The purpose of this study is to grasp the applicable extent and the general tendency of one dimensional frost growth model. The time change of frost height was calculated numerically using a prediction model with homogeneous and non-homogeneous properties in order to examine the effect of temperature and humidity in the atmosphere on frost phenomenon under the natural convection, and is was compared with the experimental result. The frost height calculated from the homogeneous model was high a little, on the other hand the value evaluated from the non-homogeneous model appeared low. Both models showed approximately a tendency equal to experimental result. Judging from the quantitative evaluation by RMS, the non-homogeneous model was closer to the experimental value, and was undoubtedly predominant. However, the problems are shown. It is necessary to grasp experimentally under the natural convection the distribution of properties in the frost growth direction, which are represented by the frost density, also, it is necessary to confirm the consistency with the calculation result. As for the frost height and density given as an initial value for the calculation, it is necessary to clarify that the value in the natural convection differs from the value in the forced convection.
This study examined transient heat transfer characteristics while frosting in order to clarify heat transfer mechanism by which a high heat flux is obtained at the frosting initial stage. For the purpose, a high-precise temperature measurement was carried out in the frost layer and outside of a horizontal cylinder. The thermocouples for the temperature measurement were composed of a chromel wire of 0.1 diameter and several constantan wires of 0.05mm diameter. The chromel wire was welded on the surface of the horizontal cylinder, and several constantan wires were welded in about a 1mm interval on the surface of the chromel wire. As a result, the surface temperature of the frost layer proportionally increased with the increase in the thickness of the frost layer. The time mean value of the measured local Nusselt number decreased in order of the value in the upper part of the cylinder, intermediate and lower parts, were respectively 11.5, 7.5 and 6.0. The measured thermal conductivity of the frost layer rose with the increase in the thickness of the frost layer, and the value was within 0.04-0.12 W/mK.
This paper deals the numerical analysis on mass transfer under forced convection cooling. The transients of frost profiles on a cooling plat in a narrow channel were calculated by use of the packaged software with built in some original subroutines. In this paper, the architecture of these subroutines and this benchmark tests were showed. The calculation results exhibit local mass transfer rates and clarified following things. On the leading edge of the cooling plate, the frost accumulation accelerates locally. For the prediction on the cooling plate temperature distribution, to take into account latent heat of sublimation is necessary. In addition, the comparison between calculation and experimental results shows below issues. Both frost distribution profiles overlap in upstream; on the contrary, they do not overlap in the downstream. This comparison result indicates that the super saturation or mist flow is not negligible in the downstream. In terms of total frost weight, both results are roughly agreed and this weight increases proportionally with a run time despite increasing of the heat resistance caused by frost layer.
The Japan Aerospace Exploration Agency has developed a hypersonic aircraft flying at Mach 5. A precooled turbojet engine is the candidate of the engine for the hypersonic aircraft. The precooled turbojet engine has a heat exchanger(precooler) which cools the breathed air by using cryogenic propellant, such as liquid hydrogen. The precooler has a problem that frost forms on the cooling tubes of the precooler, and the frost decrease the engine performance. Some approaches to deal with the frost formation problem have employed in the development. In this paper, those approaches are introduced and the results of some fundamental studies about frost are also shown.
The frosting is an unsteady phenomenon occurs simultaneously with heat and mass transfer. Both the heat and water vapor in the humid air reach the surface of the frost layer and transfer to the cold surface. The frost surface plays an important role as an interface of heat and mass transfer between air-flow and ice-air composite solid layer. However, since the frost layer surface consists of ice and air, and is rough and unsteady, any specific definition of the frost layer thickness is not found. This paper tried to give the definition. The frost layer thickness was measured by using a micro photo-sensing device combined with a light emitter and receiver traversing normal to the frost surface. During traversing the device, a peak response from the device indicates the vertical position corresponding to the maximum frost area exposed to the emitted light i.e. air around the frost inside the frost layer. This position is defined as the frost layer position and it could give an effective frost layer.
Performance of heat exchangers in appliances such as air-conditioners, showcase coolers and unit coolers can deteriorate under frosting conditions. To address this issue, the mechanism of frost deposition needs to be investigated. To achieve this goal, the frost formations process was observed and properties of frost were measured at air and cooling surface temperatures less than 0 °C using a refrigeration apparatus. Frost crystals formed on the cooling surface had a needle-like structure under the chilled condition, which represents an air temperature of 0 °C and cooling surface temperature of -10°C. On the other hand, frost crystals formed on cooling surface had a block shaped structure under the frozen condition, which represents an air temperature of -15°C and cooling surface temperature of -29°C. This work examined the relationship between density and thermal conductivity of frost, and calculated Nusselt number and Sherwood number.
