There has been a continuing effort to advance the understanding and modeling of frost formation on refrigerated surfaces during the last two decades for better design of air-to-refrigerant heat transfer equipment and effective and energy-saving control of defrosting processes. A review and comparative analysis of the available literature concerning frost properties, correlations, and mathematical models are presented in this study to provide an overview of the analytical tools for researchers, product developers, and designers. The frost research can be divided into two general groups-experimental correlations and mathematical models. In general, the properties correlated are the frost thermal conductivity, the frost average density, and air-frost heat transfer coefficient (Nusselt number). A limited operational range of these relations is observed. The mathematical models include both differential and integral approaches, which are, in general, solved numerically. These models are classified based on the geometrical configuration of cold surface. A comprehensive comparison of the models is given to assist the reader in making their decisions for design analysis. The existing gaps in the frost research are identified and recommendations are made.

To develop a better defrosting control system on finned evaporators of a refrigeration system, a study is conducted to better quantify the frosting and defrosting processes by using an infrared thermometer to determine the frost surface temperature. For gradual frost deposition a slow variation in surface emissivity is expected, while the defrosting process is characterized by a sudden change of this property. As an indicator for the defrosting initiation control mechanism, the times at which the IR signals stabilize at different conditions (Reynolds number, cold surface temperature, and ambient temperature) are reported along with the terminal mass concentration of a defined frosting process. On the other hand, the abrupt variation of surface emissivity indicates the termination of the defrosting process. Removable fins are used to measure frost weight, and a video microscope is used to determine the frost thickness. Defrosting initiation time and durations marked by melting, temperature are reported as a function of initial mass concentration and defrosting base surface temperature. The presented results could be used to design a better defrosting control system with better accuracy and energy saving features. Transactions of the ASME Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 06/01/2015 Terms of Use: http://asme.org/terms Fig. 1 Infrared sensor calibration chart, low and high temperature range Journal of Heat Transfer MARCH 2005, Vol. 127 Õ 345 Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 06/01/2015 Terms of Use: http://asme.org/terms

A series of experiments were conducted to investigate the frost formation and characteristics on a flat surface under natural convections. The process of frost formation and structures is visibly observed and measured by using of a microscope. The temperature distribution of the air around the frost layer is measured by using of the holographic interferometry technique. The influences of the main parameters are analyzed. Especially, the microscopic analysis of frost growth on the surface characteristics on the microscopic characteristics of a frost layer during the early growth period when sub-cooled droplets are formed and changed to the ice. The thickness and mass of frost layer are measured while frost formation is visualized simultaneously. The results show that the frost surface temperatures are much lower than the freezing temperature and close to the cold surface temperature at the early growth period. After the early growth period, the frost surface temperature is dominated by the convective heat transfer between the frost surface and surrounding air.

Surface frosting from atmospheric humidity under natural convection is encountered in conventional refrigeration systems, cryogenic surgery, and cryogenic stress relief of die casting metal forming applications. To advance the predictability of frost initiation and formation processes, this study reports a microscopic analysis of frost growth on a flat surface during the onset period of freezing when subcooled droplets are formed and changed to the ice phase. The onset of freezing is quantified by the mean droplet size and ice particle fractions at a critical time (when water droplet freezing point is reached) with the aid of a video microscope. An early-stage frost formation model with effective parameters is demonstrated to provide the important information for the transition to the steady-growth model. The model results are compared with the measured air-frost surface temperatures at different cooling and ambient boundary conditions, using holographic interferometry. The comparison between the model prediction and experimental results demonstrates the sensitivity of effective parameters in simulating the frost thickness and air-frost interface temperature.

In response to the need for developing a better model to predict frost formation and defrosting processes in freezer and evaporator applications, a microscopic analysis of frost growth on a flat surface is conducted to determine the microscopic characteristics of a frost layer during the early growth period when sub-cooled droplets are formed and changed to the ice. The surface characterization is performed by employing the holographic interferometry technique to determine the air-frost interface temperature, and the video microscope to determine the mean droplet size and ice particle fractions. Typical experimental results are presented to demonstrate the test technique. Preliminary experimentally determined frost thickness and air-frost interface temperature are compared with simulation results.