Koji Kunitsugu

Influence of imposed oscillatory frequency on mass transfer enhancement of grooved channels for pulsatile flow

Abstract The present experimental study describes mass transfer enhancement in grooved channels with different cavity lengths for pulsatile flow. Overall and local mass transfer rates were measured by the electrochemical method with a high Schmidt number and also the vortical motion within the groove was visualized by the electrolytic precipitation method. We especially focused on the influence of oscillation frequency on mass transport enhancement. Transport enhancement by means of fluid oscillation is found to be higher in laminar flow than in turbulent flow. There is a noticeable enhancement at intermediate Strouhal numbers, depending on the cavity length and the net flow Reynolds number. It is revealed that the mechanism for a peak transport enhancement factor against Strouhal number is not explained by the hydrodynamic resonance proposed by Patera and Mikic, under a certain condition.

Fluid mixing and mass transfer in two-dimensional cavities with time-periodic lid velocity

Fluid mixing and mass transfer in cavities with time-periodic lid velocity were examined numerically. Unsteady Galerkin finite elements computations were performed for various flow parameters. Global fluid mixing is greatly promoted when an unsteady component of the velocity is superimposed on the steady flow. There is an optimum oscillation frequency that produces the best mixing. Fluid mixing also depends on the oscillation amplitude and the geometric aspect ratio. However, the oscillation frequency has little effect on mass transfer between the walls at different concentrations. It is revealed that excellent global fluid mixing does not always lead to heat and mass transfer enhancement.

Fluid mixing and local mass transfer characteristics in a grooved channel for self-sustained oscillatory flow

Flow patterns and mass transfer rates in a periodically grooved channel were studied in the transitional flow regime. Self-sustained flow oscillations occur at a low Reynolds number. Primary flow instability arises from Tollmien-Schlichting waves triggered by a shear layer above the groove, and thus there is a fluid exchange between channel and groove parts through the shear layer. A further increase of the Reynolds number produces secondary instability causing a three-dimensional flow at the bottom of the groove. Mass transfer was performed by the electrochemical method. The transport rate at the rib increases significantly after the primary instability, but the increment of mass transfer at the bottom of the groove is small. The secondary instability leads to marked transport enhancement at the bottom of the groove

NATURAL CONVECTION SUPPRESSION BY AZIMUTHAL PARTITIONS IN A HORIZONTAL POROUS ANNULUS

Numerical studies by the Galerkin finite element method are presented for the suppression of natural convection heat transfer in a horizontal porous annulus heated from the inner surface. Azimuthal partitions are employed to reduce heat transfer, and the effects of the number of partitions and their positions are examined. For a single partition, its position has a negligible effect on heat transfer in the boundary layer regime. An increase in the number of partitions is effective in suppressing natural convection, and it is found that the Nusselt number is inversely proportional to (N + 1), where N is the number of partitions.

Pulsatile Flow in a Wavy-Walled Tube

The visualization of pulsatile flow in a wavy-walled tube for low flow rates is conducted by the aluminum dust method. We pay attention to the effect of Strouhal number on flow behaviors. It is found that, generally, the flow tends to remain stable in the accelerating phase and unstable in the decelerating phase. There exists a peak mass transfer enhancement at an intermediate Strouhal number, where inertia effects dominate the flow, while at low and high Strouhal numbers, the mass transfer enhancement is reduced, and the flows remain relatively stable.

Numerical Study of Pulsatile Flow through a Wavy Channel. Vortex Dynamics and Fluid Mixing.

Numerical analysis is performed to study the dynamical behavior of vortices generated in a sinusoidal wavy channel for pulsatile flow. This system is employed in order to enhance heat and mass transfer under laminar flow conditions. A pair of counter-rotating vortices periodically moves between the upper and lower walls with one wavelength. The vortex strength has a maximum value at a certain Womersley number, which increases with the net flow Reynolds number. A particle advection procedure also indicates that the vortex dynamics lead to efficient fluid mixing with increasing Womersley number.