Hydrodynamics of orbital shaken bioreactors

Be it to aerate a glass of wine before tasting, to accelerate a chemical reaction or to cultivate cells in suspension, the “swirling” (or orbital shaking) of a container ensures good mixing and gas exchange in a simple and intuitive way. Despite being used in such a large variety of applications, the flow generated in a container subject to orbital shaking is far from be- ing understood, and presents a richness of patterns and behaviours which has not yet been reported. While orbital shaken cell cultures are very efficient and productive at small scale, their increase in scale is hampered by several issues, some of which are thought to have their origin in the motion of the liquid medium. The present research remedies to this situation, charting the evolution of the wave behaviour with the operating parameters, highlighting the importance of the wave regimes and assessing their mixing efficiency. We present here a mathematical solution, based on the potential hypothesis and on techniques used in sloshing dynamics, predicting the shape of the free surface and the liquid motion. The validity and the limits of this model were assessed by comparison with a very large number of free surface measurement, obtained using a specifically developed automated acquisition system, and with non intrusive velocity measurements of several shaking configurations. A large variety of wave patterns (i.e. free surface shapes) were identified, ranging from single and multiple crested waves to breaking waves and waves having a shape constantly changing as they rotate. Our research revealed the importance of free surface natural modes and their sub-harmonics in the behaviour of the waves. From the results of the potential model and the measurements, we identified four dimensionless groups ensuring hydrodynamic similarity of the flow be- tween different scales. Moreover, we were able to identify the most efficient waves in terms of mixing, and to suggest optimal ranges of the operating parameters to enhance the mixing and oxygenation of the cell cultures

Surface wave dynamics in orbital shaken cylindrical containers

Be it to aerate a glass of wine before tasting, to accelerate a chemical reaction or to cultivate cells in suspension, the "swirling" (or orbital shaking) of a container ensures good mixing and gas exchange in an efficient and simple way. Despite being used in a large range of applications this intuitive motion is far from being understood and presents a richness of patterns and behaviors which has not yet been reported. The present research charts the evolution of the waves with the operating parameters identifying a large variety of patterns, ranging from single and multiple crested waves to breaking waves. Free surface and velocity fields measurements are compared to a potential sloshing model, highlighting the existence of various flow regimes. Our research assesses the importance of the modal response of the shaken liquids, laying the foundations for a rigorous mixing optimization of the orbital agitation in its applications. Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Physics of Fluids 26, 052104 (2014) and may be found at http://dx.doi.org/10.1063/1.4874612Comment: 19 pages, 7 figure

Hydrodynamic stress in orbitally shaken bioreactors

Orbitally shaken bioreactors (OSRs) of nominal volume from 50 mL to 2’000 L have been developed for the cultivation of suspension-adapted mammalian cells. Here we study the hydrodynamics of OSRs for mammalian cells. The results are expected to allow the determination of key parameters for cell cultivation conditions and will facilitate the scale-up of OSRs

On the hysteresis of cavitation incipience and desinence in hydraulic machines

The omnipresent vortical structures in hydraulic machines are extremely prone to the occurrence of cavitation. It is well known that besides the flow parameters, the incipience, development, and disappearance of cavitation within a vortex is very sensitive to the gas content. It is also known that the pressure threshold for vortex cavitation desinence may be significantly higher than that of its incipience. This hysteresis, which is not yet well understood, is the scope of the current work. The case study is made of an elliptical NACA 16020 hydrofoil, placed in the test section of EPFL high-speed cavitation tunnel. We have observed the inception and the desinence of tip vortex cavitation (TVC) for different flow conditions and gas contents. We found that the pressure threshold for the TVC desinence increases with the dissolved gas content. We have also found that this pressure threshold strongly depends on the flow parameters and may reach atmospheric pressure for specific conditions. We argue that the persistence of a cavity at pressure levels higher than the vapor pressure is due to an outgassing process that sucks air from of the surrounding supersaturated liquid to feed the cavity. The gas diffusion is likely enhanced when a laminar separation of the boundary layer is formed at the tip of the hydrofoil on the suction side

Experimental Investigation on Transient Pressure Characteristics in a Helico-Axial Multiphase Pump

