Hayato Masuda

Although natural convection is frequently encountered in various chemical processes, Rayleigh number (Ra) cannot be defined fully in shear‐thinning fluid systems. In particular, the velocity scale, which is necessary to estimate the effective viscosity of the system, should be discussed carefully. Thus, in this study, the representative velocity scale of Rayleigh‐Bénard (RB) convection, which is a typical example of natural convection, with shear‐thinning fluids was investigated based on the velocity fields obtained using computational fluid dynamics. Numerical simulations revealed that the critical temperature difference at which RB convection starts to fully develop decreases with an increase in the shear‐thinning property. The shear‐thinning property also induced subcritical bifurcation. In addition, the velocity scale of convection increases with an increase in the shear‐thinning property. Thus, the shear‐thinning property is considered to accelerate convection. Compared with several types of velocity scales used by other researchers, significant deviations from the actual scale were observed. Therefore, a new type of velocity scale, including the buoyant to viscous force ratio, arbitrary parameter, and thermal diffusivity, was proposed. The proposed velocity scale allowed an approximate estimation of the actual velocity scale. Although further investigation of the validity is necessary with varying geometries and rheological parameters, this velocity scale will be useful for controlling RB convection with Newtonian/shear‐thinning fluids.

Freezing has been used to extend the shelf life of food products. In freezing, the large ice crystals which are unfavorable for the texture and quality of food are mainly generated during a solid–liquid coexistence phase. It is well known the ice crystals produced after undergoing a supercooling state were finer than those produced without supercooling. This implies that the supercooling phenomenon affects the solid–liquid coexistence time during freezing. Therefore, in this study, the effect of the supercooling phenomenon on the solid–liquid coexistence time was investigated using a cylindrical vessel. The experimental results indicated that the solid–liquid coexistence time decreased with increasing degrees of supercooling. This result suggests that increasing the degree of supercooling is effective for producing finer ice crystals. Generally, the temperature distribution is uniform to promote the supercooled state. Thus, rotational motion of the vessel with a periodical switching the rotational direction every 180° clockwise and counter‐clockwise was imposed to relax the temperature distribution in the vessel. Below a certain rotational speed, it was observed that supercooling deepened; above that, multiple instances of supercooling release were observed owing to the inversion.