Hisashi Miyashita

Occurrence and development of double-diffusive convection during solidification of a binary system

Abstract This paper describes the occurrence and development of double-diffusive convection in the liquid phase during solidification of NH 4 Cl−H 2 O system in a confined cavity with lateral cooling. Multiple point measurements of concentration show a step change in the vertical direction, and temperature visualizations reveal an S-shaped profile of isotherms, which indicate the existence of time-dependent horizontally-stacked roll cells separated by diffusive interfaces. The cells are generated in a sequential fashion, rather than simultaneously and the thickness of each cell increases with progression of solidification. The concentration in each cell is found to remain nearly constant, except for the initial development of cells. Convection within each cell is largely controlled by the temperature field, and diffusion is dominant in the diffusive interface between two cells due to the solute field with a vertical concentration gradient. The fluid in the diffusive interface is initially stagnant, but the interface changes into a new cell under a certain condition. The criteria for cell generation are determined by the buoyancy ratio and the thermal Rayleigh number in the diffusive interface.

Oscillatory flow in a symmetric sinusoidal wavy-walled channel at intermediate strouhal numbers

Abstract Oscillatory flow in a two-dimensional symmetric furrowed channel is studied numerically and experimentally over wide ranges of flow parameters. Two-dimensional numerical computations show that the vortex strength has a peak at intermediate Strouhal numbers, regardless of the Reynolds number. Flow visualization experiments support the two-dimensional computational results at low Reynolds numbers, but deviations arise at moderate Reynolds numbers. We show the transition from two-dimensional to three-dimensional flow due to instability of the flow. The critical Reynolds number for the onset of three-dimensional flow strongly depends on the Strouhal number. The fluid motion is found to be most unstable in the limited Strouhal number range of 0.02–0.03, and this is in agreement with the prediction of the two-dimensional computation. Two distinct three-dimensional flows are discovered experimentally.

The influence of tube layout on flow and mass transfer characteristics in tube banks in the transitional flow regime

Abstract Flow and mass transfer characteristics in tube banks are investigated in the transitional flow regime at intermediate Reynolds numbers. The tube arrays considered are a staggered tube array and an in-line tube array with longitudinal and transverse pitch-to-diameter ratios of two. The flow is steady at the entrance of the tube banks, but becomes oscillatory downstream of an onset location of vortex shedding. This location moves upstream with increasing Reynolds number, and the upstream development of flow transition is much faster for the staggered array than the in-line array. Therefore, the row-by-row variation of the mass transfer rate is small for the staggered array but considerable for the in-line array. The mass transfer for the staggered array is sensitive to the unsteady wakes of the preceding row tubes, while the surface shear stress is insensitive to such wakes. However, for the in-line array the mass transfer and shear stress are both sensitive to the unsteady wakes.

Drying behavior of polymer solution containing two volatile solvents

Abstract A study on the drying of thin layers of polymer solution containing two volatile solvents is performed through experiments and numerical simulation. In the experiments, the individual drying rate of each solvent as well as the overall drying rate is obtained. The initial composition and the thickness of a sample are varied in order to examine their effects on the drying characteristics. Moreover, simulation is performed by solving a pair of diffusion equations for each solvent together with a heat balance equation. The diffusivity and the vapor pressure of solvents necessary for the simulation are estimated using free-volume theory of Vrentas and Duda and the Flory–Huggins equation, respectively. The results of simulation show qualitative agreement with experiments, while apparent mass transfer rate in the film is smaller than that estimated from diffusivity data found in the literature. The reason of underestimation is a future subject.

Visualization of temperature fields and double-diffusive convection using liquid crystals in an aqueous solution crystallizing along a vertical wall

This article presents a simple technique for temperature visualization using liquid crystals in an aqueous solution during the process of cooling and solidification. This method provides a clear picture of the role of double-diffusive convection in producing vertical compositional and density stratification in an initially homogeneous liquid during solidification.

Visualization of Temperature and Flow Fields Using Liquid Crystals in the Fluid Region During Various Solidification Processes

This article presents a simple technique for temperature visualization using liquid crystals in the liquid region during the process of cooling and solidification. This method shows interesting timewise evolution of flow and temperature fields, i. e. the process of density inversion for freezing of water and the double-diffusive process for solidification of aqueous solutions.

Heat transfer enhancement with an inclined semicylindrical turbulence promoter—The relationship between optimum clearance and film thickness

Heat transfer enhancement in a rectangular duct with a semicylindrical turbulence promoter was experimentally studied for a high Prandtl number fluid. The measurements of mass transfer coefficient and wall shear stress as well as flow visualization were performed in order to investigate how the performance of a heat transfer enhancement promoter is influenced by its inclination angle and by the clearance between the promoter and the wall surface. When the inclination angle is 0°, the variation of performance with clearance is closely related to the extent of heat transfer enhancement in the region of slipping flow. In this case, the clearance exhibiting the maximum performance is almost equal to the film thickness for a smooth duct without a promoter. As the inclination angle increases, what influences the variation of performance with clearance gradually shifts to the extent of heat transfer enhancement in the region of circulating and reattaching flows, and the clearance exhibiting the maximum performance decreases. There is no remarkable difference in the maximum performance between inclination angles examined here. It is, however, desirable to set the inclination angle to 30°, since the distribution of the local mass transfer coefficient for this angle is less curved than those for other angles.

Calculation of Nucleation and Crystal Growth Rates for Multi-Stage Continuous Crystallization.

MSMPR (Mixed Suspension Mixed Product Removal) 型晶析器を用いた連続多段晶析プロセスを対象に, 各段の結晶滞留時間および滞留量が異なり, 前段で生成した結晶スラリーがそのまま次段に供給されるという実際的な場合について各段の結晶成長速度と核化速度を算出する方法を提案した.本法では先ず各段におけるポピュレーションバランス式を誘導しその解析解を求めた後, 各段の結晶重量, 滞留時間, 粒度分布のデータのみから, パーソナルコンピューターを用いて1段目から順に各段の結晶成長速度と核化速度を区別して同時に求めるものである.本法を日産式メラミン製造プロセスにおける連続三段晶析工程に適用したところ, 満足すべき操業解析結果を得た.本解析結果と基礎実験で別途測定したメラミンの過溶解度および結晶線成長速度係数のデータを用いて各晶析器の操作過飽和度や相対操作過飽和度を算出し, 操業状況を検証した.

AUGMENTATION MECHANISM OF MASS TRANSFER AMONG TURBULENCE PROMOTERS ON WALL SURFACE IN RECTANGULAR DUCT

An experimental investigation was performed to study the mechanism for the augmentation of mass transfer by using turbulence promoters on the wall surface in a rectangular duct. The augmentation of local mass transfer among the turbulence promoters was measured by varying the diameter, the pitch of the promoters and the clearance between the promoter and the wall. In order to examine the augmentation mechanism, wall shear stress, mass transfer intensity and turbulence intensity at the wall were measured by an electrochemical method. Further, flow behaviors were measured by visualization. It was found phenomenologically that the augmentation of mass transfer with the clearance was caused by turbulence due to reattachment flow, large scale eddies and increase of shear stress due to flow jet under the promoters, and in case of no clearance, was caused by only turbulence on the wall surface.