Mass transport in a microchannel bioreactor with a porous wall.

A two-dimensional flow model has been developed to simulate mass transport in a microchannel bioreactor with a porous wall. A two-domain approach, based on the finite volume method, was implemented. For the fluid part, the governing equation used was the Navier-Stokes equation; for the porous medium region, the generalized Darcy-Brinkman-Forchheimer extended model was used. For the porous-fluid interface, a stress jump condition was enforced with a continuity of normal stress, and the mass interfacial conditions were continuities of mass and mass flux. Two parameters were defined to characterize the mass transports in the fluid and porous regions. The porous Damkohler number is the ratio of consumption to diffusion of the substrates in the porous medium. The fluid Damkohler number is the ratio of the substrate consumption in the porous medium to the substrate convection in the fluid region. The concentration results were found to be well correlated by the use of a reaction-convection distance parameter, which incorporated the effects of axial distance, substrate consumption, and convection. The reactor efficiency reduced with reaction-convection distance parameter because of reduced reaction (or flux), and smaller local effectiveness factor due to the lower concentration in Michaelis-Menten type reactions. The reactor was more effective, and hence, more efficient with the smaller porous Damkohler number. The generalized results could find applications for the design of bioreactors with a porous wall.

Velocity measurements within confined turbulent jets: application to cardiovalvular regurgitation.

An expression for centerline mean velocity distributions for circular and noncircular confined turbulent jets has been obtained by assuming self-preservation of flow downstream of the jet potential core. It was assumed that the velocity decay was not only dependent on the streamwise distance x in terms of x/d, as in the case of free jets, but also on the ratio of the orifice diameter d to the confining pipe diameter D. To validate the expression and to determine the empirical constants, measurements of the centerline velocities within the confined jets issuing from different size circular orifices and various noncircular orifices of different shapes were conducted. The results indicate that the validity of the expression is restricted to d/D < or =0.25 and is weakly dependent on the particular orifice shape. It is suggested that, as for the case of free turbulent jets reported earlier, that this expression may be used potentially to predict the valvular lesion size or to estimate the volume of valvular regurgitation for confined jets provided the value of D, which corresponds to the "atrial diameter," is known or statistically available.

Experimental investigation of pulsatile flows in tubes.

Based on cam-piston-valve arrangement, a mechanical pulsatile flow generator is designed to investigate sinusoidal flow and other types of pulsatile flow in straight rigid tube. Measurement reveals the relation between pressure gradient and flow rate. Numerical simulation using the k-epsilon turbulence model are carried out to compare the pulsatile flow produced by the generator with a sinusoidal flow and a physiological flow in a rigid tube. The results show that the pulsatile flow generated has similar dynamic properties to the physiological flow. Hence, the present setup can be used for in-vitro investigation of biofluid phenomena.

Estimation of turbulent shear stress in free jets: application to valvular regurgitation.

In an attempt to better assess the severity of valvular regurgitation, an in-vitro experiment has been conducted to estimate turbulent shear stress levels within free jets issuing from different orifice shapes and sizes by means of hot-wire anemometry. On the basis of the measured mean velocities and the jet profiles, the distributions of the normalized kinematic turbulent shear stress (uv/Um2) were estimated for different jets by using an equation available for self-preserving circular jet. The results indicate that the equation can estimate the distributions of uv/Um2 independent of the orifice shape and Reynolds number of the jet. For the range of Reynolds numbers considered, the estimation of maximum turbulent shear stress inferred from these distributions suggests that the critical shear stress level of approximately 200 N/m2, corresponding to destruction of blood cells, is exceeded for typical blood flow velocity of 5 m/s at the valvular lesion.

Pressure/flow behaviour in collapsible tube subjected to forced downstream pressure fluctuations.

An experimental investigation has been made into the pressure/flow behaviour of a collapsible tube subjected to downstream pressure fluctuations. These downstream pressure waves are observed to be transmitted upstream beyond the point of collapse. The mean flow rate is not significantly affected by the amplitude or frequency of pressure fluctuations. However, the oscillatory flow amplitude is reduced at the higher frequency. The mean flow rate also remains independent of the mean driving pressure.