability of glass exterior surface. The influence of glass inclining angle on the defogging characteristics of the area of above and nether angle is large, that is the larger the glass inclining angle is, the faster defogging on the area of above angle is, but the defogging on the area of nether angle is so slower that the defogging can not be done, there is the optimal angle. The influence of bend extent of glass curve on the performance of defogging is less. The initial fog drop radius only influence on the time of defogging and does not affect the decrease rate of fog drop radius. From the basic analysis, it can be known that when the climate condition changes, there is different in the methods of defogging in order to defog effectively, quickly, economically and comfortably.In a word, by using of the theoretical analysis, experimental research and numerical simulating calculation, the process of defogging/defrosting is analyzed. The defrosting performance of exterior surface in the windshield and defogging performance of interior surface in the windshield are studied. The characteristics of defogging/defrosting are studied to know the influence of relative factors. Furthermore, the research of defogging has leading function for the future. The research provides help and technological reference for defogging/defrosting in the windshield and lays the foundation for valid, automotive and intelligent control of defogging/defrosting in the automobile engineering.