Combustion Characteristics of a Swirl Dry Low Emission Burner Concept for Gas Turbine Application : Experiments and Simulations
Abstract: In the current global energy scenario, gas turbine can provide delicate balance between the booming worlds energy requirement and a pollutant free sustainable society. Cleaner combustion of fuel (particular natural gas), efficient, reliable, low maintenance and cost effective operation of gas turbine attracted scientific community to push the limit further (high efficiency and zero emission gas turbine). Gas turbine combustion process is complex by nature as it interacts with turbulence, chemical kinetics and thermodynamics. The combined effect directly affects the component life and cost. To gain deeper understanding and develop new eco-friendly combustion technology, continuous effort has been made from last couple of decades. In the present doctoral thesis, a downscaled prototype dry low emission technology burner was extensively investigated experimentally. The thesis also aims to compare the experimental results with numerical calculations using commercial simulation tools. The main priority of the research work was to understand the flame stabilization, flame anchoring physics, the burner operational limits and emission performance. The gas turbine burner hardware was assembled with three distinct fuel and sir supply units. Along the centerline, a primary combustion zone, the RPL (Rich-Pilot-Lean) was placed. A Pilot and Main stage was placed radially outward direction from the centerline. A secondary combustion (the main flame) zone was produced downstream of the burner throat. The primary and secondary flames were stabilized by the swirling motion of the flow. Vortex breakdown and recirculation zones assisted the steady combustion process.Several conventional measurement techniques were employed for temperature and emission (Carbon monoxide, Nitrogen oxides and unburned hydrocarbons) measurement. The experimental work in this thesis also included sophisticated optical measurement. A visually accessible liner (combustor region with diverging Quarl section) allowed optical access of the secondary flame region to analyze and record the flame characteristics. Line of sight Chemiluminescence (of hydroxyl radical), two dimensional hydroxyls radical planar laser induced fluorescence and particle image velocimetry diagnostic techniques were applied to investigate the secondary combustion (flame front and flow). All the experiments were conducted at atmospheric condition without any fuel heating. Chemical kinetic calculations were performed using CHEMKIN software for comparing the emission results. Steady and un-steady three dimensional computational fluid dynamic studies were conducted using ANSYS FLUENT.The RPL combustion produced a hot gas stream and provided radicals in to the secondary combustion zone (in the vicinity of forward recirculation zone). Initially, a dedicated experiment was conducted to explore the operability of the RPL combustor (primary zone) by varying the equivalence ratios and co-flow air properties. Results suggested that a slight rich operation could produce maximum radicals (Carbon monoxide, hydroxyls, oxygen and hydrogen radicals) from the RPL without affecting nitrogen oxides emission. The main flame (secondary combustion zone) stabilization process indicated that the secondary flame was stabilized around the inner shear layer (where the incoming reactant stream and recirculated hot gas stream interacted with each other) and near the liner wall (reactant stream impinged the liner wall). The lean and rich operability limits were identified from the full burner experiments. A sharp increase of carbon monoxide concentration was noticed in the proximity of lean blowout equivalence ratio (~ 0.40). Low frequency high amplitude flame pulsation was also observed at this operating point. Flame instability and flash back tendency was observed at higher stoichiometry (~ 0.62). The Pilot and RPL stage combustion influenced the full burner flame and emission characteristics. Interaction between Pilot stages were investigated and results suggested that rich Pilot operation was helpful for stabilizing the main flame at very lean stoichiometric combustion with an emission penalty (Nitrogen oxides concentration was increased). Lean RPL operation showed emission benefits but flame instability was increased; therefore, burner operation window was compressed. Two dimensional hydroxyls radical planar laser induced fluorescence diagnostics identified the main reaction zone (captured the super-equilibrium hydroxyl concentration) and post flame region (where relaxed hydroxyl radicals were noticed in less concentration). The maximum heat release zones were identified by the Chemiluminescence imaging. An investigation of combustor geometrical modification (aerodynamic variation) and its effect on flame characteristic was accomplished removing diverging Quarl geometry and replacing square liner with a circular cross section. The Quarl combustor arrangement demonstrated better combustion stability and wider operating window. Without Quarl, a third flame was observed from the outer recirculation zone. Outer recirculation zone flame intermittency and coupling with inner (central recirculation zone) flame structure produced high level of combustion dynamics issues. High hydrogen fuel (up to 50 % hydrogen by volume was mixed with methane) mixtures were introduced in the prototype burner. High hydrogen concentration aided a lean flame (100 K blowout benefit with 50 % hydrogen addition) operation without blowout. In addition, flow field diagnostics were carried out using two dimensional particle image velocimetry. The key flow structures (central and outer recirculation zones, shear layer, high speed swirl annular jet and vortical structures) were identified. The velocity measurement and radical concentration imaging explained the local wrinkling and dynamics of the flame structure.A preliminary effort was demonstrated to model the full burner with numerical three dimensional calculations. Different combustion (laminar flamelet and flamelet generated manifold) and turbulence model (Reynolds-averaged Navier–Stokes, Scale adaptive simulation and large eddy simulation) were implemented in ANSYS FLUENT computation. Numerical calculation added value to the experimental results by providing a detail understanding of scalar and vector fields, especially from the locations, where optical diagnostic was not possible. The computed flame structure and flow futures were compared with the experimental results. A simplified reactor based modelling was also formulated based on computational simulation results. The aim was to investigate simulation techniques conceptually that could possibly be applied in coming studies to obtain a better numerical modeling and validation activities of turbulent gas turbine combustion design and development.
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