Numerical Modeling Of The Shock Tube Flow Fields Before Andduring Ignition Delay Time Experiments At Practical Conditions

An axi-symmetric shock-tube model has been developed to simulate the shock-wave propagation and reflection in both non-reactive and reactive flows. Simulations were performed for the full shock-tube geometry of the high-pressure shock tube facility at Texas A&M University. Computations were carried out in the CFD solver FLUENT based on the finite volume approach and the AUSM+ flux differencing scheme. Adaptive mesh refinement (AMR) algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous and reactive effects. A conjugate heat transfer model has been incorporated which enhanced the credibility of the simulations. The multi-dimensional, time-dependent numerical simulations resolved all of the relevant scales, ranging from the size of the system to the reaction zone scale. The robustness of the numerical model and the accuracy of the simulations were assessed through validation with the analytical ideal shock-tube theory and experimental data. The numerical method is first applied to the problem of axi-symmetric inviscid flow then viscous effects are incorporated through viscous modeling. The non-idealities in the shock tube have been investigated and quantified, notably the non-ideal transient behavior in the shock tube nozzle section, heat transfer effects from the hot gas to the shock tube side walls, the reflected shock/boundary layer interactions or what is known as bifurcation, and the contact surface/bifurcation interaction resulting into driver gas contamination. The non-reactive model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave. Both the inviscid and the viscous non-reactive models provided a baseline for the combustion model iii which involves elementary chemical reactions and requires the coupling of the chemistry with the flow fields adding to the complexity of the problem and thereby requiring tremendous computational resources. Combustion modeling focuses on the ignition process behind the reflected shock wave in undiluted and diluted Hydrogen test gas mixtures. Accurate representation of the Shock - tube reactive flow fields is more likely to be achieved by the means of the LES model in conjunction with the EDC model. The shock-tube CFD model developed herein provides valuable information to the interpretation of the shock-tube experimental data and to the understanding of the impact the facility-dependent non-idealities can have on the ignition delay time measurements

Reduced Combustion Time Model For Methane In Gas Turbine Flow Fields

Computational fluid dynamics (CFD) modeling of the complex processes that occur within the burner of a gas turbine engine has become a critical step in the design process. However, due to computer limitations, it is very difficult to completely couple the fluid mechanics solver with the full combustion chemistry. Therefore, simplified chemistry models are required, and the topic of this research was to provide reduced chemistry models for CH4/O2 gas turbine flow fields to be integrated into CFD codes for the simulation of flow fields of natural gas-fueled burners. The reduction procedure for the CH4/O2 model utilized a response modeling technique wherein the full mechanism was solved over a range of temperatures, pressures, and mixture ratios to establish the response of a particular variable, namely the chemical reaction time. The conditions covered were between 1000 and 2500 K for temperature, 0.1 and 2 for equivalence ratio in air, and 0.1 and 50 atm for pressure. The kinetic time models in the form of ignition time correlations are given in Arrhenius-type formulas as functions of equivalence ratio, temperature, and pressure; or fuel-to-air ratio, temperature, and pressure. A single ignition time model was obtained for the entire range of conditions, and separate models for the low-temperature and high-temperature regions as well as for fuel-lean and rich cases were also derived. Predictions using the reduced model were verified using results from the full mechanism and empirical correlations from experiments. The models are intended for (but not limited to) use in CFD codes for flow field simulations of gas turbine combustors in which initial conditions and degree of mixedness of the fuel and air are key factors in achieving stable and robust combustion processes and acceptable emission levels. The chemical time model was utilized successfully in CFD simulations of a generic gas turbine combustor with four different cases with various levels of fuel-air premixing. © 2009 CAS/DICP

Butane Oxidation At Elevated Temperatures

Autoignition and oxidation of n-butane and iso-butane mixtures in air were studied in a shock tube at conditions where few data exist, namely at temperatures above 1100 K and for undiluted fuel-air mixtures. The experiments were performed in the reflected-shock region over the temperature range of 1150 - 1470 K, an average pressure of 1.45 atm, and equivalence ratios of 0.5 and 1.0. Ignition delay times were obtained for mixtures of n-C4H 10/iso-C4H10 in the ratios of 100/0, 0/100, and 50/50. Ignition was determined from the pressure trace measured at the shock-tube endwall. Effect of composition, stoichiometry and temperature were explored for the mixtures. Under all conditions, normal butane was shown to be more readily ignitable than its isomer iso-butane. The experimental results should serve as validation to chemical kinetics mechanisms containing at least C4Hx hydrocarbons that lack benchmark data, especially at elevated temperature conditions

