Efficient CFD Research Articles

Computationally efficient CFD model for scale-up of bubbling fluidized bed reactors applied to sorption-enhanced steam methane reforming

Sorption-Enhanced Steam Methane Reforming (SE-SMR) represents a novel and energy-efficient hydrogen production route with in situ CO2 capture. A comprehensive Eulerian-Eulerian CFD model of SE-SMR in a bubbling fluidized bed reactor, that uses dolomite and other solid sorbents as CO2 acceptors, has been developed. Kinetic models for steam methane reforming and CO2 capture have been implemented. In addition, a new particle drag model has been derived from customary formulas in order to reduce the computational cost. Two different scales have been studied: laboratory and semi-industrial. Results of the computation are in good agreement with literature data at both scales (SMR H2=76–78% vs. SE-SMR H2=90–96% dry basis mole fraction). Numerical simulations demonstrate that CO2 capture is the kinetic limiting step of the SE-SMR mechanism, as compared to steam methane reforming. Temperature is shown to be the key parameter of the SE-SMR chemical process at large scales, and an optimal T=625°C is estimated. Additionally, compared with the classical approaches, the new drag model provides seemingly realistic predictions within the multiple bubble regime, at a low computational cost and using a coarse grid. This represents a further advance for the scaling-up of the reactor to industrial sizes based on numerical simulation.

Development of a numerically efficient CFD model to predict transient temperature distribution of mother tubes moving translative and rotative through a gas fired furnace

In this work the heating of mother tubes in a walking beam type reheating furnace has been investigated. The tubes are heated prior to further processing into seamless tubes in a downstream stretch reducing mill. In contrast to previous works, the heating of hollow geometries moving through the furnace is considered. The main difference to existing works is the movement of the tubes in a combination of translational and rotational movement. The tubes rest on walking beams in loading bays and perform a rotational movement when passing through the furnace. The walking beams are not water cooled so that they reach the gas temperature inside the furnace. Due to these effects, it is to be expected that the influence of the skid system on tube heating is low. The model used in this work is based on two separate simulations: a steady state simulation characterizing the gas-phase combustion, and a transient simulation considering the heating of the tubes. This approach minimizes the computing power required, which is thus significantly lower than that for a full transient model. The steady state combustion simulation has been performed using the steady flamelet model (SFM). The advantage of the SFM compared to other models is the low computational effort and allows a detailed CH4 chemical mechanism, the skeletal25 to be used. The discrete ordinates model was used to solve the radiative transfer equations. To validate the model, the results of the steady state simulation are compared to process data, revealing a good agreement. A test tube, equipped with several thermocouples, provided a statement about the heating characteristic of the tubes. The data of the test tube and pyrometer measurement have been compared to the transient heating simulation, also constituting good agreements.

A resilient and efficient CFD framework: Statistical learning tools for multi-fidelity and heterogeneous information fusion

Exascale-level simulations require fault-resilient algorithms that are robust against repeated and expected software and/or hardware failures during computations, which may render the simulation results unsatisfactory. If each processor can share some global information about the simulation from a coarse, limited accuracy but relatively costless auxiliary simulator we can effectively fill-in the missing spatial data at the required times by a statistical learning technique – multi-level Gaussian process regression, on the fly; this has been demonstrated in previous work [1]. Based on the previous work, we also employ another (nonlinear) statistical learning technique, Diffusion Maps, that detects computational redundancy in time and hence accelerate the simulation by projective time integration, giving the overall computation a “patch dynamics” flavor. Furthermore, we are now able to perform information fusion with multi-fidelity and heterogeneous data (including stochastic data). Finally, we set the foundations of a new framework in CFD, called patch simulation, that combines information fusion techniques from, in principle, multiple fidelity and resolution simulations (and even experiments) with a new adaptive timestep refinement technique. We present two benchmark problems (the heat equation and the Navier–Stokes equations) to demonstrate the new capability that statistical learning tools can bring to traditional scientific computing algorithms. For each problem, we rely on heterogeneous and multi-fidelity data, either from a coarse simulation of the same equation or from a stochastic, particle-based, more “microscopic” simulation. We consider, as such “auxiliary” models, a Monte Carlo random walk for the heat equation and a dissipative particle dynamics (DPD) model for the Navier–Stokes equations. More broadly, in this paper we demonstrate the symbiotic and synergistic combination of statistical learning, domain decomposition, and scientific computing in exascale simulations.

