Session P02: Reacting Flows: Turbulent Combustion (3:10pm - 3:55pm CST)

Large Eddy Simulations of biodiesel spray flames using Flamelet Generated Manifolds

In this work, we report on Large-Eddy Simulations of biodiesel spray combustion. An Eulerian-Lagrangian approach is used to handle the motion of the gaseous and liquid phases. Further, the Flamelet Generated Manifolds (FGM) technique accounts for the turbulence-chemistry interaction. For the breakup of the fuel droplets, we utilize our modified version of the Taylor Analogy Breakup model and evaluate its impact on the overall combustion process. The numerical setup follows closely the Spray A operating condition defined by the Engine Combustion Network. However, the fuel used in our simulations is methyl butanoate, which is a suitable candidate to represent the ester content in biodiesel surrogates. By comparison with standard benchmarks, we determine the potential of the implemented numerical approach to evaluate spray combustion of new sustainable fuel types.   

Enhanced Integral Burning Rate of Turbulent Premixed Flames Through Stratification

Burning rate of turbulent premixed flames with compositionally inhomogeneous mixtures were investigated experimentally. Hydrogen-enriched methane-air turbulent flames with a global fuel-air equivalence ratio of 0.8 were tested. Two nozzles, each containing 4 fuel/air injection lobes were used in the experiments. The lobes of the first nozzle are straight, while those of the second nozzle are not, producing a swirling motion. The fuel is injected through several small diameter holes into the lobes, generating stratified conditions. Simultaneous OH and CH$_{\mathrm{2}}$O Planar Laser Induced Fluorescence (PLIF) along with Stereoscopic Particle Image Velocimetry (SPIV) were performed for the reacting conditions. SPIV and acetone-PLIF experiments were conducted to study the background turbulent flow characteristics and fuel-air mixing of non-reacting flow, respectively. The results show that stratification can lead to broadening of the preheat layer and generation of shredded-like heat release rate structures. Despite featuring a small intensity of burning rate, the shredded flame structure can feature a relatively large integral burning rate. This suggests some degree of stratification may enhance the stratified flames integral burning rate.   

Ignition and flame stabilization in turbulent premixed flames at diesel engine conditions

We assess the role of turbulence on two-stage ignition dynamics and subsequent flame stabilization at diesel engine conditions matching those of the most reactive mixture in the Engine Combustion Network's {\it n}-dodecane Spray A flame (temperature of 813 K, pressure of 60 atm, equivalence ratio of 1.3, and with $15\%$ vol. O$_2$ in the ambient gas) by performing direct numerical simulations in a simplified inflow-outflow premixed configuration. With an inflow velocity an order of magnitude larger than the laminar flame reference speed, in the absence of turbulence, ignition delays match those of the homogeneous reactor and both the cool and hot flames are spontaneous ignition fronts. Turbulence alters this picture as follows. First, the second-stage ignition delay increases, in contrast with the virtually unaffected first-stage ignition delay. Second, a sufficiently high turbulence intensity makes the cool spontaneous ignition front transition to a cool deflagration, while the hot flame is pushed further downstream, still stabilized by spontaneous ignition. Further increasing the turbulence intensity leads to both cool and hot flames transitioning to deflagrations. The mechanisms controlling this complex transition are explained and modelling challenges are discussed.   

Interactions of Turbulence and Thermodiffusive Instabilities in Premixed Lean Hydrogen Flames

A series of large-scale Direct Numerical Simulations (DNS) of turbulent lean, premixed hydrogen flames susceptible to thermodiffusive instabilities has been performed in a slot burner configuration at a jet Reynolds number of 11,000 using finite rate chemistry. The DNS comprise a variation of the flames’ Karlovitz number while the Reynolds number is kept constant to allow for a variation of flame/turbulence interactions. Since thermodiffusive instabilities result from the significantly different diffusivity of the hydrogen molecule with respect to the other species, additional DNS, where diffusivities of all species are set to the thermal diffusivity (unity Lewis number assumption), are conducted. For the flames considering realistic diffusivities, which are particularly important for hydrogen, the profiles of heat release and temperature show the characteristic behavior of thermodiffusive instabilities, e.g. local flame extinction and temperature overshoots in the burned gas, which are not existent in the unity Lewis number flames. The effects of the Karlovitz number on the thermodynamic state within the flame are discussed and the DNS are compared to two-dimensional flames in a laminar flow that feature thermodiffusive instabilities and have been performed in previous work.   

