The goal of the study presented in this paper is to analyze flame transfer functions with a new approach based on the combination of-line-of sight OH*-chemiluminescence and density fluctuation data. The OH*-chemiluminescence is acquired with a photomultiplier and an intensified camera, the density fluctuations are measured with a Laser vibrometer on a two axis traverse. In flames with forcing the acoustic fluctuations can be extracted from the data by discrimination of all contributions from combustion noise, because it is not correlated with the excitation device. Assuming rotational symmetry of the fluctuations originating from excitation, planar phase-resolved and pseudo-local OH*-chemiluminescence and density fluctuation data is obtained from the measured line-of-sight integrated signals.
In the study this technique is applied to a swirl burner configuration with FTFs from known multi-microphone measurements (MMM). In the first step, the externally premixed mode without equivalence ratio fluctuations is studied and in the second step the fuel is injected in the swirler in order to generate equivalence ratio waves. At selected frequencies the planar fields of the OH*-chemiluminescence and density fluctuations are compared to the FTFs in order to improve the understanding regarding the specific amplitude and phase values. In addition to heat release the vibrometer data reveals the periodic oscillation of the conical annular jet of the cold reactants in the combustor filled with hot products. On the global scale the amplitudes and phases of heat release expected from the MMM are satisfactorily reproduced by both methods for the premixed cases, whereas OH*-chemiluminescence data cannot be used as indicator for heat release if equivalence ratio fluctuations are present, because the amplitude of the FTF is significantly over-predicted due to the sensitivity of OH* on the local fuel-air mixture.