Wayne Strasser

CFD Investigation of Gear Pump Mixing Using Deforming/Agglomerating Mesh

A moving-deforming grid study was carried out using a commercial computational fluid dynamics (CFD) solver, FLUENT ® 6.2.16. The goal was to quantify the level of mixing of a lower-viscosity additive (at a mass concentration below 10%) into a higher-viscosity process fluid for a large-scale metering gear pump configuration typical in plastics manufacturing. Second-order upwinding and bounded central differencing schemes were used to reduce numerical diffusion. A maximum solver progression rate of 0.0003 revolutions per time step was required for an accurate solution. Fluid properties, additive feed arrangement, pump scale, and pump speed were systematically studied for their effects on mixing. For each additive feed arrangement studied, the additive was fed in individual stream(s) into the pump-intake. Pump intake additive variability, in terms of coefficient of spatial variation (COV), was >300% for all cases. The model indicated that the pump discharge additive COV ranged from 45% for a single centerline additive feed stream to 5.5% for multiple additive feed streams. It was found that viscous heating and thermal/shear-thinning characteristics in the process fluid slightly improved mixing, reducing the outlet COV to 3.2% for the multiple feed-stream case. The outlet COV fell to 2.0% for a half-scale arrangement with similar physics. Lastly, it was found that if the smaller unit’s speed were halved, the outlet COV was reduced to 1.5%.

Discrete Particle Study of Turbulence Coupling in a Confined Jet Gas-Liquid Separator

A 3D computational fluid dynamics investigation of particle-induced flow effects and liquid entrainment from an industrial-scale separator has been carried out using the Eulerian–Lagrangian two-way coupled multiphase approach. A differential Reynolds stress model was used to predict the gas phase turbulence field. The dispersed (liquid) phase was present at an intermediate mass loading (0.25) but low volume fraction (0.05). A discrete random walk method was used to track the paths of the liquid droplet releases. It was found that gas phase deformation and turbulence fields were significantly impacted by the presence of the liquid phase; these effects have been parametrically quantified. Substantial enhancement of both the turbulence and the anisotropy of the continuous phase by the liquid phase was demonstrated. It was also found that a large number (⩾1000) of independent liquid droplet release events were needed to make conclusions about liquid entrainment. Known plant run conditions and entrainment rates validated the numerical method.

Numerical study of gas-cyclone airflow: an investigation of turbulence modelling approaches

A numerical study of unsteady single-phase vortical flow inside a cyclone is presented. Two different geometric configurations have been considered, with the goal of assessing several different turbulence modelling approaches for this class of problem. The models investigated include three Reynolds-averaged Navier–Stokes models: a commonly used two-equation eddy-viscosity model, a differential Reynolds stress model (DRSM) and an eddy-viscosity model sensitised to rotational and curvature (RC) effects which was recently developed and implemented into a commercial CFD (computational fluid dynamics) code by the authors. Results were also obtained using large eddy simulation (LES). The computational results are analysed and compared with available experimental data. The RC-sensitised eddy-viscosity model shows significant improvement over the standard eddy-viscosity model. The RC-sensitised model, DRSM and LES model predictions of the mean flowfield are in good agreement with the experimental data. The results suggest that curvature- and rotation-sensitive eddy-viscosity models may provide a practical alternative to more computationally intensive approaches.

The Effects of Retraction on Primary Atomization in a Pulsating Injector

Despite its industrial relevance, the exploration of primary atomization within a transonic self-generating pulsatile three-stream injector has been minimal. Our prior experimental and computational work was centered around compressible axisymmetric (AS) models and incompressible 3-D models for the purpose of obtaining spectral content and preliminary droplet size distributions. Here, the emphasis shifts to compressible 3-D computational models involving a non-Newtonian slurry and a much more inclusive computational domain in order to further elucidate droplet size information. Effects of numerics, turbulence model, and geometric parameters are revisited. In addition, a surrogate measure for injector face erosion is introduced. Lastly, links are discovered between responses in Sauter mean diameter and trends in AS modeling metrics. As with prior air-water work and incompressible slurry simulations, higher gas inner flow rate reduced droplet size measurably. While the temporal mean droplet length scale was relatively insensitive to numerics, turbulence model, compressibility, and modeled domain size, droplet size temporal variability responded very strongly to some of these effects; compressibility dampened the droplet variability, while increased inner gas flow augmented variability, and the use of a more rigorous turbulence model showed a mixed effect. It was found that designs with less retraction (smaller pre-filming region) produced smaller droplets and allowed increased process throughputs. Newly discovered correlation equations are provided and followed similar trends as some from the earlier AS work. Interestingly, it was also shown that droplet size can be correlated with spectral information from prior companion AS studies.Copyright

