Abstract
The present work demonstrates the feasibility of heat release rate imaging using the laser-induced fluorescence (LIF) of atomic hydrogen (H-atom) and formaldehyde (CH2O) in laminar premixed flames. The product of H-atom LIF and CH2O LIF signals is evaluated on a pixel-by-pixel basis and is compared with that of the OH × CH2O technique. These results for equivalence ratio ranging from 0.8 to 1.1 are compared with computations of one-dimensional freely-propagating flames. The performance of these markers is studied based on the following two aspects: the spatial accuracy of the local heat release rate and the trend in the total heat release rate with equivalence ratio. The measured trend in the spatial distribution of radicals and the deduced heat release rate agree well with the computational values. The variation in the spatially integrated heat release rate as a function of equivalence ratio is also investigated. The results suggest that the trend in the variation of the integrated heat release rate and the spatial location of heat release rate can be evaluated by either of these markers. The OH-based marker showed certain sensitivity to the chemical mechanism as compared to the H-atom based marker. Both the OH-based and H-atom based techniques provide close estimates of heat release rate. The OH based technique has practical advantage when compared to the H-atom based method, primarily due to the fact that the H-atom LIF is a two-photon process.
Highlights
The heat release rate (HRR) is one of the important properties of the combustion process, as it characterises the extent of energy conversion from chemical potential to thermal energy
The profile marked as OH-1 is obtained from the simultaneous OH/CH2O-PLIF experiment, whereas OH-2 is obtained from simultaneous OH/H-PLIF experiment
Similar observation has been noted in a previous study [22], where the authors simulated the experimental conditions, yet the experimentally observed CH2O laser induced fluorescence (LIF) profile was found to be wider than the simulated CH2O profile
Summary
The heat release rate (HRR) is one of the important properties of the combustion process, as it characterises the extent of energy conversion from chemical potential to thermal energy. This method has been assessed by Fayoux et al [22] by comparing measured and computed HRR in counter-flow laminar premixed flames These authors [22] conclude that the product of simultaneous CH2O and OH PLIF is a reliable technique to deduce HRR. The synthetic PLIF signals were examined to assess the adequacy for their use as the heat release rate markers for MILD combustion of methane They concluded that use of only OH, CH2O or HCO PLIF signal does not reliably represent HRR, whereas the OH × CH2O PLIF based technique is found to be adequate. The conclusions of this work are summarised along with the future scope in the last section (Section 4)
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