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The electrocaloric effect in thin films of an electrocaloric material has the potential to be used for efficient cooling systems for high power electronic devices. We numerically calculated the effect of parameters in electrocaloric refrigeration with a thermal switch of fluid motion on the thermal performance. The system of changing air and water flow with the pulse generation of cold energy increased the heat transfer efficiency to 67% at a frequency of 5 Hz. The optimum time delay of water flow to increase the heat transfer efficiency was zero at low frequency and became half of the time period to change heat for a high frequency of 100 Hz. When the heat transfer efficiency was high, the final temperature change in water flow was not the maximum temperature change.

This paper discusses the system performance of a magnetic heat pump with an active magnetic regenerator. The heat-transfer coefficient and pressure drop in the AMR are performed analytically. Measurements on the experimental magnetic heat pump test device are performed using distilled water as the working fluid. The coupling of a one-dimensional thermal model is presented. The thermal model takes into consideration the magneto-caloric effect as a source term and the energy conservation between a solid and a fluid. At cyclic steady states, the temperatures of the fluid on both sides of the regenerator are calculated numerically and compared with the measurement results. A reasonable agreement between simulations and experiments confirms the validity of the proposed model.

We studied the forced convection heat transfer performance and pressure drop of high permeability metal cellular porous media in air flow using a 3-dimensional thermofluid computation code. The temperature and velocity distributions in the air flow region, local heat transfer coefficient, and local heat flux on the surface of the porous media were numerically calculated for steady air flow by changing the parameters of the pore size and air velocity. The cellular porous media were modeled by pin array, cube geometry, and truncated octahedron geometry using thin wires. The diameter of the wires was 0.1 mm, and the pore per inch (PPI) was 5–50. The relations between the Nusselt number using the volumetric heat transfer coefficient and the Reynolds number were obtained from our calculation results, and we compared them with conventionally proposed experimental correlations. Also, the pressure drop calculation result was compared with conventionally proposed experimental correlations. The following results were obtained. The local heat transfer coefficient and local heat flux on the surface of porous media were small near the joint positions of the wires of the cellular porous media because of the thermal boundary layer. The volumetric heat transfer coefficient and pressure drop agreed with conventionally proposed experimental correlations within errors of twice the volumetric heat transfer coefficient and pressure drop. The relation between the heat transfer rate per unit volume and the heat transfer area per unit volume agreed with the convection heat transfer correlation for a tube bundle.

This paper deals with the cooling characteristics of a magnetocaloric cooling technique refrigerator an active magnetic regenerator (AMR). The AMR-based refrigeration cycle, which has a thermal storage process and a regeneration process, realizes a practical magnetic refrigerator running near room temperature. The AMR cycle has four sequential processes: adiabatic magnetization, fluid flow, adiabatic demagnetization, and fluid flow. We devise an appropriate simulation model of the cyclic heat transfer process inside the particle bed as the target AMR. Then, the temperature profile inside the AMR particle bed and the cooling characteristics of the room-temperature magnetic regenerator are studied analytically. In addition, the validity of the analytical model by molecular field approximation theory is verified by comparing the experimental results with the analytical results. The results show that, when a higher magnetic field is applied to the magnetocaloric material, a greater temperature difference is obtained.

In the present study, the flow characteristics and heat transfer of a thermo-sensitive magnetic fluid, which is a multiphase-flow material, were investigated experimentally. The mini-channels considered herein have a depth of 0.5 mm, with the nominal channel width being equal to five times the depth. The channel device was constructed from a Teflon tube. The operation of the device is based on the thermo-magnetic characteristics of the fluid, a suspension of Mn-Zn ferrite particles in kerosene, the magnetization of which is known to decrease with increasing temperature. The experimental parameters were magnetic force, the position of the magnet, and the temperature of the magnetic fluid. The experimental results indicate that force convection based on the magnetic characteristics of the fluid in the mini-channel exhibits excellent cooling performance. Furthermore, the flow characteristic of the thermo-sensitive magnetic fluid was found to be strongly dependent on the magnetic condition, such as the force and the position.

