This article outlines the latent heat transport properties of the metal-to-insulator phase transition materials that our research group has been conducting. The metallic solid-solid phase change materials exhibit no change in shape before and after the phase change, and therefore require no container to maintain their shape when applied to heat sink devices. This can be a fundamental solution to the problems of increased thermal resistance due to the container or capsule, and leakage of the phase change material due to their breakage, which are apparent in conventional solid-liquid type phase change thermal storage systems. In this article, we introduce the thermal behavior of vanadium dioxide for a heat sink material.

As a candidate for latent heat storage materials, a semiclathrate hydrate (SCH, a kind of clathrate hydrate) has been expected. A remaining challenge toward the practical use of SCHs is to diminish a large degree of supercooling in the SCH crystallization. One of the effective solutions is the utilization of the hysteresis called the “memory effect”. The memory effect diminishes supercooling and/or the induction period in the clathrate hydrate recrystallization. We have elucidated that the solution structures (called clusters) remaining in the aqueous solution after complete SCH decomposition is closely related to the memory effect. In this paper, we introduce the experimental results on the residual clusters in the tetra-n-butylammonium bromide (TBAB) aqueous solution resulting from TBAB SCH decomposition, which were observed by scanning electron microscopy (SEM) with the freeze-fracture replica method, a small-angle X-ray scattering (SAXS), and a high-precision differential scanning calorimetry (μDSC).

Effective handling of reusable waste heat is one of the important fundamental technologies for reducing energy consumption as well as promoting energy conservation. Among possible ways for handling waste heat, thermal energy storage is the key technology in the thermal energy management. Basically, there are two major classes of materials for achieving thermal energy storage, which are known as phase change materials (PCMs) and chemical heat storage materials. Although some of them are familiar to our daily life and are widely used in various social systems, detailed molecular mechanism of thermal energy storage remains elusive at present. In the TherMAT project, we are now investigating the basic mechanism of thermal energy storage for both material systems based on the predictive computer simulations. As for PCM, sugar alcohols are known as one of the promising candidate for achieving large amount of thermal energy storage. Based on the molecular and crystal structures of known sugar alcohols, we have computationally designed and predicted a new organic molecular material which can achieve larger amount of thermal energy storage than ever known. At first step, we carefully analyzed molecular properties of known C4, C5 and C6 sugar alcohols based on the classical MD simulations. Then we have clarified the molecular factors that control physical properties, such as melting point and latent heat. On the basis of these detailed analyses, we proposed molecular design guidelines to achieve effective thermal energy storage; 1) linear elongation of carbon backbone, 2) separated distribution of OH groups, and 3) even numbers of carbon atoms inside the carbon skeleton. Our computational results clearly demonstrated that if we carefully designed molecular structures, non-natural sugar alcohols have potential ability to achieve thermal storage density up to 450-500 kJ/kg, which is larger than the best value of the present known organic PCMs (~350 kJ/kg).

Counter-current flow limitation (CCFL), the void fraction α, the wall friction factor f_w, and the interfacial friction factor f_i depend on the shapes of the top and bottom ends of vertical pipes, i.e., the sharp top end and round bottom end (SR), round top end and sharp bottom end (RS), and round top and bottom ends (RR). We observed flow structures with a high-speed video camera and measured CCFL (the relationship between superficial velocities of up-flow gas and down-flow liquid), pressure gradient dP/dz, and α in a RR vertical pipe with the diameter of 20 mm, and working fluids of air and water under flooding conditions, and we obtained f_w and f_i by using the annular flow model. The feature of flow structures in RR was occurrence of disturbance waves inside the vertical pipe, while they mainly appear at the bottom end in SR and RS. The liquid volume fraction (1－α), dimensionless pressure gradient －(dP/dz)^*, and fi in RR for rough film were the same as those for rough film in SR and RS. (1－α), －(dP/dz)^*, f_w, and f_i in RR for smooth film were close to those in SR for smooth film but they were larger than those in SR due to larger falling liquid volume fraction. The upward velocity of disturbance waves was also discussed.

A sub-channel analysis code is commonly used for evaluating thermal-hydraulic behaviors in a fuel assembly. Since the spatial resolution of the sub-channel analysis code is “sub-channel” level, the sub-channel analysis code can provide more detailed thermal-hydraulic information than a conventional one-dimensional nuclear reactor safety analysis code, whose spatial resolution is “reactor core” level. To predict the thermal-hydraulic behavior, it is essential to predict the distribution parameter, which significantly affects the void fraction prediction. However, existing sub-channel analysis codes assume that the distribution parameter is unity for each sub-channel or the distribution parameter obtained for the “reactor core” level is adopted. The current study developed the constitutive equation of the distribution parameter at the “sub-channel” level, based on the methodology proposed by Julia et al. (2009). The developed constitutive equation of the distribution parameter was successfully validated against the NUPEC steam-water void fraction data collected in an 8×8 rod bundle under prototypic BWR conditions.