Chunying Zhu

The viscosity distribution around a rising bubble in shear-thinning non-newtonian fluids

The viscosity distribution of the liquid around a rising bubble in carboxymethyl cellulose (CMC) aqueous solutions was measured experimentally by particle image velocimetry (PIV). The effect of the concentration of CMC solutions on the viscosity distribution around a bubble and the coupling relations between the viscosity field, flow field and shear stress field were also studied. Results indicated that the specific viscosity (non-dimensionalized by η0) decreases with the increase in CMC solution concentration due to a shear thinning effect. Within the experimental range, similar viscosity distributions of liquids around a rising bubble were found: a hollow cylindrical low viscosity region around the bubble wake and a high viscosity region in the central bubble wake.

The Enhancement of CO2 Chemical Absorption by K2CO3 Aqueous Solution in the Presence of Activated Carbon Particles

Abstract The enhancement of chemical absorption of CO 2 by K 2 CO 3 /H 2 O absorbents in the presence of activated carbon (AC) particles was investigated. The results show that the gas absorption rates can be enhanced significantly in the presence of AC particles, and the maximum enhancement factor 3.7 was observed at low stirring intensities. The enhancement factor increased rapidly with the solid loading during the initial period of absorption and then became mild gradually to a maximum value. Both the liquid-solid contact area and the probability of solid particles residing at the gas-liquid interface decreased with the increase of the particle size, leading to a negative effect on the enhancement of mass transfer. The influence of the particles on gas absorption decreased with the reaction rate. The stirring speed changed die interfacial coverage and mass transfer rate on the liquid side and consequently affected the mass transfer between the gas and liquid phases; the enhancement factor decreased with the stirring intensity. A heterogeneous two-zone model was proposed for predicting the enhancement factor and the calculated results agreed well with the experimental data.

Measurement and Correlation of Pressure Drop for Gas-Liquid Two-phase Flow in Rectangular Microchannels

Abstract The pressure drop of gas-liquid two-phase flow in microchannel is of fundamental importance in heat and mass transfer processes. In this work, the pressure drop of gas-liquid two-phase flow in horizontal rectangular cross-section microchannels was measured by a pressure differential transducer system. Water, ethanol and n -propanol were used as liquid phase to study the effects of capillary number on pressure drop; air was used as the gas phase. Four microchannels with various dimensions of 100 μm × 200 μm, 100 μm × 400 μm, 100 μm × 800 μm and 100 μm × 2000 μm (depth × width) were used for determining the influence of configuration on the pressure drop. Experimental results showed that in micro-scale, the capillary number also affected the pressure drop remarkably, and in spite of only one-fold difference in aspect ratio, the variation of pressure drop reached up to near three times under the same experimental conditions. Taking the effects of aspect ratio and surface tension into account, a modified correlation for Chisholm parameter C in the Chisholm model was proposed for predicting the frictional multiplier ϕ 2 l , and the predicted values by the proposed correlation showed a satisfactory agreement with experimental data.

Magnetofluidic control of the breakup of ferrofluid droplets in a microfluidic Y-junction

This paper is mainly focused on the investigation of the magnetofluidic control of the breakup of ferrofluid droplets in a symmetric Y-junction. The asymmetric breakup of the ferrofluid droplet or non-breakup with filtering the mother droplet into a desired branch to separate it from the satellite droplet was implemented by an external magnetic field. The breakup processes of ferrofluid droplets with and without the magnetic field were studied systematically. The influences of both the flow rate ratio between the continuous phase and dispersed phase and the magnetic flux density on the sizes of daughter droplets were determined. It was found that the attractive magnetic force shifted the mass center of mother droplet in the upstream main channel, which accordingly facilitated the asymmetric breakup of the droplet at the downstream Y-junction. A power function correlation for precisely predicting the sizes of daughter droplets was proposed by introducing the magnetic Bond number (Bom). Moreover, we also found that the controllable magnetic force could promote the pattern transition between the breakup and non-breakup of ferrofluid droplets.

The Effect of Hydrophobic Modification of Zeolites on CO2 Absorption Enhancement

Two methods of the modification of zeolite were employed: framework element modification and surface coating, and the influence of the zeolites before and after modification on the CO2 absorption was investigated. It was found that although hydrophobicity of zeolite could be obtained by means of the surficial organic coating in the method of surface coating modification, partial channel of zeolite would be plugged, as a result, leading to the surface area reducing greatly. Distinctively, the framework element modification method could maintain not only complete lattice structure and adsorption capability of zeolite, but would also obtain a good hydrophobic property. Consequently, significant enhancement on gas absorption by this modified zeolite was achieved and up to a maximum enhancement factor of 2.62. This shows that the solid particles with good enhancement role to gas absorption need not only good adsorptive capability but also certain hydrophobicity. An unsteady heterogeneous model was employed to predict enhancement factor and the calculated results agree well with the experimental data.

