Reacting Flow Simulations Research Articles

Impact of variable geometry combustor on performance and emissions from miniature gas turbine engine

The scope of this study is the experimental and theoretical research into the developed concept of variable geometry combustion chamber for miniature gas turbine engine. The aims are to investigate the combustion performance and NOX/CO emissions from combustion chamber and to demonstrate advantages of proposed design over standard, fixed geometry combustor.Experimental studies were performed on a specially designed miniature combustion chamber test stand. Standard GTM-120 combustion chamber and proposed combustion chamber with variable dilution holes area were tested. Pressures and temperatures at main combustion sections as well as NOX and CO concentrations in exhaust gases were measured.Theoretical studies involve numerical simulations of reactive flow inside standard and variable geometry combustion chambers. Detailed chemical kinetic mechanisms were used to describe the chemistry involved in the combustion process. The outputs from numerical simulations are coupled with a reduced system represented by cylindrical share flow reactor model in order to examine the impact of the gas analyzer probe and hose on the composition of the exhaust gases.Results of theoretical studies are in good agreement with results obtained experimentally. Both show significant advantages of utilization of the variable geometry combustion chamber in terms of performance and emissions reduction. The NOX reduction by over 40% in comparison to standard geometry combustion chamber and double reduction in CO emissions for variable geometry combustor were noted. Additionally, the results are compared with real GTM-120 engine performance and emissions from bench testing [3]. Based on the experimental data, a set of 3-D performance and emissions maps has been created. The maps are aimed to aid for variable area dilution zone inside a small gas turbine engine control law creation.

Numerical simulation of compressible reactive flows with high-order schemes

Numerical simulation of compressible reactive flows with high-order schemes

Numerical Simulation of Reactive Flows in Ramjet Type Combustors and Associated Validation Experiments

Twenty years ago, a review listed the main challenges to overcome to accurately predict the reactive flow fields in ramjet combustion chambers. These issues were grouped into five topics: flow organization, combustion phenomena, stability and performance, unsteady combustion and heat transfer. The aim of this paper is to give a brief overview of the numerical and experimental studies that have been carried out since this inventory in the specific program named “Research Ramjet” to improve CFD codes. First, the experimental setup designed to obtain a better understanding and to build-up an experimental database in order to validate numerical simulations is described. Then, the progress made with regard to these technical issues is presented. Significant improvements came from the implementation of Large Eddy Simulations. However, some challenges still remain, including the prediction of the overall performance parameters and the combustion instabilities.

Kinetics of carbonate mineral dissolution in CO2-acidified brines at storage reservoir conditions.

We report experimental measurements of the dissolution rate of several carbonate minerals in CO2-saturated water or brine at temperatures between 323 K and 373 K and at pressures up to 15 MPa. The dissolution kinetics of pure calcite were studied in CO2-saturated NaCl brines with molalities of up to 5 mol kg-1. The results of these experiments were found to depend only weakly on the brine molality and to conform reasonably well with a kinetic model involving two parallel first-order reactions: one involving reactions with protons and the other involving reaction with carbonic acid. The dissolution rates of dolomite and magnesite were studied in both aqueous HCl solution and in CO2-saturated water. For these minerals, the dissolution rates could be explained by a simpler kinetic model involving only direct reaction between protons and the mineral surface. Finally, the rates of dissolution of two carbonate-reservoir analogue minerals (Ketton limestone and North-Sea chalk) in CO2-saturated water were found to follow the same kinetics as found for pure calcite. Vertical scanning interferometry was used to study the surface morphology of unreacted and reacted samples. The results of the present study may find application in reactive-flow simulations of CO2-injection into carbonate-mineral saline aquifers.

About Multi-resolution Techniques for Large Eddy Simulation of Reactive Multi-phase Flows

A numerical technique for mesh refinement in the HeaRT (Heat Release and Transfer) numerical code is presented. In the CFD framework, Large Eddy Simulation (LES) approach is gaining in importance as a tool for simulating turbulent combustion processes, also if this approach has an high computational cost due to the complexity of the turbulent modeling and the high number of grid points necessary to obtain a good numerical solution. In particular, when a numerical simulation of a big domain is performed with a structured grid, the number of grid points can increase so much that the simulation becomes impossible: this problem can be overcomed with a mesh refinement technique. Mesh refinement technique developed for HeaRT numerical code (a staggered finite difference code) is based on an high order reconstruction of the variables at the grid interfaces by means of a least square quasi-eno interpolation: numerical code is written in modern Fortran (2003 standard of newer) and is parallelized using domain decomposition and message passing interface (MPI) standard.

