Browsing by Author "Karimi, Abdullah"

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    Ignition by Hot Transient Jets in Confined Mixtures of Gaseous Fuels and Air
    (Hindawi, 2016) Karimi, Abdullah; Nalim, M. Razi; Department of Mechanical Engineering, School of Engineering and Technology
    Ignition of a combustible mixture by a transient jet of hot reactive gas is important for safety of mines, prechamber ignition in IC engines, detonation initiation, and novel constant-volume combustors. The present work is a numerical study of the hot jet ignition process in a long constant-volume combustor (CVC) that represents a wave rotor channel. The hot jet of combustion products from a prechamber is injected through a converging nozzle into the main CVC chamber containing a premixed fuel-air mixture. Combustion in a two-dimensional analogue of the CVC chamber is modeled using a global reaction mechanism, a skeletal mechanism, or a detailed reaction mechanism for three hydrocarbon fuels: methane, propane, and ethylene. Turbulence is modeled using the two-equation SST -ω model, and each reaction rate is limited by the local turbulent mixing timescale. Hybrid turbulent-kinetic schemes using some skeletal reaction mechanisms and detailed mechanisms are good predictors of the experimental data. Shock wave traverse of the reaction zone is seen to significantly increase the overall reaction rate, likely due to compression heating, as well as baroclinic vorticity generation that stirs and mixes reactants and increases flame area. Less easily ignitable methane mixture is found to show slower initial reaction and greater dependence on shock interaction than propane and ethylene.
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    Jet Ignition Research for Clean Efficient Combustion Engines
    (Office of the Vice Chancellor for Research, 2013-04-05) Chinnathambi, Prasanna; Karimi, Abdullah; Rajagopal, Manikanda; Nalim, M. Razi
    Ignition by a jet of hot gas has application in lean-burn pre-chamber internal combustion engines and in innovative pressure-gain combustors for gas turbine engines. Jet ignition offers the advantage of reliable fast ignition and complete combustion of leaner mixtures. Fast burn rates due to the energetic ignition source produce multiple, distributed ignition zones, which consume the fuel-air mixture rapidly. Chemically active radicals and fast turbulent mixing in the jets create an explosion much more energetic than a spark. This high energy ignition results from the partially combusted gas from the pre-chamber products initiating combustion in the main chamber mixture. IC engines using low-cost, low-carbon natural gas need improved methods for ignition of lean mixtures to avoid nitrogen oxide emissions. This usually requires a richer mixture in the pre-chamber which is spark-ignited using a little additional gas fuel or compression-ignited with diesel fuel, possibly with a glow plug. A jet of hot reactive gas then ignites the main chamber lean mixture. Novel approaches for gas turbine engines using constant-volume, pressure-gain combustion include the multi-chamber wave rotor combustor. A wave rotor combustion chamber is best ignited with a jet of hot gas that may come from a small separately fueled pre-chamber or from a previously combusted chamber. Experiments on traversing and stationary jets have been conducted using the constant-volume wave rotor combustor established at combustion and propulsion research laboratory, IUPUI. The ignitability limit and ignition delay time for various hydrocarbon fuels (methane, ethylene and propane) have been investigated. Ignition characteristics have been analyzed using the high speed camera images and pressure data. Numerical simulations have been carried out using a hybrid eddy-break-up combustion model including finite-rate chemistry and two-equation k-ω turbulence model. Numerical and experimental results showed similar trends, with the modeling results illuminate the jet ignition process.
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    Numerical study of hot jet ignition of hydrocarbon-air mixtures in a constant-volume combustor
    (2014) Karimi, Abdullah; Nalim, M. Razi; Zhu, Likun; Anwar, Sohel
    Ignition of a combustible mixture by a transient jet of hot reactive gas is important for safety of mines, pre-chamber ignition in IC engines, detonation initiation, and in novel constant-volume combustors. The present work is a numerical study of the hot-jet ignition process in a long constant-volume combustor (CVC) that represents a wave-rotor channel. The mixing of hot jet with cold mixture in the main chamber is first studied using non-reacting simulations. The stationary and traversing hot jets of combustion products from a pre-chamber is injected through a converging nozzle into the main CVC chamber containing a premixed fuel-air mixture. Combustion in a two-dimensional analogue of the CVC chamber is modeled using global reaction mechanisms, skeletal mechanisms, and detailed reaction mechanisms for four hydrocarbon fuels: methane, propane, ethylene, and hydrogen. The jet and ignition behavior are compared with high-speed video images from a prior experiment. Hybrid turbulent-kinetic schemes using some skeletal reaction mechanisms and detailed mechanisms are good predictors of the experimental data. Shock-flame interaction is seen to significantly increase the overall reaction rate due to baroclinic vorticity generation, flame area increase, stirring of non-uniform density regions, the resulting mixing, and shock compression. The less easily ignitable methane mixture is found to show higher ignition delay time compared to slower initial reaction and greater dependence on shock interaction than propane and ethylene. The confined jet is observed to behave initially as a wall jet and later as a wall-impinging jet. The jet evolution, vortex structure and mixing behavior are significantly different for traversing jets, stationary centered jets, and near-wall jets. Production of unstable intermediate species like C2H4 and CH3 appears to depend significantly on the initial jet location while relatively stable species like OH are less sensitive. Inclusion of minor radical species in the hot-jet is observed to reduce the ignition delay by 0.2 ms for methane mixture in the main chamber. Reaction pathways analysis shows that ignition delay and combustion progress process are entirely different for hybrid turbulent-kinetic scheme and kinetics-only scheme.
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    Traversing Hot-Jet Ignition in a Constant-Volume Combustor
    (2014-04) Karimi, Abdullah; Rajagopal, Manikanda; Nalim, M. Razi
    Hot-jet ignition of a combustible mixture has application in internal combustion engines, detonation initiation, and wave rotor combustion. Numerical predictions are made for ignition of combustible mixtures using a traversing jet of chemically active gas at one end of a long constant-volume combustor (CVC) with an aspect ratio similar to a wave rotor channel. The CVC initially contains a stoichiometric mixture of ethylene or methane at atmospheric conditions. The traversing jet issues from a rotating prechamber that generates gaseous combustion products, assumed at chemical equilibrium for estimating major species. Turbulent combustion uses a hybrid eddy-breakup model with detailed finite-rate kinetics and a two-equation k-ω model. The confined jet is observed to behave initially as a wall jet and later as a wall-impinging jet. The jet evolution, vortex structure, and mixing behavior are significantly different for traversing jets, stationary centered jets, and near-wall jets. Pressure waves in the CVC chamber affect ignition through flame vorticity generation and compression. The jet and ignition behavior are compared with high-speed video images from a prior experiment. Production of unstable intermediate species like C2H4 and CH3 appears to depend significantly on the initial jet location while relatively stable species like OH are less sensitive.