Discrete Random Walk Model Research Articles

Application of computational fluid dynamics in the study of the caribbean petroleum corporation gasoline vapour dispersion

Computational Fluid Dynamics (CFD) is used to predict the dispersion of a gasoline vapour cloud. The model was validated using the results of the analysis of Buncefield incident (2005) in the UK and a new method of simulation that involves the interaction of liquid fuel and air during the dispersion was introduced. The Caribbean Petroleum Corporation (CAPECO) incident (2009) was simulated using the developed CFD modelling method.The results of CFD analysis of this incident focused on the release of flammable liquid fuels that was followed by the formation of vapour clouds. The effect of wind speed and the gasoline evaporation surface area were analysed.The CFD simulations showed that the main factors affecting the dispersion and behaviour of the vapour clouds are release surface area, wind speed, topography, turbulence and obstacles in the area. The simulation results showed similar behaviour to that observed in the real-life incidents in terms of the vapour cloud behaviour and the extent of damage.This work used the realizable k−ε model which was previously validated for the dispersion of dense gases. This turbulence model was helpful in the study of the wind speed (nil to low) characteristics of the CAPECO dispersion accident and the dispersion of the liquid gasoline droplets was successfully simulated using ANSYS FLUENT's built-in Discrete Phase Model (DPM). This work simulated evaporation of gasoline liquid and droplet dispersion in a wide area and from a large source with the presence of obstacles. Dispersion of the gasoline droplets was successfully accounted for by the inclusion of the DPM sub-models for cloud dispersion, namely, the stochastic tracking and the Discrete Random Walk model (DRW). It could be concluded that this work validates the feasibility of using the DPM sub-model application for the simulation of volatile liquids evaporation and dispersion through a domain that includes several obstacles and high levels of congestion.

Numerical Studies of Particle‐Gas Two‐Phase Flowing through Microshock Tubes

Microshock tubes are always used to induce shock waves and supersonic flows in aerospace and medical engineering fields. A needle‐free drug delivery device including a microshock tube and an expanded nozzle is used for delivering solid drug powders through the skin surface without any injectors or pain. Therefore, to improve the performance of needle‐free drug delivery devices, it is significantly important to investigate shock waves and particle‐gas flows induced by microshock tubes. Even though shock waves and multiphase flows discharged from microshock tubes have been studied for several decades, the characteristics of unsteady particle‐gas flows are not well known to date. In the present studies, three microshock tube models were used for numerical simulations. One microshock tube model with closed end was used to observe the reflected shock wave and flow characteristics behind it. The other two models are designed with a supersonic nozzle and a sonic nozzle at the exit of the driven section, respectively, to investigate particle‐gas flows induced by different nozzles. Discrete phase method (DPM) was used to simulate unsteady particle‐gas flows and the discrete random walk model was chosen to record the unsteady particle tracking. Numerical results were obtained for comparison with those from experimental pressure measurement and particle visualization. Shock wave propagation was observed to agree well with experimental results from numerical simulations. Particles were accelerated at the exit of microshock tube due to the reservoir pressure induced by reflected shock wave. Both sonic and supersonic nozzles were underexpanded at the end of microshock tubes. Particle velocity was calculated to be smaller than gas velocity, which results from larger drag of injected particles.

Improving efficiency of a square cyclone separator using a dipleg – a CFD-based analysis

The present study is mainly focused on proposing an effective way to improve the efficiency of a square cyclone separator. For this purpose, a dipleg is attached under the square cyclone to investigate its effect on the performance of square cyclone. A three-dimensional Computational Fluid Dynamics (CFD) simulation is done by solving the Reynolds-Averaged Navier-Stokes equations with the Reynolds stress model (RSM) turbulence model and applying the Eulerian-Lagrangian two-phase method. The turbulent dispersion of particles is predicted by the application of the Discrete Random Walk (DRW) model. The numerical results demonstrate that using dipleg although produced an increase in pressure drop but it positively enhances the separation efficiency of the square cyclone. Using dipleg significantly increases the separation efficiency of square cyclone especially at higher inlet velocity. This can be more obvious when using dipleg which is minimized the 50% cut size of square cyclone about 26.3%.

