A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.     

Verification and validation of CFD simulations for local flow dynamics in a draft tube airlift bioreactor

Airlift reactors have been recognized as one of the promising photobioreactors for biomass/bio-energy production, where mixing has significant impact on the reactor performance. In recent years, using CFD simulations to track microorganism cells and to generate their trajectories in the reactor for reactor performance evaluations becomes more common. However, there is a lack of systematic and rigorous verifications and validations of the reliability of CFD models in particle tracking against experimental measurements in the open literature, which is vital for the faithful application of CFD in reactor design and scale-ups. In this work, we attempt to evaluate the reliability of using CFD simulations to generate trajectories of microorganisms in a draft tube column photobioreactor. A computationally promising CFD simulation model based on CFX5.7 was validated against a benchmark experimental database reported in [Luo and Al-Dahhan, 2008a], [Luo and Al-Dahhan, 2008b] and [Luo and Al-Dahhan, 2010]. This model was then used to generate typical trajectories of microorganisms in the studied airlift column, which was further validated against experimentally measured tracer trajectories. The results indicated that the CFD model reasonably predicted the recirculation of the microorganism around the draft tube, however over-estimated the cells' residence time in the wall regions. Proper treatment for the wall region such as griding and wall function is needed to better capture the movement of microorganism cells in such bioreactors.     

Experimental and CFD analysis of two-phase cross/countercurrent flow in the packed column with a novel internal

The pressure drop and liquid hold-up for the G-L cross/counter-current flow in a packed column with a novel internal was simulated using a Eulerian/Eulerian two-fluid model solved by a commercial CFD software CFX4.4. Simulation results are in good agreement with the experimental data of pressure drop. The internal significantly increases the gas radial velocity and lower the gas axial velocity, which lowers the pressure drop and improves operational flexibility. To minimize bypass flow caused by the internal, optimum baffle thickness and width of the internal's passage are proposed.     

Modelling of dense and complex granular flow in high shear mixer granulator—A CFD approach

In the aspect of granulation process control, the numerical simulations appear to be a cost-effective and flexible tool to investigate the flow structure of granular materials in mixer granulators of various configurations and operating conditions. Computational fluid dynamics (CFD) is used in this study to model the granular flow in a vertical high shear mixer granulator. The simulation is based on the continuum model of dense-gas kinetic theory [Gidaspow, D., Bezburuah, R., Ding, J., 1992. Hydrodynamics of circulating fluidized beds, kinetic theory approach. In: Fluidization, vol. VII, Proceedings of the 7th Engineering Foundation Conference on Fluidization, Brisbane, Australia, pp. 75-82] with consideration of inter-particle friction force at dense condition [Schaeffer, D.G., 1987. Instability in the evolution equations describing incompressible granular flow. Journal of Differential Equations 66 (1), 19-50]. This study aims to verify this numerical method in modelling dense and complex granular flows, where the solids motion obtained from the simulation is validated against the experimental results of positron emission particle tracking (PEPT) technique [Ng, B.H., Kwan, C.C., Ding, Y.L., Ghadiri, M., Fan, X.F., 2007. Solids motion of calcium carbonate particles in a high shear mixer granulator: a comparison between dry and wet conditions. Powder Technology 177 (1), 1-11]. In general, the Eulerian based continuum model captures the main features of solids motion in high shear mixer granulator including the bed height and dominating flow direction (the tangential velocity). However, the continuum based kinetic-frictional model is not capable of capturing the complex vertical swirl pattern. Quantitative comparison shows over-predictions in the tangential velocity and stiff drops of the tangential velocity at the wall region. These results demonstrate the deficiency in transmitting forces in the bed of granular materials which indicate the necessity to improve the constitutive relations of dense granular materials as a continuum.     

CFD study and experimental validation of multiphase packed bed hydrodynamics in the context of Rolling Sea conditions

The necessity for a validated computational fluid dynamics (CFD)-based model to deepen our understanding about the complex hydrodynamics of gas–liquid flows in oscillating porous media has driven this experimental and simulation work. A transient three-dimensional Euler–Euler porous media CFD model using moving reference frame and sliding mesh techniques was applied to elucidate the dynamic features of gas–liquid flows of cocurrent downflow packed beds subject to tilts and oscillations reminiscent of sea conditions. Incorporation of capillary and mechanical dispersion forces besides interphase momentum exchange terms in the CFD model to achieve reliable predictions was evaluated with respect to experimental data acquired by capacitance wire-mesh sensors and differential pressure transmitter. In the light of the validated CFD model, a detailed sensitivity analysis was performed to address the interrelations between hydrodynamic parameters, influence of fluid properties and packing size on the model predictions, and additional contribution of column oscillations on multiphase dynamics. © 2018 American Institute of Chemical Engineers AIChE J, 65: 385–397, 2019     

