CFD in research for the petrochemical industry

Existing computer programs for turbulent flow simulations have been tested on their capacity to solve practical problems in engineering research for the petrochemical industry. This paper presents results of six validation studies which are all related to experimental work carried out in the Shell Group laboratories. On the basis of these studies needs for further improvement and validation of both numerical schemes and turbulence models have been formulated.The present as well as future relevance of Computational Fluid Dynamics (CFD) in engineering research is discussed in a wider context than covered by the validation studies. This analysis formed the basis for the identification of a number of CFD areas which have great potential in engineering application but need substantial further research and development to make them suitable for this purpose.     

Nonlinear analysis of the deformation and breakup of viscous microjets using the method of lines

We present a numerical model for predicting the instability and breakup of viscous microjets of Newtonian fluid. We adopt a one‐dimensional slender‐jet approximation and obtain the equations of motion in the form of a pair of coupled nonlinear partial differential equations (PDEs). We solve these equations using the method of lines, wherein the PDEs are transformed to a system of ordinary differential equations for the nodal values of the jet variables on a uniform staggered grid. We use the model to predict the instability and satellite formation in infinite microthreads of fluid and continuous microjets that emanate from an orifice. For the microthread analysis, we take into account arbitrary initial perturbations of the free‐surface and jet velocity, as well as Marangoni instability that is due to an arbitrary variation in the surface tension. For the continuous nozzle‐driven jet analysis, we take into account arbitrary time‐dependent perturbations of the free‐surface, velocity and/or surface tension as boundary conditions at the nozzle orifice. We validate the model using established computational data, as well as axisymmetric, volume of fluid (VOF) computational fluid dynamic (CFD) simulations. The key advantages of the model are its ease of implementation and speed of computation, which is several orders of magnitude faster than the VOF CFD simulations. The model enables rapid parametric analysis of jet breakup and satellite formation as a function of jet dimensions, modulation parameters, and fluid rheology. Copyright © 2010 John Wiley & Sons, Ltd.     

A computational analysis of the hydrodynamic instability of a liquid jet focused into a converging microchannel

The instability of a focused liquid jet is studied by semi-analytical methods and by methods of computational fluid dynamics. The semi-analytical approach relies on earlier work on the instability of an extending liquid thread and is based on the Stokes flow regime and small-amplitude perturbations. The evolution of different excitation modes is evaluated and compared. Through hydrodynamic focusing and the corresponding extensional flow an initially stable mode may become unstable and it depends on the position away from the inlet which mode is to be regarded as the most unstable one. When plotting a hypothetical jet decay length against the excitation wave number, a comparatively broad minimum is exhibited. The CFD simulations based on the volume-of-fluid method show that the jet may break up either in the conical focusing zone or in the attached capillary, depending on the flow velocity. When the deformation of the jet surface reaches a certain amplitude, the jet assumes a “beads-on-a-string” structure instead of a shape derived from a harmonic perturbation. A jet decay within the capillary produces elongated droplets with cusped ends. When comparing the results of the CFD and the semi-analytical model, it turns out that the CFD simulations predict more stable jets with a larger decay length. An analysis of the flow velocity field shows that the increased stability might be due to the interaction of the jet with the channel walls.     

Verification and validation of impinging round jet simulations using an adaptive FEM

This paper illustrates the use of an adaptive finite element method as a means of achieving verification of codes and simulations of impinging round jets, that is obtaining numerical predictions with controlled accuracy. Validation of these grid‐independent solution is then performed by comparing predictions to measurements. We adopt the standard and accepted definitions of verification and validation (Technical Report AIAA‐G‐077‐1998, American Institute of Aeronautics and Astronautics, 1998; Verification and Validation in Computational Science and Engineering. Hermosa Publishers: Albuquerque, NM, 1998). Mesh adaptation is used to perform the systematic and rigorous grid refinement studies required for both verification and validation in CFD. This ensures that discrepancies observed between predictions and measurements are due to deficiencies in the mathematical model of the flow. Issues in verification and validation are discussed. The paper presents an example of code verification by the method of manufactured solution. Examples of successful and unsuccessful validation for laminar and turbulent impinging jets show that agreement with experiments is achieved only with a good mathematical model of the flow physics combined with accurate numerical solution of the differential equations. The paper emphasizes good CFD practice to systematically achieve verification so that validation studies are always performed on solid grounds. Copyright © 2004 John Wiley & Sons, Ltd.     