Performance assessment of a square cyclone influenced by inlet section modifications

The main objective of the present study was to analyze the effect of varying the inlet shape on the flow field and overall performance in a square cyclone. Three different inlet shapes, namely straight, inclined, and bend inlets, were specifically proposed to improve the low separation efficiency of the square cyclone. A three-dimensional numerical simulation was done by solving the Reynolds averaged Navier–Stokes equations with the Reynolds stress model turbulence model and applying the Eulerian–Lagrangian two-phase method. The turbulent dispersion of particles was predicted by the application of the discrete random walk model. Comprehensive comparisons of pressure drop, separation efficiency, 50% cut sizes, and flow field were performed to demonstrate the mechanism of how the inlet shape influenced the square cyclone performance. The results extracted from computational fluid dynamics simulation demonstrated that although using inclined and bend inlets produced an increase in pressure drop, they positively enhanced the separation efficiency of the square cyclone. Among all inlet shapes, inclined inlet significantly helped the square cyclone to collect finer particles, as it reduced the 50% cut sizes approximately 29.4% at an inlet velocity of 28 m/s.

Prevention of surgical site infection under different ventilation systems in operating room environment.

Biological particles in the operating room (OR) air environment can cause surgical site infections (SSIs). Various ventilation systems have been employed in ORs to ensure an ultraclean environment. However, the effect ofdifferent ventilation systems on the control ofbacteria-carrying particles (BCPs) released from the surgical staff during surgery is unclear. In this study, the performance of four different ventilation systems (vertical laminar airflow ventilation (VLAF), horizontal laminar airflow ventilation (HLAF), differential vertical airflow ventilation (DVAF), and temperature-controlled airflow ventilation (TAF)) used in an OR was evaluated and compared based on the spatial BCP concentration. The airflow field in the OR was solved by the Renormalization Group (RNG) k-ε turbulence model, and the BCP phase was calculated by Lagrangian particle tracking (LPT) and the discrete random walk (DRW) model. It was found that the TAF system was the most effective ventilation system among the four ventilation systems for ensuring air cleanliness in the operating area. This study also indicated that air cleanliness in the operating area depended not only on the airflow rate of the ventilation system but also on the airflow distribution, which was greatly affected by obstacles such as surgical lamps and surgical staff.

Numerical investigation on the origin of virus-laden droplets in the respiratory tract

There have been over 17 million COVID-19 infections worldwide up to July 31, 2020, but the exact transmission route remains a subject for debate. Virus load in different-sized respiratory droplets is the key to address the above question. Investigation on the droplet deposition characteristics during exhalation inside the respiratory tract will facilitate the understanding of their origin and importance in transmitting the respiratory infection. Based on a realistic respiratory model, this study utilized the computational fluid dynamic (CFD) simulation to investigate the deposition of droplets originating from infection sites like pharynx, larynx and trachea under three flow conditions, namely 30 L/min (representing the normal breathing condition), 60 L/min (speaking) and 180 L/min (coughing). The SST k - ω turbulence model integrated in ANSYS Fluent software was utilized to obtain the flow field inside the respiratory tract by solving the continuity and momentum equations, and the Lagrangian approach was used to calculate droplet motion and deposition with the discrete random walk model to account for the turbulence fluctuation effect. Evaporation of droplets was not considered inside the saturated respiratory tract. Droplet diameter, flow rate of the exhaling air, and complexity of the respiratory tract geometry are the most important factors in determining the deposition pattern of respiratory droplets. It is revealed that the largest droplet that originates from the joint of pharynx and larynx and is able to escape out of the respiratory tract is about 20 µm (30 L/min) or 10 µm (60–180 L/min) in diameter; the so-called cut-off sizes for vocal cord and trachea originated droplets are 7 µm (30 L/min), 5 µm (60 L/min) or 3 µm (180 L/min). Larynx and joint of larynx and pharynx are the most important deposition sites due to their complexity in geometry and the existence of the laryngeal jet, whose velocity is up to 102 m/s during coughing and 32 m/s during speaking; the nasal cavity is also effective in trapping relatively small droplets as the airflow has a sudden change in direction before entering the oral cavity. Under the investigated speaking scenario (60 L/min), the escape rate of 1 µm droplets is around 50%, and the maximum escape rates of 5 and 10 µm droplets are respectively 13.1% and 1.3% (originating from the joint of pharynx and larynx). Although large droplets up to several hundred micrometers can be produced inside the oral cavity due to the atomization mechanism, they seldom carry pathogen. Thus, for COVID-19 patents in the early stage of infection who show upper respiratory symptoms, the cut-off size of virus-laden droplets released into the environment is about 20 µm; with the shifting of infection to the lower respiratory tract in the later stage, the cut-off size decreases to 7 µm. Respiratory droplets evaporate immediately after escaping into the indoor environments and shrink to about one third of their initial size, so airborne route is speculated to be important in the transmission of COVID-19. Further investigations considering realistic flow conditions together with droplet sampling experiments on human volunteers are necessary to confirm these cut-off sizes.

