Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

Abstract

Objective of this work is the incorporation of the flame stretch effects in an Eulerian-Lagrangian model for premixed SI combustion in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. To this end, effects of energy transfer from electrical circuit and turbulent flame propagation were fully decoupled. The first ones are taken into account by Lagrangian particles whose main purpose is to generate an initial burned field in the computational domain. Turbulent flame development is instead considered only in the Eulerian gas phase for a better description of the local flow effects. To improve the model predictive capabilities, flame stretch effects were introduced in the turbulent combustion model by using formulations coming from the asymptotic theory and recently verified by means of DNS studies. Experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel were used to validate the proposed approach. In the vessel, a shrouded fan blows fresh mixture directly at the spark-gap generating highly inhomogeneous flow and turbulence conditions close to the ignition zone. Experimental and computed data of gas flow velocity profiles and flame radius were compared under different turbulence, air/fuel ratio and pressure conditions.

Modeling the Effects of the Ignition System on the CCV of Ultra-Lean SI Engines using a CFD RANS Approach

“…At the ignition event, along the spark-gap centerline, a user-defined number of particles is introduced. Their temperature T p and diameter d p are initialized as function of the breakdown stage features [22,23,24,25], because only arc and glow stages of the spark-discharge are modelled in this work. Then, each particle, which represents a portion of the spark-channel and a possible ignited flame kernel, is evolved in terms of position and dimension.…”

“…Some examples can be the possibility of estimating the local laminar flame stretch as function of each kernel curvature [33], as well as the tracking of early flames generated by spark-channels whose electrical circuit support is vanished before the achievement of a fully-developed turbulent stage. Similarly to [34,25], for each particle, both mass m p conservation and radius r p variation equations are solved…”

“…7 and 8). As performed in [34,25], a dedicated submodel estimates the s plasma value through an heat conduction equation applied to a 1D axisymmetric mesh, representing a wedge of the gas region around the sparkgap centerline. The influence of the electrical circuit, which continuously provides heat during the channel evolution, is considered via s therm (see Eq.…”

“…Electrical circuit model A detailed model of a classical inductive ignition system is adopted in this work to consider with a satisfactory accuracy the effects produced by different coil types on the ignition process. The approach proposed in [25] was followed, therefore only the secondary circuit is modelled in detail, taking into account its resistance R S . The inter-electrode voltage V Ie is computed including the cathode V cf and the anode V af potential drops, as well as the gas column voltage fall which depends on the temporal evolution of the secondary circuit current i S , spark-channel length l spark and pressure p. As suggested by [38], Eq.…”

“…An estimation of the heat losses at the electrodes, in terms of efficiency η eff of the energy transfer process to the gaseous mixture, is carried out following the approach of Herweg and Maly [39,25].…”