Search

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.

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.

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

Thermal performance of electrocaloric refrigeration system composed with a thin electrocaloric material and thermal switches was numerically calculated. Two types of thermal switches were studied: a thermal switch of fluid motion and a thermal switch by changing the contact thermal conductance. The following results were obtained. For the thermal switch of fluid motion with the frequency 10 Hz, the thicknesses of the electrocaloric material 200 μm and the water flow channel 100 μm, the average heat transfer efficiency was 11%. For the thermal switch by changing contact thermal conductance with the frequency 1000 Hz, the thicknesses of the electrocaloric material 20 μm and the heat storage material 20 μm, the average heat transfer efficiency was 6% and the average heat flux transferred to the cold side of the system was 7 x 104 W/m2.

The effects of types of collector systems, operation-control methods, and locations in the world on thermal performance of solar thermal collector systems and cost to produce required amount of 90°C-hot water were studied through numerical simulations. Also performance of a hybrid photovoltaic thermal (PVT) collector system was examined. Calculation results showed that the costs for all solar thermal collector systems were much less than that without solar thermal collector system. Effect of the locations in the word on the cost was large. Total energy efficiency and exergy efficiency of PVT system were lower than those of the solar thermal collector system. The cost for PVT system was higher than that for the solar thermal collector system.