Reduced Combustion Time Model For Methane In Gas Turbine Flow Fields

Computational fluid dynamics modeling of the complex processes that occur within the burner of a gas turbine engine has become a critical step in the design process. However, due to computer limitations, it is very difficult to completely couple the fluid mechanics solver with the full combustion chemistry. Therefore, simplified chemistry models are required, and the topic of this research was to provide reduced chemistry models for CH4/O 2 gas turbine flow fields to be integrated into CFD codes for the simulation of flow fields of natural gas-fueled burners. The reduction procedure for the CH4/O2 model utilized a response modeling technique wherein the full mechanism was solved over a range of temperatures, pressures, and mixture ratios to establish the response of a particular variable such as chemical reaction time. The conditions covered were between 1000 and 2500K for temperature, 0.1 and 2 for equivalence ratio, and 0.1 and 50 for pressure. The kinetic times include the time to ignition, the time to equilibrium H2O formation, the time to equilibrium CO formation, and the time to equilibrium NO formation. The kinetic time models are given in Arrhenius type formulas as functions of equivalence ratio, temperature, and pressure; or fuel-to-air ratio, temperature and pressure. A single, global kinetics model was obtained for the entire range of conditions, and separate models for the low-temperature and high-temperature regions as well as for fuel-lean and rich cases were also derived. Predictions using the reduced model were verified using results from the full mechanism and empirical correlations from experiments. The models are intended for (but not limited to) use in CFD codes for flow field simulations of gas turbine combustors in which initial conditions and degree of mixedness of the fuel and air are key factors in achieving stable and robust combustion processes and acceptable emissions levels. The new model was applied to CFD simulations of a typical gas turbine burner with premixer with good results

A Conjugate Axisymmetric Model Of A High-Pressure Shock-Tube Facility

Purpose - An axisymmetric shock-tube model of the high-pressure shock-tube facility at the Texas A&M University has been developed. The shock tube is non-conventional with a non-uniform cross-section and features a driver section with a smaller diameter than the driven section. The paper aims to discuss these issues. Design/methodology/approach - Computations were carried out based on the finite volume approach and the AUSM + flux-differencing scheme. The adaptive mesh refinement algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous effects. The incorporation of a conjugate heat transfer model enhanced the credibility of the results. Findings - The shock-tube model is validated with simulation of the bifurcation phenomenon and with experimental data. The model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave as a result of reflected shock/boundary layer interaction or bifurcation. The pressure profiles behind the reflected shock wave agree with the experimental results. Originality/value - This paper presents one of the first studies to model the entire flow field history of a non-uniform diameter shock tube with a conjugate heat transfer model beginning from the bursting of the diaphragm while simultaneously resolving the fine features of the reflected shock-boundary layer interaction and the post-shock region near the end-wall, at conditions useful for chemical kinetics experiments. An important discovery from this study is the possible existence of hot spots in the end-wall region that could lead to early non-homogeneous ignition events. More experimental and numerical work is needed to quantify the hot spots. © Emerald Group Publishing Limited

Time-Accurate Simulation Of Shock Propagation And Reflection In An Axi-Symmetric Shock Tube

An axi-symmetric shock-tube model has been developed to simulate shock propagation and reflection in both inviscid and viscous nonreactive flows. Simulations were performed for the full shock-tube geometry of the high-pressure shock tube facility at Texas A&M University. The CFD code FLUENT was employed to simulate the shock propagation and reflection processes in the shock tube, and the flow properties behind the reflected shock wave were obtained by solving the axi-symmetric, unsteady Euler and Navier-Stokes equations. Computations were carried out based on the finite volume approach and the AUSM+ flux differencing scheme. Adaptive mesh refinement (AMR) algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous effects. The bifurcation phenomenon resulting from the interaction of the reflected shock wave and the boundary layer has been accurately simulated. Conjugate heat transfer modeling was made possible in conjunction with the viscous model which enhanced the credibility of the results. The robustness of the numerical model and the accuracy of the simulations were assessed through validations with the analytical ideal theory and experimental measurements. The model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave associated with the viscous non-ideal effects. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc