Axisymmetric Computational Fluid Dynamics Model Research Articles

Analysis of the Multiphase Flow With Condensation in the Two-Phase Ejector Condenser Using Computational Fluid Dynamics Modeling

Abstract The liquid-driven two-phase ejector condenser is the object of the numerical investigation. The spray-ejector condenser is one of the critical components of the developed gas power plant with negative CO2 emission. The task of the ejector is to entrain exhaust gas and condense steam contained in it. Computational fluid dynamics (CFD) modeling allows analyzing complex phenomena and predicting the influence of a wide range of operating parameters on the local structure of the multiphase flow with condensation. The geometrical model of the ejector was designed to provide efficient steam condensation and generate sub-pressure region at the gas inlet. The 2D, axisymmetric CFD model was created using simcenter star ccm+ software. The multiphase mixture model was used to take into account two-phase flow. Turbulent flow was computed using k–ω SST model. Direct contact condensation of steam was calculated using two different approaches: the Spalding/evaporation model and the thermally-driven boiling/condensation model. The influence of various gas inlet velocities and the presence of CO2 on the operation of the ejector condenser were investigated based on scalar fields and charts representing changes in the most important variables along the flow path. The condensation is the most intense in the suction chamber. The boiling/condensation model predicts lower suction pressure and higher condensation effectivity than Spalding/evaporation model. The CO2 considerably affects the pressure and temperature distributions and reduces the condensation rate.

Performance assessment of an improved gasifier stove using biomass pellets: An experimental and numerical investigation

This article presents an experimental and computational study of a forced draft cookstove having separate primary and secondary air fans, while utilizing pellets as fuel. A two-dimensional axisymmetric computational fluid dynamics model of the developed cookstove has been created in ANSYS Fluent to analyze the fluid flow, temperature distribution and heat loss from the different parts of the cookstove. The simulation results showed that more than one fourth of the total heat produced by the burning of fuel was being lost to the ambient environment through the outermost wall of cookstove. Also, the temperature of the outer wall of the cookstove was found to be higher than the temperature of secondary air being preheated in the annulus chamber. Therefore, the developed model was further modified by using glass wool insulation which resulted in an increment of 5.7% in thermal efficiency, while the emissions of CO and PM2.5 were reduced by 7.1% and 25.9%, respectively. The performance of the developed models have also been compared with other pellet based forced draft models available globally, and the thermal efficiency of the Mimi Moto cookstove was found to be highest followed by FD 2.2 model.

On the Hierarchy of Models for Pipe Transients: From Quasi-Two-Dimensional Water Hammer Models to Full Three-Dimensional Computational Fluid Dynamics Models

Abstract A hierarchy of models exists in the literature for the simulation of pipe transients. One-dimensional (1D) water hammer models provide a cost-effective tool for the analysis of such transients. Traditional 1D models implement a quasi-steady approximation of the frictional term, which results in poor modeling of the attenuation of the transient. To improve the modeling of the attenuation phenomenon, alternative unsteady friction models were developed for the 1D water hammer formulation. Moreover, quasi-two-dimensional (quasi-2D) water hammer models were introduced, which allow the computation of the unsteady velocity profile and hence provide improved modeling of the attenuation phenomenon. Recently, interest has developed in the use of computational fluid dynamics (CFD) models based on the Navier–Stokes equations in the simulation of fluid transients. Both axisymmetric and full three-dimensional (3D) CFD models are used in this regard. The aim of the current paper is to carry out a comparative study between the performance of quasi-2D water hammer models, axisymmetric CFD models, and full 3D CFD models. Numerical computations using the three models are performed for both laminar and turbulent flow cases. Present results show that the quasi-2D water hammer model and the axisymmetric CFD model provide near identical results in terms of computing the magnitude, phase, and attenuation of the transient. Reported results also demonstrate the computational efficiency of the quasi-2D model, which provides results that agree reasonably well with the full 3D CFD model results while using a grid density, which is an order of magnitude lower than the grid requirements for the full 3D CFD model.

