The validity of Horio’s scaling law for bubbling fluidized beds was examined experimentally by using two geometrically similar fluidized beds (internal diameter, 57 mm and 230 mm). Pressure fluctuations at the plenum and bottom section of the fluidized beds, eruption bubble size and frequency were measured, and the power spectral density and standard deviation of pressure fluctuation were examined over a wide range of superficial gas velocity and bed height adjusted to satisfy the scaling law. The validity of the scaling law for bubbling fluidized beds was discussed by use of a model based on pressure fluctuation and bubble behavior between two geometrically similar fluidized beds.
The recent attention to energy-saving housing has increased demand for high-performance heat insulators. Among several heat insulators, inorganic fibrous heat-insulators have various merits, but production costs are still high. This work proposes a fabrication process of water glass fibers by centrifugal spinning using a concentrated aqueous solution of sodium silicate as spinning solution. It was found that fiber thickness and distribution are strongly affected by the process parameters, and that larger centrifugal gravity provides thinner and more uniform fibers. In contrast, the viscosity of spinning solution scarcely affected the thickness and uniformity of fibers, and a spinning solution with viscosity of more than 3 Pa·s and less than 150 Pa·s gave fibers stably. Humidity-resistant fibers were produced by carbon dioxide gas treatment at over 100°C. Thermal conductivity measurement revealed that the optimal value was 0.027 W/(m·K), which is comparable to that of air in the absence of convection. This value was similar to that of commercially available glass wool, but the mass of fiber required was about one-half, which would be effective for cost reduction.
This study concerns the action mechanism of nucleating agents added to sodium acetate trihydrate (melting point 331 K) as a latent heat storage material in order to suppress its inherent problem of supercooling. Such nucleating agents are activated by the solidification of the trihydrate melt, but deactivated by superheating beyond a critical temperature, upper limiting temperature, during the process of heat storage. A previous study (Watanabe, 1992b) on a system of trihydrate melt with sodium oxysalts indicated that the limiting temperatures of these salts were related with their solubilities in water. The present study employed a melt system with orthophosphates in which sodium cations were replaced with H+ (partial substitution), Li+, K+, Ag+ and NH4+, and the limiting temperatures of the monovalent-cation-substituted phosphates were investigated by means of thermal cycle examination. Samples were prepared by adding an excess amount of each of the phosphates to sodium acetate trihydrate, melted by superheating at a temperature of 355 K, and then solidified by forced supercooling to 233 K. Deactivation was observed with all six kinds of phosphates, whose limiting temperatures differed from each other and fell within the range of 12–34 K above the melting point. No remarkable relationship was observed between the limiting temperature and the nucleation temperature of the melt either before or after deactivation. The degree of similarity in the lattice shapes of unit cell planes between the crystals of sodium acetate trihydrate and those of the phosphates was calculated with numerical data defined as root-mean-square deviations of their corresponding axis lengths, but no reasonable interrelation was found between the degree of similarity and the limiting temperature. On the other hand, the solubility in water of trisodium phosphate, the phosphate with the highest limiting temperature, was slightly below lower than that of the trihydrate. Consequently, in agreement with the previous study, it was concluded that an optimal solubility value for elevating the limiting temperature might exist in the melt system with monovalent-cation-substituted phosphates.
The functional mechanism of nucleating agents added to the latent heat storage material sodium acetate trihydrate (melting point 331 K) was examined. Such nucleating agents are activated by solidification of the trihydrate melt and accelerate nucleation in the melt, but superheating of an activated agent beyond a critical temperature, upper limiting temperature, during heat storage results in its deactivation. In this study, continued from the previous study (Watanabe, 2017), 14 types of cations (Mg2+, Ca2+, (CaH)3+, Sr2+, (BaH)3+, Al3+, Pb2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+ and Cu2+) were substituted for sodium ions in phosphates added as nucleating agents in the melt system, and their upper limiting temperatures were measured by thermal cycling. Deactivation was observed with 7 of these cation-substituted phosphates. The limiting temperatures of the substituted phosphates all differed within the range of 12 to 22 K above the melting point. The degree of similarity in the lattice shapes of unit cell planes between the crystals of sodium acetate trihydrate and the phosphates was quantified, but no reasonable correlation was found between the similarity and the limiting temperature. On the other hand, a clear correlation between the limiting temperature and the solubility of the phosphate in water was reconfirmed. The experimental findings suggested that the bonding force between solid surfaces of sodium acetate trihydrate crystals and the precipitated phosphate, which is mediated by the hydrating water molecules, is a physical factor controlling the limiting temperature.
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