In order to increase the coefficient of performance of heat exchangers under frosting conditions such as in air conditioners, showcases, and unit coolers, it is necessary to elucidate the frost deposition mechanism. The previous studies about frost deposition were summarized in this study. The previous studies about observation of frost deposition, properties of the frost layer, simulation methods of the frost layer, frost detection, frost studies concerning heat exchangers and heat pumps, and defrost control methods were explained. There are many conditions that have not been measured yet, though the mechanism of frost deposition and properties of the frost layer have been elucidated from the previous studies. The study of frost deposition has been invigorated in Japan recently. The understanding of frosting phenomenon is expected to be deepened in the near future.
The characteristics of shock waves and expansion waves of two-phase flow, which has essentially low sound speed, should be elucidated to aid in optimizing the design of ejector for refrigeration systems. At the mixing section of the two-phase ejector, the supersonic driving flow mixes with the subsonic suction flow which generates shock waves and expansion waves. This study focuses on the existence of two-phase oblique shock waves based on momentum and temperature relaxation. The oblique shock waves were calculated using two-dimensional CIP method. The analytical results obtained were quite different from the results using one-dimensional supersonic flow. We found out that the characteristics of high speed two-phase flow is largely affected by the momentum and thermal relaxation times.
The vapor injection refrigeration cycle using the scroll compressor driven by inverter was investigated to improve the performance of room air conditioner with R 410A. One injection hole was bored on a fixed scroll plate, and through this hole the vapor refrigeration is injected into two compression rooms of a scroll compressor at different time. Then the pressure change of the compression room and the performance of vapor injection refrigeration cycle are calculated. And the performance of the room air conditioner using vapor injection refrigeration cycle was measured. As the result, the cooling/heating compressor input was decreased to 7.4/3.2 % at capacity 4.0/5.9 kW, and the heating capacity at outdoor temperature -15 °C was increased to 11.9 %.
In the present study, we employed the recent micron-order machining technology on the cooling surface and produced the artificial micro-ordered groove pattern. The frost and defrost phenomena on the artificial surface with micro-structure are experimentally investigated. The effects of the groove depth or pattern pitch between grooves on the frost phenomenon were focused. The direct observations of the frost and defrost phenomena on the surface were carried out by using the digital microscope with high spatial resolution. It is cleared that the grooves on the surface strongly affect the frost and defrost phenomena, especially the generation of the super-cooled droplets on the surface, which can be observed at the beginning of cooling. The water drainage after frost melting was also strongly affected by the micro grooves.
In order to increase the coefficient of performance of heat exchangers under frosting conditions such as in air conditioners, showcases, and unit coolers, it is necessary to elucidate the frost deposition mechanism in low temperature environments. Frost properties were measured at air temperature below 0°C. The experimental results were analyzed and a simulation model of frost growth in low temperature environments was developed. In the simulation model, frost growth during the crystal growth period was calculated by using the ice column model of Tao et al. and during the subsequent frost growth period, another model was used; the frost growth was calculated according to diffusion into the frost layer on the basis of Fick's law and the model of Le Gall et al. The simulations were performed to calculate the frost growth on a flat plate and the influences of several parameters on frosting phenomenon were elucidated at air temperature of about 0°C.
As frost formation phenomena cause various problems, such as performance deterioration of heat exchangers in cryogenic engineering, we need to understand the frost formation mechanism in more detail. Authors have already reported on the approach for predicting the quasi-steady- frost formation, which was based on inhomogeneous frost growth model. Using this approach, the authors also reported the model could show frost growth with respect to time. In this paper, as a next step, we evaluate the effectiveness of the inhomogeneous frost growth model through various simulation results based mainly on the frost surface temperature. Through these desktop simulations, we can find that the difference between the measured data using a radiation thermometer and the simulation results is small in terms of qualitative evaluation. We also find that the initial frost height is more sensitive to these simulation results than the initial frost density.
We developed a compact snow crystal formation apparatus based on a diffusion method using a Peltier device. This apparatus does not need an assemblage and is small enough to be operated on a desk. Anyone can easily observe snow crystal formation in a normal temperature room. We adopted a diffusion method because the shape enable that several people can simultaneously observe the snow crystal formation from above. To estimate a performance of the apparatus, we investigated temperature profiles in the apparatus by measurement and simulations with (Case 1) and without (Case 2) natural convection. As results of the simulations, Case 1 and Case 2 reached a steady state. In each case, temperature stratification condition was formed in lower part of the apparatus. From the comparison of the results of measurement and simulations, finally, it is concluded that there is a natural convection, but the air current is not so strong as disturbing the temperature stratification condition in the apparatus.
This research aims to develop snow melting system around steel top of underground fire cistern by using heat pipe, for realizing quick finding of the steel top under heavy snow fall. Water in a fire cistern installed underground is heated by underground heat source, 10 ~15 °C. The iron top is put on snow melting panel made of reinforced concrete. Heat is transported from water to the snow melting panel by heat pipes, which melts snow on it. The experimental results obtained for two years show that this system can melt the snow around the steel top in winter season preferably. The numerical simulation using only weather data was found to predict temperature variations of the whole system with good agreements to the experimental data. Therefore, this simulation software can be used for designing this snow-melting system.
In order to popularize use of ice slurry, authors have been proposed application of ice slurry to cold storage of foods in place of an air conditioning. For use of the ice slurry in the wide temperature range a new harmless ice slurry to human being was developed by cooling a W/O emulsion made from tap water-edible oil mixture with small amounts of edible emulsifier and food additive. The edible emulsifier is essential to form W/O emulsion, and the food additive is used to dissolve in tap water. In this paper the optimal concentrations of emulsifiers were determined, and the fundamental characteristics such as viscosity, effective latent heat of fusion and usable temperature of ice slurry were clarified. Finally, it was concluded that new ice slurry could be fully applied to cold storage of foods in the wide temperature range because its lower limit usable temperature was about -18°C.
This study is concerned with the measurement of thermophysical properties of a urea-water mixture with the aim of adopting the mixture as a latent heat storage material for air-conditioning systems. The urea-water mixture is made of natural substances and has a good fluidity. The urea concentration in the mixture was controlled by measuring the refractive index of the mixture. Being a multi-component substance, a urea-water solution has a liquid-solid co-existent phase on a phase-diagram. Therefore, the liquidus temperature was measured to establish a relationship between the fraction of the solid-phase and temperature. Furthermore, apparent values of specific heat and coefficient of viscosity were measured in the two-phase region where the solid phase is ice. The apparent specific heat and coefficient of viscosity were measure by using an adiabatic calorimeter and a stirring torque meter respectively. The results revealed that the urea-water mixture can probably be used as a latent heat storage material of good fluidity.