Sub-grid Combustion Modeling for Compressible Two-Phase Flows

Sub-grid model;Combustion;LES;Linear eddy model;Two-phase flow

Sankaran, Vaidyanathan ; Aerospace Engineering

University:Georgia Institute of Technology

Department:Aerospace Engineering

关键词: Sub-grid model;    Combustion;    LES;    Linear eddy model;    Two-phase flow;   

Others  :  https://smartech.gatech.edu/bitstream/1853/5274/1/sankaran_vaidyanathan_200312_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

A generic formulation for modeling the sub-grid combustion incompressible, high Reynolds number, two-phase, reacting flows hasbeen developed and validated. A sub-grid mixing/combustion modelcalled Linear Eddy Mixing (LEM) model has been extended tocompressible flows and used inside the framework of Large EddySimulation (LES) in this LES-LEM approach. The LES-LEM approach isbased on the proposition that the basic mechanistic distinctionbetween the convective and the molecular effects should bepreserved for accurate prediction of the complex flow-fields suchas those encountered in many combustion systems. In LES-LEM, allthe physical processes such as molecular diffusion, small andlarge scale turbulent convection and chemical reaction are modeledseparately but concurrently at their respective time scales. Thismulti-scale phenomena is solved using a two-scale numericalapproach, wherein molecular diffusion, small scale turbulentconvection and chemical reaction are grouped as small scaleprocesses and the convection at the (LES grid) resolved scales aredeemed as the large scale processes. Small-scale processes aresolved using a hybrid finite-difference Monte-carlo type approachin a one-dimensional domain. Large-scale advection on thethree-dimensional LES grid is modeled in a Lagrangian manner thatconserves mass.Liquid droplets (represented by computational parcels) are trackedusing the Lagrangian approach wherein the Newton's equation ofmotion for the discrete particles are integrated explicitly in theEulerian gas field.Drag effects due to the droplets on the gas phase and the heattransfer between the gas and the liquid phase are explicitlyincluded. Thus, full coupling is achieved between the two phasesin the simulation.Validation of the compressible LES-LEM approach is conducted bysimulating the flow-field in an operational General ElectricPower Systems' combustor (LM6000). The results predicted usingthe proposed approach compares well with the experiments and aconventional (G-equation) thin-flame model.Particle tracking algorithms used in the present study arevalidated by simulating droplet laden temporal mixing layers.Comparison of the energy growth in the fundamental andsub-harmonic mode in the presence and absence of the dropletsshows excellent agreement with spectral DNS.Finally, to test the ability of the present two-phase LES-LEM insimulating partially premixed combustion, a LES of freelypropagating partially premixed flame in a droplet-laden isotropicturbulent field is conducted. LES-LEM along with the spray modelscorrectly captures the flame structure in the partially premixedflames. It was found that most of the fuel droplets completelyvaporize before reaching the flame, and hence provides acontinuous supply of reactants, which results in an intensereaction zone similar to a premixed flame. Some of the dropletsthat did not evaporate completely, traverse through the flame andvaporize suddenly in the post flame zone. Due to the strongspatial variation of equivalence ratio a broad flame similar to apremixed flame is realized. Triple flame structure are alsoobserved in the flow-field due to the equivalence ratiofluctuations.