Pulsatile Primary Slurry Atomization: Effects of Viscosity, Circumferential Domain, and Annular Slurry Thickness

A central theme of our prior experimental and computational work on a transonic self-sustaining pulsatile three-stream coaxial airblast injector involved obtaining spectral content from compressible 2-D models and preliminary droplet size distributions from incompressible 3-D models. The three streams entail an inner low-speed gas, and outer high-speed gas, and an annular liquid sheet. Local Mach numbers in the pre-filming region exceed unity due to gas flow blockage by the liquid. Liquid bridging at somewhat regular intervals creates resonance in the feed streams. The effects of numerical decisions and geometry permutations were elucidated. The focus now shifts to compressible 3-D computational models so that geometric parameters, modeled domain size, and non-Newtonian slurry viscosity can be more elaborately explored. While companion studies considered circumferential angles less than 45°, specific attention in this work is given to the circumferential angles larger than 45°, the slurry annular dimension, and how this annular dimension interacts with inner nozzle retraction (pre-filming distance). Additional metrics, including velocity point spectral analyses, are investigated. Two-stream experimental studies are also computationally studied.Multiple conclusions were drawn. Narrower annular slurry passageways yielded a thinner slurry sheet and increased injector throughput, but the resulting droplets were actually larger. Unfortunately the effect of slurry sheet thickness could not be decoupled from another important geometric permutation; injector geometry physical constraints mandated that, in order to thin the slurry sheet, the thickness of the lip which separates the inner gas and slurry had to be increased accordingly. Increased lip thickness reduced the interfacial shear and increased the thickness of the gas boundary layer immediately adjacent to the slurry sheet. This suppressed the sheet instability and reduced the resulting liquid breakup. Lastly, velocity point correlations revealed that an inertial subrange was difficult to find in any of the model permutations and that droplet length scales correlate with radial velocities.As anticipated, a higher viscosity resulted in larger droplets. Both the incremental impact of viscosity and the computed slurry length scale matched open literature values. Additionally, the employment of a full 360° computational domain produced a qualitatively different spray pattern. Partial azimuthal models exhibited a neatly circumferentially repeating outer sheath of pulsing spray ligaments, while full domain models showed a highly randomized and broken outer band of ligaments. The resulting quantitate results were similar especially farther from the injector; therefore, wedge models can be used for screening exercises. Lastly, droplet size and turbulence scale predictions for two external literature cases are presented.Copyright

A Significant Advancement in Understanding Self-Sustaining Pulsatile Injector Dynamics: Effects of Numerics, Nozzle Geometry, Gas Feed Rate, and Non-Newtonian Slurry

The performance of a rather large-scale self-exciting coaxial three-stream airblast injector was studied experimentally and computationally by Strasser [1] and the computational method was validated using an air-water test stand (AWTS). Frequency domain analyses revealed distinct changes in spray character and pulsations as a function of air feed rates. Since that work, efforts have progressed in studying the effects of injector geometry, including inner nozzle retraction, stream meeting angle, outer annulus gap, and nozzle diameter. Changes to retraction produce the most profound, but not always monotonic, responses in the energy content and nature of the spray pattern. Later, the use of slurry and a high-density gas (SH) as replacements for air and water is investigated, revealing that the nature of the SH flow is dramatically different from its AW counterpart. As with AW, inner nozzle retraction and stream meeting angle prove to be the most influential geometry variables. Strong geometry, materials, and gas flow rate interactions are found among the metrics considered. Estimated droplet length scales of the SH system are not much different than those of the AW system. Attempts to further stimulate the spray via modulating the inner gas provide marginal influence for both sets of flowing materials. Lastly, swirling feeds and a multitude of modeling issues are addressed.Copyright

