Flame Structure and Stability for Improved Fuel Flexibility in Practical Combustion Devices

Abstract

If it is to be achieved successfully, the rapid transition towards a low-carbon future requires the development of flexible, efficient and low-emissions combustion systems. A comprehensive understanding of fundamental combustion processes—such as flame structure and stability—is necessary to optimise the operation of such systems. While these phenomena have been investigated in detail in the past, the studies have typically been focussed on relatively “conventional” flames in open environments. There has also been significant attention towards the combustion of fuels under hot and low-oxygen conditions, such as those which would occur with intense recirculation or re-use of exhaust gases. An important concept in this regard is mild combustion, a particular combustion regime which can lead to numerous benefits such as improved fuel-flexibility and emissions reductions. While mild combustion has been studied previously, the range of conditions and fuel types investigated has remained relatively narrow, limiting the uptake of the technology in certain practical applications, such as gas turbines. To address the knowledge gaps relating to fuel-flexible combustion under conditions of practical interest, a series of experimental investigations have been performed to encompass different burner configurations, fuel types, operating pressures and boundary conditions. The first set of these experiments—upon which the first two papers of this thesis-by-publication are based—relates to a novel combustion apparatus which enables the study of jet flames issuing into a hot and low-oxygen coflow at elevated pressures. Using this configuration, the independent effects of jet Reynolds number, coflow oxygen concentration and operating pressure on the structure, stability and sooting behaviour of the flames are investigated. The flames are studied in terms of their chemiluminescence behaviour, providing a useful comparison against the results of a series of computational fluid dynamics (CFD) simulations. The validity of this CFD model in predicting the trends with increasing pressure is investigated, while the chemiluminescence mechanism is further studied via laminar flame simulations. The combustion of liquid sprays under hot and low-oxygen conditions was also studied experimentally as part of this research. A combination of laser diagnostic techniques were implemented to simultaneously image the location of key reaction zone species, fuel droplets and soot, for dilute spray flames of differing fuel compositions. A range of different boundary conditions were again investigated for these spray flames, enabling the change in stabilisation behaviour and the effect that this has on the structure of the reaction zones to be studied. The combined imaging of droplets and reaction zone structures offers unique insights regarding the complex and highly coupled interactions between chemistry, droplet evaporation and flow phenomena which influence these flames. These experiments also led to the publication of two journal papers, in which a thorough analysis of the key features of the flames and the sensitivity to changes in jet and coflow boundary conditions is performed. As an extension to the experiments based on a “jet-in-hot-coflow” configuration, a series of flames using fuel blends of predominately hydrogen were also studied experimentally. The focus of this phase of the research is on improving the visibility and radiative heat transfer characteristics of hydrogen flames, which is motivated by the potential substitution of natural gas with hydrogen for the decarbonisation of a range of processes. The results of this testing indicate a significant increase in thermal radiation and flame luminosity with the addition of toluene (a highly sooting fuel) in concentrations of up to 1% by mole. Furthermore, the spectral imaging of these flames showed a strong emission band at 589 nm, confirming the importance of sodium from the surrounding environment on the visual appearance of hydrogen flames. The fundamental reaction zone structures and stabilisation features relating to the combustion of fuels under practically relevant conditions are examined in this thesis. Particular emphasis has been placed on the experimental imaging of flames under a wide range of independently controllable boundary conditions, to isolate the effects of pressure, oxidant composition, fuel flowrate and fuel type. In addition to providing important insights in their own right, these results will enable numerical modelling capabilities to be further developed, which will in-turn facilitate the optimisation of fuel-flexible and low-emissions combustion devices.