Characterization of Bubble Shapes in Non‐Newtonian Fluids by Parametric Equations

Based on experiments with single air bubbles rising in stagnant non-Newtonian fluids, an innovative model containing the aspect ratio (E) and two parameters (alpha, beta) was proposed and proved to be capable of characterizing the bubble shape from spherical/ellipsoidal to prolate/oblate-tear with good accuracy. Several impacts on bubble deformation were investigated, involving the rheological properties of the fluids and different forces exerted on the bubble, which were quantified by multiple dimensionless numbers (e.g., Reynolds, Eotvos, and Deborah number). Within a wide range, the empirical correlations were obtained for parameter beta, and between alpha and beta. Together with the shape model, a complete system was set up for bubble shape characterization and prediction that will provide new ideas for future studies on bubble hydrodynamics

Vaccine delivery by penetratin: mechanism of antigen presentation by dendritic cells

Cell-penetrating peptides (CPP) or membrane-translocating peptides such as penetratin from Antennapedia homeodomain or TAT from human immunodeficiency virus are useful vectors for the delivery of protein antigens or their cytotoxic (Tc) or helper (Th) T cell epitopes to antigen-presenting cells. Mice immunized with CPP containing immunogens elicit antigen-specific Tc and/or Th responses and could be protected from tumor challenges. In the present paper, we investigate the mechanism of class I and class II antigen presentation of ovalbumin covalently linked to penetratin (AntpOVA) by bone marrow-derived dendritic cells with the use of biochemical inhibitors of various pathways of antigen processing and presentation. Results from our study suggested that uptake of AntpOVA is via a combination of energy-independent (membrane fusion) and energy-dependent pathways (endocytosis). Once internalized by either mechanism, multiple tap-dependent or independent antigen presentation pathways are accessed while not completely dependent on proteasomal processing but involving proteolytic trimming in the ER and Golgi compartments. Our study provides an understanding on the mechanism of antigen presentation mediated by CPP and leads to greater insights into future development of vaccine formulations

CFD simulation of the flow field in a bubble column reactor: Importance of the drag force formulation to describe regime transitions

International audienceThis paper deals with the simulation of a quasi two-dimensional bubble column. In particular, the aim is to predict regime transitions occuring in bubble column reactors, which is very important for their design. It is shown that taking into account bubble–bubble interactions through a drag coefficient depending on the local void fraction [M. Simonnet, C. Gentric, E. Olmos, N. Midoux, Experimental determination of the drag coefficient in a swarm of bubbles, Chem. Eng. Sci. 62 (2007) 858–866] allows to predict the onset of the regime transitions. This has been verified both with the Eulerian and Lagrangian approaches. It is also shown that the use of this drag correlation allows to reproduce some typical characteristics of the different regimes (velocity profiles becoming parabolic in the transition regime, typical transient phenomena, accumulation of large bubbles in the column centre in the transition and heterogeneous regimes)

An experimental study of the frequency response of electrochemical sensors in nonhomogeneous flow in packed beds

International audienceCircular electrochemical probes were mounted flush with the wall of a packed bed operating either in single-phase liquid flow or in gas-liquid downflow. Frequency analysis of the measured diffusion current indicated that the recently determined mass transfer function of the probes is approximately valid, even under spatially nonhomogeneous flow conditions, provided that the relative fluctuation intensity of the current does not exceed 0.15. Correction of the current measurements in the frequency domain by means of the mass transfer function was necessary in order to determine the fluctuating characteristics of the velocity gradient

Experimental determination of the drag coefficient in a swarm of bubbles

International audienceSimultaneous measurements of liquid velocity by laser Doppler velocimetry and bubble velocity, diameter, and void fraction by a double optical probe are performed in a bubble column to study the influence of the void fraction on the relative velocity of a swarm of gas bubbles. Bubble diameters db vary from 2 to 10mm and local void fractions αloc can reach 35%. It is found that, for αloc < 15%, the relative bubble velocity is determined by the hindrance effect and consequently decreases with the void fraction. Beyond this critical value, the aspiration of bubbles in the wake of the leading ones dominates the hindrance effect and the relative velocity thus increases suddenly. The contribution of the bubble diameters to this evolution is also determined. Finally, a drag correlation, valid for the whole range of void fraction and for pure water-air systems, is proposed

The use of microelectrodes for the determination of flow regimes in a trickle-bed reactor

International audienceThe utilization of microelectrodes in a non-conducting wall with subsequent signal analysis allowed the determination of flow regime transitions: trickling/pulsing, trickling/dispersed bubble and dispersed bubble/pulsing in a trickle-bed reactor by the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) variations as a function of liquid and gas flow rates

Hydrodynamic study of a trickle-bed reactor by means of microelectrodes: analysis of the probability densities

International audienceThe technique of microelectrodes in a non-conducting wall was used to determine the flow regimes in a trickle-bed reactor and to characterize the wall wetting by the analysis of the Probability Density Functions (PDF). The identification of flow regime transitions (trickling/pulsing, trickling/dispersed and dispersed/pulsing) and the study of the wall wetting are carried out respectively by the analysis of the variations of the fluctuation rate of the local liquid/solid mass transfer coefficient and the wetting rate as functions of the gas and liquid flow rates