Hierarchical calibration and validation of computational fluid dynamics models for solid sorbent-based carbon capture

To quantify the predictive confidence of a device scale solid sorbent-based carbon capture design where there is no direct experimental data available, a hierarchical validation methodology is first proposed. In this hierarchy, a sequence of increasingly complex “unit problems” are validated using a statistical calibration framework. This paper describes the computational fluid dynamics (CFD) multi-phase reactive flow simulations and the associated data flows within each unit problem. Each validation requires both simulated and physical data, so the bench-top experiments used in each increasingly complex stage were carefully designed to follow the same operating conditions as the simulation scenarios. A Bayesian calibration procedure is employed and the posterior model parameter distributions obtained at one unit-problem level are used as prior distributions for the same parameters in the next-tier simulations. Overall, the results have demonstrated that the calibrated multiphase reactive flow models within MFIX can be used to capture the bed pressure, temperature, CO2 capture capacity, and kinetics with quantitative accuracy. The CFD modeling methodology and associated uncertainty quantification techniques presented herein offer a solid framework for estimating the predictive confidence in the virtual scale up of a larger carbon capture device.

Impact of interaction between cavity flow and mainstream on the performance of a model trapped vortex combustor

In trapped vortex combustors, radial strut serves as a critical component for it determines the interaction between cavity flow and mainstream and thus combustion and emission performances. Aiming at this topic, a trapped vortex combustor comprising interchangeable aerodynamic multipoint fueling setup, which have different strut width but same passage area, is designed. Experimental studies are carried out to investigate the combustion efficiency and emission performance. The configuration with wider struts exhibits excellent ability of NOx reduction in all operating conditions. However, relatively higher level of carbon monoxide and hydrocarbon are produced especially under cavity-only conditions, and thus the corresponding combustion efficiencies are lower. Nonreacting computational fluid dynamics results show that it is difficult for cavity flow to rush to the tail of bluff body because slenderer struts cannot provide sufficient protection. Thus, flow pattern and mixing of cavity flow and mainstream are influenced. Residence time of cavity-fore flow is calculated using a novel approach referred to as difference of tracer-pulse response. Reacting flow simulations show a different flow structure from reacting flow. It reveals that cavity flow can partially rush to the tail of bluff body through the channel behind the slenderer struts of configuration-1. However, the mixture fraction and temperature behind the bluff body of configuration-1 are still lower, and thus the NOx formation rate is relative lower.

Transient reacting flow simulation of spouted fluidized bed for coal-direct chemical looping combustion with different Fe-based oxygen carriers

Coal-direct chemical-looping combustion (CD-CLC) is a next generation combustion technology that shows great promise as a solution for the need of high-efficiency low-cost carbon capture from fossil fueled power plants. To realize this technology on an industrial scale, the development of high-fidelity simulations is a necessary step to develop a thorough understanding of the CLC process. In this paper, simulations of multiphase flow in a CD-CLC process with chemical reactions are performed using the ANSYS Fluent CFD software. Discrete Element Method (DEM) provides the means for tracking the motion of individual metal oxide particles in the CD-CLC system as they react with the fuel and combine with CFD for capturing the solid–gas multiphase hydrodynamics. The overall results of the coupled CFD/DEM simulations using Fe-based oxygen carriers reacting with gaseous CH4 demonstrate that chemical reactions have been successfully incorporated into the CFD/DEM approach. The simulations show a strong dependence of the fluidization performance of the fuel reactor on the density of bed material and provide important insight into selection of a right oxygen carrier for the enhanced performance. This work provides the foundation for future simulations of CD-CLC systems using solid coal as fuel, which will be crucial for successful deployment of CD-CLC technology from the laboratory scale to pilot and industrial scale projects.

