Steady-state Computational Fluid Dynamics Simulations Research Articles

Steady-state multiscale CFD simulation of a circulating fluidized bed riser

Compared to transient simulation, steady-state simulation of circulating fluidized bed risers is more efficient, but is also harder to perform due to the complex scale-dependency of dense gas-solid flows. In this work, steady-state computational fluid dynamics (CFD) simulation of a riser is performed using the steady energy-minimization multi-scale (EMMS) drag. It is found that the steady state corresponds to an extremely large scale of length and time, thus the grid size required in steady-state simulation is larger than that in transient one. The time-averaged two-fluid model (TFM) coupled with the steady-state EMMS/1M drag model enables a good prediction of the S-shaped, axial solids distribution and the choking transition, whereas the two-phase turbulence and solids stress models are important in predicting the radially core-annular distribution of solids. So far as we know, this is the first time that one can predict the choking transition in a steady-state CFD simulation. Further improvement may need an EMMS modeling of the time-averaged solid stresses.

CFD wind farm evaluation in complex terrain under free and wake induced flow conditions

Massive deployment of wind energy is critical to achieving the renewable energy production targets. This requires the development and improvement of models and tools for the optimal exploitation of high altitude and complex terrain sites for wind energy installations. Predicting the wind resource assessment of these sites is very challenging, as is predicting the interaction of wind farms in complex terrain with neighbouring installations, which is necessary to maximise the efficiency of wind energy. To address these challenges, the use of high-fidelity Computational Fluid Dynamics (CFD) models is recommended. In this study, the wind resource at the complex terrain site of the CENER experimental wind farm (Alaiz) is evaluated using steady-state RANS CFD simulations performed with OpenFOAM v2212, taking into account the effects of terrain topography and vegetation. Furthermore, a virtual wind farm located at Alaiz is modelled with the Actuator Disk (AD) method to analyse the effect of topography on the the wake evolution.

Influence of posture during gliding flight in the flying lizard Draco volans

The agamid lizards of the genus Draco are undoubtedly the most renown reptilian gliders, using their rib-supported patagial wings as lifting surfaces while airborne. Recent investigations into these reptiles highlighted the role of body posture during gliding, however, the aerodynamics of postural changes in Draco remain unclear. Here, we examine the aerodynamics and gliding performances of Draco volans using a numerical approach focusing on three postural changes: wing expansion, body camber, and limb positioning. To this aim, we conducted 70 three-dimensional steady-state computational fluid dynamics simulations of gliding flight and 240 two-dimensional glide trajectory calculations. Our results demonstrate that while airborne, D. volans generates a separated turbulent boundary layer over its wings characterized by a large recirculation cell that is kept attached to the wing surface by interaction with wing–tip vortices, increasing lift generation. This lift generating mechanism may be controlled by changing wing expansion and shape to modulate the generation of aerodynamic force. Furthermore, our trajectory simulations highlight the influence of body camber and orientation on glide range. This sheds light on how D. volans controls its gliding performance, and conforms to the observation that these animals plan their glide paths prior to take off. Lastly, D. volans is mostly neutral in pitch and highly maneuverable, similar to other vertebrate gliders. The numerical study presented here thus provides a better understanding of the lift generating mechanism and the influence of postural changes in flight in this emblematic animal and will facilitate the study of gliding flight in analogous gliding reptiles for which direct observations are unavailable.

Hostile Hemodynamics in Distal Stent Graft-Induced New Entry Prior to Aortic Rupture: A Comparison of Transient versus Steady-State CFD Simulations.

Computational fluid dynamics (CFD) simulations model blood flow in aortic pathologies. The aim of our study was to understand the local hemodynamic environment at the site of rupture in distal stent graft-induced new entry (dSINE) after frozen elephant trunk with a clinically time efficient steady-flow simulation versus transient simulations. Steady-state simulations were performed for dSINE, prior and after its development and prior to aortic rupture. To account for potential turbulences due geometric changes at the dSINE location, Reynolds-averaged Navier-Stokes equations with the realizable k-ε model for turbulences were applied. Transient simulations were performed for comparison. Hemodynamic parameters were assessed at various locations of the aorta. Post-dSINE, jet-like flow due to luminal narrowing was observed which increased prior to rupture and resulted in focal neighbored regions of high and low wall shear stress (WSS). Prior to rupture, aortic diameter at the rupture site increased lowering WSS at the entire aortic circumference. Concurrently, WSS and turbulence increased locally above the entry tear at the inner aortic curvature. Turbulent kinetic energy and WSS elevation in the downstream aorta demonstrated enhanced stress on the native aorta. Results of steady-state simulations were in good qualitative agreement with transient simulations. Steady-flow CFD simulations feasible at clinical time scales prior to aortic rupture reveal a hostile hemodynamic environment at the dSINE rupture site in agreement with lengthy transient simulations. Consequently, our developed approach may be of value in treatment planning where a fast assessment of the local hemodynamic environment is essential.

