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Gas–liquid two phase flows are widely encountered in industry. The design parameters include two phase pressure drop, mixing and axial mixing in both the phases, effective interfacial area, heat and mass transfer coefficients. Currently, there is a high degree of empiricism in the design process of such reactors owing to the complexity of coupled flow and reaction mechanism. Hence, we focus on synthesizing recent advances in computational and experimental techniques that will enable future designs of such reactors in a more rational manner by exploring a large design space with high-fidelity models (computational fluid dynamics) that are validated with high-fidelity measurements (hot film anemometry (HFA), Laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc.) to provide a high degree of rigor. Understanding the spatial distributions of dispersed phases and their interaction during scale up are key challenges that were traditionally addressed through pilot scale experiments, but now can be addressed through advanced modelling. For practically complete knowledge of the fluid mechanical parameters, it is desirable to implement direct numerical simulations (DNS). However, the current computational power does not permit full DNS for real bubble columns. Therefore, we have been using simplified turbulence models (such as large eddy simulation, Reynolds stress, k – ε , etc.) which need the knowledge of turbulence parameters. For the estimation of these parameters, currently semi-empirical procedures are being used pending the knowledge of turbulence. Further, the formulation of governing equations in all the CFD models (except DNS), the knowledge of interface forces (drag, lift, virtual mass, Basset, etc.) is needed and for their estimations empirical correlations are being employed, again pending the knowledge of fluid mechanics under turbulent conditions in bubble columns. In gas–liquid dispersions, the gas is sparged in the form of bubbles. During the bubble rise, the mechanism of wake detachment creates turbulence which can be called as wake generated turbulence. In addition, energy gets transferred from the gas phase to liquid phase. The quantitative amounts are negligible when bubble motion is not hindered and the gas–liquid dispersion is homogenous. The amounts increase with an increase in the extent of hindrance. However, in the homogenous regime, even under extreme conditions, the extent of energy transfer in the bulk gas–liquid dispersions (volume other than wake volume) is fairly limited. On contrast, in the heterogeneous regime, the rates of energy transfer become sizeable. The energy received by the liquid (in both the regimes) also creates turbulent motion and termed as bulk generated turbulence. In turbulent flows a compendium of eddies (flow structures) of different length and time scales contribute towards improved/enhanced mixing, momentum transfer, heat transfer, and mass transfer (transport phenomena). Hence, a proper understanding of the dynamics of these turbulent flow structures, and their role in the transport phenomena, can bring substantial improvement in the scale-up and design procedures. The present paper brings out the current status of knowledge on bubble generated turbulence. All the published literature in experimental measurements and DNS simulations has been critically analysed and coherently presented. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: An improved coupling method is proposed to ensure mass conservation in a previously developed DPM-CFD model (C.L. Wu et al., Three-dimensional discrete particle model for gas–solid fluidized bed on unstructured mesh, Chemical Engineering Journal 152 (2009), 514–529). In enforcing the mass balance for the continuous phase in such algorithms using the single phase flow solvers, any heterogeneity in the particle concentration field (caused by clustering of particles) creates non-zero source terms. It is shown that if this is not handled properly, it can create large numerical errors (of up to 10%) and any oscillation in the flow field manifests itself in a similar error in the mass conservation. By using the conservative form of the equations in integral formulation, the coupling between the DPM and CFD models ensures mass conservation on the global scale in the entire domain, even in the presence of local heterogeneities in the flow field. The method also offers a consistent implementation of mass flow boundaries at the inlet. Numerical results show the superiority of the new coupling method over the previous one in conserving mass and handling transport processes in fluidized beds. [Copyright &y& Elsevier]

In this work, three-dimensional, unsteady Reynolds-Averaged Navier?Stokes (RANS) equations with standard k-ϵ turbulence models are solved by employing sliding mesh technique within an entire stage of a multistage diffuser pump to investigate transient flow field and pressure fluctuations due to the interaction between impeller and diffuser vanes. Sliding mesh calculation is carried out as the impeller zones slide (i.e., rotate) relative to diffuser zone along the grid interface in discrete time steps. The complicated time-periodic and spatial-periodic characteristics as well as rotor?stator interaction phenomena inside the pump stage are simulated and analysed to understand dynamic variation of the interior flow field and interference and finally to aim at optimal design. [ABSTRACT FROM AUTHOR]