Friction stir spot welding (FSSW) is a new metal-joining process, and a numerical simulation code to calculate optimal welding conditions is desired. In this paper, we analyzed temperature distribution and plastic deformation flow during the FSSW process with the fluid flow model and the elastic-plastic deformation model using the particle method. Spot welds are made with a cylindrical pin tool having flat shoulder with a fixed tool rotational speed. Simulation results predict an axisymmetric temperature distribution with the temperature below the tool in the region of 300 °C. The material flow predicted by the elastic-plastic deformation model is similar to experimental results. The model predicts the material flow at the pin periphery is in the upward direction. Near the shoulder, there are two flow patterns observed - beneath the shoulder, the material is pushed downward due to the force acting from the shoulder face, whereas on the shoulder periphery the material flows upward and outward due to extrusion of the material that is caused by the shoulder plunge. This extruded material shows up on the specimen surface as burr.

The cooling characteristics of a room temperature magnetic refrigerator using an active magnetic regenerator (AMR) is presented. The AMR system, which is based on a combination of a thermal storage process and a regeneration process, is widely applied for room temperature magnetic refrigerator. And the system has four sequential processes : adiabatic magnetization, fluid flow, adiabatic demagnetization, and fluid flow. We devise the appropriate simulation model of the heat transfer process inside the AMR particle bed. In addition, the temperature profile inside the AMR bed and the cooling characteristics of the room temperature magnetic regenerator using AMR are performed analytically. In this research, the validity of the analytical model is verified by comparing the experimental results with the analytical results. The results show the larger the fluid transfer volume, the larger the temperature difference between the hot and the cold ends. Also we can observe when the higher magnetic field is applied to the magnetocaloric material, the large temperature span is obtained.

The coupling of the electrocaloric effect in thin films with thermal switches has the potential to be used for efficient refrigeration. We studied the unsteady heat transfer performance and periodic thermal-switching behavior of a flat heat pipe to transfer cold energy from a changing heat source. The condenser of the flat heat pipe was the changing heat source and changed from −20 W to +20 W every 5 s. The temperature of the condenser surface changed in accordance with the heat generation of the heat source. The evaporator was a plate with a mesh wick attached to a water-flow pipe. Cold energy transferred from the condenser surface to the evaporator surface only when the temperature of the condenser surface was lower than that of the evaporator surface. We analyzed the unsteady temperature change and heat transfer performance of the flat heat pipe by numerical simulation. The analytical results showed that it was necessary to have two thermal switches to separate the heat energy and cold energy of the changing heat source. Also, it was important to reduce the thermal resistance and heat capacity of the evaporator surface to improve the unsteady heat transfer performance of the heat pipe. Next, we measured the unsteady heat transfer performance of the flat heat pipe experimentally. The experimental results showed that the thermal-switching behavior was observed when the heat generation of the heat source changed every 5 s.Copyright © 2015 by ASME

The purpose of this study is to understand the optimum operating condition of magnetic refrigerator at room temperature for direct air-cooling. The basic components of the target system are a magnetic circuit including two permanent magnets, a test section, an air blower, and an associated instrumentation. The test section consists of 10 test cells which enclose gadolinium chips as a magnetic working substance in a prescribed packing rate. In order to change the applied magnetic field from 0 to 0.9 T, the magnetic circuit is installed on an electric slider which generates reciprocating motion. The system performances are widely investigated both experimentally and analytically for the variety of conditions such as a volumetric flow rate of air, a packing length of magnetic working substance, and a heat exchange cycle. The results reveal that the present magnetic refrigerator has a maximum value of the cooling rate in an appropriate operating condition.

