In the present work, an experimental investigation was performed to characterize lean combustion flames in a liquid fuel burner. Two different regimes were investigated: non-premixed and partially-premixed combustion modes. The fuel mass flow rate was fixed and the air mass flow was reduced until the blow-out limit was reached. A high rate CCD and a PMT tube equipped wit1h an OH* filter were used for the acquisitions. Statistical and spectral post-processing methods were applied obtaining variance maps, trends of the averaged value of variance with respect to the equivalent fuel/air ratio and trends of the wavelets energy contents with respect to the frequency ranges. Results underline that the onset of flame instability occurs at higher fuel/air ratio in the non-premixed combustion regime compared to the partially premixed mode. Furthermore the rise of CO emissions starts at the leaner conditions in the case of partially premixed combustion. The present work also shows that imaging techniques are suitable to individuate this instability incipience using spectral and statistical parameters extracted by the temporal series of flame images, hence they might be implemented in online monitoring systems.
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