A combined computational fluid dynamics (CFD) and experimental approach to quantify the adhesion force of bacterial cells attached to a plane surface

A three‐dimensional model is developed to study the laminar shear flow past a bacterial cell attached to a plane surface. The induced hydrodynamic forces and torque exerted on the cell are computed to clarify the prevailing mechanisms involved in the detachment of model bacteria. Results are discussed in terms of drag and torque magnitude as a function of the angles defining the orientation of the cell. It is shown that reorientation and rolling of spheroid‐shaped cells are favored. It is also confirmed that rod‐shaped cells would tend to lie on the surface and become aligned with the flow. The model is used to quantify the adhesion force of spheroid Bacillus cereus spores to stainless steel, deduced from previously described experiments in a shear stress flow chamber. The magnitude of the predicted adhesion force is close to that obtained using atomic force microscopy under similar experimental conditions. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

A CFD model for predicting the flow patterns of viscous fluids in a bioreactor under various operating conditions

Computational fluid dynamics simulation is becoming an increasingly useful tool in the analysis and design of simultaneous saccharification fermentation (SSF) and saccharification followed by fermentation process (SFF). To understand and improve mixing and mass transfer in a highly viscous non-Newtonian system, it was necessary to simulate the flow behavior in this bench scale bioreactor (BioFlo 3000). This study focused on designing a high concentration medium agitation system for such a process using the commercial computational fluid dynamics package Fluent (V. 6.2.20) and its preprocessor Mixsim (V. 2.1.10). The objective of this study is to compare performance of various designs of a bioreactor and identify the flow pattern and related phenomena in the bench scale tank. The configuration of the physical model for simulating a mixing tank with a Rushton impeller consists of an ellipsoidal cylindrical tank with four equally spaced wall mounted baffles extending the vessel bottom to the free surface, stirred by a centrally located six-blade Rushton turbine impeller. Simulations were performed with the original and a modified design in which the lower bottom shaft mounted a Lightnin A200 impeller. The results suggest that there is a potential for slow or stagnant flow between top impellers and bottom of the tank region, which could result in poor nitrogen and heat transfer for highly viscous fermentations. The results also show that the axial velocity was significantly improved for the modified geometry in the bottom of the tank.     

Using positron emission particle tracking (PEPT) to study the turbulent flow in a baffled vessel agitated by a Rushton turbine: Improving data treatment and validation

Positron emission particle tracking (PEPT) is a relatively new technique allowing the quantitative study of flow phenomena in three dimensions in opaque systems that cannot be studied by optical methods such as particle image velocimetry (PIV) or laser Doppler anemometry (LDA). Here, velocity measurements made using PEPT in two sizes of baffled vessel (∼0.20 m and ∼0.29 m diameter) and two different viscosity fluids agitated by a Rushton turbine are compared for the first time directly in depth with some studies reported in the literature made by LDA for the turbulent regime in the equivalent geometry. Initially, the paper considers how the Lagrangian data obtained by PEPT can be converted into Eulerian in order to make the comparison most effective. It also considers ways of data treatment that improve the accuracy of both the raw PEPT data and the velocities determined from it. It is shown that excellent agreement is found between the PEPT and literature results, especially for the smaller vessel, except for the radial velocity just off the tip of the blade in the plane of the disc of the Rushton turbine. This difference is attributed to the very rapid changes in both magnitude and direction that occurs in that region and also to the different way of ensemble averaging in the two techniques. In addition, the results for the absolute velocities normalised by the impeller tip velocity for all the rectangular cross-section toroidal cells in each size of vessel and each fluid and a range of agitator speeds are compared in the form of frequency histograms. In this analysis, the velocities for each run are obtained from PEPT based on tracking a particle for 30 min and the mean and mode of the velocities each decrease slightly with decreasing scale and Reynolds number. The possible reasons for this variation in the mode and the mean are discussed. Overall, it is concluded that for the radial flow Rushton turbine the PEPT technique can be used to obtain accurate velocity data throughout the entire complex three-dimensional turbulent flow field in an agitated, baffled vessel except very close to the impeller in the radial discharge stream.     

