Flameless combustion is achieved by creating a strong recirculation of burnt gases inside the combustion chamber, which leads to a dilution of the fuel-oxidizer mixture. This causes the combustion region to be extended to the whole furnace, rather than concentrated on a flame front. Dilution avoids the formation of thermal hot spots, significantly reducing nitrogen oxides and carbon emissions without compromising the efficiency. The extension of the reaction zone is obviously a fundamental feature of flameless combustion. Chemiluminescence imaging of excited radicals has found considerable application in reaction zone marking. In this study, the shape and position of the reaction zone has been characterized by recording images of the chemiluminescence self-emission of OH radicals. OH radicals have been chosen because they emit at a wavelength of 307 nm (UV), a spectral region where the contribution of the wall emission is negligible. An intensified CCD camera equipped with a filter has been used. An image-processing algorithm has been developed and applied to reduce noise, correct imperfections (e.g., due to minor changes in the position and orientation of the camera) and extract relevant information from the large amount of data available. The experimental setup consisted of a 30 kW combustion chamber, a mixing unit supplying syngas from pure gas in bottles and an electrical air preheater. Two hydrogen-containing, low calorific gas mixtures (CH_4/CO/H_2/CO_2/N_2) have been tested in flameless conditions. Natural gas (NG) has been used as reference fuel. OH imagery showed that the position of the reaction zone is closer to the injection section when the fuel contains hydrogen and when the air excess is increased. CFD simulations have been performed using Ansys Fluent with an Eddy Dissipation Concept model for turbulence-chemistry interaction model and a detailed KEE mechanism (18 species, 58 reactions). The numerical prediction of the OH radical concentration has been compared with experimental results showing a satisfactory agreement. The conclusion of this study is that OH imagery is a relatively simple but extremely powerful tool for the investigation of combustion systems, provided that an optical access is available and that an automatic image processing procedure is established. The data made available by this technique can be used not only for visualization of the position and extension of the reaction zone, but can be treated to obtain quantitative information of the combustion conditions and used for CFD validation.
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