Computational fluid dynamic modelling of flow patterns in an oscillatory baffled column

Numerical simulations to date within the context of oscillatory flow in a baffled column have been limited to flows in a symmetrical regime, i.e. eddies are generated symmetrical to the central line of the column where the oscillatory Reynolds numbers are below 400. In this paper, 3-D computational fluid dynamic (CFD) simulation of flow patterns of oscillatory flow in a baffled column has, for the first time, been carried out and the results extended to all regimes of oscillatory Reynolds numbers covering from symmetric to asymmetric flows. The flow patterns simulated have also been validated by both direct flow visualisation and by digital particle image velocimetry measurements. The success of such CFD simulations opens doors for many potential studies, from optimisation of geometry for plug flow to suspension of particles, and from droplet breakage and coalescence to mass/heat transfer of particles.     

Characterisation of flexible baffles in an oscillatory baffled column

In our previous studies the volume‐averaged strain rates, of 2–22 s^?1, were obtained in an oscillatory baffled column (OBC) based on velocity measurements over a half baffled cell for oscillatory Reynolds numbers of 1000–4030. These values are very low compared with those in a traditional stirred tank vessel (at least >100 s^?1) for similar operational conditions and the same power consumption. It was also observed that the volume‐averaged strain rates in the OBC fluctuated with the phase of oscillation over any cycle, with high values coinciding with eddy generation, and low values with eddy cessation. The objective of this study is to show that such fluctuations can be attenuated by employing innovative flexible baffles, whose inside edges move with the fluid oscillation. In this paper experimental measurements of velocity vector maps and strain rate distributions using Digital Particle Image Velocimetry (DPIV) are presented for both conventional and flexible baffles in an OBC. Mixing characterisation, in terms of axial dispersion coefficients, are compared for both baffle designs. The results show that the flexible baffles can reduce the fluctuations and magnitudes of the volume‐averaged strain rates in the OBC without compromising the mixing performance. Low and uniform strain rate distributions in time and space are essential biochemical, biomedical and pharmaceutical applications where shear sensitive cultures are involved. © 2001 Society of Chemical Industry     

Characterization and optimization of an oscillatory baffled reactor (OBR) for ozone-water mass transfer

Ozone-water mass transfer was investigated using an oscillatory baffled reactor (OBR) operated as a semi-batch and as a co-current up flow continuous reactor. The effects of input ozone concentration, input gas and water flow rates, and oscillation conditions on gas hold up, volumetric mass transfer coefficient and mass transfer efficiency were determined. The same reactor was operated as a baffled column (without oscillation) and as a bubble column to assess the effect of the reactor arrangement on the mass transfer. The results show that the OBR was 5 and 3 times more efficient for ozone-water mass transfer than the baffled and bubble columns, respectively. The enhancement obtained with OBR over the baffled column reactor was found to decrease with gas flow rate due to changes in bubble flow pattern from homogenous to heterogeneous. Under continuous flow conditions, the performance of the baffled reactor and the OBR were found to be twice efficient for ozone-water mass transfer than when operating under semi-batch conditions. The mass transfer effeciency (MTE) was found to increase from 57% using the baffled reactor to 92% with OBR under continuous flow at water and gas superficial velocities of 0.3 and 3.4 cm s⁻¹, respectively.     

Numerical study of the flow pattern and heat transfer enhancement in oscillatory baffled reactors with helical coil inserts

Oscillatory baffled reactors (OBRs) are a means of process intensification as they allow processes with long residence time to be converted from batch to continuous processing. Helically baffled OBRs have only been developed at “mesoscale” so far, but at this scale have displayed significant advantages in terms of the increased range of conditions over which plug flow is achieved. Scale-up studies are underway to determine whether this is replicated at larger scales. This paper reports fluid mechanical modeling of a helically baffled oscillatory flow for the first time. Time-dependent flow structures induced in tubular reactors have been analyzed on the basis of periodic, laminar flow numerical simulation. A reversing swirled core flow and its interaction with the unsteady mechanism of vortex shedding downstream of the wires has been described. This has allowed greater understanding of the flow structures, which will underpin optimal design and scale-up. The potential for heat transfer enhancement is discussed, considering the compound effect of oscillatory motion and helical coil inserts. The results show that the heat transfer for the helical baffled tube could be enhanced by a factor of 4 compared to a smooth tube in the tested range of oscillation conditions.     

On the evaluation of droplet breakage and coalescence rates in an oscillatory baffled reactor

A numerical model is developed to evaluate droplet breakage and coalescence rates in a continuous oscillatory baffled reactor (OBR). The structure of the model is similar to that of the population balance approach, but concentrates on droplet interactions with simpler assumptions of a functional form for breakage and coalescence rate constants. The OBR is a relatively new reactor technology and offers enhanced and more uniform mixing than traditional reactors, making some model assumptions more closer to the reality. In this paper, we present the development, robustness and validation of the modelling process together with the predicted results for the OBR system.     