The main objective of the present work is to show that one can use computational fluid dynamics (CFD) to predict the flow patterns and particle impact profiles on a specimen and use such data together with phenomenological erosion models to predict both the total erosion rates and the erosion patterns on surfaces. The surface wear-out pattern from profilometry measurements and the total weight loss data of the eroded material were obtained from experiments to validate the model predictions. In the experiments, four different parameters, viz. the impingement velocity, impingement angle, sand concentration and properties of target materials were varied. Their effect on the total weight loss and the erosion patterns are investigated. Two different experimental conditions were used - viz. one in which the specimen and the jet were submerged and the other in which both were exposed (non-submerged). Experimental studies show that total weight loss has a power-law relation with respect to impingement velocity or sand concentration. The weight loss decreases as the hardness of material increases, but not in a linear fashion. Two corrosion resistance materials, 304 stainless steel and chrome white alloy, showed the same corrosion and synergism weight loss although their hardness was much different, 190 HV (Vickers hardness) for 304 stainless steel and 763 HV for chrome white alloy. This implies that the corrosion and synergism weight loss is mainly dominated by the chemical composition of the material and not by its mechanical properties. The results also differ substantially for the submerged and non-submerged cases. When the specimen and the jet were submerged, the erosion scar was typical 'W' style. But when sample and jet were not submerged, the scar was much flatter. A phenomenological erosion model that results in a simple algebraic equation has been developed, and the predicted results are in reasonable agreement with the experimental measurements. When they are used locally together with the CFD predictions of the flow field, the agreement with the erosion patterns and the total weight loss is improved significantly. [ABSTRACT FROM AUTHOR]

Computational Fluid Dynamics and Discrete Element Method are used to study the dynamics of a pseudo-2D pulsed fluidized bed, consisting of Geldart B particles. The focus is on the response of the system to variations in the mean gas velocity, V M , and the amplitude of oscillations, V A , at an imposed frequency of 3.5 Hz. Distinctive flow patterns are mapped out based on observation of simulations, pressure fluctuations and bubble granular temperature profiles. A regime transition is observed when V A is varied under constant V M. Such transition is underlined by: a peak in time-averaged pressure drop and coordination number; a change in the dynamics of the solid phase; an increase in velocity of the void fronts traversing the particle bed; and a decrease in minimum average height attained by the solid phase. This is related to the suppression of void layers at higher V A , occurring more easily at lower V M. Unlabelled Image • Regime transition is induced by amplitude of oscillation. • Flow patterns are altered by mean superficial velocity and amplitude of oscillation. • Shift in solid phase dynamics is captured by coordination number. • Stronger collapse of void layers induces longer periods of lower coordination number. [ABSTRACT FROM AUTHOR]

Abstract In this paper we present the design of a novel, passive mixer, to enhance mixing and residence time distribution for promoting transport and reaction mechanisms in such devices. The goal is to understand and optimize the flow patterns for enhancing mixing in laminar flows through toroidal helical pipes, inspired by the coiled flow inverter (CFI) (Saxena and Nigam, 1984). A toroidal helix is a smooth analytical curve winding around a torus. It is characterized by three parameters: the radius of the torus' centerline, the radius of the torus' cross-section, and the number of turns it winds around the torus. Unlike the torus or the straight helix which have constant curvatures, the toroidal helix has a spatially oscillating curvature. We found that this causes the otherwise separated pair of Dean vortices to couple together, periodically re-orient, shrink and grow within each turn of the pipe. The resultant complex streamlines span the entire cross-section and enhances mixing via advective means. By constraining the total pipe length, a meaningful comparison of mixing efficiency within a given volume of pipe can be assessed as the torus's cross-section radius and the number of helix turns are systematically varied. Reynolds number of up to 400 is explored with the number of turns of 3, 5, 7 and 8. The optimization study shows that the best mixing performance is achieved using a moderate number of turns. Apart from flow pattern and mixing efficiency, we also studied the pressure drop to monitor increased energy consumption and residence time distribution of the flow to monitor degree of mixing. [ABSTRACT FROM AUTHOR]

