Thermal conductivity of powder materials under vacuum depends on several parameters such as particle size, temperature, porosity, etc. However, there appears to be no model that quantitatively expresses these dependencies. In this study, we investigated particle size and temperature dependencies by way of thermal conductivity measurements under vacuum environment using glass beads. As a result, it was found that the conductive and radiative contributions on the effective thermal conductivity increased with increasing the particle size, which might be explained by combination of both number of contact points per unit volume and thermal resistance at interpaticle contacts and pore size between particles, respectively.

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The effective thermal conductivity with convection and radiation is analyzed by the homogenization method. This method can represent the microstructure in the packed bed precisely. In this study, Effect of parameters such as emissivity of radiation, temperature, contact area and particle size of the packed bed, on the conductivity has been estimated. For example, heat transfer by radiation is not dominant for the material with void less than 1 mm. Effect of contact area and pressure on effective thermal conductivity are negligible for thermal radiation. The homogenization method can become a powerful tool for the estimation of the effective thermal conductivity of the packed bed by considering the microscopic behavior.

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An experimental study on melting and forced-convectional heat transfer characteristics of an ice slurry around a horizontal heated cylinder has been conducted. A mixture of fine ice particles and a trehalose aqueous solution is used as the ice slurry under test. While the ice slurry forms a sub-steady flow around a heated cylinder, a constant heat flux is electrically imposed on the surface of the cylinder. Under a sub-steady state, temperatures on the heated surface are measured at different angular locations in order to evaluate the heat transfer coefficient distribution. Simultaneously, the flow behavior of the ice slurry is carefully observed to discuss the mechanism of the heat transfer. Experiments are carried out under two parameters: velocity of the ice slurry and heat flux through the surface of the cylinder. Heat transfer coefficients obtained with the ice slurry are compared to those with a single-phase solution. Based on this comparison, specific heat transfer characteristics of the ice slurry relative to those of single-phase liquids are discussed. Furthermore, the average heat transfer coefficient on the entire heat transfer area is also evaluated. The results reveal that heat transfer coefficient increases together with an increased heat flux. This tendency can be attributed to an increase in an inhibitive effect on the temperature rise at the surface due to increased latent heat absorption. In addition, it is found that the effect of ice particles on the heat transfer is greatly affected by velocity conditions, particularly in the front region of the cylinder. This fact can be explained in connection with a transition of the contact state between ice particles and the heated surface, which is brought by the action of velocity-dependent body forces on ice particles. Moreover, thermophysical properties of the testing ice slurry are estimated for a dimensional analysis to organize the effects of each factor. Finally, an experimental correlation equation is proposed.

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In hydrogen stations for fuel cell vehicles, a rapid filling process of high pressure hydrogen to a tank on the vehicle is requested, and there are some concerns that possible condensation of residual water at a low temperature section in the fuelling system leads to occlusion in pipes or instability of flow system. Systematic and reliable frost point data concerning operating conditions of actual hydrogen stations were strongly required for understanding optimal design and operational conditions of stations. In the present investigation, dew and frost point measurement system for residual water in high pressure hydrogen with pressure tight test cell utilizing cooled mirror technique with original visible image analysis had been designed and developed. The system was available for frost point temperature below -50 oC, and total pressure of system up to 10 MPa. The pressure tight test cell had extensible design available for higher pressure experiments up to 40 MPa. Frost point measurements with hydrogen-water standard gases under conditions of 5~10 MPa, 5~55 ppm were carried out systematically, and measured data by the test cell with image analysis were relatively in good agreement with data by the embedded cooled mirror sensor for verification.

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