CFD investigation on characteristics of heavy crude oil flow through a horizontal pipe

Transportation of heavy crude oil via pipelines possesses many technological issues that are inherently flow related. Accurate prediction of flow characteristics is an essential step for a reliable piping design of transporting the crude oil. A rheology-based Computational Fluid Dynamics (CFD) model of the Iraqi heavy crude oil flow through a horizontal pipe (1 m length of 3/4 in. inside diameter) was developed using the commercial software Ansys 15 Fluent. By using power law rheological model, the Iraqi heavy crude oil exhibits a non-Newtonian dilatant behavior over the examined shear rate range of 1–40 s−1. The proposed axi-symmetric CFD model identifies velocity profile and generates values of friction factor, which are validated with experimental measurements. Additionally, wall shear stress and entrance length were numerically predicted and compared with well-established correlations from the literature for Non-Newtonian flow. Detailed results of the CFD model exhibited a reliable prediction of the characteristics of heavy crude oil flow.

Computational Fluid Dynamics (CFD) Modeling and Simulation of Flow Regulatory Mechanism in Artificial Kidney Using Finite Element Method.

There is an enormous need in the health welfare sector to manufacture inexpensive dialyzer membranes with minimum dialysis duration. In order to optimize the dialysis cost and time, an in-depth analysis of the effect of dialyzer design and process parameters on toxins (ranging from tiny to large size molecules) clearance rate is required. Mathematical analysis and enhanced computational power of computers can translate the transport phenomena occurring inside the dialyzer while minimizing the development cost. In this paper, the steady-state mass transport in blood and dialysate compartment and across the membrane is investigated with convection-diffusion equations and tortuous pore diffusion model (TPDM), respectively. The two-dimensional, axisymmetric CFD model was simulated by using a solver based on the finite element method (COMSOL Multiphysics 5.4). The effect of design and process parameters is analyzed by solving model equations for varying values of design and process parameters. It is found that by introducing tortuosity in the pore diffusion model, the clearance rate of small size molecules increases, but the clearance rate of large size molecules is reduced. When the fiber aspect ratio (db/L) varies from 900 to 2300, the clearance rate increases 37.71% of its initial value. The results also show that when the pore diameter increases from 10 nm to 20 nm, the clearance rate of urea and glucose also increases by 2.09% and 7.93%, respectively, with tolerated transport of albumin molecules.

Prediction of the cohesive zone in a blast furnace by integrating CFD and SVM modelling

ABSTRACT High efficiency and clean emission operation of a blast furnace are mainly determined by its operating stability, which is closely related to the position of the cohesive zone (CZ) in the blast furnace. In order to monitor the CZ position in real time, a prediction model of the CZ was proposed by combining an off-line computational fluid dynamics (CFD) calculation and support vector machine (SVM) modelling. An axisymmetric two-dimensional steady-state CFD model was established to simulate the fluid flow, heat and mass transfer in the blast furnace shaft. The sample dataset of the CZ information was established based on the CFD off-line calculation, and the prediction of the CZ position was realized by SVM modelling. The results show that the established model can predict the CZ position in real time with high accuracy, providing effective guidance to the blast furnace operation and control.

Integrating Feedback Control and Run-to-Run Control in Multi-Wafer Thermal Atomic Layer Deposition of Thin Films

There is currently a lack of understanding of the deposition profile in a batch atomic layer deposition (ALD) process. Also, no on-line control scheme has been proposed to resolve the prevalent disturbances. Motivated by this, we develop a computational fluid dynamics (CFD) model and an integrated online run-to-run and feedback control scheme. Specifically, we analyze a furnace reactor for a SiO2 thin-film ALD with BTBAS and ozone as precursors. Initially, a high-fidelity 2D axisymmetric multiscale CFD model is developed using ANSYS Fluent for the gas-phase characterization and the surface thin-film deposition, based on a kinetic Monte-Carlo (kMC) model database. To deal with the disturbance during reactor operation, a proportional integral (PI) control scheme is adopted, which manipulates the inlet precursor concentration to drive the precursor partial pressure to the set-point, ensuring the complete substrate coverage. Additionally, the CFD model is utilized to investigate a wide range of operating conditions, and a regression model is developed to describe the relationship between the half-cycle time and the feed flow rate. A run-to-run (R2R) control scheme using an exponentially weighted moving average (EWMA) strategy is developed to regulate the half-cycle time for the furnace ALD process between batches.