Role of hydrodynamic flow parameters in lipase deactivation in bubble column reactor

The dynamic environment within the bioreactor and in the purification equipment is known to affect the activity and yield of enzyme production. In the present work, the effect of hydrodynamic flow parameters and τ_N,max) and interfacial flow parameters ( and ) on the activity of lipase has been comprehensively investigated in bubble column reactors. Lipase solution was subjected to hydrodynamic flow parameters in 0.15 and 0.385 m i.d. bubble column reactors over a wide range of superficial gas velocity (0.01<V_G<0.4-). The flow parameters were estimated using an in-house CFD simulation code based on k-ε approach. The extent of lipase deactivation in both the columns was found to increase with an increase in hydrodynamic and interfacial flow parameters. However, at equal value of any of these parameters, the extent of deactivation was different in the two columns. The rate of deactivation was found to follow first order kinetics. An attempt has been made to develop rational correlations for the extent of deactivation as well as for the deactivation constant. The rate of deactivation was found to be depending on the average turbulent normal stress and interfacial flow parameters such as bubble diameter and bubble rise velocity.     

On the development of flow pattern in a bubble column reactor: Experiments and CFD

A comprehensive analysis of the development of flow pattern in a bubble column reactor is presented here through extensive LDA measurements and CFD predictions. In the LDA measurements, the simultaneous measurements of 2D velocity-time data were carried out at several radial locations and many axial cross-sections of the column for two different spargers. The profiles of mean axial liquid velocity, fractional gas hold-up and bubble slip velocity showed excellent agreement between the predictions and the experimentally measured values. The experimental results showed that the mean tangential velocity varies systematically in the radial as well as along the axial co-ordinates. The turbulence parameters viz. turbulent kinetic energy, energy dissipation rate and eddy diffusivity were also analysed. The estimated values of local energy dissipation rate obtained using eddy isolation model were used for establishing the energy balance in the column. The experimental data were used for the estimation of normal and shear stress profiles. For the case of single point sparger, just above the sparger region, the bubble plume was seen to have a strong tangential component of motion thereby yielding higher gas hold-up slightly away from the centre. This visual observation was well captured in profiles of all the hydrodynamic parameters obtained from the experimental data. CFD simulations of the mean velocities, gas hold-up and turbulent kinetic energy compared well with the experimental results.     

Characterisation of fluid mixing in novel designs of mesoscale oscillatory baffled reactors operating at low flow rates (0.3-0.6 ml/min)

For the first time two mesoscale oscillatory baffled designs (central and integral baffles with their volumes of 5.2 ml and 4.4 ml, respectively) were experimentally characterised at net flow rates as low as 0.3 ml/min (Re_n ∼ 1.25), giving a residence time of around 15-17 min over a wide range of oscillation conditions. The purpose was to identify the lower limits of operability, thereby determining the maximum residence time per unit reactor volume for these mesoscale units. The characteristics of fluid flow were found to be strongly affected by Strouhal number at these low net flows. For the integral baffles, the oscillation conditions exhibited little influence on the fluid mixing. For the central baffles, there were three distinct flow regimes, depending on Strouhal numbers which affect the fluid characteristics differently. At two regimes of Sts, St ≥ 0.8 and 0.13 ≤ St ≤ 0.2, an increase in frequency did not alter the axial dispersion. At St ≥ 0.8, the fluid experienced less uniform mixing, representing by right-skewed residence time distribution (RTD) curves. At 0.20 ≤ St ≤ 0.13, the fluid mixing was significantly improved, indicated by narrow and symmetrical RTD curves with Re_o up to 700. At 0.4 ≤ St ≤ 0.27 and St ≤ 0.1, the degree of plug flow was a function of Re_o. The maximum number of tanks achieved at these low flow rates was in the range 30-35, occurring at a velocity ratio (Re_o/Re_n) of 39-40.     

On the effect of gap size between baffle outer diameter and tube inner diameter on the mixing characteristics in an oscillatory‐baffled column

We report our experimental investigation on the effect of gaps between baffle outer diameter and inner tube diameter on the mixing characteristics, in terms of mixing time and axial dispersion coefficient, in a batch oscillatory‐baffled column. Local concentration profiles are measured using conductivity probes at two locations along the height of the column. The mixing time was determined based on the equilibrium concentration concept, and the axial dispersion coefficient was obtained by solving the axial dispersion governing equation. Comparison of mixing time between the ‘push‐fit’ and ‘loose‐fit’ baffle arrangements was carried out and the results showed that the existence of a gap of various sizes between the baffle outer and the tube diameters lengthened the time at which the state of uniform mixing is achieved in such a device. © 1999 Society of Chemical Industry     

进气管内气体分布对塔内气体分布影响探究

在填料塔大型化、薄层化的趋势下,塔内气体初始分布显得尤为重要.因此气体分布器结构以及流场研究也成为人们研究的热点.针对某些大型塔塔径过大,气体分布器结构设计安装均困难的问题,利用计算流体力学(CFD)软件模拟的方法,探索性地对进气管内气体分布对塔内气体分布影响作了初步研究.结果表明:比较合理的进气管内构件对塔内气体分布...     

