Transient two-dimensional temperature measurement of open flames by dual-spectral image analysis

A novel optical instrumentation system has been developed for on-line continuous measurement of two-dimensional temperature distribution of a combustion flame. The system comprises a special optical assembly, a charged coupled device camera, a frame-grabber and associated software. Based on the two-colour principle, the transient two-dimensional temperature of a flame is determined from the ratio of grey levels of corresponding pixels within two images, which are simultaneously captured at two different wavelengths. Accordingly, the instantaneous two-dimensional temperature distribution of the flame is presented by pseudo-colour. Discreet considerations in the design of the instrumentation system are also described, including the choice of wavelengths, correction of location and intensity of pixels, elimination of noise, and calibration of the instrument factor. A series of experiments was conducted for butane-fired open flames on a combustion test rig. The results show that the temperature of a fuel-rich premixed flame is higher than a diffusion flame and decreases linearly with the equivalence ratio. For a diffusion flame, however, the temperature increases with the fuel flow rate.