Quantitative Assessment of Burner Flame Stability Through Digital Image Processing

Flame stability is a well-known problem in the power industry where low quality fuel is fired and combustion conditions change rapidly. Unstable flames often lead to lower combustion efficiency, higher pollutant emissions, and other operational problems. Although there are many methods available for flame monitoring and characterisation, very few are suitable for flame stability monitoring. This paper presents a method for the quantification of flame stability by introducing a term called flame stability index through digital imaging and image processing. This index combines variability of several characteristic parameters of a flame. The stability index of a premixed methane-air flame for the equivalence ratio ranging from 0.36 to 2.17 and methane-biomass flames for ten biomass fuels is measured on a laboratory-scale combustion test rig. For purpose of comparison the flame stability is also measured in terms of oscillation frequency and with two earlier methods. Results show that correlation coefficient between the stability index and the oscillation frequency is greater than 0.89, indicating that the stability index characterises the flame stability. In addition, the stability index is more sensitive to the variation in flame stability than the oscillation frequency and its quantification requires a shorter data length. The proposed method outperforms the other two earlier methods, although the latter also combines characteristic parameters of the flame through summation or mean-square calculation. Unlike the earlier methods, the stability index determined using the proposed methods is capable of quantifying flame stability regardless of combustion conditions.