Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

Michael Philip Sitte,Ellen Bach,James Kariuki,Hans-Jörg Bauer,Epaminondas Mastorakos

doi:10.1080/13647830.2016.1155756

Michael Philip Sitte, Ellen Bach + Show 3 more

Open Access

https://doi.org/10.1080/13647830.2016.1155756

Copy DOI

Abstract

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability Pign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high Pign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the ST-versus-u′ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.

Highlights

Ignition of turbulent flames is very important for a wide range of propulsion applications [1,2]
Flame A4 is very close to the blow-off point, while flames A3 to A1 are progressively farther from the blow-off condition
The movement of the flame upstream is key to successful ignition, consistent with previous work with nonpremixed [21] and spray [3] laboratory flames and single-sector gas turbine combustors [24,25]

Summary

Introduction

Ignition of turbulent flames is very important for a wide range of propulsion applications [1,2]. New insights have been developed into the fundamentals of spark ignition processes in non-premixed and spray systems [1,3]. In both experiments and modelling the key finding is that the kernels from the spark must grow and be advected by the flow towards the anchoring points of the flame for the overall burner ignition to be successful. Apart from local processes that affect the success or not of the spark to develop a kernel, the long-term fate of the flame depends on the convective pattern and on whether the flame may quench later, in other parts of the burner, as it propagates.

Methods

Results

Conclusion