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The electrocaloric effect in thin films of electrocaloric material has the potential to be used for efficient cooling systems. We numerically calculated the effect of the parameters in electrocalor...

We investigate electrokinetic behavior of colloidal particles in the vicinity of a solid-liquid interface. Colloidal liquids are expected to be used as thermal transport media for heat transfer applications such as nanofluids and phase change emulsions. They contain submicrometer-sized particles in liquid, and electrokinetic behavior of the solute particles should play an important role in the heat transfer between solid-liquid interfacing boundaries. However, experimental investigation of the behavior remains difficult due to the required spatial resolution beyond diffraction limit. We developed a measurement system based on laser Doppler principle using an interference of evanescent waves generated at total internal reflections of incident lasers at a solid wall. The system was developed for the measurement of velocities of colloidal particles at an interfacing boundary of colloidal liquid and a solid wall. The system has a unique advantage of a high spatial resolution in the direction perpendicular to the boundary due to the short penetration depth of an evanescent wave in the range of a few hundred nanometers. The principle and performance of the measurement system were investigated using a scanning probe in the measurement volume. We experimentally confirmed the validity of the measurement and characterized the uncertainty of velocity measurement. The system was further applied in a series of measurements of alumina particles dispersed in water in a square-shaped cell under induced electric fields. The measured velocities are proportional to the field strengths at different particle concentrations. The linear relationship is consistent with theoretical predictions, which demonstrates the feasibility of the system for the measurement of velocities of colloidal particles in the near wall region.

A great deal of attention has been paid to energy saving devices in place of conventional air-cooled and water-cooled devices. The thermal energy storage system that uses the latent heat of a PCM (phase change material) for air-conditioning or heating has recently become popular because it does not require high electric power and it saves energy. An emulsion dispersed nano-size particles of phase change material is produced. We discuss with the thermophysical properties, the stability of emulsion, and the heat transport characteristics as a thermal functional fluid. The testing emulsion, which has nano-size particles as the discrete phase, is produced with a d -phase emulsification method. The diameter of discrete phase in the emulsion is measured for evaluation of the long-term stability of emulsion. In addition, the DSC (differential scanning calorimetry) curve of emulsion is determined. Thermophysical properties such as viscosity and thermal conductivity of emulsions were studied in this work, and was compared with that of the base fluid. The results reveal that the emulsion with the d -phase emulsification method has the superior stability. From the differential thermal analysis, the DSC curve of present emulsion indicates a discontinuous change at the phase change temperature of phase change material due to its latent heat.

We developed a new calibration method for minimizing the uncertainty and for establishing the uncertainty traceability of laser velocimetry, especially for laser Doppler velocimetry (LDV). While former calibration methods had uncertainties larger than the lower uncertainty limit of an LDV, the new method has an uncertainty smaller than that of an LDV. In the new method, the radial uncertainty of a calibration object was reduced by using a combination of a linear precision stage and multiple scattering points placed on a rotating disk. The working principle is based on the linear relationship between the orbit radii and the resulting output signals. A linear regression provides a promising estimate of the true orbit radius and hence the calibration constant of the velocimetry. In the present work, we used multiple scattering points along a single slit aperture located on a rotating disk. The feasibility was confirmed in a series of experiments using a prototype setup of the calibration system. With the new method, the resulting calibration uncertainty is reduced down to 0.2 %, which is smaller than the general uncertainty of an LDV.

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.

Water consumption can be reduced by using a greenhouse for agriculture in desert areas. We analyzed the effect of control of ventilation, sprinkler water, and solar radiation shielding on changes of temperature and humidity in a greenhouse under various desert area conditions. We calculated the changes in temperature and humidity in a greenhouse for a whole day in four seasons, and the calculation results of water consumption with and without a greenhouse were compared. When ventilation, shielding, and sprinkler water were controlled under suitable conditions to grow orchids in a desert area, water consumption in July was only 7% of that without a greenhouse.

Microscopic observations of the cooling process in plant leaf cells have been performed using a directional solidification stage in order to study on the optimum freezing condition of living tissues. Till date, several studies on the freezing or melting of cells have been conducted. However, few researchers have studied cells near the freezing point. In this study, a temperature-controlled stage was created in order to investigate the effect of cell temperature on cytoplasmic streaming, and the cells of an aquatic plant (Egeria densa ) were used. The result showed that cytoplasmic streaming is greatly affected by ambient temperature and shift in temperature conditions. Moreover, the velocity of cytoplasmic streaming has a peak value at a particular temperature, and it does not recover soon after heating from a cold state.

