Interferometric investigation of methanol droplet combustion in varying oxygen environments under normal gravity

Non-intrusive diagnostics of droplet combustion using laser-based interferometric technique have been presented. The experiments have been conducted with methanol droplet as the model combusting material under normal gravity with oxygen-nitrogen mixtures at ambient pressure and temperature. The mixtures contained five different oxygen concentrations of 9, 13, 17, 21 and 25%. Projection data of the temperature field around the combusting droplet have been recorded using a Mach-Zehnder interferometer and the interferograms have been analyzed to retrieve the whole field flame temperature distribution around the combusting droplet. The limiting oxygen index (LOI) has been deduced based on real time interferometric images by varying the oxygen concentration levels. Results of the study showed an increase in the flame temperatures with increasing oxygen concentrations and an oxygen concentration level of 13% has been identified as the LOI for methanol droplet. The whole field temperature distribution has been used to calculate the radial temperature gradients between the flame and the droplet surface. These temperature gradients were seen to be a direct function of oxygen concentration. Based on the radial temperature gradients, instantaneous mass burning rates and burning rate constants were computed and a reasonably close agreement between the interferometric predictions and those based on the videographic method was observed. To the best of the knowledge of the authors, the present work is the first successful application of Mach Zehnder interferometry for investigating droplet combustion phenomena. In contrast to the conventional videography approach, the interferometric technique not merely acts as a visualization tool, but also provides quantitative information in the form of whole field temperature distribution, which plays an important role in the determination of mass burning rates and burning rate constants.