Emmanuel, Sunny Okara; (2021) Characterisation of Laser Induced Spark-Ignited Flame Kernels in Premixed Fuel/Air Mixtures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Gaseous alternative fuels are promising solution for today’s increasing demand for clean and reliable power. The wide number of fuel types and sources implies that engine designers need to develop fuel flexible combustors. Also, to meet tightening emission laws, these combustors would be required to operate under ultra-lean, high pressure and high temperature environments. Such extreme conditions make ignition difficult to achieve especially with current spark plugs which has been the primary ignition source during the last one hundred years. Laser ignition has been proposed as an alternative ignition system capable of providing stable combustion under these conditions. The advantages provided by laser ignition over electric spark system include: the absence of flame quenching effects of electrodes which enhances the ignition of lean mixtures, less energy requirement for ignition at higher pressures, precise timing, and choice of suitable ignition location. To explore the benefits offered by the Laser ignition in practical combustors, there is a need to characterise the propagation behaviour of the laser flame kernel since successful ignition requires the transition from an ignited spark kernel to a self-sustained flame. The present thesis contributes to existing knowledge on laser ignition through investigation of different development characteristics of the ignited flame kernel. The first investigation involves high-speed imaging of the flow field characteristics of the flame kernel based on combined 2D Laser tomography and PIV techniques. The ignition was achieved by focussing a laser beam of 1064 nm wavelength on an atmospheric co-axial straight tube burner through which stoichiometric CH4/Air was flowed. The resulting flame kernel and its flow field were visualized through laser-sheet illumination and then captured using a high-speed camera. The observed flame kernel features from the tomographic images were consistent with previous research observation and provided insight to other phenomena such as induced vortex motion in the developing kernel. Additionally, the PIV data provided insight on how the local flow field velocities were changing during development of the flame kernel. The second investigation involves direct imaging of the flame kernel chemiluminescence to understand both the fluid dynamics and chemical reactivity of the laser flame kernel. The atmospheric burner used in this setup is made of co-axial contracting nozzles in which flowing fuel/air mixtures were repeatedly ignited by a focused laser beam of 1064 nm wavelength and 2 Hz frequency. To characterise the resulting flame kernel, 2D projection images of the kernel OH* chemiluminescence was captured using intensified CCD camera. The observed geometric features of the kernel were similar to the earlier observation. Additional data on the OH* luminosity provided insight on the reactivity of the kernel at various transition points during its development and the reason for the variation in growth rate of the flame kernel at different stages. The investigation was extended to include the effects of varying physical parameters such as laser pulse energy and flow velocity. The observation showed that the effect of increasing the pulse energy within a certain threshold limit is an enhanced early kernel growth, but the ultimate effect was insignificant. Although, a higher flow velocity had no remarkable effect on the size of the kernel, it resulted in faster propagation of the flame front downstream due to the combined effect of convection and increased turbulence. In the final study, the sensitivity of the kernel characteristics to changes in the fuel thermochemical properties was investigated based on direct imaging of the OH* chemiluminescence. The investigation comprises the effect of changing equivalence ratio, variation in fuels at constant Adiabatic Flame Temperature and variation in fuels at constant Laminar Flame Velocity. The results of the analysis showed linear dependence of most characteristics with equivalence in laminar flow regime but not in turbulent flows. For both constant temperature and constant laminar velocity mixtures, the results showed differences in the flame kernel characteristics depending on the fuel. This shows that no single thermochemical property is enough to uniquely define different fuel/air mixtures. Hence, further study on the inter-dependencies of the different thermochemical properties would be necessary for development of more robust model that would characterise flame kernel propagation in flexible combustion systems.

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