Prediction of water velocities in circular aquaculture tanks using an axisymmetric CFD model

Computational fluid dynamics (CFD) software was used to construct two-dimensional axisymmetric simulations of the turbulent flow inside 1.5, 5, 9.15 and 10 m tanks containing rotating water. The water rotation was induced mathematically by either placing a rotating cover on the surface of the water or locating a tangential water inlet near the top of the side wall. Central and side drains evacuated the water from the tank when using an inlet. The rotating cover produced a forced vortex in the tanks similar to the one observed in actual rearing tanks when the majority of the flow is leaving via the side drain. The predicted flow structure in tanks with an inlet can be divided into three regions: a narrow region along the side wall where the rotating water is moving down toward the floor, a thin boundary layer along the floor where the rotating water is moving radially inward, and a large region in the bulk of the tank where radial and axial velocities are small and the tangential velocity is independent of elevation but varies with radial position. Calculations were performed with the k-ε (RNG) and Reynolds stress turbulence models. Converged solutions were easier to obtain with the k-ε (RNG) model but only the Reynolds stress model could predict the strong vortex which is experimentally observed to form in the central portion of the tank as the flow leaving via the center drains is increased. The simulations predict that the thickness of the floor boundary layer is proportional to the tank diameter and that the radial velocity within the floor boundary layer is maximum at an elevation equal to about 10% of the boundary layer thickness. At any given radial position, the maximum value of the radial velocity next to the floor is between 15 and 45% of the tangential velocity in the bulk of the tank. Predicted shear stresses along bounding surfaces were used to obtain correlations for the side-wall and floor friction coefficients in terms of the Reynolds number. These correlations were in turn substituted in an overall moment balance to obtain an analytical model for predicting the maximum tangential velocity Vθw near the side wall of multi-drain rearing tanks. The predictions of the analytical model are in excellent agreement with Vθw values reported in the literature for tanks with diameters ranging from 1.5–15 m and indicate that the drag from inlet structures is non-negligible. The results demonstrate that the axisymmetric CFD model can simulate the main features of the rotating flow inside circular tanks and provide valuable boundary-layer information that is difficult to obtain experimentally.

Model predictive control of phthalic anhydride synthesis in a fixed-bed catalytic reactor via machine learning modeling

This work proposes a general framework for linking a state-of-the-art computational fluid dynamics (CFD) solver, ANSYS Fluent, and other computing platforms using the lock synchronization mechanism in an effort to extend the utilities of CFD solvers from strictly modeling and design to also control and optimization applications. To demonstrate the effectiveness of the proposed approach, a challenging control problem in chemical engineering, i.e., maximizing the product yield and suppressing the hot-spot temperature in a fixed-bed reactor (FBR) with a highly exothermic reaction, is considered. Specifically, phthalic anhydride (PA) synthesis is chosen for this investigation because of its industrial significance and its extreme high exothermicity. Initially, a high-fidelity two-dimensional axisymmetric heterogeneous CFD model for an industrial-scale FBR is developed in ANSYS Fluent. Next, the CFD model is used to explore a wide operating regime of the FBR to create a database, from which recurrent neural network and ensemble learning techniques are used to derive a homogeneous ensemble regression model using a state-of-the-art application program interface (API), i.e., Keras. Then, a model predictive control (MPC) formulation that is designed to drive the process output to the desired set-point and suppress the magnitude of the hot-spot temperature to avoid catalyst deactivation is developed using the ensemble regression model. Subsequently, the CFD model, the ensemble regression model and the MPC are combined to create a closed-loop system by linking ANSYS Fluent to SciPy (a Python library used for scientific computing) via a message-passing interface (MPI) with lock synchronization mechanism. Finally, the simulation data generated by the closed-loop system are used to demonstrate the robustness and effectiveness of the proposed approach.

