Detailed Chemistry Simulations Research Articles

Evaluation of flamelet/progress variable model for laminar pulverized coal combustion

In the present work, the flamelet/progress variable (FPV) approach based on two mixture fractions is formulated for pulverized coal combustion and then evaluated in laminar counterflow coal flames under different operating conditions through both a priori and a posteriori analyses. Two mixture fractions, Zvol and Zchar, are defined to characterize the mixing between the oxidizer and the volatile matter/char reaction products. A coordinate transformation is conducted to map the flamelet solutions from a unit triangle space (Zvol, Zchar) to a unit square space (Z, X) so that a more stable solution can be achieved. To consider the heat transfers between the coal particle phase and the gas phase, the total enthalpy is introduced as an additional manifold. As a result, the thermo-chemical quantities are parameterized as a function of the mixture fraction Z, the mixing parameter X, the normalized total enthalpy Hnorm, and the reaction progress variable YPV. The validity of the flamelet chemtable and the selected trajectory variables is first evaluated in a priori tests by comparing the tabulated quantities with the results obtained from numerical simulations with detailed chemistry. The comparisons show that the major species mass fractions can be predicted by the FPV approach in all combustion regions for all operating conditions, while the CO and H2 mass fractions are over-predicted in the premixed flame reaction zone. The a posteriori study shows that overall good agreement between the FPV results and those obtained from detailed chemistry simulations can be achieved, although the coal particle ignition is predicted to be slightly earlier. Overall, the validity of the FPV approach for laminar pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.

Differential diffusion effects inclusion with flamelet generated manifold for the modeling of stratified premixed cooled flames

CFD predictions of flame position, stability and emissions are essential in order to obtain optimized combustor designs in a cost efficient way. However, the numerical modeling of practical combustion systems is a very challenging task. As a matter of fact, the use of detailed reaction mechanisms is necessary for reliable predictions, especially for highly diffusive fuels. Unfortunately, the modeling of the full detail of practical combustion equipment is currently prohibited by the limitations in computing power, given the large number of species and reactions involved. The Flamelet Generated Manifold (FGM) method reduces these computational costs by several orders of magnitude without loosing too much accuracy. Hereby, FGM enables the application of reliable chemistry mechanisms in CFD simulations of combustion processes. In the FGM technique the progress of the flame is generally described by a few control variables. For each control variable a transport equation is solved during run-time. The flamelet system is computed in a pre-processing stage, and a manifold with all the information about combustion is stored in a tabulated form. In the present paper, the FGM model is implemented for the analysis of partially premixed non-adiabatic flames, including the effects of differential diffusion. Subsequently, a computational analysis of partially premixed non-adiabatic flames is presented. In this scope, a series of test simulations is performed using FGM for a two dimensional geometry, characterized by a distinctive stratified methane/air inlet, and compared with detailed chemistry simulations. The results indicate that detailed simulations are well reproduced with the FGM technique.

Sound generation by premixed flame annihilation with full and simple chemistry

This paper presents a numerical study of sound generation by premixed laminar flame annihilation with full and simple chemistry. Planar annihilation is examined from lean (ϕ=0.5) to rich (ϕ=3) conditions, which correspond to (effective) Lewis numbers ranging from 0.5 to 2.0 respectively. The full chemistry simulations use a well known detailed mechanism for hydrogen-air mixtures [1,2]. The simple chemistry simulations are tuned such that the freely propagating laminar flame speed matched those from the detailed chemistry simulations at the same equivalence ratio.These simulations examine two, related issues. First, they are used to confirm whether the Lewis number plays a significant role in the sound generation process, as our previous, simple chemistry studies have found [3,4]. More generally, these simulations seek to determine whether the use of simple chemistry to model sound generation is reasonable. The importance of the Lewis number is confirmed, and the use of simple chemistry is found to be reasonable in some cases.

Impact of acoustic pressure on autoignition and heat release

A feedback mechanism for thermoacoustic pulsations in gas turbine reheat combustors is proposed and investigated, namely the impact of acoustic pressure waves on the reaction kinetics of autoignition. An analytical model framework is developed, which represents the combustor as a plug flow reactor. This analogy allows one to derive modulations of the heat release rate as a result of the history of pressure perturbations during the autoignition process. Numerical studies are conducted on homogeneous reactors with detailed chemistry simulations, in order to assess quantitatively the pressure sensitivity of the reaction kinetics of autoignition. From such sensitivities, flame transfer functions of autoignition flames are derived. The expressions obtained are successfully compared with time-domain simulations in one-dimensional space, and used in acoustic network models for stability predictions. The wider applicability of the model is demonstrated by extending it to nonlinear dynamics, transverse modes and technical premix conditions. The results obtained indicate that in general the feedback mechanism results in a positive contribution to the acoustic source term, in particular at elevated frequencies.