带制退器的膛口射流噪声数值模拟与实验研究

为了研究膛口装置对膛口噪声气动特性的影响,对带膛口制退器的某小口径武器的膛口射流噪声进行了数值模拟和实验研究。采用计算流体力学CFD (computational fluid dynamics)-计算气动声学CAA (computational aeroacoustics)耦合算法对膛口噪声进行数值模拟,即对膛口流场进行瞬态CFD模拟,获取流场数据,然后利用所得到的结果采用声学方程模拟声源信息求解声场。基于数值模拟结果,分析了膛口流场变化及噪声的指向性分布,并与实验结果进行了对比。研究表明:膛口制退器的安装改变了膛口流场结构,影响了膛口射流噪声的指向性分布。计算结果与实验结果的误差小于9%,验证了该计算方法的可行性。研究结果可为膛口射流噪声的预测及膛口制退器的设计提供一定的参考。     

Computation of a drastic flow pattern change in an annular swirling jet caused by a small decrease in inlet swirl

This paper investigates the flow pattern change in an annular jet caused by a sudden change in the level of inlet swirl. The jet geometry consists of an annular channel followed by a specially designed stepped‐conical nozzle, which allows the existence of four different flow patterns as a function of the inlet swirl number. This paper reports on the transition between two of them, called the ‘open jet flow high swirl’ and the ‘Coanda jet flow.’ It is shown that a small sudden decrease of 4% in inlet swirl results in a drastic and irreversible change in flow pattern. The objective of this paper is to reveal the underlying physical mechanisms in this transition by means of numerical simulations. The flow is simulated using the unsteady Reynolds‐averaged Navier–Stokes (URANS) approach for incompressible flow with a Reynolds stress turbulence model. The analysis of the numerical results is based on a study of different forces on a control volume, which consists of the jet boundaries. The analysis of these forces shows that the flow pattern change consists of three different regimes: an immediate response regime, a quasi‐static regime and a Coanda regime. The simulation reveals that the pressure–tangential velocity coupling during the quasi‐static regime and the Coanda effect at the nozzle outlet during the Coanda regime are the driving forces behind the flow pattern change. These physical mechanisms are validated with time‐resolved stereo‐PIV measurements, which confirm the numerical simulations. Copyright © 2008 John Wiley & Sons, Ltd.     

Investigation of the velocity field in a full-scale model of a cerebral aneurysm

Due to improved and now widely used imaging methods in clinical surgery practise, detection of unruptured cerebral aneurysms becomes more and more frequent. For the selection and development of a low-risk and highly effective treatment option, the understanding of the involved hemodynamic mechanisms is of great importance. Computational Fluid Dynamics (CFD), in vivo angiographic imaging and in situ experimental investigations of flow behaviour are powerful tools which could deliver the needed information. Hence, the aim of this contribution is to experimentally characterise the flow in a full-scale phantom model of a realistic cerebral aneurysm. The acquired experimental data will then be used for a quantitative validation of companion numerical simulations. The experimental methodology relies on the large-field velocimetry technique PTV (Particle Tracking Velocimetry), processing high speed images of fluorescent tracer particles added to the flow of a blood-mimicking fluid.First, time-resolved planar PTV images were recorded at 4500 fps and processed by a complex, in-house algorithm. The resulting trajectories are used to identify Lagrangian flow structures, vortices and recirculation zones in two-dimensional measurement slices within the aneurysm sac. The instantaneous inlet velocity distribution, needed as boundary condition for the numerical simulations, has been measured with the same technique but using a higher frame rate of 20,000 fps in order to avoid ambiguous particle assignment. From this velocity distribution, the time-resolved volume flow rate has been also derived. In this manner, a direct comparison between numerical simulations and PTV measurements will be possible in the near future, opening the door for highly accurate computational predictions.     

