Browsing by Author "Chellam, Shankararaman"

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    Fluid mechanics and particle transport in a channel with one porous wall: Application to membrane filtration
    (1991) Chellam, Shankararaman; Wiesner, Mark R.
    Fluid mechanics of a channel with one porous wall was studied from first principles as the initial step towards understanding polarization phenomena in membrane modules. A regular perturbation method was used to solve the steady-state Navier-Stokes equations for an incompressible, constant property fluid in two dimensions with uniform suction and slip at the permeable boundary. The effects of solute and hydrodynamic parameters on concentration polarization during potable water treatment applications are investigated numerically. Inertia dominated and permeation drag dominated particle transport is discussed. Experimentally determined residence time distributions of particles in a microporous channel are interpreted in the light of inertial and permeation forces. Inertial lift theory is shown to predict initial particle transport. Experimentally observed long trailing edges in particle residence time distributions indicate the importance of other transport mechanisms even in dilute suspension mechanics. It is seen that inertial effects are negligible under conditions typical of microfiltration.
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    Laminar fluid flow, particle transport and permeate flux behavior in crossflow membrane filters
    (1996) Chellam, Shankararaman; Wiesner, Mark R.
    Similarity solutions for axial and lateral velocity profiles, pressure gradients and wall skin friction are derived for the laminar, isothermal single phase flow of incompressible fluids in channels having porous boundaries. Results from a finite difference solution to the vorticity-stream function formulation of the Navier-Stokes equations are compared with previously reported perturbation, asymptotic, similarity and infinite series solutions. Initial transport of non-interacting particles suspended in laminar flow in the membrane far-field is reported to be accurately predicted by trajectory theory. RTDs obtained in response to pulse inputs in slow axial crossflows and high permeation rates appear to reveal a minimum in back-transport for 7 $\mu$m particles in the range of experimental conditions investigated here. Back-transport of smaller particles is due to Brownian diffusion whereas shear-induced diffusion appears to control the behavior of larger macrocolloids. The effects of suspension concentration, shear rate, Particle Size Distribution (PSD) and initial permeation rate on permeate flux are reported. Existing transient models based on shear-induced diffusion and particle adhesion as well as the steady state inertial lift model are found inadequate in predicting experimental observations of the specific permeate flux during the laminar crossflow filtration of narrow PSD suspensions. Under the range of experimental conditions investigated here, smaller particles deposit preferentially in the cake. Also, under identical experimental conditions higher permeate fluxes are obtained during the filtration of suspensions with a higher average particle size. Hence, pretreatment aimed at coagulating smaller particles could have a beneficial impact on permeate flux production. In all cases, specific resistances of cakes are higher in the crossflow mode compared to the dead-end mode. Also, cake specific resistances increased with shear and decreased with increasing permeation rate. Cumulative resistance to permeation is reported to increase on application of shear even without particle feed. Thus, even though cake mass decreases with increasing shear, it may not result in higher permeate flux. Therefore, pilot scale testing may still be necessary to evaluate the fouling potential of feed waters as well as in optimizing the operation of existing crossflow membrane filters.