Kendrick Aung

Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure

Abstract Effects of positive flame stretch on the laminar burning velocities of hydrogen/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements were based on motion picture shadowgraphy, while numerical simulations were based on typical contemporary chemical reaction mechanisms. Flame conditions studied included hydrogen/air flames having fuel-equivalence ratios in the range 0.3–5.0 at normal temperature and pressure. Both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers over the test range (Karlovitz numbers up to 0.4), yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. Markstein numbers were in the range −1 to 6, with unstable (stable) preferential-diffusion conditions observed at fuel-equivalence ratios below (above) roughly 0.7. Present stretch-corrected laminar burning velocities were in reasonably good agreement with other determinations of laminar burning velocities at fuel-lean conditions where Markstein numbers, and thus effects of stretch, are small. In contrast, the stretch-corrected laminar burning velocities generally were smaller than other measurements in the literature at fuel-rich conditions, where Markstein numbers, and thus effects of stretch, are large. Finally, predicted unstretched laminar burning velocities and Markstein numbers were in reasonably good agreement with measurements, although additional study to improve the comparison between predictions and measurements at fuel-rich conditions should be considered.

Measured and predicted properties of laminar premixed methane/air flames at various pressures

Abstract Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements based on motion picture shadowgraphs, and numerical simulations based on typical contemporary chemical reaction mechanisms, were used to find the sensitivities of the laminar burning velocities to flame stretch, characterized as Markstein numbers, and the fundamental laminar burning velocities of unstretched flames. Reactant conditions included methane/air mixtures having fuel-equivalence ratios of 0.60–1.35 and pressures of 0.5–4.0 atm, at normal temperatures. Both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied significantly from unity (in the range 0.6–2.3) even though present stretch levels did not approach quenching conditions. Absolute values of Markstein numbers increased with increasing pressure, while the transition from unstable to stable preferential-diffusion conditions with increasing fuel-equivalence ratio shifted from an equivalence ratio of 0.6 at 0.5 atm to 1.2 at 4.0 atm, suggesting increased unstable flame behavior due to preferential-diffusion effects at the elevated pressures of interest for many practical applications. Finally, predictions using two contemporary chemical reaction mechanisms were in reasonably good agreement with present measurements of both Markstein numbers and unstretched laminar burning velocities.

Effects of pressure and nitrogen dilution on flame/stretch interactions of laminar premixed H2/O2/N2 flames

Effects of positive flame stretch on the laminar burning velocities of H2/O2/O2 flames at normal temperatures and various pressures and nitrogen dilutions were studied both experimentally and computationally. Measurements and numerical simulations considered freely (outwardly)-propagating spherical laminar premixed flames at both stable and unstable preferential-diffusion conditions with fuel-equivalence ratios in the range 0.45–4.00, pressures in the range 0.35–4.00 atm, volumetric oxygen concentrations in the nonfuel gas in the range 0.125–0.210, and Karlovitz numbers in the range 0.0–0.6. For these conditions, both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular pressure and reactant mixture. Measured Markstein numbers were in the range −4 to 6, implying strong flame/stretch interactions. For hydrogen/air flames, the neutral preferential-diffusion condition shifted toward fuel-rich conditions with increasing pressure. Predictions of stretch-corrected laminar burning velocities and Markstein numbers, using typical contemporary chemical reaction mechanisms, were in reasonably good agreement with the measurements.

Properties of Laminar Premixed Hydrocarbon/Air Flames at Various Pressures

Outwardly propagating spherical laminar premixed e ames were experimentally and computationally used to e nd the sensitivities of laminar burning velocities to e ame stretch, represented by Markstein numbers, and the fundamental laminar burning velocities of unstretched e ames. Conditions considered included ethane, ethylene, and propane/air e ames at fuel-equivalence ratios of 0.8 ‐1.6, and pressures of 0.5‐4.0 atm at normal temperatures. Predictions were limited to unstretched (plane) e ames using mechanisms based on GRI-Mech, e nding reasonably good agreement between measurements and predictions. An interesting experimental e nding was that Markstein numbers tended to become negative over broader ranges of fuel-equivalence ratios as the pressure increased, suggesting a greater propensity toward unstable combustion because of preferential-diffusion effects at the elevated pressures of interest for most practical applications.