Improved Discrete Random Walk Stochastic Model for Simulating Particle Dispersion and Deposition in Inhomogeneous Turbulent Flows

Abstract The performance of different versions of the discrete random walk models in turbulent flows with nonuniform normal root-mean-square (RMS) velocity fluctuations and turbulence time scales were carefully investigated. The OpenFOAM v2−f low Reynolds number turbulence model was used for evaluating the fully developed streamwise velocity and the wall-normal RMS velocity fluctuations profiles in a turbulent channel flow. The results were then used in an in-house matlab particle tracking code, including the drag and Brownian forces, and the trajectories of randomly injected point-particles with diameters ranging from 10 nm to 30 μm were evaluated under the one-way coupling assumption. The distributions and deposition velocities of fluid-tracer and finite-size particles were evaluated using the conventional-discrete random walk (DRW) model, the modified-DRW model including the velocity gradient drift correction, and the new improved-DRW model including the velocity and time gradient drift terms. It was shown that the conventional-DRW model leads to superfluous migration of fluid-point particles toward the wall and erroneous particle deposition rate. The concentration profiles of tracer particles obtained by using the modified-DRW model still are not uniform. However, it was shown that the new improved-DRW model with the velocity and time scale drift corrections leads to uniform distributions for fluid-point particles and reasonable concentration profiles for finite-size heavy particles. In addition, good agreement was found between the estimated deposition velocities of different size particles by the new improved-DRW model with the available data.

CFD-DEM simulation of pneumatic conveying in a horizontal pipe

In this paper, the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) is used to study pneumatic conveying in a horizontal pipe. A new three-dimensional modified wall roughness (WR) model and the discrete random walk (DRW) model are introduced into the CFD-DEM approach to simulate the effect of wall roughness and the random effect of turbulence on particle dispersion respectively. The interaction between particles and fluid of the pneumatic conveying in a horizontal pipe is calculated by both the one-way coupling method and the two-way coupling method. The flow characteristics of the pneumatic conveying in a horizontal pipe, including the particle velocity, the particle mass flux, and the particle trajectory are obtained by the simulations. The particle mass flux predicted by the CFD-DEM with WR-DRW is quite different with that predicted by the traditional CFD-DEM approach although the particle horizontal velocities obtained by two different approaches are similar, and the distribution of particles across the pipe cross section predicted by the CFD-DEM with WR-DRW is more uniform than that predicted by the traditional CFD-DEM approach. The accuracy of the CFD-DEM with WR-DRW is validated by the corresponding experiments from literatures. Comparing to the computational fluid dynamics - discrete phase model (CFD-DPM) without considering collision between particles, the CFD-DEM with WR-DRW developed in this work can be applied to study the pneumatic conveying in a horizontal pipe under both dilute and dense phase conditions.