In oil and gas exploitation, the multiphase pump is a vital piece of equipment to guarantee production with high efficiency and stable operation. The complicated pattern of multiphase flow in the multiphase pump affects the pump performance; for this reason, the multiphase performance and the inner flow should be sufficiently investigated. In the present work, a liquid-gas multiphase pump is designed and manufactured, and then tested in a specialized test rig to reveal the transient pressure characteristics of the multiphase pump. Results show that the dominant frequency under water and water-air conditions is the blade passing frequency, which is induced by the rotor stator interaction. In the downstream of the impeller, the pressure fluctuation is obviously weakened, because the splitter-blade design could improve the control ability of flow pattern in the downstream region. In comparison with water condition, the pressure fluctuation of water-air condition greatly increases, and the multiples of impeller rotating frequency are stimulated due to the movement and merging of air bubbles. Finally, the correlation of transient pressure and phase distribution in impeller is revealed by using a high-speed camera. With the gradual pressure rise from impeller inlet to outlet, the relative movement and separation of two phases induce violent pressure fluctuations

Suppressing tip vortex cavitation by winglets

Despite the numerous remedies prescribed so far, tip vortex cavitation (TVC) remains a major issue in design and operation of diverse applications. In this paper, we experimentally investigate the effectiveness of winglets in suppressing TVC. An elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at +/- 45 degrees and +/- 90 degrees dihedral angles. To better focus on the physics of the problem, we have intentionally avoided any optimization on the geometries and our winglets are only smooth non-planar extensions of the original cross-section. Modifying no more than 3.7% of the lifting surface, lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not substantially different from the baseline. Nevertheless, cavitation inception-desinence tests reveal that undeniable advantages are achieved by the winglets in TVC alleviation. It is found that the 10%-bent 90 degrees winglets are more effective than the 45 degrees cases, with - 90 degrees (bent down toward the pressure side) performing superior to + 90 degrees. For instance, the 90 degrees-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14 degrees incidence angle. In addition, the study on the bending length effect conducted for the 90 degrees configurations shows that the 5%-bent winglets are not as striking as the 10% ones. Employing Stereo-PIV technique, the influence of winglets on non-cavitating flow structures is examined. For the most effective winglet (10%-bent 90 degrees-downward), we observe that the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC

On the physical mechanism of tip vortex cavitation hysteresis

Inception and desinence thresholds of tip vortex cavitation (TVC), generated by an elliptical NACA 16-020 hydrofoil, are measured at different flow conditions for various gas contents. It is observed that TVC often disappears at cavitation indices significantly higher than the inception thresholds introducing large hystereses. Our measurements reveal that TVC desinence pressure increases with gas content and, under specific flow conditions, may reach to atmospheric pressure. When the pressure of the cavitating core is below the initial saturation pressure of the dissolved gases, water flowing adjacent to the interface becomes supersaturated, which leads to the diffusion of air molecules into TVC. To estimate the outgassing rate, a simple diffusion model is proposed and analytically solved. In addition, we demonstrate that the extent of the delay in desinence due to outgassing is also dictated by the bulk flow parameters, i.e., the incidence angle and freestream velocity. Owing to flow visualizations, we assert that formation of a laminar separation bubble of appropriate size and shape at the hydrofoil tip is a necessary condition for a delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core by forcing the incoming flow to wrap around the axis. By roughening the hydrofoil tip, we demonstrate that the hysteresis is completely suppressed once the laminar separation bubble is destroyed. Moreover, our velocity measurements show that at near-wake, the incidence angle associated with delayed desinence is accompanied by a jet-like axial velocity profile while a wake-like profile is observed for the low-hysteresis case

Efficient and reproducible mammalian cell bioprocesses without probes and controllers?

Bioprocesses for recombinant protein production with mammalian cells are typically controlled for several physicochemical parameters including the pH and dissolved oxygen concentration (DO) of the culture medium. Here we studied whether these controls are necessary for efficient and reproducible bioprocesses in an orbitally shaken bioreactor (OSR). Mixing, gas transfer, and volumetric power consumption (P(V)) were determined in both a 5-L OSR and a 3-L stirred-tank bioreactor (STR). The two cultivation systems had a similar mixing intensity, but the STR had a lower volumetric mass transfer coefficient of oxygen (k(L)a) and a higher P(V) than the OSR. Recombinant CHO cell lines expressing either tumor necrosis factor receptor as an Fc fusion protein (TNFR:Fc) or an anti-RhesusD monoclonal antibody were cultivated in the two systems. The 5-L OSR was operated in an incubator shaker with 5% CO(2) in the gas environment but without pH and DO control whereas the STR was operated with or without pH and DO control. Higher cell densities and recombinant protein titers were obtained in the OSR as compared to both the controlled and the non-controlled STRs. To test the reproducibility of a bioprocess in a non-controlled OSR, the two CHO cell lines were each cultivated in parallel in six 5-L OSRs. Similar cell densities, cell viabilities, and recombinant protein titers along with similar pH and DO profiles were achieved in each group of replicates. Our study demonstrated that bioprocesses can be performed in OSRs without pH or DO control in a highly reproducible manner, at least at the scale of operation studied here