This investigation focuses on the premixed conical flame, a mainstay of combustion diagnostics. While extensive studies have explored high-frequency oscillations in the canonical Bunsen flame, little attention has been given to low-frequency oscillations affecting flame fronts. This thesis aims to explain the origins of these oscillations and proposes mitigation strategies. The goal is to improve high-accuracy diagnostics of standard conical flames by generating stable flame plumes through oscillation reduction.
The thesis explores two theories for low-frequency oscillations: buoyancy variations and flow rate fluctuations. Image chemiluminescence and CARS spectroscopy validate these hypotheses. Spatial oscillations are detected using flame front edge detection algorithms, while CARS analysis indirectly confirms fluctuations. Preliminary experiments establish reference data sets, followed by a primary experimental campaign spanning Reynolds numbers from 500 to 1000 for constant equivalence ratios. This isolates the effect of buoyancy variations. The report concludes by suggesting simple design changes for the flame holder to reduce instability amplitudes.

There is a tremendous push to explore alternative fuels such as hydrogen or synthetic fuels to combat the effects of climate change. An important step in the development of alternative fuels is the characterisation of its combustion characteristics. There are various diagnostic tools which aid in the determination of flame properties. Optical diagnostics play a crucial role by providing a non-invasive form of measurement. In the current project the idea is to utilise the passive chemiluminescence from flames to determine flame properties. The focus is on the emission from the OH* radical present in flames. This radical is responsible for radiation in the ultraviolet region ranging from 280 nm to 350 nm. This radical was also chosen as it is a radical common to both hydrocarbon as well as hydrogen flames. The objective of the project was to develop an emission spectra model of the OH¤ radical and to test its feasibility in predicting flame temperature by comparing it with emission from a laminar premixed methane/air Bunsen flame. The underlying physics regarding the various aspects of the project are discussed in detail in the initial part of the report. In the latter part, the work done for the project is explained. An emission model was developed for the OH* radical in MATLAB. The model is capable of calculating the emission spectra at different temperatures and line shapes. Then a spectrometer was built to capture the radiation from the flames in the UV region. The obtained data was post processed to get the flame spectra. The flame spectra obtained from the flame was compared against the emission model to determine the best temperature fit. It was observed that the temperatures were much higher than the gas temperatures observed in methane/air flames. This is attributed to a non thermal population distribution in the excited molecules. There however exists a unique mapping to the equivalence ratio which can be utilised for equivalence ratio determination when properly calibrated. There are many possible directions uncovered to take the work further by exploring the applicability of the tool for other fuels and flame regimes.