Effect of surfactant on homogeneous regime stability in bubble column

Experiments were carried out to investigate the effect a surface active agent on homogeneous-heterogeneous flow regime transition in a laboratory scale bubble column. Air and water with various amount of CaCl₂ were the phases. The (voidage e) - (gas flow rate q) dependence was measured. The critical point where the homogeneous regime loses stability and the transition begins was evaluated by several methods. These methods are based on the slip speed concept and the drift flux model. The critical values of voidage and gas flow rate were taken as the quantitative measures of the homogeneous regime stability. They were plotted against the surfactant concentration. It was found that the surfactant has a dual effect on both the voidage and the regime transition: low concentration stabilizes and larger concentration destabilizes the homogeneous bubble bed. At present, we do not have an explanation to these observations. Possible physical mechanisms of the surfactant effect are expected to be revealed by further experiments, which are currently under way.     

A Langrangian Study of Solids Suspension in a Stirred Vessel by Positron Emission Particle Tracking (PEPT)

A technique of Positron Emission Particle Tracking (PEPT) was used to obtain information on the flow behavior of coarse particles suspended in pseudoplastic liquids agitated by axial‐hydrofoil Lightnin impellers A320 and A410. PEPT enables the position of a 600 μm radioactive particle tracer inserted inside one of the suspended particles to be detected many times per second and its full trajectory followed inside the vessel. Particle trajectory analysis yielded information on particle circulation, velocity distribution, and spatial occupancy. The minimum speed for complete particle suspension, N_js, was also determined. The well‐known Zwietering correlation failed to predict the measurements by a substantial margin, suggesting that it is inadequate for viscous non‐Newtonian liquids.     

Effect mechanism of multiple obstacles on non-Newtonian flow in ceramic 3D printing (arcuate elements)

Direct ink writing (DIW), as an additive manufacturing method, can effectively mould ceramic parts. The single screw extruder is used here to extrude viscous SiC slurry. The deposits caused by the low viscosity and the agglomerations resulting from the nonuniform mixing, form the obstacles in the channel, which affect the normal theoretical flow of the slurry, and interaction with other printing parameters. Therefore, it is necessary to explore the effect mechanism of the obstacles on the flow. The obstacles are always irregular, which makes it difficult to directly analyse them. The irregular geometries are always composed of linear and/or arcuate elements; therefore, the obstacles can be simplified into regular geometries. In the previous work, linear elements have been analysed first. As the continuous work, arcuate elements are investigated in the current research. To conduct the required simulations, an improved MRT LBM (multiple relaxation time lattice Boltzmann method) with a pseudo external force is proposed. The above numerical method is combined with the rheological model to analyse cases with two obstacles, and the obtained results are used to reveal the general mechanism in cases with multiple obstacles. The results show that the central angles, radii, and positions of the obstacles are important factors affecting the flow. To obtain a stable and controllable slurry flow, it is recommended that the central angle and the radius should be small enough. The number of obstacles should be minimized, and the position of the last obstacle is expected to be far away from the outlet to avoid the negative velocity. In addition, the adjacent obstacles should maintain a certain distance to ensure the full development of the vortex and avoid affecting the following obstacles or back vortices.     

Effect mechanism of multiple obstacles on non-Newtonian flow in ceramics 3D printing (linear elements)

Direct ink writing (DIW) provides a new way to mould ceramic parts. When a single screw extruder is used to extrude SiC slurry, the deposits caused by low viscosity and the agglomerations resulting from the nonuniform mixing, form the obstacles in the channel, which affect the normal flow of the slurry, theoretical outlet velocity, and interaction with other printing parameters. Therefore, it is necessary to study the mechanism responsible for the effects of the obstacles on the flow. The obstacles are always irregular, which makes it difficult to directly analyse them. Irregular geometries are always composed of linear and/or arcuate elements; therefore, the obstacles can be simplified into regular geometries. In the present work, linear elements are analysed first. Then, an improved MRT LBM (multiple relaxation time lattice Boltzmann method) with a pseudo external force is proposed for the flow analysis. The improved MRT LBM is combined with rheological test data to investigate cases with two obstacles, and the results are applied to reveal the general mechanism in cases with multiple obstacles. The results show that the angles, sizes, and positions of the obstacles are three important factors influencing the flow. To obtain a stable and controllable slurry flow, it is recommended that the first angle θ₁ be an acute angle. In addition, the number of obstacles should be minimized, and the position of the last obstacle should be far away from the outlet.     

