Fundamental Aspects of Jet Ignition for Natural Gas Engines

Abstract

Large-bore natural gas engines may use pre-chamber ignition. Despite extensive research in engine environments, the exact nature of the jet, as it exits the pre-chamber orifice, is not thoroughly understood and this leads to uncertainty in the design of such systems. In this work, a specially-designed rig comprising a quartz pre-chamber fit with an orifice and a turbulent flowing mixture outside the pre-chamber was used to study the pre-chamber flame, the jet, and the subsequent premixed flame initiation mechanism by OH* and CH* chemiluminescence. Ethylene and methane were used. The experimental results are supplemented by LES and 0D modelling, providing insights into the mass flow rate evolution at the orifice and into the nature of the fluid there. Both LES and experiment suggest that for large orifice diameters, the flow that exits the orifice is composed of a column of hot products surrounded by an annulus of unburnt pre-chamber fluid. At the interface between these layers, a cylindrical reaction zone is formed that propagates in the main chamber in the axial direction assisted by convection in the jet, but with limited propagation in the cross-stream direction. For small orifice diameters, this cylinder is too thin, and the stretch rates are too high, for a vigorous reaction zone to escape the pre-chamber, making the subsequent ignition more difficult. The methane jet flame is much weaker than the one from ethylene, consistent with the lower flame speed of methane that suggests curvature-induced quenching at the nozzle and by turbulent stretch further downstream. The velocity of the jet is too high for the ambient turbulence to influence the jet, although the latter will affect the probability of initiating the main premixed flame. The experimental and modelling results are consistent with ongoing Direct Numerical Simulations at ETH Zurich.

Simultaneous Negative PLIF and OH* Chemiluminescence Imaging of the Gas Exchange and Flame Jet from a Narrow Throat Pre-Chamber

“…In Bunce's work mentioned above [20], the authors speculated that there must be a mass transfer from the PC to the MC between the pressure rise in the PC and the appearance of the visible flame jet. Mastorakos et al [22] also inferred that the flame jet emerging from the PC should be surrounded by an atmosphere of unburned PC mixture. In a recent metal PCC engine experiment, Hlaing et al [23] reported a "postcombustion" phenomenon that could be recognized on the heat release curve after the main combustion when running the engine with λ=1.…”

Pre-Chamber Ignition Mechanism: Simulations of Transient Autoignition in a Mixing Layer Between Reactants and Partially-Burnt Products

“…An experimental study investigating these phenomenon is given in Ref. [8]. A related observation is that an explosion may be transmitted through a gap between two plates, as may be present in flanges in pressure vessels, even if the gap is significantly below the nominal quenching distance of the mixture and hence only a fast jet of hot gases escape the vessel [12].…”

“…Hence, for instance, a non-premixed counterflow layer between hot air and cold fuel may not autoignite if the strain rate is too high [ 4 ], the MIE in an already premixed homogeneous mixture will increase if there is turbulence in the flow [ 3 , 5 ], and a cylindrical ”spark” (i.e. a hot gas column), as when a hot inert gas jet is injected into premixed reactants, may not lead to ignition if its diameter or velocity is too high [ 6 – 8 ].…”

Transient Ignition of Premixed Methane/Air Mixtures by a Pre-chamber Hot Jet: a DNS Study

“…As is seen, 2 L ∕d > 1 in Cases A-C and Cases E-B3D. They should thus correspond to the "jet ignition"-regime according to Mastorakos et al (2017), while Case D corresponds to the "flame ignition"-regime. In Table 1, Case A is a reference configuration to investigate the effect of different parameters; Case B is chosen to study the effect of a different spark location.…”

“…A global Damköhler number was defined; the limiting Damköhler number for a CH 4 /air flame is found to be around 140, below which ignition probability is almost 0. To reveal the details of the flow and flame through the orifice, both experiments and Large-Eddy Simulations (LES) were conducted in Mastorakos et al (2017). It was found that the jet flow is composed of a column of hot products surrounded by an annulus of unburnt pre-chamber gases.…”