One of the main concerns regarding in-vessel retention (IVR) during a severe accident is guaranteeing sufficient cooling performance to avoid the melt-through of the pressure vessel. In such an event, the vessel is submerged in water, and boiling is occurred to remove the heat. However, the main problem is that there is a limit to the pool boiling heat transfer at the outer surface of the reactor vessel due to occurrence of critical heat flux (CHF) conditions. Therefore, CHF enhancement is important to increase safety margin of IVR. In previous studies, CHF was enhanced approximately twice compared to bare surface by attaching honeycomb porous plate (HPP) to the heated surface. In order to establish the IVR, the cooling technology should be considered in forced flow boiling with the downward heat transfer surface. Moreover, we proposed a two-layer structured HPP for further CHF enhancement. Therefore, in the present paper the effect of the single and two layers HPPs on the critical heat flux under flow boiling with the downward heat transfer surface was investigated experimentally. As a result, the coalescent vapor bubbles cover the heated surface for a long period depending on the flow pattern under the flow boiling condition, resulting in liquid drying inside a HPP and reduction of CHF. Even in this situation, a two-layer structured HPP can be applied to CHF enhancement in flow boiling with the downward heat transfer surface.
Interest in the room temperature magnetic refrigerator, which is an environmentally friendly refrigeration system, is increasing. Previous studies have revealed that the maximum temperature span of the magnetic refrigerator is expanded by layered beds of multiple materials with different curie temperatures in AMR(Active Magnetic Regenerator). But, the effective factors of the layered beds configuration of multiple materials are not revealed. In this study, we investigated with one dimensional heat transfer numerical analysis the performance of layered beds of manganese(Mn) based compounds which has a larger magnetic entropy change than conventional gadolinium(Gd) based compounds. Analysis results show that when the AMR duct length and the curie temperature of the cold end and hot end materials are fixed, the achieved temperature span of multi layered beds is not affected by the curie temperature interval, while its cooling power is increased with the decreasing of the curie temperature interval. In the layered beds of multiple materials with the preferable curie temperature interval, the achieved cooling power is determined by the cold side materials, on the other hand the temperature span is expanded by the other materials.
In selecting a refrigerant, it is necessary to consider not only ODP and GWP but also the impact on global warming from various aspects. It is therefore indispensable to continue to search for better refrigerants that minimize impact on the climate to the total CO2 equivalent. Recently, a new refrigerant R 452B was reported as a high-efficiency refrigerant. There is a possibility that excellent performance may be exhibited depending on the use load of the air conditioner. We carried out cooling performance comparison in wide capacity range for R 452B and R 32. As a result, it was found that the performance difference becomes large in high ambient temperature and high heat load. We also analyzed the causes.
Frost formation in heat exchangers is an important problem because frost lowers energy efficiencies of refrigeration and air conditioning systems. In this study, we investigated frost formation on a flat plate under forced convection with air temperatures of -45 to -5 ℃. After frost growth for 3 to 130 min, the cooling surface was heated stepwise below the melting point of frost and the occurrence of a separation phenomenon between the frost layer and the cooling surface was verified. It is conjectured that sublimation of the frost layer was caused by the heated plate and then mass transferred to the upper part of the frost which is cooled by low temperature air. This phenomenon is thought to reduce adhesion of frost, thereby causing separation. This paper introduces the separation phenomenon and the boundary conditions under which it occurs. It is expected that this phenomenon will become an effective defrost method for air coolers in cold storage rooms.
Herein, the distribution characteristics of a vertical header exceeding 30 branches were experimentally clarified. In our experiments, the flow rate and the quality were changed in a plurality of shapes and refrigerant state. Our experimental findings indicated that the liquid refrigerant maldistribution downward to the lower part, the non-reachable part to the upper part, the drift to the upper part, the flow almost uniformly. Additionally, it was clarified that the quality does not reach the upper part of the header depending on the conditions, and flow may become be maldistribution. Furthermore, the header diameter corresponding to the flow rate and the quality of the refrigerant can be estimated by creating a relational expression between the reaching height ratio H of the liquid refrigerant and the Wallis dimensionless number C.
