Simulations of vitiated bluff body stabilized flames

LES;Bluff body;Combustion

Smith, Andrew Gerard ; Menon, Suresh Aerospace Engineering Seitzman, Jerry Lovett, Jeffery Sankar, Lakshmi Jagoda, Jechiel ; Menon, Suresh

University:Georgia Institute of Technology

Department:Aerospace Engineering

关键词: LES;    Bluff body;    Combustion;   

Others  :  https://smartech.gatech.edu/bitstream/1853/55582/1/SMITH-DISSERTATION-2016.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

Bluff bodies have a wide range of applications where low-cost, light weight methods are needed to stabilize flames in high-speed flow. The principles of bluff body flame stabilization are straightforward, but many details are not understood; this isespecially true in vitiated environments where measurements are difficult to obtain. Most work has focused on premixed flames but changing application requirementsare now driving studies on non-premixed gaseous and spray flames. This thesis aimsto improve the understanding of vitiated, bluff body stabilized flames, specifically onnon-premixed, spray flames, through the use of Large Eddy Simulation (LES).The single flameholder facility at Georgia Tech was chosen as the basis for thesimulations in this thesis. The flameholder was a rectangular bluff body with anaerodynamic leading edge with discrete liquid fuel injectors embedded just upstreamof the trailing edge in a configuration described as “close-coupled.” The liquid phasewas modeled using a Lagrangian particle approach where discrete fuel droplets wereinjected into the domain. Experimental data was used to tune model parameters aswell as the stripped droplet velocities and sizes. The discharge coefficient needed tobe taken into account to achieve the correct fuel jet penetration.The experiments were conducted over a range of global equivalence ratios; leanequivalence ratios, φ global ≈ 0.5, exhibited symmetric flame shedding and converselylarge scale sinusoidal B ́ernard/von-K ́arm ́an shedding was observed when the equiva-lence ratio was near unity. Reacting flow LES were computed at these two fuel flowrates to improve understanding of the different flame dynamics. LES were first com-pleted using a quasi-laminar subgrid turbulence-chemistry interaction model. Span-wise averaging of instantaneous and time-averaged LES results were compared with experimental high- and low-speed imaging and showed the LES was in qualitativeagreement at both fuel flow rates. At phi_global ≈ 0.5, the fuel jet did not penetrate asfar into the crossflow compared to phi_global ≈ 0.95 and thus more fuel was delivered tothe shear layers of the bluff body resulting in higher heat release in the shear layersfor the low fuel flow rate. The heat release damped the large sinusoidal structuresvia gas expansion and baroclinic torque generation. Higher fuel jet penetration in thephi_global ≈ 0.95 case meant less fuel was delivered to the shear layers and so less heatrelease occurred directly behind the bluff body so the large scale sinusoidal sheddingwas not damped. The impact of the subgrid turbulence-chemistry interaction modelon the flame dynamics was tested by comparing the quasi-laminar LES with LESusing the subgrid linear eddy model (LEMLES). The flame structure predicted withLEMLES matched that of the quasi-laminar LES, at both fuel flow rates in the near-field behind the bluff body but deviated farther downstream. A flame edge analysisshowed little sensitivity to the choice of subgrid model in the region x < 4D.A high-order hybrid finite-difference solver with consisting of a WENO upwindmethod and compact central scheme was implemented to assess the effects of thenumerical method. A series of test cases was used to verify, validate and compareseveral of the available spatial and temporal methods before the high fuel flow ratebluff body case was run. For the simple test cases the higher-order methods wereclearly more efficient but for more complex cases the differences between the second-order and high-order methods are smaller.To test the hypothesis that the fuel jet penetration was the main factor in the flamedynamics another configuration with a modified fuel injector diameter was simulated.The injector size was chosen to match the spray penetration of phi_global ≈ 0.5 casewhile maintaining the fuel flow rate of the phi_global ≈ 0.95 case. The results confirmedthe hypothesis as the flame dynamics of this configuration match the original low fuelflow rate case.