In this work, we conduct a computational study on the loading of cryoprotective agents into cells in preparation for cryopreservation. The advantages of microfluidics in cryopreserving cells include control of fluid flow parameters for reliable cryoprotectant loading and reproducible streamlined processing of samples. A 0.25 m long, three inlet T-junction microchannel serves as an idealized environment for this process. The flow field and concentration distribution are determined from a computational fluid dynamics study and cells are tracked as inert particles in a Lagrangian frame. These particles are not confined to streamlines but can migrate laterally due to the Segre-Sildeberg effect for particles in a shear flow. During this tracking, the local concentration field surrounding the cell is monitored. This data are used as input into the Kedem-Katchalsky equations to numerically study passive solute transport across the cell membrane. As a result of the laminar flow, each cell has a unique pathline in the flow field resulting in different residence times and a unique external concentration field along its path. However, in most previous studies, the effect of a spatially varying concentration field on the transport across the cell membrane is ignored. The dynamics of this process are investigated for a population of cells released from the inlet. Using dimensional analysis, we find a governing parameter α, which is the ratio of the time scale for membrane transport to the average residence time in the channel. For α<=0.224, cryoprotectant loading is completed to within 5% of the target concentration for all of the cells. However, for α>0.224, we find the population of cells does not achieve complete loading and there is a distribution of intracellular cryoprotective agent concentration amongst the population. Further increasing α beyond a value of 2 leads to negligible cryoprotectant loading. These simulations on populations of cells may lead to improved microfluidic cryopreservation protocols where more consistent cryoprotective agent loading and freezing can be achieved, thus increasing cell survival. [ABSTRACT FROM AUTHOR]

DEM–CFD simulations of a gas–solid fluidized bed are presented and compared to experimental results of the small scale challenge problem (SSCP) from the National Energy Technology Laboratory. It has been well recognized that the pressure drop and the solid velocity profile across the bed may be over-predicted with the widely used constitutive relations for gas–solid flows in the Eulerian two-fluid frame work. An attempt is made to address this problem using energy and force balances across a bubbling fluidized bed to attenuate the drag coefficient while preserving the correct velocity profile. This is done with the DEM model for particulate phase and the new drag law is applied to the Lagrangian–Eulerian model for particulate flows. We show that a drag model used in simulating gas–solid fluidized bed should account for the energy dissipation in both a particle-rich and particle-lean (bubbles and slugs) regions of the fluidized bed. The new drag model proposed in this work has taken into account the additional energy dissipation from forming heterogeneous structures and performs better than the standard closure models used for bubbling fluidized bed simulations in the selected cases we have examined against NETL data. Such approaches to closure model building enforce the macroscopic conservation principles while attempting to still accommodate local variations in drag due to cluster formation mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an under-development tubular micro-solid oxide fuel cell was mathematically modeled. The cell was ∼3mm in diameter and ∼300μm thick, with Ni/YSZ and LSM/YSZ composite electrodes and YSZ electrolyte. A steady-state, 2-D axi-symmetric, computation fluid dynamics model was developed. The model included mass and momentum transport within the gas channels and the porous electrodes. Charge transfer was modeled in details for the electrodes and the electrolyte, assuming volumetric current generation. The model was validated and was found to be consistent with the experimental results. Particular attention was paid to the physical property parameters in the model. Some of the parameters were measured experimentally, as it was found that correlation commonly used in the literature may not be particularly accurate, especially when new fabrication methods are used. The model was used to gain a thorough insight into the fuel cell’s operation, and the contribution of each performance loss mechanism. In addition, the sensitivity of the simulation results to variations in the physical property parameters was investigated. [ABSTRACT FROM AUTHOR]

Most of the CFD codes are parallelized under distributed-memory parallel computing environments. For this reason, many attempts have been made to develop parallel DPM/DEM algorithms for dense particulate flows under the same parallel architecture. For such a development, it is very difficult to achieve efficient load balancing of processors due to the heterogeneous particle spatial distribution that often characterizes dense particulate systems. In this work, parallel CFD–DEM algorithms have been developed under the distributed memory environment with a fluid flow solver based on finite volume method and arbitrary 3D unstructured meshes. An implicit two-phase coupling scheme was proposed to enhance numerical stability for complex dense particulate flow problems. Parallelization-generated numerical difficulties such as void fraction calculation, two-phase momentum exchange, and contact force calculations for particles at irregular and arbitrary partition boundaries were efficiently addressed. The load-balancing difficulty due to heterogeneous particle distribution was partly overcome by the introduction of multi-threading. An efficient algorithm is proposed to handle data-exclusive access of the shared-memory by multi-threads on a compute node. The developed parallel DPM model has been successfully used to simulate many important applications such as bubbling fluidized bed, granular Rayleigh–Taylor instability, and particle swarm dynamics. [ABSTRACT FROM AUTHOR]