Flame/stretch interactions in laminar and turbulent premixed flames

The flame/stretch interactions of laminar and turbulent premixed flames are considered both experimentally and computationally. Potentially strong effects of flame/stretch interactions due to preferential-diffusion phenomena within practical turbulent premixed flames were suggested by experiments and numerical simulations of spherical outwardly propagating laminar premixed flames. These considerations were limited to conditions where ignition disturbances, pressure variations, intrinsic unsteadiness of propagating spherical flames, and radiative heat losses were small. Flame reactants consisting of H 2 /O 2 /N 2 and several light hydrocarbon/air mixtures were studied for various fuel-equivalence ratios and pressures of 0.5-4.0 atm at normal temperature (298±3K). The measurements and predictions yielded several interesting results, as follows: Flame response to stretch was linear using a local conditions hypothesis to define characteristic flame length and time scales, yielding constant Markstein numbers for given flame conditions; effects of stretch were surprisingly strong with up to 100 percent variations of laminar burning velocities resulting from rather modest stretch rates well below extinction conditions (i.e., Karlovitz numbers less than 0.5); there was a progressive tendency for greater ranges of unstable preferential-diffusion conditions (negative Markstein numbers) as pressures were increased for all reactant mixtures studied; and several contemporary detailed treatments of multicomponent transport and chemical reaction mechanisms yielded reasonably good predictions of laminar burning velocities and their sensitivity to flame stretch due to preferential-diffusion effects. The predictions suggest that the strong sensitivity of the present flames to stretch is mainly caused by preferential diffusion of light radicals and stable species relative to typical stable reaction products and heat, with increased preferential-diffusion instability at elevated pressures resulting from reduced radical concentrations in the reaction zone due to increased radical recombination rates. The potential practical importance of flame/stretch interactions was examined by considering the properties of strongly turbulent premixed flames. These measurements involved premixed H 2 /O 2 /N 2 and C 3 H 8 /air flames propagating in the thin wrinkled flamelet regime within isotropic turbulence. Test conditions included unstable, near-neutral, and stable flames with respect to effects of preferential-diffusion. The experiments yielded several interesting observations, as follows: 1) Rates of turbulent flame propagation progressively decreased as flame stability with respect to preferential-diffusion effects increased even through unstretched laminar burning velocities and turbulence properties were the same; 2) Distortion of the flame surfaces by turbulence as the flames grew caused their fractal dimensions to progressively increase from a value of 2.0, appropriate for a smooth surface, to asymptotic values in the range 2.3-2.4, irrespective of preferential-diffusion stability conditions; 3) Other parameters characterizing the extent of distortion of the flame surfaces showed no tendency to approach asymptotic values for available observation times, however, raising questions about the existence of steady turbulent flame propagation properties for the present test conditions; and 4) The extent of flame surface distortion progressively increased at a given flame diameter, but decreased at a given time of propagation, as preferential-diffusion stability was progressively increased even though unstretched laminar burning velocities and turbulence properties were the same. These flame/stretch interactions in turbulent flames can be explained by noting that stable (unstable) preferential-diffusion conditions tend to retard (enhance) distortion of the flame surface by turbulence for outwardly propagating spherical turbulent premixed flames in much the same way that preferential-diffusion effects interact with small disturbances to yield either stable (unstable) flames for nonturbulent conditions.

Performance Predictions of a Hydrogen-Enhanced Natural Gas HCCI Engine

The characteristics of HCCI engines were numerically investigated by CHEMKIN software, CHEMKIN 4.0. Cylinder temperature and pressure, ignition delay time, peak temperature and pressure, indicated work, indicated power and IMEP were performed for the analysis the HCCI combustion of Methane and Hydrogen. The natural gas can be represented as the Methane (CH4 ), which is the main ingredient. The simulation evaluations were done by increasing the initial concentrations of H2 , changing the initial temperature (inlet temperature), or varying the equivalence ratio. The numerical simulations were accomplished using CHEMKIN Suite from Reaction Design and the results are focused on ignition time, peak temperature, and indicated power. The effect of hydrogen addition to methane increases peak temperature and pressure, decreases ignition delay time, and increases indicated power.Copyright

Markstein numbers and unstretched laminar burning velocities of wet carbon monoxide flames