Machine Learning-Based Model Predictive Control of Distributed Chemical Processes

Abstract This work proposes a general framework for linking of a state-of-the-art computational fluid dynamics (CFD) solver, ANSYS Fluent, and other computing platforms using the lock synchronization mechanism in an effort to extend the utilities of CFD solvers from strictly modeling and design to also control and optimization applications. Specifically, phthalic anhydride (PA) synthesis is chosen for this investigation because of its industrial significance and its extreme high exothermicity. Initially, a high-fidelity two-dimensional axisymmetric heterogeneous CFD model for an industrial-scale FBR is developed in ANSYS Fluent. Next, the CFD model is used to explore a wide operating regime of the FBR to create a database, from which recurrent neural network and ensemble learning techniques are used to derive a homogeneous ensemble regression model using a state-of-the-art application program interface. Then, a model predictive control (MPC) formulation that is designed to drive the process performance to the desired set-point and to avoid catalyst deactivation is developed using the ensemble regression model. Subsequently, the CFD model, the ensemble regression model and the MPC are combined to create a closed-loop system by linking ANSYS Fluent to SciPy via a message-passing interface (MPI) with lock synchronization mechanism. Finally, the simulation data generated by the closed-loop system are used to demonstrate the robustness and effectiveness of the proposed approach.

Effect of cooling pattern on centrifugal casting mold stress–strain distribution through computational fluid dynamics and finite element method analysis

A two-dimensional axisymmetric computational fluid dynamics model is developed to predict the transient temperature distribution in the ductile iron pipe as well as in the rotating mold during the horizontal centrifugal casting process. The transient temperature data of the mold generated from the computational fluid dynamics model has been applied as input to the finite element method based mechanical analysis to predict the stress and strain acting on the mold during cyclic thermal loading. The simulations have been performed for five different cases varying the cooling pattern of the mold to optimize the water flow rate and minimize the thermal strain developed in the mold. In the first three cases simulated, the water flow has been decreased only after extracting the cast pipe from the mold. In case 4, the flow rate has been decreased at the start of the casting, increased to 100% after 10 s and again decreased after the pipe extraction till the next cast. The water flow rate has been tripled in case five after the solidified pipe extraction from the mold. The computational fluid dynamics results have been validated against the measured temperature with an optical pyrometer at the plant, and the tangential stress calculated by finite element analysis model has been reasonably validated against the data available in the literature. From the results of computational fluid dynamics and finite element method analyses, case 4 experiences lower von Mises equivalent stress range and plastic equivalent strain at the critical region of the mold near socket section compared to other cases.

Melting of graphene based phase change nanocomposites in vertical latent heat thermal energy storage unit

The present study deals with the investigation of melting phenomena in single vertical shell-and-tube latent heat thermal energy storage unit. A two dimensional axi-symmetric computational fluid dynamics model based on the enthalpy-porosity method was developed to investigate the melting behaviour. Organic alkane n-eicosane and n-eicosane/graphene nanosheets with different volume fractions were considered as the phase change materials (PCMs). Water was considered as the heat transfer fluid (HTF) flowing inside the tube and the PCM is filled in the shell side of thermal energy storage unit. A variety of numerical simulations were performed for different heat transfer fluid inlet temperatures and varying loadings of graphene nanosheets. Numerical calculations show that higher inlet temperature of the heat transfer fluid decreases the melting time due to accelerated natural convection. We also show that the inclusion of graphene nanosheets significantly decreases the melting time due to the enhanced thermal conductivity of PCM. At 2vol% graphene laoding, melting time reduces significantly by ∼41% when the HTF temperature is 60°C and ∼37% when the HTF temperature is 70°C.

Jet pumps for thermoacoustic applications: Design guidelines based on a numerical parameter study

The oscillatory flow through tapered cylindrical tube sections (jet pumps) is characterized by a numerical parameter study. The shape of a jet pump results in asymmetric hydrodynamic end effects which cause a time-averaged pressure drop to occur under oscillatory flow conditions. Hence, jet pumps are used as streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional axisymmetric computational fluid dynamics model is used to calculate the performance of a large number of conical jet pump geometries in terms of time-averaged pressure drop and acoustic power dissipation. The investigated geometrical parameters include the jet pump length, taper angle, waist diameter, and waist curvature. In correspondence with previous work, four flow regimes are observed which characterize the jet pump performance and dimensionless parameters are introduced to scale the performance of the various jet pump geometries. The simulation results are compared to an existing quasi-steady theory and it is shown that this theory is only applicable in a small operation region. Based on the scaling parameters, an optimum operation region is defined and design guidelines are proposed which can be directly used for future jet pump design.