This paper reports both the heat transfer performance and the flow characteristics of slush ice flow between horizontal heated plates. A mixture of fine ice particles and ethylene-glycol aqueous solution was adopted as a testing slush ice and was heated by the top or the bottom wall of the flow channel. Experiments were carried out to investigate the effects of heat flux, velocity of slush ice flow, and channel height of test section on the melting heat transfer characteristics. Both the velocity distribution and the ice packing factor distribution in the flow channel were measured in detail. The results revealed that the forced-convection heat transfer and flow characteristics of slush ice were markedly affected by both the velocity distribution and the ice packing factor distribution near the heated wall.

Experiments were carried out to determine the effect of flow separation on freezing heat transfer characteristics along a horizontal cooled flat plate immersed within a rectangular duct. The flow separation was induced by a vertically situated thin plate placed against the flow at the leading edge of a uniformly cooled plate. The degree of flow separation was altered by employing vertical plates of various heights. The Reynolds number and the cooling temperature ratio ranged from 5.6 x 102 to 8.4 x 104 and from 3.0 to 10.0, respectively. The measurements showed that the flow separation exerts a marked influence on the local ice formation characteristics. Two kinds of ice formation of convex and concave profiles were observed to occur as a function of the Reynolds number, as well as of the cooling temperature ratio. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 43–56, 1998

Experiments were performed to determine the effect of salinity level on the melting heat transfer characteristics of a horizontal ice cylinder immersed in quiescent saline water. Emphasis was placed on interpreting the heat transfer mechanism which dominates the solid-liquid interface situation. Measurements were carried out for saline water of 0.5–3.5 wt% in salinity, while the ambient temperature ranged from 1.8 to 24.0°C. Flow visualization was employed to investigate the transient flow patterns and corresponding solid-liquid interface locations. It was found that the flow patterns around the ice cylinder were a strong function of the saline water concentration, which then considerably affected the local heat transfer coefficient along the melting ice cylinder.

An experimental and numerical study of solidification characteristics of a droplet on a horizontal cooled wall is reported. Pure water and molten salt were utilized as the testing liquids. The droplet was cooled under a variety of conditions such as wall temperature and initial liquid temperature in a static atmosphere. Extensive observations of both solidification characteristics and morphologies of the droplet were made. Numerical calculations based on the Landau method considering the effects of both surface tension at the droplet surface and the density inversion at 4°C within the droplet were carried out. It was found that the numerical results are qualitatively in good agreement with the experimental results except for the terminating period of solidification

We experimentally study the thermal conductance of single-tube and loop heat pipes for a solar collector. The evaporator of the heat pipe is 1 m long, 6 mm in diameter and has 30° inclination. The thermal conductance is defined as the heat transfer rate divided by the temperature difference between the evaporator-wall and the condenser-wall. Effects of heat transfer rate, saturation temperature of the working fluid, liquid filling ratio, inclination angle, and position of the evaporator on the thermal conductance are examined. We found that the thermal conductance of the 30°-inclined loop heat pipe with an upper-evaporator is 40–50 (W/K), which is 1.8 times higher than that of the vertical loop type and 3 times higher than that of the single-tube type. Thus, the inclined loop heat pipe is preferable for a solar collector. There is an optimum liquid filling ratio. When the liquid filling ratio is too small, a dry-out portion appears in the evaporator. When the liquid filling ratio is too large, the liquid flows in the condenser to decrease heat transfer area. Also we numerically analyze the thermal conductance of a vertical loop heat pipe.

This paper discusses a pulsating heat pipe (PHP) using a self-rewetting fluid. Unlike other coommon liqids, self-reweffing flinds have the properfy that the surface tension other common liquids, self-rewetting fluids have the property that the surface tension increases with temperature. The increasing surface tension at a higher temperature can cause the liquid to be drawn toward a heated surface if a dry spot appears and thus to improve boiling heat transfer. In experiments, 1-butanol and 1-pentanol were added to water at a concentration of less than I wt % to make self-rewetting fluid. A pulsating heat pipe made from an extruded multiport tube was partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability at different input power levels. The experiments showed that the maximum heat transport capability was enhanced by a factor of 4 when the maximum heater temperature was limited to 110°C. Thus, the use of a self-rewetting fluid in a PHP was shown to be highly effective in improving the heat transport capability of pulsating heat pipes.