Parallelized numerical modeling of the interaction of a solid object with immiscible incompressible two-phase fluid flow

PurposeA numerical method is developed to capture the interaction of solid object with two-phase flow with high density ratios. The current computational tool would be the first step of accurate modeling of wave energy converters in which the immense energy of the ocean can be extracted at low cost.Design/methodology/approachThe full two-dimensional Navier–Stokes equations are discretized on a regular structured grid, and the two-step projection method along with multi-processing (OpenMP) is used to efficiently solve the flow equations. The level set and the immersed boundary methods are used to capture the free surface of a fluid and a solid object, respectively. The full two-dimensional Navier–Stokes equations are solved on a regular structured grid to resolve the flow field. Level set and immersed boundary methods are used to capture the free surface of liquid and solid object, respectively. A proper contact angle between the solid object and the fluid is used to enhance the accuracy of the advection of the mass and momentum of the fluids in three-phase cells.FindingsThe computational tool is verified based on numerical and experimental data with two scenarios: a cylinder falling into a rectangular domain due to gravity and a dam breaking in the presence of a fixed obstacle. In the former validation simulation, the accuracy of the immersed boundary method is verified. However, the accuracy of the level set method while the computational tool can model the high-density ratio is confirmed in the dam-breaking simulation. The results obtained from the current method are in good agreement with experimental data and other numerical studies.Practical/implicationsThe computational tool is capable of being parallelized to reduce the computational cost; therefore, an OpenMP is used to solve the flow equations. Its application is seen in the following: wind energy conversion, interaction of solid object such as wind turbine with water waves, etc.Originality/valueA high efficient CFD approach method is introduced to capture the interaction of solid object with a two-phase flow where they have high-density ratio. The current method has the ability to efficiently be parallelized.

3D Transient CFD Simulation of Scroll Compressors with the Tip Seal

A new template simulation tool is developed for scroll compressors/expanders capable of modelling tip seal leakages. This scroll template generates a high quality 3D multiblock structured mesh from user-input stationary and orbiting scroll surfaces. The mesh movement is then automatically calculated to account for every position of the orbiting scroll, maintaining good grid quality and smooth movement throughout the whole revolution. A state- of-the-art efficient CFD solver is used to solve Navier-Stokes equations, capable of simulation with both real gas and ideal gas. A case study is presented for a generic scroll compressor with refrigerant R410A. The case was run with and without the tip seal volumes. Comparisons are made to show the impact of tip seals on the compressor performance.

Numerical simulation of deflagration-to-detonation transition in large confined volumes

A methodology for the computationally efficient CFD simulation of hydrogen-air explosions (including transition to detonation) in large volumes is presented. The model is validated by means of the largest ever conducted indoor DDT experiments in the RUT facility. A combination of models is proposed with a particular focus on the influence of flame-instabilities, especially of thermal-diffusive nature, which are crucial for very lean mixtures. Excellent agreement is achieved in terms of flame acceleration. The quality of DDT predictions itself depends on the underlying mechanism. Whereas DDT by shock-focusing is successfully simulated on under-resolved meshes, DDT by local explosions in the vicinity of the turbulent flame brush remains a challenge. Adaptive mesh refinement therefore emerges as a key technique to resolve more of the essential phenomena at reasonable computational costs affordable by industry. Finally, a generic case demonstrates the influence of mixture inhomogeneity, which can promote flame acceleration and ultimately DDT.

Modeling of the building local protection (shelter – in place) including sorbtion of the hazardous contaminant on indoor surfaces

Purpose. Chemically hazardous objects, where toxic substances are used, manufactured and stored, and also main lines, on which the hazardous materials transportation is conducted, pose potential sources of atmosphere accidental pollution.Development of the CFD model for evaluating the efficiency of the building local protection from hazardous substantives ingress by using air curtain and sorption/desorption of hazardous substance on indoor surfaces. Methodology. To solve the problem of hydrodynamic interaction of the air curtain with wind flow and considering the building influence on this process the model of ideal fluid is used. In order to calculate the transfer process of the hazardous substance in the atmosphere an equation of convection-diffusion transport of impurities is applied. To calculate the process of indoors air pollution under leaking of foul air Karisson & Huber model is used. This model takes into account the sorption of the hazardous substance at various indoors surfaces. For the numerical integration of the model equations differential methods are used. Findings. In this paper we construct an efficient CFD model of evaluating the effectiveness of the buildings protection against ingress of hazardous substances through the use of an air curtain. On the basis of the built model a computational experiment to assess the effectiveness of this protection method under varying the location of the air curtain relative to the building was carried out. Originality. A new model was developed to compute the effectiveness of the air curtain supply to reduce the toxic chemical concentration inside the building. Practical value. The developed model can be used for design of the building local protection against ingress of hazardous substances.

