Ho Nam Chang

THE EFFECT OF TURBULENCE PROMOTERS ON MASS TRANSFER-NUMERICAL ANALYSIS AND FLOW VISUALIZATION

Abstract Numerical investigations are made on mass transfer in a 2-dim. flow between two parallel plates with turbulence promoters fixed to the lower and/or the upper walls. The ‘zigzag-type’ and ‘cavity-type’ geometries are used, for which mass transfer and flow visualization studies are conducted in the laminar flow regime. The numerical results show that turbulence promoters enhance the mass transfer by forming a recirculating flow which causes a convective mass transfer effect and increases the wall shear stress in the main stream. The flow separations observed by visualization studies are in good agreement with the predictions made by the numerical analysis at Reynolds numbers ranging from 20 to 150. Also the visualization shows that the unsteady flow in the channel begins at Reynolds numbers of 250–300 for both geometries with the aspect ratio of 5.

Velocity field of pulsatile flow in a porous tube

This paper describes velocity fields for fully developed periodic laminar flow in a rigid tube with a porous wall. We obtained an analytical solution of the flow by the linear approximation of the Navier-Stokes equation. Unlike the previous works with a constant seepage rate along the axis, we used a wall velocity which contained hydraulic permeation constant Lp. The axial velocity profile shows a local maximum velocity near the wall at a large Womersley number alpha. This suggests that concentration polarization in porous tubular membrane may be reduced at high frequencies if a membrane device is operated under pulsatile flow conditions. The magnitude of wall permeation velocity decreases linearly along the tube axis because the damping of the pressure difference between the inside and the outside of the tube is very small.

Experimental study of mass transfer around a turbulence promoter by the limiting current method

Abstract Local mass transfer rates around a turbulence promoter in a channel having the geometry of zigzag-type or cavity-type characterized by the location of promoters have been measured by the limiting current technique. The experimental results showed that turbulence promoters are effective in breaking the concentration boundary layer and thus increasing the mass transfer rate. In the present experiment the Reynolds numbers were varied from 10 to 200 for the different aspect ratios of 3.5, 5 and 7. The experimentally obtained Sherwood numbers were compared with those numerically obtained by Kang and Chang. As a result, they were found to be in agreement to within an error of 20%.

Effect of external laminar channel flow on mass transfer in a cavity

Abstract To investigate the enhancement of mass transfer by vortices induced in a cavity due to external channel flow, numerical analysis has been carried out for various aspect ratios, Reynolds numbers and Schmidt numbers. As the aspect ratio increases the number of vortices increase but only the primary and the secondary vortices have such a strength as to enhance mass transfer. For each aspect ratio the mean Sherwood number was expressed in the form Sh m = K Re x Sc y . For an aspect ratio of one, the results have been compared with experimental data measured by the limiting current method and good agreement, within an error range of 20%, was obtained.

Mass transfer in a three-dimensional net-type turbulence promoter

Abstract Local mass transfer rates in a three-dimensional net-type turbulence promoter were measured using a limiting current technique and flow patterns were visualized with an ink tracer. The model promoter consisted of upper and lower rectangular flow channels which overlapped at a given angle to form an interfacial contacting and mixing region for the two layers of fluid streams coming through each channel. Local and overall mass transfer rates using 16 segmented electrodes were measured to study the effects of the Reynolds number ( Re ) and the ratio of two average velocities in the lower and upper channels by varying cross flow angle and the ratio of channel height to width. The highest mixing efficiency was obtained in tie promoter with 135° cross flow angle and progressively less in those with 90° and 45° cross flow angles. Flow visualization showed that in the three-dimensional promoter a vortex created by two interfacial cross flows was the main mechanism in breaking a concentration boundary layer in contrast to the recirculating flows in the two-dimensional promoters studied by Chang and co-workers (1983, 1984).

Pressure drop and mass transfer around perforated turbulence promoters placed in a circular tube

Abstract The friction factors and mass transfer coefficients in a tube with fins of e/D = 0.1, 0.2, 0.24 and 0.3 are investigated both experimentally and theoretically in the wide range of Reynolds numbers up to 8000. As expected the friction factor decreased linearly with Reynolds number up to Re = 200 and remained nearly constant at Re > 500 for all cases of e/D. The distribution of pressure between the fins is measured in detail. The perforated fins are proved to be effective both in reducing loss and in enhancing mass transfer.

MASS-TRANSFER IN THE U-TURN OF AN ELECTRODIALYZER

Abstract As a link in the chain of the hydrodynamic theories on electrodialysis, the mass transfer rate in the U-turn of a tortuous spacer was investigated. Besides, the solutions by Sonin and Probstein (1968) for a straight channel geometry were modified using a concentration profile approximated with a third order polynomial. The flow in a U-turn geometry at low Reynolds numbers was analyzed with the aid of the potential flow theory, and the related mass transfer problem was solved in connection with the flow field solution for the U-turn and the revised concentration distribution. Theoretical analysis showed that the desalting efficiency in the U-turn was nearly the same as that in the straight channel for low polarization, but the efficiency was lower for high polarization. Experimental results for the U-turn geometry were in good agreement with the theory for the case of low polarization. However the discrepancy between the two was noticeable for the case of high polarization. Also the disagreement became more pronounced at higher flow velocities possibly owing to the formation of a secondary flow caused by centrifugal forces.

Flow Pattern and Mixing in the Multi‐Turbine Agitated Bioreactor Filled with Poly(γ‐Glutamic Acid) Solution

We investigated the flow pattern and mixing behavior of a poly(γ‐glutamic acid) (γ‐PGA) solution in a bioreactor equipped with two Rushton turbines by simulation and experiment. Computational fluid dynamics (CFD) is used to solve the three‐dimensional hydrodynamics in the bioreactor and to obtain the flow patterns and tracer concentration at every point. The flow circulation patterns by inter‐impeller clearance and viscosity and their effects on overall mixing time were studied. Based on the results we can conclude that the impeller clearance should not be larger than 0.2 D for the efficient mixing under non‐aerated condition when the liquid viscosity is above 20 cp, which corresponds to concentrations of 20 g/L or above for γ‐PGA.

Numerical study on the effect of inert region on mass transfer by the segmented electrodes in limiting current method

The effect of inert region on mass transfer has been studied numerically for the cathode package that is used to measure the local mass transfer rate to a solid surface in a flow system. The inert region introduces a considerable error in the limiting current method as the region increased.The overall mean Sherwood number on the cathodic electrodes with the inert region was correlated as follows.Shm = 13.96 Re0.339(1 + RT)0.444 when RT is the ratio of the inert region to the active region. If RT is less than 0.25, the error of theoretical value to Leveque solution is less than 12%.