Numerical Investigation Of Near-critical Fluid Injection And Combustion Instability Of Bluff Body Stabilized Flame

Mixing of fuel and oxidizer and combustion stability are of great importancein a combustor. Well mixing of fuel and oxidizer can improve the e(?)ciency of thecombustor. Stable combustion can also enhance the combustion e(?)ciency and moreimportantly, keep the combustor running safely.In high pressure combustors, the liquid fuel is injected into a high pressure andhigh temperature environment. The state of the fuel transits from subcritical to su-percritical. The thermodynamic and transport properties of (?)uid are highly variableand exhibit anomalies in the near-critical regime. These anomalies can cause distinc-tive e(?)ects on heat transfer and hydrodynamics. Moreover, it brings new challengesfor numerical methods especially for the boundary condition treatment. A boundarycondition treatment capable of handling problems at all (?)uid states and all (?)ow speedsis developed based on the method of characteristic. The method is tested against avariety of validation cases and satisfying results are obtained. In addition, the use ofthe boundary condition shows to improve the computational e(?)ciency.Nitrogen injection, including cryogenic high density jet, neutral jet and hot lowdensity jet, under conditions in the close vicinity of liquid-gas critical point is studiedthrough numerical simulation. To focus on the in(?)uence of the highly variable (?)uidproperties and avoid the di(?)culties encountered in modeling high Reynolds number(?)ows, a relatively low injection Reynolds number of 1750 is adopted. Reference caseswith the same con(?)guration, Reynolds number and density ratio are simulated inthe ideal gas regime. Full conservation laws, real-(?)uid thermodynamic and transportphenomena are accommodated in the model. Results reveal that the (?)ow featuresof the near-critical (?)uid jet are signi(?)cantly di(?)erent from the ideal gas case for thecryogenic jet and neutral jet while the di(?)erence is not so obvious for the hot jet.For the neutral jet, the near-critical (?)uid jet spreads faster and mixes better withthe ambient (?)uid compared to the ideal gas jet. It is identi(?)ed that vortex pairingprocess develops faster in the near-critical case than in the ideal gas case. Detailedanalysis at di(?)erent streamwise locations including both (?)at shear layer region andfully developed vortex region reveals the important e(?)ect of volume dilatation andbaroclinic torque in the near-critical (?)uid case. The volume dilatation e(?)ect disturbsthe shear layer and makes it more unstable. The volume dilatation and baroclinice(?)ects strengthen the vorticity and stimulate the vortex rolling up and pairing process.For the cryogenic jet, the near critical jet is more stable than the ideal gas jet. Thereason is that sharper density strati(?)cation in the near critical case stabilizes theshear layer and makes the jet stable. The volume dilatation e(?)ect in the near criticalcase is smaller than the ideal gas case due to the smaller temperature variation. Forthe hot jet, the development of the jet in two cases are very similar even though thedensity strati(?)cation is di(?)erent. The reason is that the dynamics of hot jet is nolonger dominated by the shear layer instability but by a absolute instability relatedto the jet dimension and density ratio.Blu(?) body (?)ame stabilizer is widely used in industry combustors. The dynamics of a (?)ame stabilized by a triangular blu(?) body in a straight channel is simulatedusing a large eddy simulation technique along with a level-set (?)amelet approach.Both acoustically non-re(?)ecting and re(?)ecting inlet boundary conditions are treated.The physical processes responsible for driving combustion instabilities in the chamberare identi(?)ed and quanti(?)ed, including the mutual coupling between acoustic wavemotion, vortex shedding, (?)ame oscillation and heat release. The mechanism of energytransfer from chemical reactions in the (?)ame zone to acoustic oscillation in the bulkof chamber is investigated systematically. Strong resonance occurs between the shearlayer separated from the blu(?) body and one of the chamber acoustic modes, when thenon-re(?)ecting inlet boundary condition is enforced. As a consequence, a close loop ofacoustic wave, vortex shedding, (?)ame oscillation, and heat release is formed, whichexplains the observed unstable combustion with large pressure oscillation.