Analysis of chemiluminescence, density and heat release rate fluctuations in acoustically perturbed laminar premixed flames

Laser interferometric vibrometry (LIV) has recently been proposed as an alternative mean to obtain timeresolved density and heat release rate measurements at relatively low cost and experimental effort. This technique is sensitive to fluctuations of the refractive index of gases resulting from density and composition changes along the laser beam intersecting the reacting flow. It yields a line-of-sight integrated signal of the probed flow from which density and heat release rate disturbances may be inferred. The link between these signals with chemiluminescence is examined in the present study by first considering a theoretical analysis to determine the relationships between the LIV, density and heat release rate perturbation signals in a multi-species reactive mixture of gases. For air combustion systems interacting with sound waves, low frequency density perturbations in the flame zone, result mainly from heat release rate fluctuations below a certain frequency threshold. An experimental analysis is then conducted with confined conical laminar premixed flames submitted to harmonic flow modulations. Measurements are presented for methane-air mixtures at different equivalence ratios 0.8 <= phi <= 1.2 and thermal powers. It is shown that fluctuations of the chemiluminescence signal examined in different wavelength bands, including the OH*, CH* or the entire visible emission bands, always capture the same dynamics. This indicates that heat release rate fluctuations can be deduced without specific filters for the laminar premixed methane-air flames investigated. It is then shown that heat release rate measurements deduced from LIV and chemiluminescence data match well. A proportional relation is found that does not depend on the measurement position, modulation frequency and modulation level for fixed injection conditions. This linear relation slightly depends on the mean flow operating conditions partly due to the difficulty to interpret chemiluminescence emission for rich flames. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.