Figure 1.
Experimental configuration for PIV measurement in spinner flask bioreactor.
A) The impeller consisted of a thin rectangular Teflon sheet and a magnetic stirring bar. The middle height of the magnetic rod was placed at the fluid level throughout the experiment. B) Top view of the impeller. The impeller rotates in counter-clockwise direction throughout the experiment. The spinner flask was placed inside a square tank filled with water to eliminate the lensing effect due to the flask curvature. The working fluid was seeded with 31 µm fluorescent particles and agitated by the impeller driven by a stepper motor. A cylindrical lens was used to create a laser sheet to illuminate the imaging region. The seeded flow was imaged with a high-speed camera (IDT Y4). C) In the meridional plane imaging configuration, the laser sheet was aligned vertically and placed at the center of the flask. D) For measurements in the azimuthal plane, the laser sheet was aligned to the horizontal plane and a mirror was placed underneath the setup to allow view access to the illuminated region.
Figure 2.
Azimuthal plane measurement locations.
A non-dimensional parameter, z/H, is used to show the location of the measurement, where z is the distance from the bottom wall and H is the height of the fluid free surface. In our experiment, H = 19 mm for 50 mL working fluid. A) Three measurements were conducted at z/H = 0.15, 0.50 and 0.85. Velocity, shear stress and vorticity analyses were conducted at each measurement. B) Two measurements were conducted near the bottom wall. A parabolic function was fitted between the measurements at z/H = 0.06 and z/H = 0.04. Then, the velocity gradient near the bottom wall was calculated.
Figure 3.
Velocity magnitude contour for flow in spinner flask in meridional plane.
The velocity increases with increasing agitation speed. Furthermore, the fluid accelerates radially outwards at highest magnitude behind the flat impeller.
Figure 4.
Vorticity evolution at 3 angular positions for 25, 35 and 45 RPM.
The main vortex slowly reduces its strength as the plane of measurement shifts from 93 degrees to 135 degrees.
Figure 5.
Shear stress distribution in azimuthal plane at varying phases.
Significant shear stress can be seen at the bottom wall, caused by a high velocity gradient due to the interaction of the fluid with the wall.
Figure 6.
Shear stress distribution for varying rotational phase.
A) Mean and B) maximum shear stresses. In both plots, the shear readings spike at 90 degrees, where the flat impeller is in the imaging plane. C) Histogram of shear stress for various speeds between 20 and 45 RPM. D) Shear stress distributions over meridional plane at two extreme cases; 20 RPM at 180 degrees and 45 RPM at 93 degrees.
Figure 7.
Maximum and 99th percentile shear stress plots in the meridional plane at varying spinning rates.
The plots increase linearly with the agitation rates. However, the shear stress magnitudes at 99th percentile are marginally lower than the maximum values at each speed.
Figure 8.
Flow visualization in azimuthal plane.
A) Velocity and B) shear stress contours at z/H = 0.15, 0.50 and 0.85. Highest velocity and shear stress magnitudes are achieved around the flat impeller region in the z/H = 0.85 plane. The farthest plane from the impeller shows homogeneous distribution flow characteristics in both velocity and shear plots.
Figure 9.
Plots of maximum shear stress when the bioreactor was spun at 20 to 45 RPM.
Although at each height the shear varies linearly with the spinning rates, the slopes differ from one to another.
Figure 10.
Results of fluid interaction at bottom surface of the flask.
A) The velocity distribution at 40 RPM at z/H = 0.04. B) Shear stress near the bottom surface is higher than other azimuthal measurements. C) The mean and maximum shear stress increase linearly to the agitation speed. D) Shear stress distribution is skewed to higher shear magnitude at increasing speed.
Figure 11.
Normalized cell count over culture period for static and suspension culture method.
A cell count achieved in the 20 RPM bioreactor was superior to the static culture over 7 days. The plot also shows that declining cell counts were obtained for 28 and 30 RPM suspension cell culture.