Numerical exploration of mixing and combustion in ethylene fueled scramjet combustor

Numerical simulations are performed for full scale scramjet combustor of a hypersonic airbreathing vehicle with ethylene fuel at ground test conditions corresponding to flight Mach number, altitude and stagnation enthalpy of 6.0, 30km and 1.61MJ/kg respectively. Three dimensional RANS equations are solved along with species transport equations and SST-kω turbulence model using Commercial CFD software CFX-11. Both nonreacting (with fuel injection) and reacting flow simulations [using a single step global reaction of ethylene-air with combined combustion model (CCM)] are carried out. The computational methodology is first validated against experimental results available in the literature and the performance parameters of full scale combustor in terms of thrust, combustion efficiency and total pressure loss are estimated from the simulation results. Parametric studies are conducted to study the effect of fuel equivalence ratio on the mixing and combustion behavior of the combustor.

On the Finite Differences Used in Reacting Flow Simulations

AbstractThere exist many spatial discretization schemes that are well able to provide accurate and stable approximations for isothermal turbulent flows. Comparatively little analysis has been made of the performance of these schemes in the presence of temperature gradients driven by combustion. In this paper, the effects of temperature gradients on numerical stability are explored. A surprising result is that temperature gradients in the flow have a tendency to impinge on left half plane (LHP) stability of the spatial discretization scheme. Reasons for this tendency are explored and two remedies are proposed: one based on the particular class of finite difference schemes, and one based on an alternative method of boundary condition specification.

X-ray tomography-based analysis of transport and reaction in the catalyst coating of a reformer

A multi-scale analysis of the diffusion and reaction in the catalyst coating of a reformer is done based on virtual reconstructions of the porous catalyst coating. Pore structure can have significant impact on reaction and transport processes in the catalyst coating and consequently on the reformer performance. In this work, tomography-based simulation of the methane steam reforming in the microstructure of a nickel–alumina spinel coating is presented. X-ray nano- and micro-computed tomography are done to reconstruct the catalyst pore structure at different length scales and to consider the effect of both intra- and inter-particle pores on the reacting flow in the coating. Results of the reacting flow simulation in the coating microstructure based on tomographic imaging are compared with the simulation using numerically reconstructed microstructure based on a random packing of spheres with similar morphological characteristics. The comparison shows that the rate of hydrogen production, in general, is higher in the particle packing due to better inter-particle pore connectivity.

Transesterification of Waste Cooking Oil: Kinetic Study and Reactive Flow Analysis

Homogenous catalyst transesterification of biofuel into a biodiesel is a multiple, reversible reactions process. It is steered by several parameters including residence time, the reactants quality, proportions, state of mixing, as well as catalyst amount and temperature. In this work the chemical kinetics of a simple and robust tubular semi-continuous reactor of methoxide and biomass lipid is carried out in the presence of NaOH (sodium hydroxide) homogenous catalyst. The kinetic parameters are evaluated for the mixed waste cooking oil (WCO). Results demonstrated that higher conversion and product yield is achieved at relatively higher temperatures and methanol to triglyceride molar ratios, and longer residence time. The kinetic study is carried out at the optimal conversion metrics. The obtained kinetic data was found to be in line with the published kinetic studies and inferior to published data for virgin oil under the similar process parameters. The evaluated rate constants and their activation energies are implemented in high fidelity reactive flow simulation and resulted in a worthy reaction trend that brought more insight to several reactor and flow parameters.

Reducing the memory footprint in Large Eddy Simulations of reactive flows

RAM/core is a critical value for coupled CFD chemistry applications.Two implementation options are proposed to share chemistry LUTs on compute nodes.Two test cases show: memory usage is reduced significantly using any implementation.Runtime is larger using MPI-3 based implementation compared to MMAP system call. CFD simulations of reactive flows couple the domains of flame chemistry and computational fluid dynamics. Solving the chemistry domain in-situ is extremely demanding. It is therefore calculated beforehand and stored into a Look-up table (LUT), which is loaded in each MPI-based CFD application process in a parallel simulation. These chemistry database files can become very large when including many variables and solution values, putting a limit on the number of CFD mesh points as well as the solver instances which can be kept in RAM at the same time. In the paper we approach this problem using dynamic memory managing techniques for single core and parallel applications effectively reducing the memory requirements per core, while keeping the same database resolution. Different implementation options for shared memory on compute nodes provided by the MPI-3 standard and MMAP as a POSIX-compliant system call are analyzed and evaluated using two test cases.

