Kalyanasundaram Seshadri

arXiv2024

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

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.

physics.chem-ph↗

arXiv2024

Experimental and Computational Investigation of the Influence of Ethanol on Auto-ignition of n-Heptane in Non-Premixed Flows

Experimental and computational investigations are carried out to elucidate the influence of ethanol addition on n-heptane auto-ignition in counterflows. An axisymmetric stream of air, temperature gradually increased, is directed onto the surface of an evaporating pool of a liquid fuel. The air-stream temperature at auto-ignition is measured at various strain rates for n-heptane, ethanol, and various n-heptane/ethanol mixtures. Critical conditions for auto-ignition are predicted employing San Diego Mechanism for both fuels and fuel mixtures, and the results are compared with measurements. Measurements and predictions show that low-temperature chemistry (LTC) plays a significant role in promoting auto-ignition of n-heptane at low strain rates, but there is insufficient residence time at high strain rates for LTC to take place, so auto-ignition is promoted by high-temperature chemistry. Experimental and computational results show addition of ethanol inhibits LTC of n-heptane. To identify the responsible elementary steps, computations are performed to identify those dominate O2 consumption and contribute to the temperature rise in the reaction zone for n-heptane and n-heptane/ethanol mixtures at low strain rates. For n-heptane, O2 is consumed primarily by the low-temperature steps that result in ketohydroperoxide; the temperature rise is produced by subsequent LTC steps. For the mixtures, a key step consuming O2 is O2 + CH3CHOH = HO2 + CH3CHO, and the heat release occurs through the classical high-temperature reaction mechanism. Thus, the inhibition of auto-ignition that is observed to occur when ethanol is added to n-heptane arises from the competition for O2 between this step and the LTC addition of O2 to the heptyl radical and to the radical arising from the subsequent isomerization, for n-heptane.

physics.chem-ph↗