Large Eddy Simulation of Stratified and Sheared Flames of a Premixed Turbulent Stratified Flame Burner Using a Flamelet Model with Heat Loss

This paper presents large eddy simulations (LES) of the Darmstadt turbulent stratified flame burner (TSF) at different operating conditions including detailed heat loss modeling. The target cases are a non-reacting and two reacting cases. Both reacting cases are characterized by stratification, while one flame additionally features shear. In the regime diagram for premixed combustion, the studied flames are found at the border separating the thin reaction zones regime and the broken reaction zones regime. A coupled level set/progress variable model is utilized to describe the combustion process. To account for heat loss, an enthalpy defect approach is adopted and reformulated to include differential diffusion effects. A novel power-law rescaling methodology is proposed to integrate the enthalpy defect approach into the level set/progress variable model which is extensively validated in two validation scenarios. It is demonstrated that the LES with the newly developed model captures the influence of heat loss well and that the incorporation of heat loss effects improves the predictions of the TSF-burner over adiabatic simulations, while reproducing the experimentally observed flame lift-off from the pilot nozzle.     

Heat transfer investigation of an array of jets impinging on a target plate with detached ribs

This combined experimental and numerical study focuses on impingement jet cooling in combination with detached rib turbulators on a flat target for turbomachinery applications. The investigated impingement array consists of an impingement plate with 9 × 9 jet holes with diameter D and a target plate with detached ribs installed beneath the jet hole. The effects of different separation distances (H/D=3-5), jet Reynolds numbers (15,000-35,000) and rib clearances (0.3D and 0.08D) are investigated. The heat transfer is investigated experimentally by the transient liquid crystal (TLC) method. A computational fluid dynamics (CFD) model is carried out within the software package ANSYS CFX. This model uses a steady-state three-dimensional Reynolds-Averaged Navier-Stokes (RANS) approach with the Shear Stress Transport (SST) turbulence model. Numerical simulations allow detailed insight into the fluid mechanics of the complex flow field and complement experimental measurements. Detached ribs in the impingement channel have a strong influence on the flow field and can increase the global Nusselt number by up to 4% if the ribs have adiabatic boundary conditions. The usage of the detached rib reduces the relative discharge coefficient by up to 11% compared to a smooth target.     

An investigation into the interaction of closely-spaced starting jets

The transient, three-dimensional scavenging flow inside a novel two-stroke engine has been investigated both experimentally in a scaled water model as well as numerically using a commercial CFD code incorporating an unsteady Reynolds averaged Navier–Stokes (URANS) formulation. The scavenging flow consists of 16 round jets in close proximity of each other and the cylinder wall, developing from the top of the combustion chamber down towards the exhaust ports located along the wall at the bottom of the cylinder. Flow visualization of the scavenging flow was performed using a scaled fixed-piston water model and was used as a means of validating the URANS simulations themselves. The flow visualization experiments provided insight into the complex jet–jet and jet–wall interactions within the engine cylinder. These interactions were not as well predicted by the CFD simulations. In fact, the CFD simulations were found to significantly under-predict the turbulent mixing between the jets. This suggests that unsteady-RANS formulations are incapable of reproducing the large-scale and unsteady mixing structures associated with the vortex shedding between the closely-spaced jets.     

Trade-off analysis of high-aspect-ratio-cooling-channels for rocket engines

High performance liquid rocket engines are often characterized by rectangular cooling channels with high aspect ratio (channel height-to-width ratio) because of their proven superior cooling efficiency with respect to a conventional design. However, the identification of the optimum aspect ratio is not a trivial task. In the present study a trade-off analysis is performed on a cooling channel system that can be of interest for rocket engines. This analysis requires multiple cooling channel flow calculations and thus cannot be efficiently performed by CFD solvers. Therefore, a proper numerical approach, referred to as quasi-2D model, is used to have fast and accurate predictions of cooling system properties. This approach relies on its capability of describing the thermal stratification that occurs in the coolant and in the wall structure, as well as the coolant warming and pressure drop along the channel length. Validation of the model is carried out by comparison with solutions obtained with a validated CFD solver. Results of the analysis show the existence of an optimum channel aspect ratio that minimizes the requested pump power needed to overcome losses in the cooling circuit.     

