An Investigation Of Mechanism For Frosting And Frost Restraint As Well As Their Simulations

The phenomenon of frosting widely exists in nature and many engineering fields. When the moist air blows in a surface with the temperature lower than its dew point, the moisture will condense on the surface. In particular, frost will form if the surface temperature is below the freezing point of water. Previous studies show that frost is undesirable in most cases since it will worsen the operating characteristics of equipments and even cause the abnormal running. Therefore, it can benefit the design of engineering equipments running under frost conditions to predict the frost formation and growth. Moreover, it has great implications in engineering fields to study the mechanism of the frost formation in order to alleviate or even prevent the frost formation and eventually eliminate the negative effects of frost. This paper has studied the mechanism of the frost formation, established a new mathematical model for the prediction of the frost formation and numerically simulated the unsteady frost growth with the hope of helping the most reasonable design of engineering equipments. Also, several methods to prevent the frost formation have been proposed, the validity of which has been proved by numerical simulations in this paper.From the macroscopic perspective, the frost growth can be categorized into three periods when the temperature is dropped rapidly. They are:the period of the ice crystal formation and growth, the frost growth period and the period of the full frost development. Three factors are mainly associated with the frost growth. They are:ice nucleus’forming, the adhesion of the ice nucleus on the solid surface, and the rate of the ice branch growth. As for the first factor, the crystallization mechanism under three nucleation conditions (i.e. homogeneous nucleation, heterogeneous nucleation and nucleation with nucleation promoters) has been analyzed on the basis of the classical nucleation theory. As a result, the critical radius and the radius growth rate of the nucleus were obtained. With regard to the second factor, the results of this paper show that the adhering force of the ice nucleus on the surface can be attributed to the adsorption between molecules. For the last factor, previous models about the growth of the ice branch have been summarized and analyzed.Then, based on the mechanism of the frost formation and growth, a new model is proposed to predict the frost formation and growth with the benefit of the macroscopic CFD (Computational Fluid Dynamics) and the classic nucleation theory. This model was applied to describe the frost formation on the cold surface when the moist air blows in it. The simulation results show a good agreement with the experimental data and previously reported simulation results, which demonstrates the validity of the new model. Furthermore, the calculation methods for frost layer properties, such as density and thermal conductivity, have been provided as well, and the time- and space-dependent frost properties can be predicted for the first time. The new model was expanded into a three dimensional one and applied to investigate the performance of fin-and-tube heat exchanger under frost condition. The transient local frost formation has been obtained. The average frost thickness, heat exchanger coefficient and pressure drop on air-side has been analyzed. The influence factors have also been discussed, such as fin pitch, relative humidity, and air flow rate and evaporating temperature of refrigerant. These simulation results are meaningful for the engineering design.Finally, the frost restrain methods have been proposed based on the phase equilibrium theory. In the practical engineering applications, both increasing the contact angle and decreasing the contact area between the liquid and solid surfaces can effectively restrain the frost formation and growth. This paper suggests that rough elements with a "composite contact" arrangement on the super-hydrophobic surface can satisfy the abovementioned purposes. In this paper, the Lattice Boltzmann Method (LBM), an numerical approach on the mesoscopic level, was employed to investigate the kinetics of the clash of the liquid drop against a horizontal solid surface, the flowing characteristics of the gas-liquid displacement and the heat-transfer characteristics of the high temperature fluid flowing in the low temperature channel. The simulation results show that the "composite contact" can be formed if the height of the rough element is greater than the entrapping depth of the droplet into grooves. An improved enthalpy method-based LB model, the solidification model, has been proposed to simulate the freezing process of the flowing fluid. The new model was also used to simulate the unsteady freezing of the saturated liquid on the low temperature solid surface. The results show that the freezing rate is slowly, the contact angle of the ice nucleus is large and the contact area is quite small on the super-hydrophobic surface. Besides, the freezing rate can be reduced further when the smooth super-hydrophobic surface is displaced with the rough super-hydrophobic one. Our results qualitatively verified that the frost restraint effectiveness of the rough super-hydrophobic surface.