Modelling blood flow in patients with heart valve disease using deep learning: A computationally efficient method to expand diagnostic capabilities in clinical routine.

The computational modelling of blood flow is known to provide vital hemodynamic parameters for diagnosis and treatment-support for patients with valvular heart disease. However, most diagnosis/treatment-support solutions based on flow modelling proposed utilize time- and resource-intensive computational fluid dynamics (CFD) and are therefore difficult to implement into clinical practice. In contrast, deep learning (DL) algorithms provide results quickly with little need for computational power. Thus, modelling blood flow with DL instead of CFD may substantially enhances the usability of flow modelling-based diagnosis/treatment support in clinical routine. In this study, we propose a DL-based approach to compute pressure and wall-shear-stress (WSS) in the aorta and aortic valve of patients with aortic stenosis (AS). A total of 103 individual surface models of the aorta and aortic valve were constructed from computed tomography data of AS patients. Based on these surface models, a total of 267 patient-specific, steady-state CFD simulations of aortic flow under various flow rates were performed. Using this simulation data, an artificial neural network (ANN) was trained to compute spatially resolved pressure and WSS using a centerline-based representation. An unseen test subset of 23 cases was used to compare both methods. ANN and CFD-based computations agreed well with a median relative difference between both methods of 6.0% for pressure and 4.9% for wall-shear-stress. Demonstrating the ability of DL to compute clinically relevant hemodynamic parameters for AS patients, this work presents a possible solution to facilitate the introduction of modelling-based treatment support into clinical practice.

First insights into the role of wall shear stress in the development of a distal stent graft induced new entry through computational fluid dynamics simulations.

Distal stent graft induced new entry (dSINE) is an emerging complication after frozen elephant trunk (FET) procedure. The aim of this computational fluid dynamics (CFD) study was to investigate the role of wall shear stress (WSS) after the development of dSINE based on hemodynamic changes. Aortic diameter and WSS of five patients who developed a dSINE after FET procedure were retrospectively analyzed before and after the occurrence of dSINE. Patient-specific 3-dimentional surface models of the aortic lumen were reconstructed from computed tomography angiographic datasets (pre dSINE: n=5, dSINE: n=5) to perform steady-state CFD simulations with laminar blood flow and zero pressure outlet conditions. WSS was calculated at the level of the stent graft (SG), the landing zone of the SG and at a location further distal to the SG, as well as on the outer and inner curvature of the aorta from SG center to its distal end. Post dSINE occurrence, median WSS increased significantly from 0.87 [interquartile range (IQR): 0.83-1.03] to 1.55 (IQR: 1.09-2.70) Pa, (P=0.043) within the SG and from 1.22 (IQR: 0.81-1.44) to 1.76 (IQR: 1.55-3.60) Pa, (P=0.043) at the landing zone of the SG. A non-significant increase from 1.22 (IQR: 0.59-3.50) to 2.58 (IQR: 1.16-3.78) Pa, (P=0.686) further downstream was observed. WSS at the outer curvature of the SG was significantly higher compared to WSS at the inner curvature for dSINE. Adverse hemodynamic conditions in the form of elevated WSS consist inside and at the distal end of the SG as well as at the outer curvature of the aorta, which may contribute to weakening of the aortic wall. These new findings emphasize the relevance and potential of WSS in dSINE for additional adverse events, such as aortic rupture. Further prospective studies are warranted to explore if the combination of clinical parameters with WSS might be useful to decide which patients require an urgent reintervention in terms of a SG extension.

