title: Combustion aerodynamics and pollution formation in gas-fired furnaces
Creators: name: Kenbar, Asaad M. A.
Publisher: name: University of Glasgow
Year: 1991

description: This thesis presents a combined experimental and theoretical study of the combustion aerodynamics and pollutant formation in confined swirling flames. The fuel used in this study was natural gas. In the experimental part of the work, two fuel injection modes are examined as alternatives to the conventional central axial fuel injection mode. These alternatives are (a) introducing the fuel around the periphery of the swirled air jet, and (b) injection radially outwards from the central axis, across the entering swirled air flow. The measurements were performed in a semi-industrial size furnace with a movable-block swirl generator. Four swirl settings were examined, covering swirl number range of 0 to 2.25. The flow patterns (as defined by three time-averaged velocity components and static pressure), combustion patterns (as defined by temperature and species concentrations) and pollutant formation (CO and NOx) were investigated for these two alternative injection modes as well as for the conventional central axial mode to assess the merits of the three systems. The formation of NOx has been studied in greater detail in these three systems. For the flow and combustion patterns, measured along the furnace, the main input variable was swirl, while for the pollutants, measured in the stack, the main input variables were fuel equivalence ratio and swirl. The investigations showed that with the radial fuel injection system, a stable flame was achieved without swirl, while for the peripheral and central axial injection systems, a minimum swirl number of 0.8 was required to establish a stable flame. By introducing some of the combustion air radially outward through a central gun with the peripheral fuel injection system, a stable flame was achieved without swirl. With the central radial and peripheral fuel injection modes, complete combustion can be guaranteed with 5% excess air, while for the central axial fuel injection at least 10% excess air was required to achieve complete combustion. The results of flow and combustion patterns demonstrate that the highest rates of mixing, combustion efficiency and heat transfer from the flame were achieved with the peripheral fuel injection. Increasing the degree of swirl was found to improve these characteristics by producing a more uniform and intense flame. The measurements of NOx at lean and rich conditions showed that this system offers wider scope for NOx reduction through lean combustion and staged combustion (ie both air and fuel staging). The radial fuel injection has also produced much improved mixing and combustion efficiency compared with the central axial fuel injection. However, with this system and with the central axial injection, only air staging can be used to reduce NOx formation. In all fuel injection modes, the strong dependence of NOx generation on flame temperature confirms that its principal formation is by the thermal mechanism. Locally, the effect of swirl on NOx formation was significant, however, its effect on the overall values was small. In the theoretical part of the work, predictions of the overall NOx formation are made using a well-stirred reactor model based on the extended Zeldovich mechanism. The model takes account of the fluctuations of the concentrations of fuel and oxidant in NOx reaction zone. A stochastic analysis has been introduced by the author to calculate the effect of these fluctuations on the NOx formation rate. The results of these predictions compare satisfactorily with the experimental measurements for the three methods of fuel injection. As part of the validation process of existing computational fluid dynamics (CFD) codes, an assessment is made of the ability of a CFD code to model swirling flames. The peripheral fuel injection mode applied to natural gas swirling flame is a novel test case (Beltagui and Maccallum (1988)). The predictions were made using the PHOENICS code, with turbulence and combustion represented by the k-e and the 'eddy break-up' models respectively. The main changes in the combustion patterns caused by switching from central axial to peripheral fuel injection were qualitatively well predicted. For the peripheral fuel injection system, the predicted flow patterns were in reasonable agreement with those previously measured. The quantitative agreement for combustion patterns, however, was good for the non-swirled flow only. This is attributed to the simplified turbulence and combustion models used.