In order to clarify the frost adhesion area on the inner wall of freezer compartment, defrost simulation technique for prediction the vapor change from the frost of the heat exchanger was developed. As a frost melting modeling, physical properties were only considered, and the volume and shape change of frost layer were ignored. This simulation technique was verified by the experiment under the defrost of heat exchanger by forced convection and the defrost condition of refrigerator. Calculated time variation of heat exchanger pipe surface temperature is agreed with experimental one. In addition, calculated time variation of air temperature and absolute humidity at the outlet of the heat exchanger are generally agreed with those for experiment with the progress of melting of frost. However, at the beginning of frost melting, increasing of the above mentioned calculated values is a little bit faster than those for experiment. It is considered that the difference between them is caused by not considering of changing of sectional air duct area between fins.
In the conventional showcase, the timer-control has been widely used for the defrost operation. However, the timer-control requires the unnecessary energy consumption, because the defrost operation is executed at fixed intervals irrespective of the quantity of the frost in the showcase. Therefore, the quantity of the frost in the showcase should be detected to avoid the unnecessary defrost operation. In this research, the parameters to detect the quantity of the frost are selected based on the temperatures of the specified positions in the showcase, and a new control method is developed, by which the defrost operation is executed only when a large quantity of the frost is produced in the showcase.
Characteristics of the heat transfer with solidification of phase change material (PCM) around horizontal tubes immersed in working fluid were studied by numerically and experimentally. In this study, variations in the specific heat of the PCM are incorporated in order to account for the phase change. The body fitted coordinate system is used to compute arbitrary shape of tubes. Numerical results were compared with experimental results. Results are presented as transient freezing front contour of PCM, velocity field, temperature distribution of water and freezing fraction ratio.
In this study, the ice slurry producing system, which was introduced in our previous study, was used and the effect of sublimation of ice was investigated. In the system, ethanol solution, as a thermal storage material, is evaporated under low-pressure condition, and the remaining solution is cooled and partially frozen as a consequence of the latent heat of evaporation. In our previous study, it was clarified that the ethanol concentration of vapor phase became lower during the ice producing process of this system, than that under the condition of vapor-liquid equilibrium, in which ice is not generated in the liquid phase. The reason for this is expected to be the effect of the sublimation of ice generated in the liquid phase. In this study, the effect of sublimation of ice on the composition of the vapor phase was investigated. Using the results, the COP and non-dimensional ice production rate of this system was calculated, and the effects of sublimation of ice on them were also evaluated.
Research on invention of the function ice is actively advanced in the field of perishable foodstuff saving, deodorization, sterilization and cleaning in addition to levelization of electric power in summer. When oxygen low-temperature plasma is generated in a decompressed furnace, a high-dense ozone gas is secondarily formed in the furnace. By utilizing the instant freezing of liquid introduced in the furnace, the purpose of this study is to efficiently store the ozone gas in the ice. The ozone content in the ice was evaluated for each liquid introduction method of a water spray system, a water droplet adsorption system and a gas involvement system on the surface of the water vessel in the furnace.
This paper reports the freezing characteristics of sand molded with "the freeze mold method". The freeze mold method is the casting process where little water is added to cast sand then they are frozen and become harden as a mold. Instead of using organic hardener, this molding technology possibly reduces resources and environment loading. We aimed at the practical application of the freeze mold method and chose cold airflow as the medium taking heat away from the mold. At first the sand with water was filled into a rectangular container that is the test section. Then cold air flowed into the container. Consequently, the mold was cooled and frozen by the cold airflow. The freezing behavior of the sample by cold airflow was investigated experimentally under the conditions of added water amount, superficial velocity, inflow air temperature and fixed bed height. As a result, the freezing completion time becomes long as the added water amount increases. However, an increase in the added water amount doesn't influence the pressure loss of the test section so much. Moreover, the empirical equation was derived to predict the freezing completion time.
The production characteristics of ice particles treated with surfactant additives and brines in the case of using vacuum ice production system have been investigated. In the present study, cetyl dimethyl betaine was used as surfactants and the results were compared with those in the case when poly(oxyethylene) sorbitan monooleate used in the previous studies was tested. From the results, it was found that ice particles size produced by using a vacuum system becomes much smaller than that made by the scraper ice production system used in the previous study. It was also found that the size of ice particles treated with the present surfactants without brine still remains small. Additionally, the fluidity of ice slurry treated with the present surfactants was enough high though the drag reduction could not be observed due to the small diameter of the present test pipe. From these results, a combination of the present surfactant treatment without brine and the vacuum ice production system was concluded to have an advantage for the production of fine ice particles.
Time variation characteristics of ice particles in a surfactant solution have been investigated. The effect of surfactants on corrosion characteristics was also studied. The results were compared with those treated with poly(vinyl alcohol). From the results, the present surfactant, cetyl dimethyl betaine was not found to be effective on preventing Ostward ripening of ice particles as poly(vinyl alcohol) showed. Then, it was concluded some effective technology has to be installed with surfactants when this surfactant treatment is realized. On the corrosion characteristics, it was found that the present surfactant shows the same level as tap water.