A Multi‐Block Approach to Obtain Angle‐Resolved PIV Measurements of the Mean Flow and Turbulence Fields in a Stirred Vessel

Two‐dimensional Particle Image Velocimetry (PIV) measurements have been used to characterize the complex turbulent flow generated by a T/3 45° pitched‐blade down‐flow turbine, operated at Re ≈ 5 · 10⁴, in a fully turbulent stirred vessel. To maintain high spatial resolution when viewing the whole vessel, a multi‐block approach has been developed, which combines data from different fields of view into a composite flow map. Using 500 measurements of instantaneous u and v velocity fields, angle‐resolved mean velocity maps and turbulence properties, such as the RMS velocities and the turbulence kinetic energy, have been estimated near to the blade, as well as in the bulk of the vessel, at a spatial resolution of between 1 and 2 mm. Vorticity maps have also been calculated to help visualize the trailing vortex structures close to the impeller blades and integral length scales have been estimated from the two‐dimensional spatial auto‐correlation function. It is shown than the common assumption that the integral length scale is about half the blade width is an overestimate close to the impeller and an underestimate far from the impeller.     

An experimental and CFD study of liquid jet injection into a partially baffled mixing vessel: A contribution to process safety by improving the quenching of runaway reactions

Thermal runaway remains a problem in the process industries with poor or inadequate mixing contributing significantly to these incidents. An efficient way to quench such an uncontrolled chemical reaction is via the injection of a liquid jet containing a small quantity of a very active inhibiting agent (often called a stopper) that must be mixed into the bulk of the fluid to quench the reaction. The hazards associated with such runaway events mean that a validated computational fluid dynamics (CFD) model would be an extremely useful tool. In this paper, the injection of a jet at the flat free surface of a partially baffled agitated vessel has been studied both experimentally and numerically. The dependence of the jet trajectory on the injection parameters has been simulated using a single-phase flow CFD model together with Lagrangian particle tracking. The comparison of the numerical predictions with experimental data for the jet trajectories shows very good agreement. The analysis of the transport of a passive scalar carried by the fluid jet and thus into the bulk, together with the use of a new global mixing criterion adapted for safety issues, revealed the optimum injection conditions to maximise the mixing benefits of the bulk flow pattern.     

Response surface study of the performance of lean NOx storage catalysts as a function of reaction conditions and catalyst composition

NO_x storage and reduction (NSR) catalysts containing Pt, Ba and Fe were studied as a function of reaction conditions and catalyst composition using response surface methodology combined with high-throughput experimentation. The concentrations of the reactant gases and the reactor temperature were varied to probe their effect on catalyst performance, as quantified by lean NO_x storage and N₂O production. An empirical model relating the catalyst performance to five reaction condition variables and three metal weight loading variables has also been developed. It was found that the temperature and the concentrations of the reducing agents, i.e. carbon monoxide and ethylene, had the strongest effect on the lean NO_x storage. It was also found that the Pt and Ba weight loadings had a much greater effect than Fe weight loadings on the performance of NSR catalysts. This model provides insight about the factors controlling the NO_x conversion by NSR catalysts and also predicts the optimum catalyst composition for given reaction conditions and vice versa. As an additional study, the relationship between sulfur poisoning, nitrous oxide production, and exotherm generation was also explored.     

A new triaxial apparatus to study the mechanical and fluid flow aspects of carbon dioxide sequestration in geological formations

Climate scientists are practically unanimous in the belief that anthropogenic greenhouse gas contributions have added to the thickness and thus the effectiveness of the greenhouse gas layer, leading to a warming of the planet (IPCC, 2005 [1]). Engineers and scientists around the globe are researching and developing measures to reduce greenhouse gas emissions. These measures have included proposals to sequester carbon dioxide (CO₂) in deep geological formations (Perera et al., in press [18]). For CO₂ sequestration in deep geological reservoirs to become a feasible strategy to reduce greenhouse gas emissions, a sound understanding of the manner by which mechanical properties and permeability changes with the introduction of CO₂ to the geological reservoir will influence the stability of that reservoir is required. Thus there is a need to develop laboratory equipment capable of simulating the CO₂ injection and storage process for deep geological CO₂ sequestration under the expected in situ pressure (confinement and fluid) and temperature conditions. Triaxial experiment has been identified as the best method for this purpose (Perera et al., 2011b [19]). Therefore, we present a new high-pressure triaxial apparatus which can provide the high confining and fluid injection pressures and elevated temperatures expected for deep geological CO₂ sequestration. The new setup can be used to conduct mechanical and permeability testing on intact or fractured natural rock samples or synthetic rock samples subjected to high-pressure injection of up to three fluid phases (gas and/or liquid) at high pressures and temperatures corresponding to field conditions. The equipment is capable of delivering fluids to the sample at injection pressures of up to 50 MPa, confining pressures of up to 70 MPa and temperature up to 50 °C and will continuously record fluid injection and confining pressures, axial load and displacement, radial displacement and independent outflow rates for liquid and gas fluid phases (under drained conditions).Leakage tests have confirmed the effectiveness of the device at pressures up to its maximum capacities. Additionally the temperature-pressure relationship for the hydraulic oil used to apply confining pressure to the sample has been calibrated to account for the influence of changes in temperature on confining pressure. Several permeability tests (using N₂ and CO₂ as the injection fluid and 10 MPa confining pressure) and one strength test are reported for black coal samples from the Sydney Basin, New South Wales. According to the results of the permeability tests, coal mass permeability decreases with increasing effective stress for both gases. However, the permeability for N₂ gas is much higher than CO₂. Moreover, test results are consistent with matrix swelling due to the adsorption of CO₂ in coal. The strength testing results are in agreement with the results of testing carried on similar black coal samples from literature, certifying the ability for the new device to accurately measure strength and deformation properties of rock under deep ground conditions.     

