Initiation of Ignition of a Combustible Mixture in a Flow by Local Energy Supply

A direct numerical simulation of the influence of a local energy release region on the development of the combustion propagation process in a high-speed flow of homogeneous fuel-air mixture (Н2 + air) is carried out. The mathematical model of the energy source describes the physical phenomena characteristic for pulsed energy supply using laser radiation. The energy supply and ignition are modeled taking into account a given kinetic scheme. The results on the spatial and temporal evolution of the ignition region are presented. The dependence of the position of the ignition region on the flow velocity is investigated.

Two-Step Concept for Low-Pressure Direct Hydrogen Injection

In this paper, a low-pressure hydrogen direct-injection solution is presented that entails low storage residual pressure (∼12 bar). The injection is realised in two steps. First, hydrogen is simply metered by an electro-injector (a conventional one for Compressed Natural Gas - CNG application) that feeds a small intermediate chamber. Next, hydrogen enters the cylinder by means of a mechanically-actuated valve which allows higher flow than any electro-injector. Injection must end early enough to allow good charge homogeneity and, in any case, before in-cylinder pressure rise constraints hydrogen admission. Backfire is avoided by starting injection at intake valve closing. A prototype has been realised modifying a single-cylinder 650 cc production engine with three intake valves. The central one has been modified and properly timed to in-cylinder inject hydrogen from the intermediate chamber. Hydrogen injection through different-shape poppet valves in a quiescent, constant volume has been simulated in order to investigate the effects of valve and seat-valve geometries in controlling fuel-air mixing in the cylinder. Additional predictions for the actual engine configuration indicate that an acceptable fuel distribution can be obtained in the combustion chamber at the spark timing, with equivalence ratios in the ignition region that are inside the flammability range of the mixture for all the operating conditions (loads and speeds) that have been considered.

Noise Generated by a Lifted Flame in a Vitiated Co-Flow

The acoustic field generated by a lifted flame is studied by a hybrid approach. First, Large Eddy Simulations (LES) are used to compute the flow and the acoustic sources. Next, an inhomogeneous wave equation is solved to obtain the resulting acoustic field. The flow computations show good agreement with experimental data. The dominant acoustic sources are found to be located in the ignition region and at the tip of the flame. The acoustic computations revealed the presence of low-frequency waves radiated in the far-field. The shape of the most energetic acoustic modes are identified by POD analysis to be axial modes.

Oscillatory ignitions and cool flames in the oxidation of butane in a jet-stirred reactor

The oxidation of butane ([C 4 H 10 ] : [ O 2 ] = 1.13:1.00) has been studied over the temperature and pressure ranges 371 ⩽ T/ K ⩽ 675, 226 ⩽ P /Torr ⩽ 489 in a jet stirred reactor with a residence time of 9.4 s (1 Torr ≈ 133.3 Pa), The gas temperature and pressure were probed and phase diagrams constructed delineating regions of oscillatory ignitions and cool flames, and high- and low -temperature stationary states. On heating at an initial pressure of 400 Torr from 570 K sharp transitions were observed, first to an oscillatory ignition and then to an oscillatory cool flame region, followed by a smooth transition to a high-temperature stationary state via a supercritical Hopf bifurcation. On cooling from this high - temperature stationary state, oscillatory cool flames were observed with a sharp extinction at 542 K, without any entry to the oscillatory ignition region. The latter could be entered, however, by suddenly cooling the system from the oscillatory cool flame region by temporarily substituting nitrogen for oxygen in the gas streams. Complex waveforms, consisting of bursts of oscillatory cool flames interspersed with periods of monotonic cooling, were also observed at lower pressures. A Nd : YAG pumped dye laser was used to probe laser induced fluorescence from form aldehyde in the oscillatory ignition region. Variations in the internal surface of the reactor demonstrated the significance of surface reactions. An outline mechanism, based on detailed numerical simulations, is presented to account for the shape of the ignition profiles and the transition from multiple ignitions to oscillatory cool flames.