Ignition by Hot Free Jets

This paper describes some of our experimental studies on the re-ignition caused by jets of hot gas that interact with unburned fuel/air mixtures. The problem is approached from two complementary sides: On the one hand, phenomenological studies are conducted, which ask for the conditions under which a hot jet may cause ignition. A dedicated experiment is described which allows to create well-controlled exhaust gas jets and ambient conditions. In this experiment, parameters influencing the ignition process are varied, and the dependence of jet behavior on these parameters (i.e. pressure ratio, diameter and length of the gap through which the exhaust gas has to pass before getting into contact with ambient fuel/air) is studied. In particular, the frequency of a jet causing re-ignition in the ambient gas is studied. On the other hand, we also perform studies which are more “analytical” in nature. These attempt a more in-depth understanding, by first decomposing the hot jet ignition phenomenon into the underlying physical processes, and then studying these processes in isolation. This approach is applied to measurements of mixture fraction fields. First, non reacting isothermal variable density jets are studied. Here, the density of the gas mixture varies as to mimic the density of hot exhaust gas at varying temperatures. A laser-based non-intrusive method is introduced that allows to determine quantitative mixture fraction fields; although applied here to cold jets only, the method is also applicable to hot jets. The results show the effect of turbulence on the mixing field in and at the free jet, and allow to derive quantities that describe the statistics of the turbulent jet, like probability density functions (PDFs) and geometrical size of fluctuations.

Ignition by Hot Transient Jets in Confined Mixtures of Gaseous Fuels and Air

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 SSTk-ωmodel, 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.

Traversing Hot-Jet Ignition in a Constant-Volume Combustor

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.