CFD simulation of a transpiring‐wall SCWO reactor: Formation and optimization of the water film

A two‐dimensional axisymmetric computational fluid dynamics model of a transpiring wall reactor for supercritical water oxidation was developed using the commercial software Fluent 6.3. Numerical model was validated by comparisons with experimental temperature profiles and product properties (total organic carbon and CO). Compared with the transpiration intensity, the transpiring water temperature was found to have a more significant influence on the reaction zone. An assumption that an ideal corrosion and salt deposition inhibitive water film can be formed when the temperature of the inner surface of the porous tube is less than 374°C was made. It was observed that lowering transpiring water temperature is conducive to the formation of the water film at the expense of feed degradation. The appropriate mass flux ratio between the total transpiring flow and the core flow was determined at 0.05 based on the formation of the water film and feed degradation. © 2015 American Institute of Chemical Engineers AIChE J, 62: 195–206, 2016

Investigation of Natural Convection in Heat Generating Molten Nuclear Fuel and Assessment of Core Damage Propagation

Total instantaneous blockage (TIB) is a severe subassembly accident in a sodium cooled fast reactor. During such an accident, a heat generating fuel pool is formed which is bounded by six neighboring subassemblies which are force-cooled by sodium. The molten fuel pool attacks the walls of the neighboring hexcan, melting them layer by layer. The rate of propagation of such damage and the temperature rise in sodium due to heat transfer from fuel pool through hexcan wall are investigated by a two-step mathematical approach. In the first step, natural convection in the fuel pool is studied by a 2D axisymmetric computational fluid dynamic model and correlations for effective conductivity as a function of internal Rayleigh number and aspect ratio have been developed. In the second step, rate of damage propagation to the hexcan wall and sodium temperature rise are predicted by a 1D transient enthalpy model. It is found that rate of damage propagation is accelerated by natural convection inside the pool. Further, the rate of heat transfer to neighboring subassembly sodium also increases due to natural convection in the pool. Eventually, the residual thickness of hexcan at the time of reactor trip is found to be insensitive to the presence/absence of natural convection in the pool.

Computational fluid dynamics simulation of hydrodynamics in USP apparatus 3-the influence of dip rate.

This study investigated the influence of dip rate on USP Apparatus 3 hydrodynamics in the presence of a solid dosage form (e.g. tablet) using Computational Fluid Dynamics (CFD) simulations. The primary variables of interest were the liquid phase velocity in the computational domain and wall shear stresses on the tablet surfaces. Geometry building and model setup were based on a number of simplifying assumptions. Computational grid-independent solutions were achieved for dip rates ranging from 5 to 10 dips per minute (dpm). For all cases studied, the hydrodynamics exhibited a periodicity dictated by the dip rate. Cycle-to-cycle variations were found to be negligible. Higher velocities were predicted in the wake of the tablet and they peaked at midway positions both during the up- and downstrokes of the cylinder. Three sub-regions of velocity were identified inside the reciprocating cylinder. Results also showed localized vortices/recirculations specific to the up- and downstroke, in addition to local stagnation zones. The wall shear stresses and velocity magnitudes scaled proportionately with increasing dip rates while exhibiting qualitatively similar behavior in their spatial and temporal distributions. Based on the predictions of the 2D axisymmetric CFD model, the hydrodynamics in USP Apparatus 3 is characterized by complex and periodic flow structures.