Stochastic reactor modelling of multi modes combustion with diesel direct injection or hydrogen jet ignition start of combustion

Recent papers [1–5] have proposed two different systems to more efficiently and more rapidly burn the fuel in highly boosted, high compression ratio, directly injected internal combustion engines permitting multi-mode combustion operation. In a first system, a second direct injector is coupled with the standard Diesel direct injector and glow plug. The second direct injector introduces the most of the fuel while the Diesel direct injector only introduces a minimum amount of fuel to control the start of the combustion about top dead centre. The fuel injected before the Diesel ignition injection burns premixed, the fuel injected after the Diesel ignition injection burns diffusion. This design permits combustion premixed gasoline-like if all the fuel is injected before the Diesel ignition injection, diffusion Diesel-like if all the fuel is injected after the Diesel ignition injection (as done in the Westport High Pressure Direct Injection concept [12]), and mixed gasoline/Diesel like injecting the fuel before and after the Diesel ignition injection. The premixed gasoline-like mode is actually a homogeneous charge compression ignition (HCCI)-like mode, where an amount of fuel smaller than the threshold value producing top dead centre auto ignition is then ignited at top dead centre by the Diesel ignition injection in a more robust, stable and repeatable operation unaffected by small changes in properties and composition of the fuel and air mixture. In an alternative design, the glow plug is replaced by a jet ignition devices feed preferably with H2. In this case, a spark ignition ignites the stoichiometric H2-air mixture within the jet ignition pre-chamber. The jets of hot reacting H2-air combusting gases then ignite the main chamber premixed mixture in the gasoline-like operation or create suitable conditions for the fuel subsequently injected to burn diffusion in the Diesel-like operation or perform both duties in the mixed gasoline/Diesel-like operation. A single main chamber direct injector is generally needed (for example with H2, CH4 or C3H8 fuels). With NH3, a second main chamber direct injector with H2 is also used to limit the volume of the jet ignition pre-chamber. In this short communications, the results of detailed chemistry simulations with the SRM (Stochastic Reactor Model) suite, a sophisticated engineering tool combining conventional 1D or 3D fluid dynamics approaches are presented to further support these two engine concepts working with fuels H2, CH4, C3H8, NH3, I-C8H18 and N-C7H16 and adopting two different mechanisms for chemical kinetics. Within the limits of the present simulations (a very accurate chemical kinetic for combustion of I-C8H18 and N-C7H16 but a much less accurate chemical kinetic for the other fuels and especially for NH3, unavailability of variable composition and variable properties multiple injections), the Diesel injection ignition and the hydrogen jet ignition are proved to permit combustion modes leading to indicated thermal efficiencies up to 10% better than the latest Diesels at high loads within the same peak pressure and peak temperature constraints.

Computational Fluid Dynamics Modeling for Urea Hydrolysis in a Batch Reactor for Flue Gas Conditioning

AbstractA computational fluid dynamics (CFD) model is proposed to simulate urea hydrolysis for ammonia synthesis as a safe feed stock to flue gas conditioning in thermal power plants. A series of parametric studies to investigate flow rates, thermal boundary conditions, and reactor geometry was performed and operating conditions and reactor geometry were optimized. Detailed 3D flow, heat, and chemistry simulations of ammonia were carried out with predicted conversions comparable to measurements and the dependence of the experiments on the reaction parameters was evaluated. Through simulation under the same conditions the output was generated and compared to the experimental plot. Profiles of temperature and flow patterns were successfully achieved through simulation.

A Detailed Chemistry Simulation of the SI-HCCI Transition

A Stochastic Reactor Model (SRM) has been used to simulate the transition from Spark Ignition (SI) mode to Homogeneous Charge Compression Ignition (HCCI) mode in a four cylinder in-line four-stroke naturally aspirated direct injection SI engine with cam profile switching. The SRM is coupled with GT-Power, a onedimensional engine simulation tool used for modelling engine breathing during the open valve portion of the engine cycle, enabling multi-cycle simulations. The model is initially calibrated in both modes using steady state data from SI and HCCI operation. The mode change is achieved by switching the cam profiles and phasing, resulting in a Negative Valve Overlap (NVO), opening the throttle, advancing the spark timing and reducing the fuel mass as well as utilising a pilot injection. Experimental data is presented along with the simulation results. The model is used to investigate key control parameters and their effects on parameters that are difficult to measure experimentally. The effect of the spark in the first HCCI cycles is found to have a major impact on the stability of the transition.