The experimental discussion was performed to the process of ice formation and melting in the ice thermal energy storage cell employing the resin tube-mat for the heat transfer surface. The experiments were carried out by varying the size of resin tube-mat and the arrangement of the tubes by measuring the ice formation rate and the melting rate, and by measuring the temperature change of the brine. As a result, in the solidification process, the steady performance for the storing of cold energy was achieved during about 4-6 hours by the brine of - 5°C and in the melting process, the performance for the utilizing of cold energy was achieved during about 3-5 hours by the brine of 10°C. It was found that the heat transfer characteristics becomes better for the case of the thin pipe thickness and the large pipe diameter.
Characteristics of ice formation and melting for latent heat thermal energy storage cell using resin tube-mat to prevent destruction by a volume increase of ice and corrosion of heat exchanger tube was examined by the numerical analysis. The numerical analysis is carried out with the enthalpy method does not need the special handling by the introducing latent heat term. The numerical results agree well with the experimental results. Influence on ice formation and melting was considered by the difference between the size (thickness of tube and diameter, etc.) and the physical properties of resin tube. Quantity of ice formation and melting is almost proportion at time. Thickness of tube greatly influences ice formation and melting. Ice formation and melting quicken as the thickness of tube thins. Ice formation speed increases as the inner diameter of tube grows. However, increase proportion at formation or melting speed becomes small as the inner diameter of tube grows. As a result, a basic characteristic of latent heat thermal energy storage cell using resin tube-mat was clarified.
In a cold storage warehouse, by developing the thermal energy storage technique using cheap electric powerin the night, it is necessary to construct a high-efficient and energy-saving-type refrigeration system in which air conditioning is possible at 0 degrees c. We created a brine iced water (ice slurry) cooled under 0 degreesc by a closed supercooling ice making method. For a practical application, the brine iced water was directly sent to the load side, and it was utilized as the secondary refrigerant for the heat exchange. As a result, by replacing the pure water with a marketed propylene glycol solution, it was proven that the conventional closed supercooling ice making method could be similarly utilized for the ice making. However, it is necessary to control the evaporation temperature in the refrigerator, because the freezing temperature changes with the brine concentration. In the refrigerator entrance, it is necessary to heat at a constant temperature so that the inflow brine may not freeze. In case of the brine iced water, the fluidity of the brine iced water is high, and the ice particle is carried away by the flow. Therefore, it is necessary to prevent runoff of the ice particle from an intake of the thermal storage tank in case of thebrine water. This proposal system was confirmed that there was practically no problem by an operation of a 15kW refrigerator system.
A feasibility study was carried out to develop a technique for detecting the occurrence of eutectic solidification of electrolyte solutions, which is a potential cause of cellular damage during freezing of cells and tissues. The electric impedance of a sodium chloride or a potassium chloride aqueous solution between two parallel electrodes was measured at subzero temperatures as a function of the frequency ranged from 1 to 105 Hz. The impedance increased abruptly with decreasing temperature near the eutectic point, suggesting the possible detection of eutectic solidification. Some extent of supercooling was observed before the eutectic solidification, while the impedance drop agreed well with the eutectic temperature during thawing.
Ice slurries have been used as environmentally-friendly secondary refrigerants. In addition to such ice slurries, aqueous solutions in slurry-state have also been put to practical use at temperatures below 0 oC. Urea-water mixture is a multi-component substance that has a eutectic point. If we can form a two-phase fluid substance by the liquid-solid phases at the eutectic point, it can be used as a fluid latent heat storage material, which will maintain the secondary refrigerant in a heat exchanger at constant temperature. In the present study, we propose a urea-water mixture as a novel functional thermal fluid that can be used as a fluid latent heat material. To demonstrate its feasibility, we first measured the latent heat and density of a urea-water mixture, and then used a counter-flow double tube heat exchanger to produce a liquid-solid two-phase flow of the urea-water mixture. This work demonstrates that it is possible to make a fluid latent heat storage material continuously from an aqueous solution at the eutectic point by flowing it through a double tube heat exchanger equipped with a stirrer.
The objective of this study was to clarify the mechanism of injury caused by acid snow stress in wintering plants. In this study, influence of simulated acid snow stress on leaf tissues of wintering plant was conveniently estimated by extracellular freezing tests under acid conditions in vitro. The survival rates of leaf tissues after freeze-thawing with 0.3 ml of sulfuric acid solution of pH 2.0 were significantly decreased, compared with the survival rates under acid condition of pH 3.0 or pure water. In this study, the initial volume of sulfuric acid solutions was the same in the treatments. Therefore, it is thought that the more acidic the initial pH of sulfuric acid solution becomes, the greater the volume of residual unfrozen solution with concentrated sulfuric acid in the extracellular part at a subzero temperature would be, consequently, the survival rate of leaf tissues were decreased. When leaf tissues were freeze-thawed with a large volume of sulfuric acid solution of pH 3.0, the survival rate of leaf tissues was comparable to the survival rate using pH 2.0. These results suggest that an increase in the volume of acid meltwater derived from snow cover will enhance the damage to wintering plants even the mild acidity of the acid snow.