Impinging flame ignition and propagation visualisation using Schlieren and colour-enhanced stereo imaging techniques

An innovative 3D flame visualisation and digital reconstruction technique was developed on the basis of digital flame colour enhancement and conventional stereo reconstruction methods. This combined methodology, together with Schlieren visualisation technique, was utilized to investigate the ignition process of propane impinging diffusion flame. Through the application of flame colour analysis, it was shown that majority of the flame feature observed during the initial stage of the ignition process resembles a bluish colouration similar to the colouration induced by visible chemiluminescence observed from lean premixed hydrocarbon flames. Digital flame colour analysis principle was applied to identify and enhance the weak chemiluminescence signals from the raw image data. The colour-enhanced images were able to clearly depict the details of local flame structures and allow the application of 3D stereo reconstruction on these flame features. The results from 3D reconstruction and direct high speed imaging showed that the initial flame kernel, which led to the eventual flame establishment, was very sensitive to the ignition location. Furthermore, this flame feature expands outward with time. The combined colour-enhanced stereo technique has the advantage over traditional stereo method in enabling the visualisation and reconstruction of a wider range of flame structures and shows great potential for application in future combustion research.