Analysis of Flame Structure and Interactions Between Chemical Reactions, Species Transport and Heat Release in Laminar Flames

TL;DR

The paper presents a Zurada-inspired sensitivity framework to quantify how changes in species concentrations and reaction-rate constants affect the heat-release rate $\dot{Q}$ in counterflow diffusion flames, with a focus on complex fuel blends exhibiting low-temperature chemistry (LTC). It decomposes the problem into reaction-rate-change, heat-release-rate-change, and species-concentration-change analyses, using first- or second-order Taylor expansions and explicit expressions for $\dot{\omega}_n$, $\dot{Q}$, and species balance equations. Demonstrations on n-heptane–ethanol mixtures under low strain reveal that LTC inhibition by ethanol reduces diffusion of LTC intermediates like CH$_2$O and C$_2$H$_4$ to the high-temperature zone, lowering $\dot{Q}$ and elevating auto-ignition temperatures in certain compositions, while ethanol-dominated flames enhance heat release via ethanol-derived radicals such as CH$_3$CHOH, CH$_2$CH$_2$OH, and CH$_3$CH$_2$O. The method provides a quantitative lens to compare kinetic and diffusive influences across temperature zones and could inform mechanism reduction and fuel-blend design for laminar diffusion flames.

Abstract

A novel method for analyzing counterflow diffusion flames, inspired by Zurada's sensitivity approach for neural networks, is proposed to identify critical species influencing the heat release rate in combustion. By further analyzing concentration changes of selected key species and radicals, this method reveals complex interactions among them across regions with temperature. To illustrate this approach, the study investigates the mechanisms of auto-ignition of n-heptane and ethanol mixtures in a counterflow configuration under low strain rates. In mixtures where n-heptane is dominant, the inhibition of low-temperature chemistry (LTC) by addition of ethanol impacts the heat release rate in regions where the temperature is higher through the diffusion of specific radicals such as CH2O, C2H4, C3H6, and H2O2. In mixtures where ethanol is dominant, the high ethanol fractions in the mixture increase the heat release rate, primarily due to ethanol decomposition and its subsequent reactions. This method effectively quantifies and compares the influence of both chemical kinetics and species diffusion effects, providing detailed insights into the interactions among species across the reactive field when analyzing the counterflow configuration of complex fuel mixtures.

Figures (31)

• Figure 1: Auto-ignition temperature at strain rate of 95 s$^{-1}$.
• Figure 2: Heat release rate in the high-temperature zone [T$_{\text{ox}}$=1060 K]
• Figure 3: Distribution of heat release rate change from 100%n-heptane to 90%n-heptane
• Figure 4: Heat release rate change from 50% n-heptane to 20% n-heptane
• Figure 5: CH2O Species equation terms