CFD simulation of a novel design of square cyclone with dual-inverse cone

The objective of the present study is to propose a novel design to improve the separation efficiency of the conventional square cyclone. For this purpose, the conical section of the conventional square cyclone with single-cone is modified to dual inverse-cone. In addition, the effect of second-cone length on the performance of cyclone is considered. A three-dimensional numerical simulation is done by solving the Reynolds averaged Navier-Stokes equations with the Reynolds Stress Model (RSM) turbulence model and applying the Eulerian-Lagrangian two-phase method. The turbulent dispersion of particles is predicted by the application of the Discrete Random Walk (DRW) model. The numerical results demonstrate that dual inverse-cone square cyclone although produces higher pressure drop but its separation efficiency is higher than the square cyclone with single-cone. This is due to a smaller separation zone and shorter path of particle movements which force the particles exit from the outlet section of the cyclone. Finally, using dual-inverse cone square cyclone reduces the 50% cut size about 10% and 30% for inlet velocities of 12 m/s and 28 m/s, respectively.

CFD–DEM simulation of pneumatic conveying in a horizontal channel

In this paper, the pneumatic conveying in a horizontal channel is simulated by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM). A modified wall roughness model and the Discrete Random Walk (DRW) model are introduced into the CFD–DEM approach to investigate the effect of the wall roughness and the random effect of the turbulence on particle dispersion respectively, and this combined approach is named as the improved CFD–DEM approach. The interaction between the particles and the fluid is calculated by both the one-way coupling method and the two-way coupling method. The accuracy of the improved CFD–DEM approach is validated by corresponding experiments from the literatures. By using the improved CFD–DEM approach, several dense channel gas–solid flows are simulated. The simulation results indicate that with the increase of mass loading rate, the particle agglomeration is more significant, and the trend of increasing the particle concentration near the upper and lower walls is more obvious. This research shows that the improved CFD–DEM approach developed in this work is an effective tool for studying the particle phase properties under both dilute and dense phase conditions.

Simulation of Particle Resuspension Caused by Footsteps

In order to explore the cause of particle resuspension by footsteps, a footstep motion model was developed. The transient calculation method is used to simulate the resuspension of particles around footsteps. The particles are tracked by Discrete Random Walk (DRW) model based on Lagrange method. The Discrete Phase Model (DPM) is used to release particles in the simulation. The vortex, velocity vector, streamline diagram and particle motion were analyzed by numerical simulation. The results show that the footsteps cause resuspension of particles. At high footstep velocity, vortices are generated. The vortices occur at the periphery of the footsteps, and the air flow in the whole space is more symmetrical. Over time, during the whole footsteps, the vortices experience the process of occurrence, growth and annihilation. Due to the vortex in the air, the particles originally deposited on the ground will resuspend.

The Interaction between Erosion Particle and Gas Stream in High Temperature Gas Burner Rig for Thermal Barrier Coatings

Abstract Thermal barrier coatings (TBCs) as a kind of temperature-resistance materials have been widely applied in super high temperature components in aircraft engines. However, TBCs are subjected to harsh service environment such as high temperature oxidation and erosion, which lead to the coating failure. It is important to investigate the effect of fire temperature, angle and velocity of particle on erosion to understand the failure mechanism. In this paper, the temperature and velocity distributions of erosion particles in high temperature gas burner rig are investigated by using the fluid–solid coupling method with the discrete random walk model. The results show that a non-uniform distribution of temperature appears in different positions of the central axis, and the temperature of particle is affected obviously by the gas stream and particle size. The trajectory of particles and velocity diagrams under different particle size are determined by coupling the continuous phase with the erosion particles.