CFD Study of an Evaporative Trickle Bed Reactor

A numerical study was carried out to investigate steady-state and transient phase distribution, evaporation, and thermal runaway in a large-scale high-pressure trickle bed reactor operating in the low interaction regime. The thermal inertia of the catalyst particles proved to be a significant contribution to the overall energy balance. A cooling recycle stream, containing reaction products and a fresh feed, was included via a closed loop calculation. It was found that, as expected, phase distribution in the catalyst bed had a substantial impact production rate; a faulty feed distribution system can cost approximately 20% in overall steady-state product conversion. Grid resolution effects were quantified and were found to have minimal impact on macroscopic measures. Also, most results were insensitive to the extent of the modeled domain and the commercial solver version. In the event that the cooling recycle stream is lost, the external reactor shell temperature can exceed its design intent. It was found that reducing the quantity of fresh reactant feed in this situation can dramatically reduce the potential for vessel damage.Copyright

Application of a Hybrid RANS-LES CFD Methodology to Primary Atomization in a Coaxial Injector

Non-zonal hybrid RANS-LES models, i.e. those which do not rely on user-prescribed zones for activating RANS or LES, have shown promise in accurately resolving the energy-containing and highly anisotropic large-scale motions in complex separated flows. In particular, the recently proposed dynamic hybrid RANS-LES (DHRL) approach, a method which relies on the continuity of turbulence production through the RANS-to-LES transition zone, has been validated for several different compressible and incompressible single phase flow problems and has been found to be accurate and relatively insensitive to mesh resolution. Time-averaged source terms are used to augment the momentum balance. An added benefit of the DHRL is the ability to directly couple any combination of RANS and LES models into a hybrid model without any change to numerical treatment of the transition region. In this study, an attempt is made to extend the application of this model to multiphase flows using two open literature coaxial two-stream injectors involving non-Newtonian liquids. For the first time, the new model has been successfully implemented in a multiphase framework, combining the SST RANS model with MILES LES approach. Favre averaging is used to ensure consistency between the momentum equations and the density fluctuations. It was found that the momentum source terms must be density weighted in order to ensure stability of the solution. Primary atomization findings with a stable model are encouraging. The spray character with the new model was somewhere between that of a RANS model and the LES result. Droplet sizes, which are indicative of the shear layer energy, for the RANS model were greater than the hybrid results, which were comparable to the LES result and matched the experimental expectation. Additionally, the new approach showed a liquid core breakup length close to that expected from the literature.Copyright

Wall temperature considerations in a two-stage swirl non-premixed furnace

It is desired to keep the walls of a heat transfer medium (HTM) swirled-burner furnace warm enough to prevent corrosion. A computational study was carried out in order to assess the normal and lowest possible wall temperatures, specifically those of the sheet metal layers making up the outer wall. Various combustion models, radiation parameters, and operating conditions were considered. Field-measured values matched CFD results closely. It was found that the walls were sufficiently warm to prevent corrosion under all reasonable modelling approaches and conceivable operating circumstances. A dramatic computational time savings can be realised by employing a thermal-only solution for certain modelling permutations.

Numerical and Experimental Investigations of Non-Newtonian Polymer Heating in a Static Mixer

A non-Newtonian polymer is heated by co-current hot water in an insulated jacketed pipe. Both the viscosity and density depend on the temperature and shear rate. The polymer passes through two static mixers which are manufactured by KOFLO and are intended to accelerate the heating process. Each static mixer’s length to diameter ratio is approximately 16. An experiment was carried out in which the temperature of the polymer was measured directly downstream of the first static mixer with a thermocouple. The results from a 3D computational model were compared with those from an experiment. It was found that the polymer’s temperature rise with the simulation was within 6.5% of the experiment. The heating performance and residence time were also compared with a regular pipe.Copyright