Numerical simulation of a turbulent flow with droplets injection in annular heated air tube using the Reynolds stress model

This work presents computational fluid dynamics (CFD) simulations of single-phase and two-phase flow. The droplets are injected in annular heated air tube. The numerical simulation is performed by using a commercial CFD code witch uses the finite-volume method to discretize the equations of fluid flow. The Reynolds-averaged Navier–Stokes equations with Reynolds stress model were used in the computation. The governing equations are solved by using a SIMPLE algorithm to treat the pressure terms in the momentum equations. The results of prediction are compared with the experimental data.     

Implementation of rotational resistance models: A critical appraisal

Contact models that simulate rotational resistance at particle contacts have been proposed as a means to capture the effect of shape in DEM simulations. This contribution critically explores some key issues related to the implementation of rotational resistance models; these include the need for physically meaningful model parameters, the impact of the model on the overall numerical stability/critical time increment for the DEM model, model validation, and the assessment of model performance relative to real physical materials. The discussion is centred around a rotational resistance model that captures the resistance provided by interlocking asperities on the particle surface. An expression for the maximum permissible integration time step to ensure numerical stability is derived for DEM simulations when rotational resistance is incorporated. Analytical solutions for some single-contact scenarios are derived for model validation. The ability of this type of model to provide additional fundamental insight into granular material behaviour is demonstrated using particle-scale analysis of triaxial compression simulations to examine the roles that contact rolling and sliding have on the stability of strong force chains.     

Mixing Characteristics in a Supersonic Combustor with Gaseous Fuel Injection Upstream of a Cavity Flameholder

Non-reacting experiments and numerical simulations have been performed to investigate the mixing characteristics in a supersonic combustor with gaseous fuel injection upstream of a flameholding cavity in a supersonic vitiated air flow with stream Mach number 1.7. Using helium as simulated fuel, the acetone vapor is adulterated into the fuel jet. The fuel distribution in spanwise and streamwise direction is imaged by the planar laser-induced fluorescence (PLIF) measurement. According to the similarity of experimental observations with different cavities, the typical L/D = 7 cavity with aft wall angle 45° is chosen and the flowfield with the injection is calculated by Large Eddy Simulation. Experimental and numerical results have shown that most of the fuel flow away upon the open cavity with the lifting counter-rotating vortex structures induced by the transverse jet. Only a small portion of the fuel is convected into the cavity shear layer by the vortex interaction of the jet with cavity shear layer, and then transported into the cavity due to the cavity shear layer motion and the interaction of the shear layer with the cavity trailing edge.     

Impact of turbulence modelling on external supersonic flow field simulations in rocket aerodynamics

This paper contains a three-dimensional study on the influence of different turbulence models for external supersonic flow field simulations, aiming at reaching the best accuracy in rocket aerodynamics. A well-studied test case -a slender body- has been used for the validation process, which involves the major turbulence models available. The SST k–ω model has been selected as the most suitable one for this kind of flows. Good agreements between numerical, theoretical and experimental results are obtained, which are used to set up some guidelines regarding the configuration of Reynolds-averaged Navier–Stokes Computational Fluid Dynamics (CFD) supersonic models for these flight regimes.     