Metal 3D printing(additive manufacturing) has shown promise as a method of fabricating high-performance heat sinks, owing to its ability to produce optimized structures of any design. In this study, computational fluid dynamics was initially performed to investigate the heat transfer performance of heat sinks with lattice structures. The heat sinks were then fabricated using 3D printing, and their actual heat transfer performance was compared with the findings from the numerical computational fluid dynamics. Additionally, X-ray CT scanning was used to measure the three-dimensional shape and surface area of the fabricated objects, and computational fluid dynamics accounting for inherent surface roughness of 3D printed objects was performed on the 3D shapes derived from these measurements. This analysis revealed that lattice structural heat sinks have a more effective heat transfer coefficient than finned heat sinks. Further, while the surface area of the fabricated structures was greater than the design values owing to the roughness in the surface profile, it was evident that this surface roughness degraded the heat transfer performance.
An Active Magnetic Regenerator (AMR) cycle that employs magnetocaloric materials (MCMs) as regenerators is generally used to realize the effective temperature span for magnetocaloric heat pump systems. The MCMs of the first-order phase transition materials have gained attention because of the latent heat involved in their phase transitions. However, because the range within which the MCM exhibits an magnetocaloric effect (MCE) is narrow near its Curie temperature, only a narrow temperature span exists in the case of the AMR with a single MCM. Therefore, there is a demand for the AMR system with material layers made of a couple of MCMs. However, only few studies have investigated the influence of the properties of MCMs on magnetocaloric heat pumps using material-layered systems. In the present study, the refrigeration capacity and the temperature span of the magnetocaloric heat pump with a material-layered bed of manganese-based compounds, which are first-order phase transition materials, were determined. It was found that the temperature span can be extended by optimizing the volumetric flow rate and the number of the layers that constitute the AMR. In addition, the number of the layers and the refrigeration capacity were found to be related.
The purpose of this research is to evaluate the performance characteristics of two-stage compressed gas injection cycle with respect to the pressure pulsation experimentally. The pressure pulsation is generated by the cyclic flow between two compressor chambers. It has been thought that pressure pulsation affect not only the pressure difference between both ends of the injection pipe but also flow rate in this pipe. However, these effects and performance characteristics caused by pressure pulsation are not clear. This paper shows that the pressure wave is transmitted from the injection port of the compressor to the gas-liquid separator through the injection pipe and COP is varied by the length of injection pipe cyclically. Appropriate injection pipe length can be calculated using the resonance wavelength caused by compressor rotational speed.
Room-temperature magnetic refrigeration is attracting attention as a next-generation CFC-free refrigeration technology. However, the magnetic refrigeration is not put into practical use because of a low cooling capacity. The aim of this work is to increase the cooling capacity of the magnetic refrigerator by focusing on an improvement of the refrigeration cycle based on the Active Magnetic Regenerator (AMR). By using the hill-climbing method, we searched for the optimum refrigeration cycle depending on the temperature span and the AMR aspect ratio. The cooling capacity is calculated by the one-dimensional model of the AMR. The results of this study show that there are better cycles in obtaining high cooling capacity than conventional ones such as Brayton cycle. These cycles are constructed by the combination of Brayton and Ericsson cycles.
The effect of pre-treatment at subzero temperature of grains on the grinding process was investigated in respect of physical properties of particle. The average particle size of ground soybean and black soybean powders decreased as pre-treatment temperature decreased. The theoretical model that described grinding characteristics revealed that the freezing as pre-treatment is effective on grinding process. In all grain samples, the Bond’s constant and work index showed lower values as the pretreatment temperature decreased. The scanning electron microscopy was used for observation of surface damages on the particles by grinding process. Some cracks were seen on the surface of particles of soybean powder ground with freezing pretreatment. On the other hand, the particles of black soybean powder showed no fractures. The freezing as pre-treatment of grains prior to grinding process is effective to controlling their grinding characteristics and microstructure damages.
Vapor pressure and saturated liquid density for 1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone were measured in the temperature range from 300 K to 400 K at intervals of 10 K by the extraction method. The author obtained 11 data points of vapor pressure in the range of 42 kPa to 825 kPa and saturated liquid density in the range of 1210 kg‧m-3 to 1598 kg‧m-3. On the basis of the present data, correlations were formulated. The maximum deviations from correlations were 0.8 kPa for vapor pressure and 0.4 kg‧m-3 for saturated liquid density. The normal boiling point and acentric factor were determined to be 321.95 K and 0.465, respectively. In addition, the saturated vapor pressure and vaporization of heat were also calculated.