We studied the reduction of natural convection heat loss from a solar thermal collector by placing a high-porosity porous medium above the collector plate in a solar thermal collector system. It is known that natural convection can be diminished in a porous medium. In order to use a porous medium in a solar thermal collector, it is necessary to minimize the shading effect of solar radiation caused by the porous medium. In this work, we used a series of offset wire screens made of fine nylon fishing lines of 0.05 mm in diameter with 2-mm pitch and a porosity of 0.999. The experimental apparatus consisted of a copper 300 mm × 300 mm collector plate with a selective absorption film on the surface. We measured the reduction rate of the convection heat loss from the collector plate and the shading effect for actual sun radiation by changing the condition of the porous medium, the temperature of the collector plate, and the inclined angle of the collector plate. Experimental results of the Nusselt number of natural convection in the high-porosity porous medium agreed with the equation proposed by Gupta et al. The effect of the inclined angle on the Nusselt number was small. The net reduction rate of natural convection heat loss was 7% by placing the high-porosity porous medium above the collector plate when the temperature of the collector plate was 100 °C.

The primary objective of this study was to investigate the fundamental phase change characteristics of a nanoemulsion using differential scanning calorimetry (DSC). Tetradecane, which has a slightly higher melting point than water, was utilized as the phase change material for the nanoemulsion. The melting point of the nanoemulsion, the melting peak temperature, and latent heat were examined in detail. Regarding the fundamental phase change characteristics of the nanoemulsion, it was found that its phase change characteristics were strongly affected by the temperature-scanning rate of the DSC. Moreover, it was confirmed that the phase change behavior does not change with repeated solidification and melting.

Ice slurry, which is a mixture of fine ice crystals and liquid water, is utilized in ice storage systems owing to its good flowability and large latent heat of fusion. In the present study, a method was developed to generate ice slurry without the deposition of an ice layer on a cooled surface. This method is based on freezing-point depression of the aqueous solution, which is maintained under highpressure conditions. To control the timing for solidification and to generate ice slurry, we investigated the relationships among the pressure, temperature, and concentration of the aqueous solutions. We developed an ice slurry generator based on the new method that controls the pressure and temperature of the aqueous solution. Experimental results showed that the characteristics of the ice slurry generation were closely related to the pressure and initial stage temperature of the test fluid.

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.

Enhancement of evaporation heat transfer performance is important for cooling of electronic devices. Evaporation heat transfer coefficient of two types of porous surfaces supplied with water liquid film was studied experimentally. First porous surface was multi layers of meshed plates with micro-channel of 0.3 mm width. Thin water liquid film was supplied to the porous surface by bubbles. Evaporation heat transfer coefficient of the porous surface with one or two layers of meshed plates was 8×104 W/m2K and it was higher than pool boiling heat transfer coefficient. Second porous surface was 1 mm thickness of glass-beads of 0.4 mm in diameter on a heated cupper plate. Water was supplied to the porous layer at the center of the plate. Evaporation heat transfer coefficient was 1000 W/m2K after water was supplied. Evaporation heat transfer coefficient increased to 7000 W/m2K before appearance of dry portion on the plate, because water film thickness in the porous layer was thin.

An experimental study was carried out to investigate the freezing characteristics of supercooled water droplets impacting on solid surfaces. Each water droplet was supercooled on the inside surface of a waterrepellent guideway in a cooled air environment and then blasted horizontally by a jet of pressurized cooled air to impact on the vertical surface of a test block. The deformation and freezing of the droplets were observed using a high-speed camera. The droplets were frozen under varying conditions of air temperature, surface temperature, impact velocity and droplet volume. The results indicated that the freezing behavior of a droplet was strongly affected by the condition of the solid surface. Further, the results revealed that the supercooling solidification process consisted of two steps with two greatly different time scales. Finally, it was shown that the freezing behavior of an ice layer was affected by the presence of a liquid-like thin film.

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.

The effect of tool geometry on the plastic flow and material mixing during friction stir spot welding (FSSW) is investigated using the particle method approach. For spot welds made with a cylindrical pin tool with flat shoulder, the model predicts the material flow at the pin periphery to be in the upward direction and the material is pushed downward beneath the shoulder giving rise to the resultant hook geometry. Other pin geometries evaluated include tapered pin, inverse tapered pin, triangular pin, convex shoulder, and concave shoulder. With good correlation with experimental trials, this model is then used to predict the material flow for spot welds. The material flow, and thereby the resultant hook formation, is quantified using numerical methods and is expressed as standard deviation of the particle movement. A triangular pin with a concave shoulder is the preferred tool geometry from the current study that results in high strength spot welds.

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.

Ice slurry has attracted a great deal of attention as a coolant for direct contact cooling. In this study, we generated ice slurry by the method of pressure shift freezing (PSF), which is based on the freezing-point depression of an aqueous solution at high-pressure conditions. As a result, the basic characteristics of the ice slurry generation are clarified. Moreover, the physical properties of the ice slurry indicate that the shape of an ice particle in the ice slurry is strongly affected by both the supercooling degree and the aqueous solution concentration.

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.