Modeling of chemical and mixing effects on methane autoignition under direct-injection, stratified charged conditions

Abstract

Fundamental experiments and direct numerical simulations indicate that reaction rates during the ignition delay of direct-injection, stratified charge mixtures are determined by both chemical reaction and mixing rates. Consequently, the commonly used, purely chemistry-dependent approach for predicting ignition is no longer valid when the mixing rate limits the reaction rates. This work proposes an improved approach for predicting the reaction rates during ignition delay which accounts for both chemistry and mixing. Initially, reaction rates are determined using only the chemical reaction rate. Subsequently, a transition is made to the use of a modified eddy dissipation concept in which reaction rates are determined based on interaction between chemical reaction rates and mixing rates. To illustrate and validate the model, compression ignition under stratified charged conditions is considered for methane/air mixtures with temperatures ranging from 1200 to 1500 K. The predictions agree well with the experimental results qualitatively as well as quantitatively. Furthermore, by comparing our approach against a chemistry-only model, conditions where mixing plays an important role in predicting reaction rates during the ignition period are identified.

Ignition of turbulent non-premixed flames

“…Modern variants of the Eddy Dissipation Concept (EDC) [142] have been used to give the mean reaction rate in compressionignition engines [143]. Cabra et al [110] have also used this model for their hydrogen jet in vitiated air experiment, with further results [144] exploring the sensitivity of the predictions to the boundary conditions.…”

Impact of detailed chemistry and transport models on turbulent combustion simulations

“…These findings are attributed to the fact that an increasing penetration of the fuel jet has a cooling effect on the mixture formed. This model has been extended [200] to describe the transition from ignition to diffusive combustion, using a progress variable. When a certain value of this progress variable is attained, the Eddy Dissipation Concept combustion model is activated to calculate the chemical source terms of the main species and the CHEMKIN calculations are stopped.…”

Quasi-Dimensional Modeling of a CNG Fueled HCCI Engine Combustion Using Detailed Chemical Kinetic

“…Kong et al (2002) proposed a similar approach up to the point of ignition, while after ignition they introduced a reaction rate incorporating the effects of both chemical kinetics and turbulent mixing through characteristic timescales. Hong et al (2002) proposed a more computationally demanding model to simultaneously account for the effects of detailed chemistry and mixing on ignition delay within the KIVA-3V CFD code. In this study, an in-house quasi dimensional code has been developed which simulates the processes occurred in a HCCI engine fueled with CNG.A detailed chemical kinetic scheme developed by Bakhshan and Shahrir (2009) constituting of 1701 elementary reactions and 310 species and with its modifying to CHEMKIN format, has been used.…”