Optimization of ammonia production from urea in continuous process using ASPEN Plus and computational fluid dynamics study of the reactor used for hydrolysis process

The present study addresses the methods and means to safely produce relatively small amounts (i.e., up to 50 kg/h) of ammonia. The optimization and simulation study conducted for continuous process and effect of operation conditions like reaction temperature, initial feed concentration and pressure on ammonia production carried out using ASPEN Plus. Also, a computational fluid dynamics (CFD) model was proposed to simulate the hydrolysis of urea for synthesis of ammonia. A series of parametric studies to investigate flow rates, thermal boundary conditions and reactor geometry was performed for hydrolysis of urea and the optimized operating conditions and reactor geometry were obtained. Detailed three-dimensional flow, heat and chemistry simulations of ammonia, carbon dioxide and ammonium carbamate. The study demonstrates that simulation is a useful tool for diagnosing hydrolysis reactor mixing pathologies and for identifying practical countermeasures that could improve process performance.

Simulation of partial oxidation of natural gas with detailed chemistry: Influence of addition of H2, C2H6 and C3H8

The partial oxidation (POX) of natural gas to produce acetylene was studied using detailed chemistry simulation to determine the influences of adding H2, C2H6 and C3H8 to the CH4 feed. Four detailed chemistry mechanisms, codenamed GRI 3.0, GRI 486, Petersen, and Curran, for describing the POX of natural gas under fuel-rich conditions were evaluated by comparing calculated results with ignition delay time and homogeneous oxidation data. The Curran mechanism gave the best performance, and was further used to examine the influences brought about by changes in the natural gas composition due to the addition of H2, C2H6 and C3H8. The addition of H2, C2H6 and C3H8 reduced the consumption of natural gas per ton of acetylene produced, and would enhance the economy of the POX process and help use up excess H2, C2H6 and C3H8 from other processes in chemical plants. For natural gases that contain small amounts of higher hydrocarbons, the adding of H2, C2H6 and C3H8 significantly decreased the ignition delay time, and the residence time of the feed in the mixing zone has to be reduced when adding these species to avoid uncontrolled combustion.

Transported PDF modeling of high-Reynolds-number premixed turbulent flames

The transported PDF approach, closed at the joint composition–enthalpy level, is applied to model premixed turbulent flames at a wide range of Reynolds numbers. The initial aim of the study is to establish the impact of closure approximations for the scalar dissipation rate upon the relationship between turbulence fluctuations and predicted turbulent burning velocities. The cases considered feature stoichiometric methane–air flames with the chemical source term extracted from a detailed chemistry simulation of the corresponding unstrained laminar flame. The transported PDF approach is subsequently combined with a systematically reduced C/H/O mechanism featuring 142 reactions and 14 solved and 15 steady-state species and applied to piloted premixed stoichiometric methane–air flames investigated experimentally by Chen et al. [Combust. Flame 107 (1996) 223–226]. The cases considered here feature Re = 24 , 200 (flame F3) and 52,500 (flame F1) and Damköhler numbers approaching unity. The effects of variations in the time-scale ratio ( 2 ⩽ C ϕ ⩽ 8 ) and heat losses to the burner were investigated, along with the impact of an extended algebraic relationship for the scalar dissipation rate that accounts for small-scale properties. Comparisons with experimental data show that the modified Curl's model and the extended scalar dissipation-rate closure produce turbulent burning velocities in close agreement with measurements. The study further indicates that a closure at the joint scalar level combined with comprehensive chemistry has the potential to reproduce the detailed chemical structure of premixed turbulent flames. The importance of boundary conditions and comprehensive scalar statistics, including the scalar dissipation rate, is also emphasized by the study.

A computational model for supercritical water oxidation of organic toxic wastes

A computational fluid dynamics (CFD) model was proposed to simulate the supercritical water oxidation (SCWO) process for organic wastes. The SUPERTRAPP code (by NIST) for thermodynamic and transport properties of hydrocarbon mixtures was incorporated into the commercialized general purpose CFD code CFD-ACE for flow under supercritical pressure. The global kinetic models for supercritical water oxidation of methanol and methane (developed at MIT Energy Lab) were implemented and validated in the CFD-ACE framework against experimental data from a tubular reactor. A series of parametric studies to investigate flow rates, thermal boundary conditions and reactor geometry was performed for methane SCWO in a co-axial reactor and the optimized operating conditions and reactor geometry were obtained. Detailed three-dimensional flow, heat and chemistry simulations of methanol SCWO in the CSTR at MIT Energy Lab were also performed with predicted conversions comparable to measurements.

Repetitive Electron-Beam Controlled Switching

Previous investigators have demonstrated the feasibility of using an ionizing electron beam to control the conductivity of a gaseous volume-discharge switch. We consider the possibility of using such switches repetitively at high-power levels (up to 1010 W), with switch opening and closing times as short as several nanoseconds. An analysis of the relevant gas chemistry indicates that these requirements can best be met by using a nonelectronegative base gas with a high electron mobility, diluted with a small percentage of an electronegative gas. Detailed chemistry simulations, using the nonelectronegative gas N2 and the electronegative gas O2, are presented to support the analysis.