Experimental and Numerical Study of Flame Kernel Formation Processes of Propane-Air Mixture in a Pressurized Combustion Vessel

Abstract

Fuel lean combustion and exhaust gas dilution are known to increase the thermal efficiency and reduce NOx emissions. In this study, experiments are performed to understand the effect of equivalence ratio on flame kernel formation and flame propagation around the spark plug for different low turbulent velocities. A series of experiments are carried out for propane-air mixtures to simulate engine-like conditions. For these experiments, equivalence ratios of 0.7 and 0.9 are tested with 20 percent mass-based exhaust gas recirculation (EGR). Turbulence is generated by a shrouded fan design in the vicinity of J-spark plug. A closed loop feedback control system is used for the fan to generate a consistent flow field. The flow profile is characterized by using Particle Image Velocimetry (PIV) technique. High-speed Schlieren visualization is used for the spark formation and flame propagation. To support the experimental activity and to better understand the effects of local flow and turbulence on the combustion process, CFD simulations were carried out at both reacting and non-reacting conditions using the OpenFOAM code with suitable libraries (Lib-ICE) developed for combustion modeling. The full vessel geometry was considered and the rotation of the fan, used to generate turbulence and velocity fields, was modeled. In this way it was possible to identify the expected combustion regimes and to clarify the effects of the spark-plug geometry on the flame propagation process

Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

“…In fact Bianchi's model, as Reitz and Colin's ones, exploits the coupling Lagrangian particles -Eulerian flame model only to improve the ignition stage description, leaving the prediction of the further flame propagation to the main Eulerian model (for example the Extended Coherent Flame Model -ECFM [5], used with success by all described models). At the beginning of the 2010s, Dahms [6,7,8] and Lucchini [9][10] used similar techniques to manage the ignition stage in their comprehensive models for SI combustion, confirming the strength of the coupled Lagrangian-Eulerian approach. Both models exploit a linear path of particles, positioned along the spark-gap centerline, to simulate in more detail the shape of the plasma channel.…”

“…Therefore, the comprehensive model proposed by Lucchini [9][10] was chosen as basic strategy to model combustion. This choice was achieved because its Eulerian-Lagrangian approach, joint with several sub-models, allows to consider correctly all the involved effects and to model the plasma arc as close as possible to its original geometry.…”

“…As in previous implementations [9][10], among the three characteristic stages of ignition in SI engines, namely breakdown, arc and glow discharge, only the last two were modeled because of the negligible duration of the first one. At spark time, and after any possible restrike event, a set of 10 Lagrangian particles were introduced along the spark-gap centerline, with the purpose to model properly the spark-channel geometry and its interaction with the local flow conditions.…”

“…Moreover, recent DNS studies [14] about premixed flames propagation underline that the flame speed in presence of multidimensional and unsteady flows is a local property, and consequently the related stretch effects, which take a role in its computation, should be evaluated at the flame front position. This paper describes a modified strategy to include flame stretch effects into the Eulerian-Lagrangian model for premixed SI combustion proposed by Lucchini [9][10], in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. This is performed by fully decoupling the effects of the electrical circuit, in terms of thermal energy transferred to the gaseous mixture, from the computation of the laminar/turbulent flame speed.…”

“…In addition, the electrical circuit modeling was modified, in order to predict properly restrike events and obtain the consequent current and voltage trends. Finally, the proposed approach was validated on experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel [10]. In this configuration, a shrouded rotating fan coupled with a guide tube generates directly to the ignition zone a jet of fresh mixture with highly inhomogeneous flow and turbulence conditions.…”