ATMOSFERİK GEÇİŞ YAPAN ARAÇ ETRAFINDA NAVIER-STOKES DENKLEMLERİ İLE ÜÇ BOYUTLU HİPERSONİK AKIŞ ANALİZİ

ÖZETBu çalışmada hipersonik akışlar için kullanılabilecek isabetli ve etkin bir hesaplamalı akışkanlar dinamiği kodu geliştirilmesi amaçlanmaktadır. Akış analizi üç boyutlu Navier-Stokes denklemlerini temel almaktadır. Bu denklemler Newton metoduyla çözülmekte ve Jacobian matrislerini hesaplamak için analitik metot kullanılmaktadır. Model olarak alınan AS-202 Apollo uzay aracı geometrisi üzerinde akış parametreleri ve taşınımla ısı transferi analiz edilecektir. Ayrıca nümerik olarak stabil olan ve genelde hipersonik akış uygulamalarında iyi tahminler sağlayan Spalart-Allmaras türbülans modeli kullanılarak hipersonik akış için türbülanslı akış analizi yapılacaktır.

Statistical Evaluation of Rans Simulations Compared To Experiments for A Model Premixer

This paper investigates the ability of computationally time efficient CFD models to predict mixing of hydrogen and air in a model premixer with a complex 3D geometry. Axial and radial injection of hydrogen into swirling and non-swirling air streams at standard and elevated pressures and temperatures conditions are considered. Statistical analysis is applied to the results of comparison between experimental measurements and numerical simulations. The analysis of variance suggests that, for non-swirling flows, reasonable numerical predictions can be made if appropriate modeling parameters, such as turbulence model and turbulent Schmidt number, are considered. Comparing the results at standard conditions to those at elevated pressures and temperatures suggests that the conclusions at standard conditions may hold if appropriate scaling parameters are utilized.

Methods for Computationally Efficient Structured CFD Simulations of Complex Turbomachinery Flows

This research presents more efficient computational methods by which to perform multiple-block structured CFD simulations of turbomachinery, thus enabling higher-fidelity solutions of complicated geometries and their associated flows. This computational framework offers flexibility in allocating resources to balance process count and wall-clock computation time, while facilitating research interests of simulating axial compressor stall inception with more complete gridding of the flow passages and rotor tip clearance regions than is typically practiced with structured codes. The scheme presented herein expedites CFD simulation of previously impractical geometries and flows. These methods are validated and demonstrate improved computational efficiency when applied to complicated geometries and flows.

Impact of computational physics on multi-scale CFD and related numerical algorithms

Development of accurate computational methods for the constitutive relation that plays a role as the bridge between microscopic and macroscopic physics becomes a key issue in a continuum approach for describing rarefied and micro-scale gas flows. The mathematical form of the constitutive relation dictates the resulting computational methods and related algorithms. It is, therefore, vital to develop proper computational models on the basis of a correct understanding of the multi-scale physics inherent in non-equilibrium gases. In this study the computational issue is discussed by considering two benchmark multi-scale problems: the compression-dominated shock structure and velocity shear-dominated gas flows. Special emphasis is placed on efficient CFD algorithms within the finite volume formulation. In addition, the verification and validation issue of the multi-scale methods is discussed and a simple verification method based on basic physical laws is proposed.

Ｌａｇｒａｎｇｅ 粒子付Ｅｕｌｅｒ 型スキームによる波浪中耐航性能評価のためのＣＦＤツールの開発

We have developed an efficient and robust CFD method to compute a strongly nonlinear interaction between wave and ship,such as heave and pitch motions in a severe wave condition, resultant slamming on the surface of a ship and an impact pressure due to green-water phenomena. The proposed method is based on coupling the Eulerian method for fluid phase with the Lagrangian method for solid phase to compute fluid-structure interaction. In this model, two kinds of Lagrangian particles are employed. One is free surface particle located near the free surface to capture a water surface accurately. The other one is SPH particle to compute ship motions in 3D. A fully nonlinear phenomenon with wave breaking is treated on a fixed Eulerian grid with the free surface particles to rebuild density function for capturing an interface between two phases in a filamentary under resolved region. In this study, the proposed model was applied to ship-wave interaction for evaluating of seakeeping of a ship in waves and their results were compared with experimental results and strip theory.

Simultaneous pseudo‐time stepping for aerodynamic shape optimization

AbstractIn this paper we discuss about numerical methods for aerodynamic shape optimization problems. These problems require efficient CFD techniques to solve the state (as well as costate) equations and fast algorithms for solving the optimization problems. Both of these are independent active areas of research since long time. Wide range of applications in science and engineering involve solution of optimization problems where the governing PDEs appear as constraints. Therefore, merging the two for the purpose of practical applicability is relatively new. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Implementation of a conditional moment closure for mixing sensitive reactions

An implementation of the conditional moment closure (CMC) of Bilger (Phy. Fluids A 5 (1993) 436) and Klimenko (Fluid Dyn. 25 (1990) 327) has been carried out in the commercial CFD software FLUENT. A new implementation method is suggested that uses the CMC-equations to calculate an unconditional reaction rate that is subsequently inserted in a Reynolds averaged species equation. The objectives are to ensure conservation of reacting species by elimination of the spurious term that usually contaminates a CMC solution and to obtain convergence on a coarser grid than is usually required in CMC. The implementation has been successfully carried out for two sets of well-known mixing sensitive reaction pairs (parallel competitive and competitive consecutive) that both have one instantaneous and one fast irreversible reaction. Calculations with different discretizations of the CMC-equations in mixture fraction space have been performed and compared to experiments, full/finite-mode PDF models and the interpolation model of Baldyga. It is shown that the suggested implementation can achieve sufficient numerical accuracy with a much coarser mixture fraction space discretization than standard CMC implementations, which allows for more efficient CFD calculations.

