Mechanism of Cold-spot Autoignition in a Hydrogen/Air Mixture

TL;DR

This study investigates the mechanism of cold-spot autoignition in hydrogen/air mixtures within a controlled reactor, revealing how pressure wave interactions and thermal stratification contribute to detonation initiation.

Contribution

It provides a detailed analysis of how pressure oscillations and thermal stratification influence cold-spot autoignition, advancing understanding of detonation development in hydrogen engines.

Findings

Pressure waves synchronize to increase reactivity periodically.

Average temperature in cold-spot exceeds other regions, promoting autoignition.

Reactivity accelerates due to enhanced reaction rate constants in the cold-spot.

Abstract

When designing high-efficiency spark-ignition (SI) engines to operate at high compression ratios, one of the main issues that have to be addressed is detonation development from a pre-ignition front. In order to control this phenomenon, it is necessary to understand the mechanism by which the detonation is initiated. The development of a detonation from a pre-ignition front was analyzed by considering a one-dimensional constant-volume stoichiometric hydrogen/air reactor with detailed chemistry. A spatially linear initial temperature profile near the end-wall was employed, in order to account for the thermal stratification of the bulk mixture. A flame was initiated near the left wall and the effects of its propagation towards the cold end-wall were analyzed. Attention was given on the autoignition that is manifested within the cold-spot ahead of the flame and far from the end-wall, which is followed by detonation. Using CSP tools, the mechanism by which the generated pressure waves influence the autoignition within the cold-spot was investigated. It is found that the pressure oscillations induced by the reflected pressure waves and the pressure waves generated by the pre-ignition front tend to synchronize in the chamber, increasing the reactivity of the system in a periodic manner. The average of the oscillating temperature is greater in the cold-spot, compared to all other points ahead of the flame. As a result, the rate constants of the most important reactions are larger there, leading to a more reactive state that accelerates the dynamics of the cold-spot and to its autoignition.