Improving efficiency of conventional and square cyclones using different configurations of the laminarizer

In the present study, effects of different configurations of the laminarizer including inlet, vortex finder and both inlet and vortex finder on the separation efficiency, pressure drop, tangential velocity, and 50% cut size are analyzed. The computational fluid dynamics (CFD) method is applied to predict and demonstrate the flow field inside both conventional cylindrical style and square cyclones. The Reynolds stress model (RSM) is used to simulate the turbulent flow field. Particle trajectories are calculated via discrete phase model (DPM). The discrete random walk (DRW) model is used to model the turbulent dispersion of particles. The results show that installing both inlet and vortex finder laminarizers has the most effect on increasing separation efficiency compared to other configurations of the laminarizer. The cyclone with both inlet and vortex finder laminarizers has the smallest 50% cut size in both conventional and square cyclones. Installing the laminarizer caused a minor increase in the pressure drop for all cases. By installing the laminarizer at inlet and vortex finder simultaneously, a minor increase of about 4.6% and 6.4% is obtained for conventional and square cyclones at the inlet velocity of 18 m/s, respectively. Also, installing the laminarizer at both inlet section and vortex finder of conventional and square cyclones reduced the 50% cut size at inlet gas velocity of 18 m/s about 8% and 27%, respectively. This reduction proved that using the laminarizer in the square cyclone was more effective than the conventional cyclone.

Effect of Turbulence Modeling on the Melt Flow and Inclusions Transport in a Steel Filtration Experiment

The current article deals with the effect of turbulence modeling on inclusions transport and melt flow in an induction crucible furnace (ICF), which was employed in order to investigate the efficiency and the performance of a ceramic filter. Furthermore, the influence of the discrete random-walk dispersion model on the behavior of inclusions in the ICF was investigated. Different turbulence models were employed in order to predict the turbulent melt flow. The numerical results show that the flow field is affected by the turbulence modeling method. Moreover, the distribution of the turbulent kinetic energy depends considerably on the choice of the turbulence model. In addition, the turbulence model and dispersion model also affect the inclusion transport in the melt. The filtration rate is also affected by the choice of the turbulence model.

Numerical study of fluid flow and particle dispersion and deposition around two inline buildings

In the present study, turbulent airflows and particle deposition around two inline buildings were investigated numerically. A computational modeling approach including the RNG k-ε and Realizable k-ε turbulence models were used for these simulations. The Lagrangian particle tracking approach was implemented for evaluating dispersion and deposition of spherical dust particles. For simulation of turbulence fluctuations, an improved discrete random walk (DRW) model that includes the near wall correction was implemented into several user-defined functions (UDFs) that were linked to the ANSYS-Fluent code. It was shown that the new improved DRW stochastic model led to results that are more accurate compared to the standard model. The improved DRW model was then used for simulating turbulent deposition and dispersion of dust around building models. The presented results showed that putting buildings on elevated supports reduces dusts deposition from downstream of single and inline buildings at short distances, particularly for small particles of about 1 μm. It is also shown that particle deposition fractions around buildings on rough ground are higher than those for smooth ground. Deposition fractions were also predicted by the improved model for faces of single and inline buildings.

Effects of Turbulent Dispersion on Water Droplet Impingement Based on Statistics Method

Research on the water droplet motion and impact on surface is the basis of aircraft icing prediction and design of ice protection system. First, a new computational method based on statistical theory was developed in the Euler–Lagrange framework to calculate the local water impingement coefficient on the surface, which could be used to study the effect of turbulent dispersion on the water droplet motion and impingement characteristics. Second, based on the RANS model for the air flow field, taking engine cone surface with or without slot as examples, the influence of turbulent dispersion on the water impingement coefficient on the surface was comparatively analyzed with the discrete random walk model. For the cone without film slot, turbulence causes a little impact on water droplet impingement. However, for the slotted cone, water droplet collection calculation must consider the effect of turbulence due to the stronger turbulence induced by the jet air. Further studies show that the effect is enhanced with the decrease of water droplet diameter. Finally, discussion about turbulent effects was proposed. This research about the influence of turbulent dispersion will be helpful to improve the accuracy of the water droplet impingement calculation for some complex geometries.