入口流速对间断微通道内流型演变及流量分配特性影响

微通道内气液两相流动规律是影响微通道换热器换热系数和流场温度均匀性的主要因素。以N2和H2O为工质，对间断、并联矩形微通道换热器内气液两相流型的起始、发展、稳定过程的演化以及并联通道内流量分配不均匀特性进行了数值模拟研究。结果表明，不同的进口Re对微通道内流型的演变过程和流动周期有重要影响，当进口Re为450时，气相工质在均流腔内以离散的散团状形态脉动扩散至微通道中，并联通道内气相工质从弹状流型态逐渐转变为泡状流型态；当进口Re增至为1600时，气相工质在均流腔内以较连续的椭圆状型态扩散至微通道中，并联通道内气相工质从环状流型态逐渐转变为泡状流型态。通道结构还将影响并联通道间的流量分配的均匀性，间断微腔的存在使微通道内工质质量流量分布均匀性提升38.7%，通过研究通道内压力分布规律，发现通道内静压的分布不均匀是导致两相工质从均流腔进入微通道时发生不均匀分配的重要原因。     

Effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column

An experimental investigation is reported on the effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column. Different combinations of 1 and 3 mm Rayon fibers are used to simulate different fiber length distributions. At a constant total fiber mass fraction, gas holdup generally decreases with increasing mass fraction of the 3 mm Rayon fiber while other conditions remain constant. Crowding factors estimated using four different methods (N_c=N_c,A, , N_c,L, and N_c,M) and the parameters and are tested on their performance to quantify the overall effects of fiber mass fraction and fiber length and its distribution on gas holdup. and provide the best characterization of the fiber effects on gas holdup in the cocurrent air-water-fiber bubble column. The crowding factor estimated using the model-based average fiber length (N_c,M) also provides a good characterization and is better than the other crowding factor definitions.     

Effect of Low Temperatures on the Performance of an Anaerobic Baffled Reactor (ABR)

The effect of a decrease in operating temperature on the performance of two 10 dm³ anaerobic baffled reactors (ABR) was examined in terms of steady state chemical oxygen demand (COD) removal efficiency. To minimise variations, and have a totally biodegradable feed, a synthetic carbohydrate (sucrose)–protein (meat extract) substrate was used. The reactors were operated at 20 h hydraulic retention time (HRT), 4 g dm⁻³ COD, and 35°C as a base-line condition. Because of their different histories, the reactors responded differently to a decrease in operating temperature to 25°C. Reactor 1 remained stable at 97% COD removal, whereas Reactor 2 decreased to 93% removal, but rose to 97% after adding an effluent recycle of 0·25. At 15°C, the efficiency of Reactor 1 dropped to 75%, while the removal of Reactor 2 declined to 83%, and no improvement in efficiency occurred with an effluent recycle at 0·25. At 25°C, the decreased rate of catabolism of the slow-growing syntrophs and methanogens resulted in a shift of the volatile fatty acids (VFA) peak to the second compartment. However, the biomass present in the reactor prevented VFAs breaking through in the effluent. Nevertheless, at 15°C VFAs were present in the effluent, perhaps due to the lower rates of metabolism and an increase in the K_s for VFAs. Finally, at 15°C part of the increase in the effluent COD was due to the enhanced production of soluble microbial products (SMP), or a decrease in their metabolism, with these compounds constituting some 10% of the inlet COD. © 1997 SCI.     

Effect of large amplitude oscillatory shear (LAOS) on the dielectric response of 1,4-cis-polyisoprene

The dielectric response of 1,4-cis-polyisoprene under applied mechanical oscillatory shear with various shear amplitudes was investigated. For this purpose, a special setup was constructed which enables to measure dielectric spectra under the influence of large amplitude oscillatory shear (LAOS); the setup is explained in detail. A strong influence of the shear amplitude on the dielectric relaxation strength was observed if the dielectric normal mode was mechanically affected. With increasing amplitude the relaxation strength decreased while the mean relaxation time, the width and asymmetry basically remained unchanged. We interpret this process as an orientational phenomenon of the end-to-end-vectors which results in a decreasing fluctuation amplitude of the polarization fluctuations.     

Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

Electrical resistance tomography (ERT) for flow and velocity profile measurement of a single phase liquid in a horizontal pipe

Electrical resistance tomography (ERT) is a novel, simple, and robust method of process imaging which uses non-invasive sensors located on the periphery of vessels to reconstruct a cross-sectional image of the vessel interior. This method of imaging when conducted on two adjacent planes on a pipe provides the ability to extract flow information. Previous studies have investigated this application on multi-phase flows in which the secondary phase provided the required pulse conductivity variation. In these studies the velocity profile and flow regime were identified using the common cross-correlation technique.