Multiple-point ignition as a driver of flame acceleration at the early stage of burning in channels

Abstract

In the present study, flame acceleration driven by an array of multiple ignition kernels, equally spaced in the axial direction of a channel closed at one end, is studied numerically. In order to demonstrate the effects of the proposed ignition configuration on the flow dynamics, free-slip wall boundary condition is adopted. It is demonstrated that the burning of multiple flame kernels generates a powerful upstream flow, propelling the flame kernels, with the leading kernel’s tip undergoing a strong exponential acceleration. In a channel with smooth walls, such a powerful acceleration process is limited in time. The present study is mainly focused on the dynamics of the flame during the exponential stage of acceleration. The dependence of the acceleration rate, total acceleration time, and the maximum flame velocity on the initial distance between the kernels, the number of ignition kernels, and the thermal gas expansion coefficient, is quantified. Remarkably, the initial distance between the kernels has a weak influence on the above mentioned characteristics of flame dynamics as long as it is sufficiently large. It is observed that the acceleration rate increases with the kernel number. Notably, a significantly large maximum flame tip velocity can be achieved using the multi-kernel configuration as compared to the single-point ignition scenario. The acceleration rate demonstrates a nearly linear dependence on the thermal expansion coefficient, thus, the increase in thermal expansion results in a considerably stronger flame acceleration process.

Novelty and Significance Statement

For the first time, the flame acceleration process upon simultaneous ignition of a multi-point array of hot kernels equally spaced at the centerline of a channel was systematically studied. The work emphasizes the potential for achieving an extremely high flame speed within a very short time as compared to the single-ignition method. The acceleration rate, total acceleration time, and maximum achievable flame velocity were quantified. In a multi-point ignition scenario, we demonstrate that the burning of the fresh mixture between the kernels increases an overall cumulative gas expansion, propelling the leading tip. This result suggests a method for creating a powerful flame acceleration, which is an essential step in deflagration-to-detonation transition. Notably, the powerful acceleration is achieved in channels with smooth walls, which is important for DDT applications, in which using obstructed channels may be associated with a range of operational problems. The simplicity of the setup is also emphasized.