Spectral Analysis of Turbulent Diffusion Effects in Premixed Ammonia/Hydrogen/Nitrogen-Air Flames

Multi-scale spectral analysis of the contributions of turbulent diffusion relative to molecular diffusion is presented with recent DNS of turbulent premixed ammonia/hydrogen/nitrogen-air premixed flames in intense sheared turbulence. Damk\"{o}hler's second hypothesis states that the turbulent flame speed is proportional to the square root of the sum of molecular and turbulent diffusivities, and where to leading order, the turbulent diffusivity scales as the product of the relevant velocity fluctuation and turbulence length scale. The spectral analysis enables clarification of the influence of different eddy scales that are most effective at turbulent mixing through the diffusion of heat and mass, including the effects of mean shear. The relative importance of differential diffusion of hydrogen and turbulent diffusion of heat and mass internal to the flame and their scale dependency is also determined. For this study, the spectral densities are defined as Fourier transformations of density-weighted two-point correlations suggested by Kolla et al ({\it J. Fluid Mech.}, 2014, {\bf vol 754}, 456-487). The preliminary results show that the dominant eddy scale for turbulent diffusion and return-to-isotropy rate vary among different species.   

Kinetic Energy Backscatter in Turbulent Reacting Flows and Implications for Large Eddy Simulations

The accuracy of Large Eddy Simulations (LES) depends on the accurate modelling of the subgrid scale (SGS) closure terms. Present LES models are based on assumptions of the universality of small scales and the cascade of kinetic energy from large to small scales. It has been shown that for reacting flows, direction of kinetic energy cascade is reversed near the flame region, and is directed from small to larger scales on average. In the present work, we demonstrate this backscatter for flows with higher Mach number and Reynolds number in a realistic experimental setting. As a result, for such flows other approaches towards SGS modelling are required, one of which is to obtain the LES closures directly by simulating the small scales in an embedded direct numerical simulation (eDNS). We demonstrate a method to force such an eDNS calculation by assuming a power law function for the spectral kinetic energy density in the LES, calculating the exponent based on narrow band pass filtered velocities at various scales, and using it to obtain the energy injection rate for the eDNS. Results indicate that the embedded calculation forced in such a way gives a spectrum that is in good agreement with a full DNS simulation for temporally evolving complex unsteady turbulence.   

Measurements of local statistics in a premixed turbulent Bunsen flame

Interaction between propagating flames and surrounding flow turbulence is critical in controlling flame dynamics in engines widely used for power generation, transportation, and propulsion. In this study, we present an experimental investigation of flame-turbulence interaction in a newly constructed premixed Bunsen burner. For two different turbulence levels and two flame temperatures, the flow-field with and without the presence of the flame was characterized by High-speed Particle Image Velocimetry, while the flame edges were identified from the Mie-Scattering images. Several analyses have been performed to assess the effect of flame on the flow and vice-versa. A comparison between the local turbulence intensity adjacent to the flamefront and the cold flow measurements confirmed that the flame imposes a weakening effect on the turbulence, while the degree of such weakening inversely depends on Karlovitz number (\textit{Ka}). Probability distribution functions of different components of stretch rate were analyzed to extract the effect of \textit{Ka}, on the stretching and wrinkling of the flame segments. The joint probability distribution functions showed unique shapes, which were further analyzed to demonstrate that the three components of the stretch rates are pairwise linked.   

Combustion Studies of MMA/$\text{GO}_\text{x}$ for a Hybrid Rocket Motor

Poly(methyl methacrylate) (PMMA) is the synthetic polymer of methyl methacrylate (MMA), used as a solid fuel in hybrid rockets. PMMA undergoes pyrolysis into predominantly gaseous MMA ($C_5 H_8 O_2$), which then undergoes combustion with an oxygen stream in the combustion chamber. Experimental studies of this combustion chamber have been performed in literature, and the current study performs simulations, which can access more data in the combustion chamber. Simulations of laminar and turbulent non-premixed flames are performed using NGA, in a cylindrical domain, with gaseous MMA introduced through the cylinder walls. The rate of inflow of MMA is controlled by the temperature field in the combustion chamber, and the results from the 3D simulation are compared with that of experimental setups of hybrid rocket motors. The fuel regression rate and the chemical composition in the combustion chamber are calculated and compared, between simulations and experiments. Different models are used for the chemistry and combustion, and the results from these different simulations are compared and contrasted.