Axial mixing in pipe flows: turbulent and transition regions

In the present work, the flow pattern in pipe flow has been simulated using low Reynolds number k-ε model. The CFD model has been extended to simulate the axial dispersion phenomena in both the transition and turbulent regions. An extensive comparison of the predicted axial dispersion coefficient with the experimental data has been presented along with the predictions of various models published in the literature. The proposed CFD model for axial mixing was found to give an excellent agreement with the experimental measurements

CFD simulation of mixing and dispersion in bubble columns

Liquid phase mixing time was measured in 0.2 and 0.4 m i.d. columns over a wide range of superficial gas velocity (0.07-0.295 ms-1) and height-to-diameter ratio (1-10). A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The predictions agree favourably with all the experimental data published in the literature. Complete energy balance was established in all cases. The validated CFD model was extended for the simulation of the macro-mixing process by incorporating the effects of both the bulk motion and the eddy diffusion. Excellent agreement was observed between the CFD predicted and experimental values of the mixing time over the entire range of D, VG and HD/D covered. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (DL). The predicted values of DL were found to agree very well with all the experimental data published in the literature

Axial mixing in laminar pipe flows

A computational model has been developed to simulate the axial dispersion phenomena in laminar pipe flows. Peclet number (Pe=au<SUB>0</SUB>ID<SUB>m</SUB>) and dimensionless time (τ=D<SUB>m</SUB>t/a<SUP>2</SUP>) have been covered over a wide range of 1-10<SUP>5</SUP> and 10<SUP>−8</SUP> to 10<SUP>2</SUP>, respectively. An extensive comparison of predicted mean concentration profiles with the experimental data has been presented. The proposed computational model for concentration profiles was found to give an excellent agreement with the experimental measurements. Further, the results compare well with other analytical and experimental studies. The solution has been shown to be valid at both the short and long times and for all values of the Peclet number

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 (PG/V, εmax, τ̅S, τS, max, τ̅N and τN,max) and interfacial flow parameters ( ε̅G, a̲ and kLa̲) 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&lt;VG&lt;0.4-ms−1). 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

CFD Modeling of Gas-Liquid Bubbly Flow in Horizontal Pipes: Influence of Bubble Coalescence and Breakup

Modelling of gas-liquid bubbly flows is achieved by coupling a population balance equation with the three-dimensional, two-fluid, hydrodynamic model. For gas-liquid bubbly flows, an average bubble number density transport equation has been incorporated in the CFD code CFX 5.7 to describe the temporal and spatial evolution of the gas bubbles population. The coalescence and breakage effects of the gas bubbles are modeled. The coalescence by the random collision driven by turbulence and wake entrainment is considered, while for bubble breakage, the impact of turbulent eddies is considered. Local spatial variations of the gas volume fraction, interfacial area concentration, Sauter mean bubble diameter, and liquid velocity are compared against experimental data in a horizontal pipe, covering a range of gas (0.25 to 1.34 m/s) and liquid (3.74 to 5.1 m/s) superficial velocities and average volume fractions (4% to 21%). The predicted local variations are in good agreement with the experimental measurements reported in the literature. Furthermore, the development of the flow pattern was examined at three different axial locations of L/D = 25, 148, and 253. The first location is close to the entrance region where the flow is still developing, while the second and the third represent nearly fully developed bubbly flow patterns

CFD simulation of homogeneous reactions in turbulent pipe flows-Tubular non-catalytic reactors

An analysis of turbulent reactive flows in tubular non-catalytic reactors is presented for various reaction orders and rate constants. A CFD model has been developed to predict the flow pattern in pipe flow using low Reynolds number k-ε model. Particular emphasis is placed upon analyzing the phenomena near the wall region. The CFD model has been extended to simulate the axial dispersion phenomena in turbulent regions. Further, the CFD model has been extended to obtain changes in the radial and axial concentration distributions. For the case of thermally neutral reactions and the isothermal conditions, it was observed that the lower order reactions cause a more rapid decrease in the axial concentrations. The effect of Reynolds and Schmidt numbers on the conversion levels is also discussed

CFD simulation of sparger design and height to diameter ratio on gas hold-up profiles in bubble column reactors

In the present work, a two-dimensional CFD model has been developed for the prediction of flow pattern in bubble column reactors. The model has been validated using available experimental data and extended to simulate the effect of the sparger design and height to diameter ratio on radial gas hold-up profiles. The predictions were compared with experimental measurements for a 0.385 m i.d. bubble column. The complete energy balance was established in all the cases. The simulations were carried out for three different gas-liquid systems (air-water, air-aqueous solution of butanol and air-aqueous solution of carboxyl methyl cellulose). A comparison has been presented between the predicted and the experimental data over a wide range of superficial gas velocity and for three gas-liquid systems. In all these cases, the CFD model has been found to predict the variation of hold-up profiles with respect to the column height and the sparger design

CFD simulations of bubble column reactors: 1D, 2D and 3D approach

CFD simulations have been carried out for the predictions of flow pattern in bubble column reactors using 1D, 2D and 3D k-ε models. An attempt has been made to develop a complete correspondence between the operation of a real column and the simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. All the models showed good agreement with the experimental data for axial liquid velocity and the fractional gas hold-up profiles. However, as regards to eddy diffusivity, only the 3D model predictions agree closely with the experimental data. The CFD model has been extended for the estimation of an axial dispersion coefficient (DL) using 1D, 2D and 3D models. Excellent agreement was found only between the experimental values and the 3D predictions. The 1D and 2D simulations, however, yielded DL values, which were lower by 25-50%. For this, a mechanistic explanation has been provided

Liquid phase axial mixing in bubble columns operated at high pressures

Liquid phase axial mixing was measured in a 100 mm i.d. bubble column operated in the pressure range of 0.1-0.5 MPa. Water, ethanol and 1-butanol were used as the liquid phase and nitrogen as the gas phase. The temperature and superficial gas velocity were varied in the range of 298-323 K and 0.01-0.21 m/s, respectively. The axial dispersion coefficient increased with an increase in the gas density due to pressure. The temperature had surprisingly a small effect. A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (DL). The predictions of axial dispersion coefficient agree favorably with all the experimental data collected in this work as well as published in the literature. The model was extended for different gas-liquid systems. The predicted values of axial dispersion coefficient were found to agree very well with all the experimental data

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