Parametric Study of Panel PCM–Air Heat Exchanger Designs

Heat exchangers, devices for the transfer of heat between two or more working fluids, are extensively used in cooling applications and heating applications. Heat exchangers in buildings are typically components of space-conditioning systems, as well as of water-heating applications. Heat exchangers are also sometimes used in applications that require storage and release of energy at specific times. Phase change materials (PCMs) enhance these heat-exchange processes, given their ability to melt and solidify at a fixed range of temperatures, absorbing or releasing significant amounts of latent heat. Five different configurations of PCM–air heat exchangers for thermal control in buildings are analyzed in this work. The heat exchangers were fitted with PCM encapsulated in plastic and composite pouches of various shapes, and packaged in stackable panel layers. Three-dimensional computational fluid dynamics (CFD) modeling of coupled incompressible fluid and conjugate heat transfer were performed on the designs. The phase change process was numerically modelled using the apparent heat capacity method. Steady-state CFD simulations provided quantification of pressure drop as a function of air flow velocity. Transient simulation results describe the thermal evolution of PCM in the pouches, helping to determine the best performing configuration with respect to total thermal charging time.

Research on the effect of visceral artery Aneurysm's cardiac morphological variation on hemodynamic situation based on time-resolved CT-scan and computational fluid dynamics

Background and ObjectiveMuscular arteries and related aneurysms keep deforming during the cardiac cycle. However, current patient-specific computational fluid dynamics (CFD) analyses of aneurysms are usually based on individual cardiac phase images. The cardiac deformation and displacement characteristics of muscle arteries and aneurysms, as well as their impact on CFD results, have not been adequately explored. The present study tried to illustrate the cardiac morphological variation of visceral muscular arteries (VMAs) & aneurysms (VAAs) and evaluate its influence on the hemodynamic situation at lesion locations. MethodsFour-dimensional computed tomography angiogram (4D-CTA) images of six patients with VAAs were acquired. Medical image registration is used to capture cardiac variations of VMAs. The steady-state CFD simulation is performed on twelve different time-phase geometries. Deformation, displacement, wall shear stress (WSS), velocity, and pressure values at pathological locations are compared to illustrate the deforming characteristics of VAAs and their influence on CFD simulation results. ResultsThe deformation and displacement characteristics of lesion locations for six specific patients show a pulsatile pattern. Maximum displacements are always less than 4 mm. The ratio fluctuations of endovascular cavity volume and vascular inner wall surface area, which were employed to depict cardiac deformation, are always less than 20%. According to CFD simulations based on deformed VMAs, WSS has a larger coefficient of variation (COV) than velocity and pressure. Except for one patient's WSS, the COVs of different hemodynamic parameters obtained from simulation results are always less than 10%. ConclusionsBased on 4D-CTA images, we confirmed that cardiovascular circulation has a periodic impact on the morphologic characteristics of VMAs. A wave that has extended throughout the studied region is observed. It has a dominant influence on the displacement of VMAs. According to CFD results, the influence of the VMAs' deformation and displacement on different hemodynamic parameters is distinct. The variance in WSS is more prominent compared to pressure and velocity. On most occasions, the influence of the VMAs’ periodic deformation and displacement on simulation results is insignificant. However, the variant simulation results induced by deforming VMAs cannot be simply ignored.

Airflow temperature and humidity patterns in a screenhouse with a flat insect-proof screen roof and impermeable sloping walls – Computational fluid dynamics (CFD) results

Insect-proof screenhouses are commonly used to grow plants in warm climates. However, there is relatively little literature on their microclimate compared to greenhouses. This study presents computational fluid dynamics (CFD) results of airflow, temperature, and humidity ratio patterns in a screenhouse with a roof consisting of a large flat insect-proof screen and impermeable walls. First, vertical profiles of velocity, temperature, and humidity at the center of the screenhouse were obtained by 2D steady-state CFD simulations and validated by experimental results. Root mean square error (RMSE) values were used to measure the differences between the two. The lowest RMSE values among simulations with different turbulence models were 0.49 K, 1.26 g kg −1 , and 0.05 m s −1 (with the RNG turbulence model) for temperature, humidity ratio, and air velocity, respectively. The main deviation of the CFD results from the experimental results was observed with the air velocity in the upper region of the screenhouse. Inflow and outflow in the leeward and windward parts of the flat roof were observed, respectively. This resulted in large-scale airflow within the screenhouse opposite the outside wind direction at the canopy level. The results suggested that the leeward section of the screenhouse is warmer than the windward one and has a lower humidity ratio. Large-scale rotating airflow formed in the center of the screenhouse, close to the roof, a large area with a humidity ratio similar to ambient conditions. • CFD simulations are used to determine airflow, temperature and humidity patterns. • CFD and experimental results were in agreement except of velocity near screenhouse top. • Airflow within the canopy of an insect-proof screenhouse was opposite to wind direction. • From windward to leeward ends the temperature increased and humidity decreased.