Bubble Formation in Non-Newtonian Fluids Using Laser Image Measurement System

Abstract A self-developed laser image measurement system was established to study the behavior of bubble formation at a single orifice in non-Newtonian polyacrylamide (PAAm) solutions. Images of bubbles were captured by a CCD camera and volumes of bubbles were digitally analyzed online. The effects of rheological property of PAAm solution, orifice, reservoir, and gas flowrate on bubble formation were studied experimentally. It is found that the volume of bubble increases with the concentration of PAAm solution, the diameter of the orifice, and the gas flowrate, respectively, whereas little effect of reservoir is observed in experiments.

Dynamics of bubble breakup at a T junction.

The gas-liquid interfacial dynamics of bubble breakup in a T junction was investigated. Four regimes were observed for a bubble passing through the T junction. It was identified by the stop flow that a critical width of the bubble neck existed: if the minimum width of the bubble neck was less than the critical value, the breakup was irreversible and fast; while if the minimum width of the bubble neck was larger than the critical value, the breakup was reversible and slow. The fast breakup was driven by the surface tension and liquid inertia and is independent of the operating conditions. The minimum width of the bubble neck could be scaled with the remaining time as a power law with an exponent of 0.22 in the beginning and of 0.5 approaching the final fast pinch-off. The slow breakup was driven by the continuous phase and the gas-liquid interface was in the equilibrium stage. Before the appearance of the tunnel, the width of the depression region could be scaled with the time as a power law with an exponent of 0.75; while after that, the width of the depression was a logarithmic function with the time.

The Drag Coefficient and the Shape for a Single Bubble Rising in Non-Newtonian Fluids

The dynamical characteristic of a single bubble rising in non-Newtonian fluid was investigated experimentally. The bubble as-pect ratio and rising velocity were measured by high speed cam-era. The shape regimes for bubbles in non-Newtonian fluids wasplotted by means of Reynolds number Re, Eo¨tvo¨s number Eo andMorton number Mo. The effects of bubble shape and liquid rheo-logical property on the total bubble drag coefficient were studied.A new empirical drag coefficient correlation covering sphericalbubble and deformed bubble was proposed, the predicted resultsshows good conformity to experimental values over a wide rangeof 0.05<Re<300. [DOI: 10.1115/1.4007073]Keywords: bubble, drag coefficient, non-Newtonian fluids, bubbleshape

Towards the Mechanism of Mass Transfer of a Single Bubble

This paper focuses on the mechanism of interfacial mass transfer of a single bubble, based on the chemical potential driving force, an approach for calculating interfacial concentration in practical process is proposed. The absorption processes of bubble under both quiescent and mobile conditions are analyzed and discussed respectively. For a stationary absorption, only in the case of liquid bulk concentration near saturated value, the interfacial concentration could close to the equilibrium value; For a moving bubble, under ordinary operating condition (Yo>1), the interfacial concentration is far from its equilibrium. Only under bulk concentration near saturated value or a smaller Yo(Yo<0.1) which may involve the complication of additional resistance at interface induced by surface contamination or surfactant added, the interfacial concentration could be approximate to equilibrium value. The interfacial concentration close to the interface on liquid side for a single CO2 bubble absorbed by methanol is measured using a modern optical instrumentation in which the laser holographic interference method is adopted with a real-time and amplification technique. Experimental results show that the interfacial concentration decreases significantly with increasing Re and is far from the equilibrium one in a larger Re range. Experiments validate the proposed model.

Removal of SO2 using ammonium bicarbonate aqueous solution as absorbent in a bubble column reactor

In this work, the removal of SO2 from gas mixture with air and SO2 by ammonium bicarbonate aqueous solution as absorbent was investigated experimentally in a bubble column reactor. The effects of the concentration of ammonium bicarbonate, the SO2 inlet concentration of gas phase and the gas flow rate on the removal rate of SO2 were studied. The results showed that the higher the SO2 inlet concentration and the gas flow rate, the shorter the lasting time of SO2 completely removed in gas outlet, and then the faster the decrease in the removal rate of SO2. The lasting time of SO2 completely removed in gas outlet increased with increasing ammonium bicarbonate concentration. During the process of SO2 absorption, there was a critical pH of solution. When the solution pH was less than the critical pH, it would sharply fall, resulting in a rapid decrease of the SO2 removal rate. A theoretical model for predicting the SO2 removal rate has been developed by taking the chemical enhancement and the sulfite concentration in the liquid phase into account simultaneously.