Influence of incoming air conditions on fuel spray evaporation in an evaporating chamber

The main purpose of this article is to investigate the effects of pressure and temperature of incoming air into an evaporating chamber on fuel spray evaporation and fuel droplets characteristics. Before that, the influences of secondary break-up and coupling between droplets and gas phase on the present spray modeling are studied. An Eulerian-Lagrangian approach and two-way coupling model are implemented for modeling spray and atomization of liquid fuel in air. The SSTk-ω turbulence model is applied to estimate eddy viscosity, whilst the discrete random walk model is employed to trace the droplets in a turbulent flow. With increasing inlet air temperature from 373 to 573 K, the lifetime and penetration depth of droplets are reduced by about 70% and 46%, respectively. The influence of incoming air pressure on droplets lifetime is not remarkable. The penetration of droplets is reduced by almost 61% while augmenting the air pressure from 1.0 to 10.0 atm.

Evaluation of the turbulence models for gas flow and particle transport in URANS and LES of a cyclone separator

Gas-solid separators have played an important role for process or emission control of many facilities including fluidized bed reactors. The flow field and collection efficiency were investigated for a Stairmand- type cyclone separator by Reynolds stress transport model (RSTM) and large eddy simulation (LES) in this work. RSTM and LES both showed good agreement with measured mean tangential velocity, whereas LES reproduced unsteady flow in the core with better agreement of the mean axial velocity profile. The particle dispersion model is required to obtain an accurate overall collection efficiency in both RSTM and LES. Anisotropy in velocity fluctuation should be taken into account, whereas its effect is more significant in RSTM than in LES. The continuous random walk (CRW) model gave more accurate results than the discrete random walk (DRW) model in RSTM, whereas there was no significant difference in LES resolving most large scale eddies. LES made better prediction of the overall particle collection efficiency than RSTM, although further investigation may be required for deviation in the maximum inlet flow case of the cyclone separator.

Coupling of CFD and semiempirical methods for designing three-phase condensate separator: case study and experimental validation

This study presents an approach to determine the dimensions of three-phase separators. First, we designed different vessel configurations based on the fluid properties of an Iranian gas condensate field. We then used a comprehensive computational fluid dynamic (CFD) method for analyzing the three-phase separation phenomena. For simulation purposes, the combined volume of fluid–discrete particle method (DPM) approach was used. The discrete random walk (DRW) model was used to include the effect of arbitrary particle movement due to variations caused by turbulence. In addition, the comparison of experimental and simulated results was generated using different turbulence models, i.e., standard k–ε, standard k–ω, and Reynolds stress model. The results of numerical calculations in terms of fluid profiles, separation performance and DPM particle behavior were used to choose the optimum vessel configuration. No difference between the dimensions of the optimum vessel and the existing separator was found. Also, simulation data were compared with experimental data pertaining to a similar existing separator. A reasonable agreement between the results of numerical calculation and experimental data was observed. These results showed that the used CFD model is well capable of investigating the performance of a three-phase separator.

Numerical study on polydisperse particle deposition in a compact heat exchanger

In this paper, the effect of particle size on deposition in a compact heat exchanger is investigated numerically. The effect of flow velocity and particle mass on the deposition for different particle size is also studied and discussed. Particle motions are simulated using the Lagrangian approach and discrete particle model (DPM). The deposition is modeled applying user defined function (DEFINE_DPM_EROSION). Flow simulation is performed using the Eulerian approach by solving Reynolds-Averaged Navier-Stokes (RANS) equations. Turbulence is simulated with the Realizable k-epsilon model applying the Discrete Random Walk (DRW) model to account turbulent velocity fluctuations. Also, enhanced wall treatment is used with turbulence model. A computational study is done on a 3D five fin channels of a compact heat exchanger. The air flow is entered from upstream with velocity over a range from 1m/s to 5m/s and six polydisperse particles groups with various diameters from 1μm to 1500μm, are introduced into the computational domain. The results show that most of the particles deposited on the front of channels and also on the first, second and last edges of fin channels. Besides, deposition ratio increased with the increase of particle diameter, up to a maximum value, and then it decreased. The effect of injecting small particles with larger size particles is also studied, and their results showed an increase in particle deposition. Results show that velocity increase can promote or hinder particle deposition.