Effects of positive flame stretch on the laminar burning velocities of wet carbon-monoxide/air flames at normal temperature and pressure were studied both experimentally and computationally. Measurements and numerical simulations considered outwardly-propagating spherical laminar premixed flames having both stable and unstable preferential-diffusion behavior. Test conditions included concentrations of hydrogen in the fuel mixture of 3-50% by volume, fuel equivalence ratios of 0.6-5.0, Karlovitz numbers of 0-0.91 and laminar burning velocities corrected to unstretched (plane) flame conditions of 130-1730 mm/s. Both measured and predicted ratios of unstretched (plane) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers in the range -6.5 to 7.6, reaching largest values near limits for the largest hydrogen concentrations in the fuel mixture. Effects of stretch on laminar burning velocities were modest at low hydrogen concentrations but approached earlier observations for hydrogen/air flames as hydrogen concentrations increased. Present and earlier stretchcorrected measurements of the laminar burning velocities and Markstein numbers for unstretched (plane) flames were generally in good agreement, aside from some exceptions at conditions where Markstein numbers were large. Predicted and measured unstretched laminar burning velocities and Markstein numbers were in fair agreement, using a chemical reaction mechanism due to Kim et al. (1994); nevertheless, additional development of the mechanism is needed to improve predictions of Markstein numbers over the test range, and unstretched laminar burning *Visiting Scholar from Helwan University, Cairo, Egypt. Graduate Student Research Assistant **Professor, Fellow AIAA. Copyright

Analysis on the Effect of Input Impact Profiles in Drop Test

The objective of this paper is to develop an analytical or mathematical predicative model for the evaluation of dynamic response of a structural element in a microelectronic or an optoelectronic product to an impact load occurring as a result of drop or shock test. Closed-form theoretical solution was obtained to simulate the board level drop test. The block diagram based SIMULINK analysis was introduced to determine the response with various impact configurations for the system level drop test as well. This study will help reliability engineers to design the impact input profiles and obtain the desired responses, and to calibrate and validate finite element analysis results quickly for both board level and system level drop test. It was found that time durations of the input profiles play an important role in the dynamic response. The system response can be designed by carefully choosing the impact time duration. Certain input pulse time results in the response with very low ringing after first or second peaks.Copyright

Numerical Simulations of Dynamics of a Tunnel Fire

Study of fire in a tunnel is very important for fire safety. Increasing concerns over terrorism put a lot of focus on the fires in tunnels as they are used extensively in mass transit systems all over the world. A lot of experiments have been carried out to study the fire hazard, smoke movement, and the effects of ventilation on fire behavior. In this paper, dynamics of a ventilated tunnel fire have been simulated using Computational Fluid Dynamics (CFD) Software, CFX 5.6, from Ansys Inc. Simulations considers different models of turbulence and radiation heat transfer. Combustion of methane is modeled using the chemical reaction schemes available in the CFX software. Two turbulent models, k–e and Shear Stress Transport, are considered. Radiant heat exchange between the species is modeled using P1 model available in CFX 5.6. The results of the simulation have highlighted the effects of ventilation on the fire and movement of harmful gases such as carbon monoxide and nitrogen oxide. Comparison of simulated temperature fields and flame shape with the experimental data has shown good agreement.Copyright

Effects of Multi-Grade Oils in Modeling Non-Newtonian Rheology Between Piston and Cylinder Surfaces in Engine Initial Start Up Conditions

This paper presents the results of a transient analysis of hydrodynamic lubrication between piston and cylinder surfaces in engine Initial startup conditions with a Non Newtonian lubricant under oscillatory motion. Effects of different multi-grade oil viscosities are also investigated in the simulation. The time dependent Reynolds equations use a Maxwell type model to analyze fluid rheology. A perturbation scheme is used to derive coupled non linear partial differential equations to obtain the fluid velocity. The oil film profile is predicted by solving the two-dimensional Reynolds equations using the finite difference computational method. The piston velocities in engine secondary motion are adjusted by using fourth order Runge-Kutta technique. Using different oil viscosities, the effect of viscoelasticity on lubricant velocity and pressure fields is examined and the influence of film thickness on lubricant characteristics is investigated. Numerical simulations show that piston eccentricities and film thickness profiles vary under different multi grade oils at engine start up conditions.Copyright