New thermal-hydraulic correlations for printed circuit heat exchangers (PCHEs) with zigzag channels under high Reynolds numbers

PurposeThe purpose of this paper is to investigate the impact of semi-circular zigzag-channel printed circuit heat exchanger (PCHE) design parameters on heat transfer and pressure drop of flows under high Reynolds numbers and provide new thermal-hydraulic correlations relevant to conditions encountered in natural gas processing plants.Design/methodology/approachThe correlations were developed using three-dimensional steady-state computational fluid dynamics simulations with varying semicircular channel diameter (from 1 to 5 mm), zigzag angle (from 15° to 45°) and Reynolds number (from 40,000 to 100,000). The simulation results were validated by comparison with experimental results and existing correlations.FindingsThe results revealed that the thermal-hydraulic performance was mostly affected by the zigzag angle, followed by the ratio of the zigzag channel length to the hydraulic diameter. Overall, smaller zigzag angles favored heat transfer intensification while keeping reasonably low pressure drops.Originality/valueThis study is, to date, the only one providing thermal-hydraulic correlations for PCHEs with zigzag channels under high Reynolds numbers. Besides, the broad range of parameters considered makes the proposed correlations valuable PCHE design tools.

Hydrodynamic Evaluation of Five Influent Distribution Systems in a Cylindrical UASB Reactor Using CFD Simulations

UASB reactors are a promising option for environmentally friendly wastewater treatment due to their reduced carbon footprint and their capacity to treat a variety of wastewater strengths, among other recognized advantages over alternative wastewater treatment systems. The Influent Distribution System (IDS) is a critical structure for generating granules in a UASB reactor since it provides the required flow hydrodynamics for their formation. Thus, the objective of this study was to evaluate and compare the efficiency of five IDS configurations to generate ideal granulation conditions using Computational Fluid Dynamics (CFD) simulations. The IDS configurations were as follows: (C1) single radial inflow, (C2) upward axial inflow, (C3) downward distributed axial inflow, and two novel configurations in the form of (C4) double opposite radial inflow and (C5) downward tangential inflow. The hydrodynamic response of configuration C1 was validated in a physical model with dynamic Froude similitude. The granulation measurement was velocity-based in the reactor reaction zone using steady-state CFD simulations. The novel IDS configuration C4 was the one that resulted in the highest granulation volume, with up to 45.5% of the potential granulation volume of the UASB reactor, in contrast to the IDS C2 that obtained the lowest granulation with only 10.8%. Results confirm that the IDS directly impacts the hydrodynamics of the reactor and that model-based design can be used to ascertain IDS configurations that better promote granulation in UASB reactors.

Geometrically induced wall shear stress variability in CFD-MRI coupled simulations of blood flow in the thoracic aortas

Aortic aneurysm is associated with aberrant blood flow and wall shear stress (WSS). This can be studied by coupling magnetic resonance imaging (MRI) with computational fluid dynamics (CFD). For patient-specific simulations, extra attention should be given to the variation in segmentation of the MRI data-set and its effect on WSS.We performed CFD simulations of blood flow in the aorta for ten different volunteers and provided corresponding WSS distributions. The aorta of each volunteer was segmented four times. The same inlet and outlet boundary conditions were applied for all segmentation variations of each volunteer. Steady-state CFD simulations were performed with inlet flow based on phase-contrast MRI during peak systole.We show that the commonly used comparison of mean and maximal values of WSS, based on CFD in the different segments of the thoracic aorta, yields good to excellent correlation (0.78–0.95) for rescan and moderate to excellent correlation (0.64–1.00) for intra- and interobserver reproducibility. However, the effect of geometrical variations is higher for the voxel-to-voxel comparison of WSS. With this analysis method, the correlation for different segments of the whole aorta is poor to moderate (0.43–0.66) for rescan and poor to good (0.48–0.73) for intra- and interobserver reproducibility.Therefore, we advise being critical about the CFD results based on the MRI segmentations to avoid possible misinterpretation. While the global values of WSS are similar for different modalities, the variation of results is high when considering the local distributions.

An orifice shape-based reduced order model of patient-specific mitral valve regurgitation

Mitral valve regurgitation (MR) is one of the most prevalent valvular heart diseases. Its quantitative assessment is challenging but crucial for treatment decisions. Using computational fluid dynamics (CFD), we developed a reduced order model (ROM) describing the relationship between MR flow rates, transvalvular pressure differences, and the size and shape of the regurgitant valve orifice. Due to its low computational cost, this ROM could easily be implemented into clinical workflows to support the assessment of MR. We reconstructed mitral valves of 43 patients from 3D transesophageal echocardiographic images and estimated the 3D anatomic regurgitant orifice areas using a shrink-wrap algorithm. The orifice shapes were quantified with three dimensionless shape parameters. Steady-state CFD simulations in the reconstructed mitral valves were performed to analyse the relationship between the regurgitant orifice geometry and the regurgitant hemodynamics. Based on the results, three ROMs with increasing complexity were defined, all of which revealed very good agreement with CFD results with a mean bias below 3% for the MR flow rate. Classifying orifices into two shape groups and assigning group-specific flow coefficients in the ROM reduced the limit of agreement predicting regurgitant volumes from 9.0 ml to 5.7 ml at a mean regurgitant volume of 57 ml.

High-performance building: Sensitivity analysis for simulating different combinations of components of a two-sided windcatcher

Traditional windcatchers with massive structures were used to capture the local wind stream to improve occupant thermal comfort. To obtain smaller configurations with enhanced/similar performance metrics, the individual components of a conventional windcatcher, i.e. inlet opening, divider, outlet, and height are widely studied in the literature. This work aims to (1) use an in-house developed and verified numerical model to explore the impact of combinations of components of a windcatcher on its performance and (2) introduce the implementation of nozzles with different configurations in obtaining a flexible design for an extended range of applications. A 3-dimensional steady-state computational fluid dynamics simulation with a shear-stress-transport k-ω turbulence model is used to explore the flow characteristics. The airflow distribution throughout the system and mass flow rate at the outlet of a reference case, a well-studied two-sided windcatcher design, are validated with published wind tunnel experimental data. The verified model is then used for the following analyses. The results highlight the role of different windcatcher individual components and their combinations on the developed pressure gradients inside the windcatcher. A desired combination of components to improve the aerodynamic performance of the reference case is introduced: the integration of a convergent-divergent nozzle with finned-curved inlet openings along with the longest dividers increase the induced air mass flow rate up to 13%. The maximum flow velocity is increased by 45% at the nozzle throat area for the identified windcatcher configuration. The outcomes of this research can be used by engineers and researchers to enhance building-integrated windcatcher designs.

Multi-zone indoor CFD under limited information: An approach coupling solar analysis and BIM for improved accuracy

Abstract It is important to monitor the indoor air quality and thermal comfort of an office environment for the wellbeing of its occupants, and, to do so, computational fluid dynamics simulation is more cost-effective than measuring an entire floor. Computational fluid dynamics simulation has been used by previous studies for single rooms and partitioned spaces, but not for office floors with multi-zone ventilation systems, and air infiltrations between different zones through closed doors have been neglected. Also, since it is often not possible to take measurements across an entire floor due to concerns of tenant privacy, few studies have used the limited obtainable field measurements to validate multi-zone computational fluid dynamics simulations. This study describes a methodology to conduct indoor multi-zone steady-state computational fluid dynamics simulation, with improved accuracy, on a typical office floor where there is limited information on carbon dioxide concentrations and temperatures. Heat and mass conservation equations were used to compensate for the lack of information. The mechanical ventilation and air conditioning layout was considered along with the sources of heat and carbon dioxide emissions. To improve the accuracy of the simulation on temperature, a solar analysis, based on building geometry, orientation, materials, location, and weather, was conducted to estimate any solar heat gain and distribution through curtain walls. Building information modeling supported the solar analysis and provided geometric information for the computational fluid dynamics simulation. The methodology was validated by a real case of a commercial building, where the accuracy of the temperature simulation improved by 9.9%.

Purge flow effects on rotor hub endwall heat transfer with extended endwall contouring into the disk cavity

Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines inject bleed air from the compressor into the stator/rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. The purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. This complex interaction causes non-uniform and jet-like penetration of the purge flow into the main flow field, hence affecting the endwall heat transfer on the rotor. To improve the understanding of purge flow effects on rotor hub endwall heat transfer, an unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal was tested. The measurements were conducted in the 1.5-stage axial turbine facility LISA at ETH Zurich, where a state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert was used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state computational fluid dynamics simulations were performed to complement the experiments. It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side, which is caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow locally reduces the heat transfer rate. Changes in adiabatic wall temperature and heat transfer (depending on purge rate) are observed from the platform start up to the cross passage migration of the secondary flow structures.

