Turbulence/chemistry interactions in a ramp-stabilized supersonic hydrogen–air diffusion flame

Abstract

Hybrid large-eddy / Reynolds-averaged Navier-Stokes simulations of turbulence / chemistry interactions occurring within a ramp-injected, hydrogen-fueled scramjet combustor are presented in this work. The experimental geometry is one of several studied at the Universty of Virginia as part of the National Center for Hypersonic Combined Cycle Propulsion and consists of an isolator, a combustor, and an extender section. Data collected includes coherent anti-Stokes Raman spectroscopy (CARS) measurements of major species composition and temperature at several streamwise planes, stereoscopic particle image velocimetry (PIV) measurements, hydroxyl planar-induced fluorescence (OH-PLIF) imagery, wall pressure distributions, and line-of-sight profiles of temperature and water concentration obtained using tunable diode laser spectroscopy (TDLAS). This paper focuses on an equivalence ratio of 0.17, which does not produce enough heat release to force a shock train into the isolator. The computational methods utilize a hybrid fourth-order central-difference / upwind strategy to enable accurate resolution of turbulent structures and employ a nine-species hydrogen oxidation mechanism. Generally accurate predictions of temperature, velocity, and nitrogen mole fraction are achieved through a 'laminar chemistry' assumption for the filtered species production rates, though results do improve slightly with the use of a simple turbulence / chemistry subgrid closure model. The predictions are most sensitive to the choice of isolator inflow boundary condition, with the use of a recycling / rescaling technique to sustain turbulent fluctuations resulting in an 'over-mixing' effect immediately downstream of the fuel injector. Turbulence-chemistry interactions in the flameholding region are examined from the standpoint of laminar flamelet theory. A region of high scalar dissipation rate, coincident with the breakdown of the fuel plume and the interaction of a reflected shock wave with the plume, inhibits flame propagation, forming a 'hole' in the flame. Advection of cooler fluid downstream into regions of moderate scalar dissipation enlarges the 'hole', but eventually the flame reconnects. These results point to one potential disadvantage of fuel-air mixing technologies that enhance axial vorticityeven if conditions for combustion are favorable, high strain rates generated by the interaction and breakdown of vortex pairs can lead to flame suppression.

Toward finite-rate chemistry large-eddy simulations of sooting swirl flames

“…While this approach is not applicable to (U)RANS (Duwig et al, 2011), some authors report that it can be a reasonable approximation for LES if the flame is 'adequately' resolved (Duwig et al, 2011;Edwards et al, 2012;Fulton et al, 2016;Hodzic et al, 2017;Lourier et al, 2015;Potturi and Edwards, 2015;Ranjan et al, 2016;Strakey and Eggenspieler, 2010;Zhang et al, 2015Zhang et al, , 2010. Fulton et al 2016for example obtained reasonable LES-QLC results using an average mesh spacing which was half the laminar flame thickness.…”

Comparative study on chemical kinetic schemes for dual-fuel combustion of n-dodecane/methane blends

“…Hence, the work in Publications II-III is mainly focused on autoignition phenomena. Furthermore, multiple LES studies with this approach have shown a rather good agreement against experiments when a relatively high grid resolution is applied [182,196,197]. However, author suggests careful consideration and grid resolution sensitivity tests if the present approach is utilized in e.g.…”

Coherent Anti-Stokes Raman Spectroscopy of a Hydrogen Diffusion Flame in a Ramjet

“…Results of measurements with the combustor fueled at ER=0.18 and 0.49, and operating in scramjet mode and ramjet mode, have been previously reported. [18,22] The five CARS measurement planes were at x/H=-10.3, 6.1, 18.1, 38, and 56.1 relative to the point of fuel injection and denoted planes 0, 1, 2, 3, and 4 respectively ( Figure 2). Plane 0 (the entrance plane) was used to quantify the upstream boundary condition to the combustor, while the four downstream planes were to study of the fuel-air mixing and combustion downstream of the injector.…”

“…[21] Computational studies of configuration C flow using LES-RANS methods and comparisons to the OH-PLIF, PIV and CARS measurements considered modeling of turbulence-chemistry interactions in the flame. [22] 2. Test facility and dual-mode scramjet…”

“…Greater-than-chemical-equilibrium mole fraction of minor species are predicted in the hottest part of the plume (less than 0.06 atomic H, 0.01 atomic O, 0.02 OH, while the remaining minor species are negligible), a maximum at plane 1 and decline substantially downstream. [22] The non-resonant susceptibility of the minor gas constituents for the mixture of minor gases (assumed to be H, O, and OH in the above ratios) is 9.1×10 -18 cm 3 /erg amagat. [30] The buffer gas susceptibility used to fit the scramjet data is the same as used in Ref.…”