Transient Reacting Flow Simulation of Spouted Fluidized Bed for Coal-Direct Chemical Looping Combustion

Coal-direct chemical-looping combustion (CD-CLC) is a next generation combustion technology that shows great promise as a solution for the need of high-efficiency low-cost carbon capture from fossil fueled power plants. To realize this technology on an industrial scale, the development of high-fidelity simulations is a necessary step to develop a thorough understanding of the CLC process. In this paper, simulations for multiphase flow of the CD-CLC process with chemical reactions are performed using ANSYS Fluent computational fluid dynamics (CFD) software. The details of the solid–gas two-phase hydrodynamics in the CLC process are investigated using the Lagrangian particle-tracking approach called the discrete element method (DEM) for the movement and interaction of the solid oxygen carrier particles with the gaseous fuel. The initial CFD/DEM simulation shows excellent agreement with the experimental results obtained in a laboratory scale fuel reactor in cold-flow conditions at Darmstadt University of Technology. Subsequent simulations using 60% Fe2O3 supported on MgAl2O4 reacting with gaseous CH4 demonstrate successful integration of chemical reactions into the CFCD/DEM approach. This work provides a strong foundation for future simulations of CD-CLC systems using solid coal as fuel, which will be crucial for successful deployment of CD-CLC technology from the laboratory scale to pilot and industrial scale projects.

Reduced Chemical Kinetics for the Modeling of TiO2 Nanoparticle Synthesis in Flame Reactors

Flame synthesis represents a viable technique for large-scale production of titanium dioxide (TiO2) nanoparticles. A key ingredient in the modeling of this process is the description of the chemical kinetics, which include Ti oxidation, hydrocarbon fuel combustion, and chlorination. While detailed chemical mechanisms have been developed for predicting TiO2 nanoparticle properties by West et al. (e.g., Combust. Flame 2009, 156, 1764), their use in turbulent reacting flow simulations is limited to very simple configurations or requires significant modeling assumptions to bring their computational cost down to an acceptable level. In this work, a reduced kinetic scheme describing the oxidation of TiCl4 in a methane flame is derived from and validated against the predictions of a detailed mechanism from the literature. The reduction procedure uses graph-based methods for unimportant kinetic pathways elimination and quasi-steady-state species selection. Reduction targets are chosen in accordance with previous modeling results that showed the importance of temperature and overall concentration of titanium-containing species in both nucleation and surface growth rates. The resulting reduced scheme is thoroughly evaluated over a wide range of conditions relevant to flame-based synthesis, and the capability of the reduced model to adequately capture the process dynamics at a much lower computational cost is demonstrated.

Numerical Simulation of Reactive Flows in Overexpanded Supersonic Nozzle with Film Cooling

Reignition phenomena occurring in a supersonic nozzle flow may present a crucial safety issue for rocket propulsion systems. These phenomena concern mainly rocket engines which use H2gas (GH2) in the film cooling device, particularly when the nozzle operates under over expanded flow conditions at sea level or at low altitudes. Consequently, the induced wall thermal loads can lead to the nozzle geometry alteration, which in turn, leads to the appearance of strong side loads that may be detrimental to the rocket engine structural integrity. It is therefore necessary to understand both aerodynamic and chemical mechanisms that are at the origin of these processes. This paper is a numerical contribution which reports results from CFD analysis carried out for supersonic reactive flows in a planar nozzle cooled with GH2film. Like the experimental observations, CFD simulations showed their ability to highlight these phenomena for the same nozzle flow conditions. Induced thermal load are also analyzed in terms of cooling efficiency and the results already give an idea on their magnitude. It was also shown that slightly increasing the film injection pressure can avoid the reignition phenomena by moving the separation shock towards the nozzle exit section.

