Koji Kunitsugu

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.

Abstract: 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.

Abstract: The present experimental study deals with mass-transfer enhancement and fluid dynamic behavior in a wavy-walled tube for pulsatile flow Three flow parameters are considered here: the net flow Reynolds number, the oscillatory fraction of the flow rate, and the Strouhal number Among them, we especially focused on the effect of Strouhal number on mass-transfer enhancement under the condition of no reverse flow Time-averaged transport enhancement by means of imposed fluid oscillation is found to vary with the net flow Reynolds number and with the oscillatory fraction of the flow rate The most effective mass-transfer enhancement is registered when the net flow falls on just before the transitional flow regime for steady flow Furthermore, there is an optimum Strouhal number, corresponding to the maximum transport enhancement, that depends on the oscillatory fraction of the flow rate The results of time variation for flow behavior show that stable and unstable flow states exist during one oscillation cycle The duration of the unstable flow state, corresponding to the optimum Strouhal number, is found to be the longest, and it leads to a remarkable fluid exchange between the mainstream and the recirculation zone, thus contributing to the mass transfer enhancement These results indicate that the transport enhancement mechanism for the wavy-walled tube is quite different from the resonant transport enhancement observed in the two-dimensional (2-D) wavy-walled channel © 2004 American Institute of Chemical Engineers AIChE J, 50: 762-770, 2004

Abstract: 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

Abstract: The dissolution of gypsum or plaster of Paris has been widely used as an inexpensive integral measure of ‘water motion’ in the field and in laboratory tanks for studies of physical‐biological interactions. Commonly, gypsumdissolution rates have been calibrated to steady flow speed or velocity in the laboratory and the calibrations have been applied to dissolution (i.e., mass-transfer) rates in the field or in tanks. We evaluated the gypsum-dissolution technique in a steady-flow, a fluctuating-flow, and a mixed-flow environment by comparing dissolution rate to direct flow measurements with an acoustic Doppler velocimeter. We found that dissolution rates were related to steady flow and to fluctuation intensity in the exclusively steady-flow and fluctuating-flow environments, respectively. The relationships were weak in the mixed-flow environment. Finally, dissolution and thus mass-transfer relationships were different in each flow environment, and the effects of steady flow and fluctuation intensity were not additive. Providing that it is rigorously checked and appropriately calibrated, the dissolution technique can be used to measure steady flow speed or fluctuation intensity in a steady-flow or fluctuating-flow environment, respectively. However, comparisons of dissolution rates between steady-flow, fluctuating-flow, and mixed-flow environments or within environments that change over time to determine water motion will be misleading. The gypsum-dissolution technique can be used as a good direct indicator of mass-transfer rates. However, mass-transfer rates are different in different flow environments. The gypsum-dissolution technique is not a universal integrator of ‘water motion.’ The dissolution of gypsum or gypsum-based plaster of Paris from ‘‘plaster balls’’ or ‘‘clod cards’’ was first introduced by Muus (1968) and Doty (1971) as an inexpensive technique to measure water flow in the absence of expensive field instrumentation for direct measurements of water velocity. Effects of temperature, salinity, and water volume on

Abstract: The effect of buoyancy ratio on the flow structure is investigated numerically for a binary mixture gas in a rectangular enclosure subject to opposing horizontal thermal and compositional buoyancies. The following conditions were considered: RaT = 105, Pr = 1, Le = 2 and N = 0.0–2.0 for A = 2. The numerical solution predicts that oscillatory double-diffusive convection with the secondary cell flow structure occurs for a certain range of buoyancy ratio. The key mechanism for oscillatory flow is that the unstably stratified region of species shifts from the central part of the enclosure to the upper and lower parts, and vice versa in a time-periodic sense, due to the interaction of heat and mass transfer with different diffusivities near the vertical walls. Bifurcation structures of the oscillatory flow in the present system are discussed.

Abstract: Experimental work has been performed to examine the combined effects of rib-grooved turbulators on the turbulent forced convection heat transfer and friction characteristics in a rectangular duct under a uniform heat flux boundary condition. In the experiments, three types of rib-groove arrangements: rectangular-rib and triangular-groove (RR–TG), triangular-rib and rectangular-groove (TR–RG) and triangular-rib with triangular-groove (TR–TG), were examined. Measurements were carried out for the duct of one aspect ratio, AR = W/H = 20 and duct height, H = 9 mm with rib height, e = 3 mm at three pitch ratios, PR = P/e = 6.6, 10 and 13.3. Experiments were conducted for the Reynolds number range of 3000 to 10,000. Influences of rib-groove arrangements on the Nusselt number and friction factor have been discussed and compared with smooth duct results under similar test conditions. Isothermal friction factors were also taken and presented. The obtained results of the smooth duct are in good agreement with the previous studies found in the literature. Experimental results also show that the duct with RR–TG arrangement provides maximum heat transfer rate and friction factor than others. On the other hand, the thermal enhancement index obtained at constant pumping power reveals that the TR–TG provides the highest values for all pitch ratios studied. Finally, correlations for the heat transfer (Nu), friction factor (f) and the enhancement index (η) have been developed as a function of pitch ratio (PR) and Reynolds number (Re).

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.