Flow through diffusion cell method: A novel approach to study in vitro enzymatic degradation of a starch‐based ternary semi‐interpenetrating network for gastrointestinal drug delivery

A ter‐polymeric semi‐IPN has been synthesized by aqueous polymerization of methacrylamide in the presence of polyethylene glycol (PEG) and natural polysaccharides starch, and its enzymatic degradation has been studied in the phosphate buffer medium of pH 6.8 at the physiological temperature 37°C. With the increase in content of enzyme in the external solution and starch in the hydrogel, the degradation is enhanced while the extent of degradation is lowered with the increase in the amount of PEG in the hydrogel. The initial water content also affects the degradability of the polymer matrix. The degradation follows Michaelis–Menten kinetics and K_M was found to be 3.92 × 10^?5 mol dm^?3. The hydrogel exhibits different degradation behavior when studied by “traditional degradation method” (TDM) and “flow through diffusion cell” (FTDC) method. The degradability is suppressed in FTDC method because of the absorption of amylase molecules onto filler particles. Finally the nature and size of the filler particles also affects the degradation behavior of hydrogels. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2975–2984, 2006     

Numerical simulation of the dynamic flow behavior in a bubble column: A study of closures for turbulence and interface forces

Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, C_S, as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737-1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79-95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965-2000] to account for the bubble-induced turbulence in the k-ε model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with C_S=0.08 and 0.10 agree well with the measurements. When C_S is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio (H/D=3 and H/D=6), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22-24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results.     

Investigation on strategies for optimizing process definition in rubber processing: A study on mechanical and chemical properties of vulcanizates as basis for the development of a new calculation model for quality prediction

Rubber compounds may exhibit significant batch variations due to multiple different ingredients mixed in one compound. Hence, defining the manufacturing process for constant part quality can be challenging. Common strategies in considering batch variations in rubber processing include the determination of reaction kinetics, and the definition of process parameters according to normalized vulcanization isotherms. Thereby, maintenance of the degree of cure is targeted. With this path, information on the mechanical properties of vulcanizates is lost, despite its visibility from the kinetic data and part quality assurance is missed. This contribution points out the differences obtained for parts produced to the same degree of cure at various temperatures and intends to emphasize new strategies in process definitions. Therefore, compression molded parts were produced from styrene-butadien rubber, which was then characterized with mechanical and chemical methods. Each of the methods revealed a significant difference in part behavior, which were manufactured to the same degrees of cure but at different temperatures. It was concluded that a temperature-dependent reaction rate should be considered when quality maintenance is targeted in the production. Only then will it be possible to predict the properties adequately, with simultaneous effect of enhancing sustainability policies in rubber processing.     

A systematic approach to the study of multicomponent polymerization kinetics: butyl acrylate/methyl methacrylate/vinyl acetate. III. Emulsion homopolymerization and copolymerization in a pilot plant reactor

A systematic study of the terpolymerization of butyl acrylate/methyl methacrylate/vinyl acetate (BA/MMA/VAc) is being conducted. In this third stage of the study,

1 Parts 1 and 2: Dubé, M. A. & Penlidis, A., Polymer, 36 (1995) 587. Dubé, M. A. & Penlidis, A., Macromol. Chem. Phys., 196 (1995) 1101.

emulsion homopolymerizations and copolymerizations of the monomers comprising the BA/MMA/VAc system were performed in a 5 litre stainless steel pilot plant reactor, mainly for troubleshooting purposes and as a precursor to the detailed terpolymerization experiments to follow. First, a search for a stable emulsion recipe was conducted. At the same time, experimental procedures were established for the 5 litre pilot plant reactor along with product characterization techniques. Finally, selective emulsion homopolymerizations and copolymerizations were run for each of the three monomers and each combination of the three monomers, respectively. The polymers produced were characterized for conversion, composition, molecular weight and particle size. Although the emphasis of the experiments was to establish recipes, techniques, and procedures for emulsions terpolymerization, several useful observations were made regarding the kinetics even from these troubleshooting experiments.