Experimental Investigation On The Frosting Characteristics And The Effects On Refrigeration System For Air Cooler

The phenomenon of frosting widely exists in nature as well as refrigeration, air conditioning, chemicals, aerospace and other engineering fields. If the cold surface temperature is below the dew point temperature of wet air and at the same time below the triple point temperature of water, water vapor in the wet air will condense into frost on the cold surface. In the refrigeration and air conditioning systems, the frosting on the surface of an evaporator causes the heat exchange capacity decreased and leads to severe damage to the refrigeration equipment in some serious cases. Therefore, it has very important significance on ensuring the refrigeration system safety to study frosting characteristics of air coolers and find the way to alleviate and eliminate the loss of frosting and defrosting for the refrigeration system. In this paper, based on the analysis of frosting mechanism, an in-depth systematic study on frosting characteristics of air coolers is carried out, a refrigeration system simulation model is proposed, and a test rig with photomicrography is set up to validate the model accuracy by testing different structures of air coolers. Also an annual cost analysis model of air cooler under frosting condition is built to study the effects of frost on the operation economy of refrigeration system, which provides a strong theoretical basis for optimized design of the system.Derivation of quantitative nucleation driving force for saturated water vapor condenses in the cold wall is done according to the thermodynamics and kinetics of phase transformation. The results show that the subcooling degree and saturation ratio of wet air increases, the driving force of the phase transition increases as well. A statistical thermodynamic model of the nuclear density and droplet growth containing saturated ratio, subcooling degree and surface contact angle of the wall is established. The analysis found that the contact angle has a greater impact on the nucleation density within the range of50-120, the effect of subcooling degree on nucleation density weakens when the subcooling degree is more than25K. As the subcooling degree increases, the growth rate of the droplet increases, and the radius of the droplet is smaller, the effect of subcooling degree on the growth rate of droplet is greater. When the radius of droplet is larger than0.05mm, the promoting of subcooling degree on growth rate of droplet levels off. When the radius of droplet is larger than0.5mm, the growth rate of droplet is stabilized; the supercooling degree has no significant effect on it.The smaller contact angle of the wall is, the greater growth rate of droplet is. The effect of contact angle of wall on the growth rate of droplet is little when the contact angle is more than90°. The effect of contact angle of wall on the growth rate of droplet is much smaller than the effect of subcooling degree with the similar radius of droplet. An experimental system for air cooler under frosting condition is set up, in which photomicrography method is used for observing the frosting online and image processing method is used for calculating thickness of frost layer, to research the frosting characteristics of air cooler and explore the effects of structural parameters of air cooler and inlet air parameters on the frosting characteristics. Experimental results show that the smaller the fin spacing is, the earlier frosting occurs, the faster the frost grows and the larger frost quantity, but the lower frost density. The frosting can be delayed by coating the hydrophilic material over fin surfaces; however, the frost quantity is not obviously affected. The lower the evaporating temperature is or the larger the subcooling degree is, the larger driving force of frost formation is, the earlier frosting occurs and the thicker of the frost layer is. The lower the evaporation temperature is, the smaller diameter of the initial droplet is. Under the condition of large air relative humidity, the frost thickness and frost growth rate increases. Under the test condition, the occurrence of the frosting is delayed by increasing the air flow rate but the frosting rate is larger. When the temperature of inlet air is above the triple point temperature of water, defrosting on the frost surface occurs by increasing the air flow rate during the frost formation.A dynamic simulation model is developed to simulate a refrigeration system under frosting condition, the growth circumstances of frost layer and the changes of refrigeration capacity, COP, evaporating temperature and condensing temperature during frosting are calculated using this model, then a series of test are carried out to validate the proposed model through the visualization test platform. The effects of air maldistribution at windward side and the growth of the frost layer on the air flow rate distribution at the air side are considered fully in the model to improve the accuracy of the calculation. The simulation calculation method can effectively reflect the dynamic changes of the refrigeration system operating parameters under frosting conditions, the deviations between the calculated value and the experimental value for the evaporating temperature and condensing temperature are less than3%.This proposed model can effectively guide the engineering system design.On the basis of evaporator model, combined with annual cost analysis, the economic analysis for the evaporator under frosting condition is analyzed t in order to design optimization. Through calculating and comparing analysis, we can draw the following conclusions:The annual cost per unit refrigeration capacity of the evaporator reduces by optimizing the structure design. By considering the impacts of fin space, diameter of pipe and the row of pipe on the annual cost, the annual cost is minimum when the fin spacing is10mm, the outer diameter of heat exchanger pipe is9.52mm and the rows of pipe is6.