Pulsating heat pipes (PHPs) are complex heat transfer devices whose thermal performance is governed by a strong thermohydrodynamic coupling. Recently, PHPs have attracted attention as novel electronic cooling devices. In this study, we used a self-rewetting fluid and obtained new experimental results for the improvement of the heat transport efficiency in PHPs. In contrast to the case of common liquids, the surface tension of self-rewetting fluids increases with temperature. Because of the increase in the surface tension at high temperatures, these fluids tend to flow toward the dry spot appearing on a heated surface, and thus, the boiling heat transfer is improved. We constructed PHPs from multiport extruded aluminum tubes with square channels. The PHPs consisted of a heating section, an adiabatic section, and a condensation section with a heat sink. We investigated the effect of the type of working fluid and the fluid fill ratio on the device performance. The working fluids employed were a self-rewetting fluid, water, and ethanol. The thermophysical properties of the working fluid affected the device performance, which also depended strongly on the boundary conditions employed during the PHP operation. In particular, the use of a self-rewetting fluid in the PHPs helped enhance the heat transport efficiency to a considerable extent.Copyright © 2010 by ASME

In order to elucidate the cooling characteristics and to improve the cooling performance of a room temperature magnetic refrigerator using air as a heat transfer fluid, experimental and analytical study concerned with the heat transfer characteristics of an active magnetic regenerator (AMR) are conducted. The basic components of the target system are a magnetic circuit, a test section, a fluid supplying system and an associated instrumentation. Spherical gadolinium particles are packed in the test section as a magnetic working substance. Temperature profiles under several operating conditions for the Air-AMR are measured and calculated. Also, those for the Water-AMR are investigated in order to compare with the Air-AMR. The results show that the AMR cycle is very effective for improvement of the cooling performance of a room-temperature magnetic refrigerator for both heat transfer fluids, because increase in the temperature span between the hot-end and the cold-end due to regeneration is realized. Furthermore, it is revealed that a high flow rate of air is necessary in order to improve the cooling performance of the Air-AMR refrigerator, compared with the case of the Water-AMR. It is also shown that thermophysical properties of the heat transfer fluid have a significant effect on the cooling characteristics of an AMR refrigerator.

In this study, a flat-parallel-plate active magnetic regenerator made of gadolinium is investigated. The coupling of a one-dimensional thermal model with a two-dimensional magnetic model is presented. The thermal model takes into consideration the magnetocaloric effect as a source term and the energy conservation between a solid and a fluid. The magnetic model considers the space distribution of the internal magnetic field strength, and thus it includes the demagnetization effect. Measurements on an experimental magnetic refrigeration test device are performed using distilled water as the working fluid. At cyclic steady states, the temperatures of the fluid on both sides of the regenerator are calculated numerically and compared with the measurement results. The inability to provide a constant internal magnetic field during the heat transfer process with a permanent magnet is demonstrated. A reasonable agreement between simulations and experiments confirms the validity of the proposed model. (C) 2013 Elsevier Ltd. All rights reserved.

The purpose of this study was to understand the optimum operation condition of magnetic refrigerator at room temperature for direct air cooling. The basic components of the target system were a magnetic circuit including two permanent magnets, a test section, an air blower, and an associated instrumentation. The test section consisted of ten test cells which enclosed gadolinium chips as a magnetic working substance in prescribed packing factor. In order to change the applied magnetic field from 0 to 0.9 T, the magnetic circuit was installed on an electric slider which generated reciprocating motion. The system performances were widely investigated both experimentally and analytically for the variety of operation conditions such as a volumetric flow rate of air, a packing mass of magnetic working substance, and a heat exchange cycle. The results reveal that the maximum value of the cooling capacity of the present magnetic refrigerator is obtained in a certain operating condition.