Simulation of the primary breakup of a high-viscosity liquid jet by a coaxial annular gas flow

The conversion of low-grade fossil and biogenic energy resources (petcoke, biomass) to a synthesis gas in a high pressure entrained flow gasification process opens a wide spectrum for high efficient energy conversion processes. The synthesis gas can be used for production of methane (SNG), liquid fuels (BtL, CtL) or as fuel for operation of a gas turbine in a combined cycle power plant (IGCC). The production of a tar free high quality syngas is a challenging objective especially due to the fact that typical liquid or suspension fuels for entrained flow gasifiers feature viscosities up to 1000 mPas. Fuel droplet conversion at typical entrained flow gasification conditions is characterized by heat up, evaporation and subsequent degradation of the vapour phase. To guarantee a high fuel conversion rate in the gasifier an efficient atomization of the fuel is required. Mainly twin-fluid burner nozzles are used for atomization of those typically high viscous fuels. The present study is focused on the assessment of the accuracy of CFD computations for the primary breakup of high-viscosity liquids using an external mixing twin fluid nozzle. In a first step experiments were performed with a Newtonian glycerol-water-mixture featuring a liquid viscosity of 400 mPas. Jet breakup was investigated using a high speed camera as well as PIV and LDA-System for a detailed investigation of the flow field. In a second step the experimental results serve as reference data to assess the accuracy of CFD computations. Compressible large eddy simulations (LES) were performed to capture the morphology of the primary breakup as well as the important flow field characteristics. A Volume of Fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Comparison of experimental and numerical results showed good agreement with respect to breakup frequency, velocity fields and morphology.     

湍流冲击射流流动与传热的数值研究进展

湍流冲击射流由于其冲击表面时具有很高的局部传热率和冲击力,被广泛应用于如表面的加热、电子元件的冷却、纸张的干燥和材料的切割等工程应用和工业过程中.由于其流动的复杂性,也常被作为一种理想的测试实例来评价湍流模型的性能.此外,湍升力射流与地面之间的空气动力作用对V/STOL (垂直或短距离起落)飞机的性能具有很大的影响.长期以来,人们从理论分析、实验测量和数值模拟方面对冲击射流进行了广泛而系统的研究,积累了丰富的资料.本文在分析了湍流冲击射流的数值研究现状的基础上,对近年来有关湍流冲击射流流动与传热的数值研究方面的文献有选择地进行了综述,重点评述了不同湍流模型对冲击射流流动与传热的预测能力,讨论了存在的问题并对该领域今后的研究方向进行了展望.      

Study on the influences of interaction behaviors between multiple combustion-gas jets on expansion characteristics of Taylor cavities

The purpose of this study is to investigate means of controlling the interior ballistic stability of a bulk-loaded propellant gun(BLPG).Experiments on the interaction of twin combustion gas jets and liquid medium in a cylindrical stepped-wall combustion chamber are conducted in detail to obtain time series processes of jet expansion,and a numerical simulation under the same working conditions is also conducted to verify the reliability of the numerical method by comparing numerical results and experimental results.From this,numerical simulations on mutual interference and expansion characteristics of multiple combustion gas jets(four,six,and eight jets) in liquid medium are carried out,and the distribution characteristic of pressure,velocity,temperature,and evolutionary processes of Taylor cavities and streamlines of jet flow Held are obtained in detail.The results of numerical simulations show that when different numbers of combustion gas jets expand in liquid medium,there are two different types of vortices in the jet flow field,including corner vortices of liquid phase near the step and backflow vortices of gas phase within Taylor cavities.Because of these two types of vortices,the radial expansion characteristic of the jets is increased,while changing numbers of combustion gas jets can restrain Kelvin-Helmholtz instability to a certain degree in jet expansion processes,which can at last realize the goal of controlling the interior ballistic stability of a BLPG.The optimum method for both suppressing Kelvin-Helmholtz instability and promoting radial expansion of Taylor cavities can be determined by analyzing the change of characteristic parameters in a jet flow field.     

Environmental design of outdoor climate based on CFD

The difficulties in computational fluid dynamics (CFD) analysis of outdoor environments are described and new techniques in CFD for overcoming these difficulties are developed. A ‘Software Platform’ is proposed that integrates various numerical analysis tools and that is able to give a complete evaluation of outdoor climates. A number of case studies on designing outdoor climates are reported based on this newly developed Software Platform.