An efficient CFD approach for simulating unsteady hypersonic shock–shock interference flows

An efficient CFD approach based on higher order accurate numerical schemes has been developed for simulating unsteady hypersonic viscous flows strongly associated with shock–shock interference flow phenomena. The principal concept of the present approach is to employ the higher-resolution finite difference schemes based on the fourth-order accurate compact MUSCL TVD scheme coupled with the AUSM-based scheme in space discretization and based on the maximum second-order accurate LU-SGS scheme modified using the Newton iteration in time integration. The hypersonic shock–shock interference flow which is well known as the Type IV is numerically investigated. The obtained numerical results are showing extremely complicated flow structures, such as an unsteady supersonic jet, a jet-bow-shock, and unsteady supersonic shear layers. The reliability of the calculated results is checked by comparison with the existing experimental and numerical results.

CFD optimization of a theoretical minimum-drag body

This article describes a methodology behind coupling a fast, parabolized Navier-Stokes flow solver to a nonlinear constrained optimizer. The design parameters, constraints, grid refinement, behavior of the optimizer, and flow physics related to the CFD calculations are discussed. Pressure drag reduction in the supersonic regime of a theoretical minimum-drag body of revolution is performed. Careful selection of design variables allows the optimization process to improve the aerodynamic performance. A calculation including nonlinear and viscous effects produces a different minimum drag geometry than linear theory and results in a drag reduction of approximately 4%. Effect of grid density on the optimization process is also studied. In order to obtain accurate optimization results, CFD calculations must model physical phenomena that contribute to the optimization parameters. OR the past two decades, many different approaches have been developed to design aircraft with better aerodynamic performance.1 Some of these techniques fall into the general category of inverse design methods.2-3 The quality of the opti- mized shape obtained from this method depends on the distri- bution, usually pressure, it is required to match. Therefore, this approach depends on the knowledge of the designer to establish a desirable optimum. In addition, the inverse method does not lend itself to the imposition of constraints. A different approach is called shape perturbation method.4~6 In this method, an anal- ysis code is coupled with a numerical optimizer to find a shape that optimizes the objective function. This method may be com- putationally expensive because of the gradient evaluations, which require CFD calculations. Recently, intelligent methods such as the one-shot method7-8 and the control-based method9 have merged that yield rapid convergence to the design shape. Un- fortunately, these two methods make use of the governing equa- tions of the CFD code; therefore, receding is needed for dif- ferent objective functions, boundary conditions, and flow solvers. The methods cannot immediately take advantage of existing validated analysis codes that have already been developed. A shape perturbation method is chosen for optimization in the present study. An efficient CFD flow solver is coupled with an optimizer for use as a tool in aerodynamic design. Careful selection of design variables allows fast convergence in the optimization process and yields improvements in aero- dynamic performance. The present method takes advantage of a Fourier sine series that defines the original body. The Fourier coefficients are convenient, physically relevant design variables for the problem studied here. As a test case, the Haack-Adams1012 (H-A) theoretical minimum-drag body of revolution is chosen. The H-A body is selected in this study because it is a classic aerodynamics problem for which validating experimental data13 are avail- able. Because of its simple geometry, running large numbers of cases in a grid refinement study is still relatively inexpen- sive. Since the geometry ends in a finite base, it is particularly

Dynamic Force Performance of Annular Gas Seals at Off-Center Conditions©

A numerical analysis to predict the static and dynamic performance characteristics of gas annular seals at off-center conditions is presented. Isothermal rotor and stator surfaces are assumed according to conditions observed on an existing test rig. Heat flow to the rotor and stator is modeled by bulk-flow heat transfer coefficients. The non-linear differential equations for the turbulent bulk flow in the annular seal are solved using an efficient CFD algorithm. A perturbation method for calculation of the zeroth- and first-order flow fields determines the seal steady-state response and dynamic force coefficients. Numerical predictions for a test seal show that heat transfer from the bounding solid surfaces to the fluid film is important to correctly predict the temperature field. An adiabatic flow assumption leads to unrealistically large temperature drops across the seal length. The seal leakage and the direct stiffness coefficients are insensitive to the position of the rotor center in the seal, while the seal forces, the cross-coupled stiffness, and the direct damping coefficients increase with the rotor static eccentricity.