Improving the energy storage capability of hot water tanks through wall material specification

Domestic hot water tanks represent a significant potential demand side management asset within energy systems. To operate effectively as energy storage devices, it is crucial that a stratified temperature distribution is maintained during operation; this paper details experimental and numerical work conducted to understand the influence that wall material specification has on de-stratification within domestic hot water tanks. A 2d axisymmetric CFD (Computational Fluid Dynamic) model was consistent with experiments which showed that switching from copper to stainless steel resulted in a 2.7 fold reduction in useable hot water loss through reduced de-stratification for a 74 L UK domestic hot water tank over a 48 h period. During simulation, a counter rotating convection system, with peak velocities of 0.005 m/s, was observed above and below the thermocline. Minimizing de-stratification, through appropriate wall material selection, increases the performance of hot water tanks and scope for their use in demand side management applications. Given the inconclusive evidence surrounding copper's efficacy as a sanitizing agent, along with the low tensile strength of polyethylene, this paper advocates the use of stainless steel in hot water tank walls and further exploration of alternative materials and composites which have low cost and low thermal conductivity along with high strength and manufacturability.

A new settling velocity model to describe secondary sedimentation

Secondary settling tanks (SSTs) are the most hydraulically sensitive unit operations in biological wastewater treatment plants. The maximum permissible inflow to the plant depends on the efficiency of SSTs in separating and thickening the activated sludge. The flow conditions and solids distribution in SSTs can be predicted using computational fluid dynamics (CFD) tools. Despite extensive studies on the compression settling behaviour of activated sludge and the development of advanced settling velocity models for use in SST simulations, these models are not often used, due to the challenges associated with their calibration. In this study, we developed a new settling velocity model, including hindered, transient and compression settling, and showed that it can be calibrated to data from a simple, novel settling column experimental set-up using the Bayesian optimization method DREAM(ZS). In addition, correlations between the Herschel-Bulkley rheological model parameters and sludge concentration were identified with data from batch rheological experiments. A 2-D axisymmetric CFD model of a circular SST containing the new settling velocity and rheological model was validated with full-scale measurements. Finally, it was shown that the representation of compression settling in the CFD model can significantly influence the prediction of sludge distribution in the SSTs under dry- and wet-weather flow conditions.

Investigating the production of liquid fuels from synthesis gas (CO + H2) in a bench-scale packed-bed reactor based on Fe–Cu–La/SiO2 catalyst: experimental and CFD modeling

A computational fluid dynamic (CFD) and experimental study of Fischer–Tropsch Synthesis (FTS) process in a fixed-bed reactor is presented. The reactor was a 1.2 cm diameter and 80 cm length steel tube in which ceramic particles were employed to dilute the catalyst bed and thus prevent the emergence of hot spot in it. An axi-symmetric CFD model with an optimized mesh of 22,016 square cells was developed to model hydrodynamics, chemical reaction, and heat and mass transfer in the reactor. Thermodynamic non-ideal behavior of the gas mixture was modeled using Peng–Robinson equation of state. Kinetic models for FTS and water–gas-shift reaction rates based on Langmuir–Hinshelwood type for each of the species were employed. Good agreement was achieved between the bench experimental data and the model. Performing reactions inside packed-bed reactor due to high pressure and temperature is very difficult and expensive, and CFD simulations are considered as numerical experiments in many cases. A sensitivity analysis was run to find the effect of temperature, pressure, GHSV and H2/CO ratio on the reactor performance. It was concluded that the obtained results from CFD analysis give precise guidelines for further studies on optimization of FTS fixed-bed reactor performance.

Modelling of Heat and Mass Transport Phenomena and Chemical Reaction in Underground Coal Gasification

A two-dimensional axi-symmetric computational fluid dynamic model of an underground coal gasification cavity partially filled with an ash bed has been developed. The model is used to simulate the combined effects of heat and mass transport and chemical reaction during the gasification process. Simulations have revealed that when bottom injection of the oxidant is applied, the flow in the void space above the ash bed is dominated by a single buoyant force due to temperature gradients established by combustion. Optimum oxygen injection rates can be found which maximise the production of chemical energy in the product gas. When the oxidant is injected into the cavity from the top, most of the valuable gasification products are oxidized leading to a product gas with a high temperature and a low calorific value. The simulations eludicate the important transport and reaction processes occurring in the underground cavity and the results are in qualitative agreement with observations from field trials of underground coal gasification.