Two-stage ignition and NTC phenomenon in diesel engines

Two-stage ignition and NTC phenomenon in diesel sprays is investigated by performing 3-D two-phase reacting flow simulations in a dual-fuel engine. Spray processes modeled include fuel atomization, droplet distortion, droplet drag, turbulent dispersion, droplet interactions in terms of collision and coalescence, vaporization, and spray–wall interaction. A validated reaction mechanism is implemented in the CFD solver, which has previously been validated for both evaporating and reacting sprays. For single-fuel cases, the effect of temperature on two-stage ignition is examined by varying the start of injection (SOI). While results indicate global similarities between the two-stage ignition processes in diesel sprays and spatially homogeneous mixtures, there are also noticeable differences between them due to temporally and spatially evolving temperature and species fields in the spray case. For instance, both the first- and second-stage ignition delays are higher for the spray cases compared to homogeneous mixtures. Second, while ignition delay for homogeneous mixtures exhibits a NTC region, that for sprays indicate a ZTC region. Moreover, the first- and second-stage ignitions for the spray occur over a wide φ range and at multiple locations in the spray, implying a spatially wide ignition kernel. Additionally, while the chemical ignition delays are strongly influenced by the injection timing, the physical delays are essentially independent of this parameter. Results with dual fuel indicate that the two-stage ignition behavior remains intact even at high molar fractions of methane. The addition of methane increases ignition delays for both sprays and homogeneous mixtures, and can be attributed to the reduction in O2 and the chemical effect of methane. The sensitivity analysis indicated that the chemical effect is primarily due to reaction CH4+OH=CH3+H2O.

Assessment of a methodology to include differential diffusion in numerical simulations of a turbulent flame

A newly developed methodology to incorporate differential diffusion effects in CFD simulations of turbulent reactive flows is applied to the ‘H3’ benchmark flame of the Turbulent Non-premixed Flames (TNF) workshop series. The fuel consists of species with vastly different mass diffusivities. The accuracy of the proposed methodology is assessed and the influence of inclusion of differential diffusion is examined. Good agreement is obtained between numerical simulation results and experimental data if differential diffusion effects are considered. Differential diffusion effects are shown to be significant in the region near the inlet and have a great influence on the stabilization mechanism of the flame, as well as on the predicted profiles of temperature and species concentration. If differential diffusion is modeled, temperatures above the adiabatic flame temperature are recovered. Ignoring differential diffusion, this is not the case and large discrepancies between numerical simulations and experiments are observed.

A mesh partitioning algorithm for preserving spatial locality in arbitrary geometries

A space-filling curve (SFC) is a proximity preserving linear mapping of any multi-dimensional space and is widely used as a clustering tool. Equi-sized partitioning of an SFC ignores the loss in clustering quality that occurs due to inaccuracies in the mapping. Often, this results in poor locality within partitions, especially for the conceptually simple, Morton order curves. We present a heuristic that improves partition locality in arbitrary geometries by slicing a Morton order curve at points where spatial locality is sacrificed. In addition, we develop algorithms that evenly distribute points to the extent possible while maintaining spatial locality. A metric is defined to estimate relative inter-partition contact as an indicator of communication in parallel computing architectures. Domain partitioning tests have been conducted on geometries relevant to turbulent reactive flow simulations. The results obtained highlight the performance of our method as an unsupervised and computationally inexpensive domain partitioning tool.

Emission Reduction in a GDI Engine with Different Injection Timings – A CFD Investigation

Spark ignition dominates the personal transportation segment. These engines have been well developed over 100 years. In order to meet the stringent emission norms, the performance of the engines has to be enhanced. In this regard CFD plays an important role in designing a better engine. In this paper the results of a three dimensional reactive flow simulation conducted on a commercial SI engines are presented. The simulations were conducted at an engine speed of 1000rpm with fuel injected directly into the cylinder. Three modes of fuel injections are analyzed. They are during opening of intake valve (case-1), during peak valve lift (case-2) and after the closure of intake valve (case-3). The injected fuel quantity is such that when all the injected fuel evaporates completely it would produce a stoichiometric mixture in the combustion chamber. The early injection (case-1) gives better performance in terms of higher peak pressure, early heat release, better homogeneous mixture and lowest emission compared to the other two cases. Compared with case-1, case-2 have 1.91% less maximum pressure, 4.3% lower CO2, 7.62% higher CO and 64.42% higher NOx. Whereas in case-3, 38.63% less maximum pressure, 34.78% lower CO2, 35.32% higher CO and 56.5% higher NOx.