Optimization of dilution ventilation layout design in confined environments using Computational Fluid Dynamics (CFD)

Dilution ventilation systems have been widely used to control the airborne toxic and explosive material in confined spaces. Layout design of dilution ventilation is critical to industrial hygiene control and ventilation efficiency. A properly designed dilution ventilation system can significantly improve the safety of confined workshops and maintain a comfortable work condition. In this work, Computational Fluid Dynamics (CFD) has been used to analyze the performance of dilution ventilation system in the confined workshop environment. Seven different ventilation layouts are proposed to evaluate ventilation performance of different installation layouts. Carbon monoxide (CO), which has the similar density as air, is selected as the sample contaminant to conduct steady-state CFD simulations. The simulation results of different layouts are examined and compared to get the optimal layout design for the best contaminant control. Results have shown that the layout with two opposite inlets has the highest ventilation efficiency among seven proposed layouts. This work can serve as a reference to increase dilution ventilation efficiency and minimize the energy cost in general confined areas.

WearGP: A computationally efficient machine learning framework for local erosive wear predictions via nodal Gaussian processes

Computational fluid dynamics (CFD)-based wear predictions are computationally expensive to evaluate, even with a high-performance computing infrastructure. Thus, it is difficult to provide accurate local wear predictions in a timely manner. Data-driven approaches provide a more computationally efficient way to approximate the CFD wear predictions without running the actual CFD wear models. In this paper, a machine learning (ML) approach, termed WearGP, is presented to approximate the 3D local wear predictions, using numerical wear predictions from steady-state CFD simulations as training and testing datasets. The proposed framework is built on Gaussian process (GP) and utilized to predict wear in a much shorter time. The WearGP framework can be segmented into three stages. At the first stage, the training dataset is built by using a number of CFD simulations in the order of O(102). At the second stage, the data cleansing and data mining processes are performed, where the nodal wear solutions are extracted from the solution database to build a training dataset. At the third stage, the wear predictions are made, using trained GP models. Two CFD case studies including 3D slurry pump impeller and casing are used to demonstrate the WearGP framework, in which 144 training and 40 testing data points are used to train and test the proposed method, respectively. The numerical accuracy, computational efficiency and effectiveness between the WearGP framework and CFD wear model for both slurry pump impellers and casings are compared. It is shown that the WearGP framework can achieve highly accurate results that are comparable with the CFD results, with a relatively small size training dataset, with a computational time reduction on the order of 105 to 106.

Flux-dependent anisotropic pellet diffusivity in particle-resolved CFD simulations of fixed beds

The implementation of multicomponent diffusion in porous catalyst particles into resolved-particle fixed bed computational fluid dynamics (CFD) presents challenges. Usually an isotropic effective diffusivity is employed for each species, which includes molecular diffusion, Knudsen diffusion and viscous flow. To obtain this quantity, constant and isotropic molar flux ratios are assumed. In fixed bed models, pellets composed of isotropic materials are situated in non-isotropic fluid-phase gradients in the bed, which call the assumptions into question. In this paper we present an extended model using a non-isotropic effective diffusivity tensor, which allows evaluation of the flux assumptions. The method is illustrated for resolved particle steady-state CFD simulations of ethylene oxidation in small test beds of spheres and four-hole cylinders. Comparisons to the original constant molar flux ratio model are made, which show that the flux changes affect reaction, species and temperature profiles in the particles, and are stronger in particles with longer diffusion paths. Larger differences between results from the two diffusion models are also seen when the effective diffusivity is decreased by reducing porosity and increasing tortuosity, typical of a reaction system where deactivation increases by carbon deposition.

Drying Uniformity Analysis in a Tray Dryer: An Experimental and Simulation Approach

This study aimed to use a Computational Fluid Dynamics (CFD) simulation of a tray dryer to investigate the possible causes of non-uniformity in the absolute final product humidity after the drying process in an empirically constructed tray dryer. In the experimental part, turmeric, which is the rhizome of a perennial herbaceous plant with interesting properties for the cosmetic and food industries, was dried. Pieces of turmeric with identical dimensions were placed in different zones of the dryer and their initial and final humidity values were empirically measured. In the simulations, the momentum and energy equations were solved with ANSYS software (Student v.17.2). The K-epsilon model was used to model the turbulence and the boundary and process conditions were identical to those used in the experiment. Although the steady-state CFD simulations show a homogenous local temperature distribution in the dryer, the local turbulence intensity profiles display dead zones with low turbulence values. Furthermore, these dead zones are consistent with areas where the product exhibits higher humidity. Low local turbulence values can negatively affect the local mass transfer coefficient, which may explain the experimental results of this study.