Vaidyanathan Sankaran

LES of spray combustion in swirling flows

The ability to predict turbulence-chemistry interactions in realistic full-scale gas turbine combustors has not been feasible until now due to the lack of simulation models and computing power. Her...

Structure of premixed turbulent flames in the thin-reaction-zones regime

In practical combustors, the structure of a turbulent premixed flame has many facets due to the widely varying turbulence-chemistry interactions that can occur. Premixed combustion in the flamelet, the corrugated flamelet, and the distributed reaction (recently called the thin reaction zones) regimes can coexist within the same device. Models using large-eddy simulation (LES) methodology to simulate premixed combustion in practical systems must therefore, be able to predict these space- and time-varying flame structure and propagation characteristics without requiring ad hoc changes. Here, the linear-eddy model (LEM), developed earlier for the flamelet regime, is used without any modifications to simulate premixed flames over the entire parameter space. A 15-step, 19-species, methane-air mechanism has been used in the in situ adaptive tabulation (ISAT) procedure to investigate premixed flame structure from the flamelet to the thin-reaction-zones regime. Qualitative and quantitative comparisons with experimental observations show that the LEM is capable of accurately capturing the flame structure in both flamelet and thin-reaction-zones regimes. These simulations confirm that in the thin-reaction-zones regime the preheat zone thickness increases with increase in the Karlovitz number, Ka . However, the reaction zone remains very thin and of the order of the laminar flame thickness. These results, along with earlier studies of LEM in non-premixed and flamelet premixed combustion, confirm the viability of LEM as a practical combustion subgrid model for use in reacting LES.

Vorticity-scalar alignments and small-scale structures in swirling spray combustion

The effect of droplets, heat release, and swirl on the fine-scale structure of the turbulence in a full-scale gas turbine combustor is studied using large-eddy simulation. Alignment of vorticity and scalar gradient with the strain-rate field is examined in detail. Results indicate that the most likely strain state is axisymmetric extension corresponding to one of the two positive strain rates. Examination of the isotropic part of the strain tensor indicates that volumetric dilatation due to heat release significantly alters the strain-rate field. Analysis also shows that the vorticity tends to align with the intermediate strain rate, whereas the scalar gradient aligns with the most compressive strain rate. These results are in agreement with those obtained in isotropic turbulent flows. The magnitude of these alignments is found to decrease in the presence of droplets and with heat release and/or an increase in swirl. Probability density functions of the strain-rate eigenvalues and flow visualization are used to characterize the geometrical structure of the small scales. It is shown that both tube-like and sheet-like structures exist in the combustor and their relative abundance (or the lack thereof) is a function of spatial location and swirl magnitude. Tube-like structures are found to coincide with regions of intense vorticity gradients, whereas regions of increased scalar gradients form sheet-like structures that in turn wrap around the tubular vortical structures. The implication of these results on fuel/air mixing and combustion in a two-phase system is also discussed.

Subgrid Mixing Modeling for Large Eddy Simulation of Supersonic Combustion

Large eddy simulations of compressible mixing layers have been conducted to investigate scalar mixing at two convective Mach numbers at 0.25 and 0.62. Two sub-grid scalar transport models, one based on a conventional gradient di usion and the other based on the Linear-Eddy Mixing (LEM) model are used to predict scalar mixing in supersonic ows. It is found that the mixing layer growth is reduced signi cantly with increase in compressibility, which is consistent with past observations. Numerical predictions obtained using LES-LEM compares very well with the experimental measurements of scalar properties, whereas, the gradient di usion closure shows signi cant di erences from the measured values. Flow visualizations of density, temperature and mass fraction contours reveal the delay in the formation of the large structures and the growth of the mixing layer as the convective Mach number is increased. Statistics such as mean and the RMS of the velocity and the scalar eld exhibit self similarity in the far eld. PDFs of the species mass fraction in the supersonic stream become narrow as the compressibility increases, indicating the reduction in mixing.

Measurements and Modeling of the Flow Field in an Ultra-Low Emissions Combustor

The flowfield of a novel combustor design that can operate stably even at high flowrates and very lean conditions is studied. This Stagnation Point Reverse Flow (SPRF) combustor consists of a central injector at the single open end of a cylindrical chamber, with the injector inlet area much less than the open area of the combustor through which the exhaust products leave. Thus the flowfield can be characterized as a confined jet in an opposed flow. Experiments with Particle Image Velocimetry (PIV) as well as computations employing Large Eddy Simulations (LES) have been used to characterize the nonreacting and reacting flowfields within the combustor for premixed and nonpremixed modes of operation. Both nonreacting and reacting cases exhibit a “stagnation” region with local average and high fluctuating velocities. The reacting flows exhibit higher mean and fluctuating velocities than the nonreacting flow. The nonreacting flow stagnates earlier than the reacting flow due to the effects of gas expansion in the reacting flow case. Consequently, the jet decay rates are higher for the reacting flows. The high shear between the forward and reverse flows causes significant recirculation, resulting in enhanced entrainment and mixing of the returning hot product gases into the incoming reactant jet. Comparison of the instantaneous flowfields reveals that the reacting jets exhibit significant lateral motion and distortion compared to the nonreacting case. This parallels the large increase in fluctuating velocities and turbulence intensities that coincide witht the region of high heat release. Nonpremixed and premixed reacting flowfields at the same fuel and air mass flow rates are found to be very similar except in the near field region of the jet, due partly to the lack of heat release there in the nonpremixed case.

Large-Eddy Simulation of a Swirl-Stabilized, Lean Direct Injection Spray Combustor

Large-eddy simulation (LES) of a lean-direct injection (LDI) combustor is reported in this paper. The full combustor and all the six swirl vanes are resolved and both cold and reacting flow simulations are performed. Cold flow predictions with LES indicate the presence of a broad central recirculation zone due to vortex breakdown phenomenon near the dump plane and two corner recirculation zones at the top and bottom corner of the combustor. These predicted features compare well with the experimental non-reacting data. Reacting case simulated a liquid Jet-A fuel spray using a Lagrangian approach. A three-step kinetics model that included CO and NO is used for the chemistry. Comparison of mean velocity field predicted in the reacting LES with experiments shows reasonable agreement. Comparison with the non-reacting case shows that the centerline recirculation bubble is shorter but more intense in the reacting case.Copyright

Turbulence-Chemistry Interactions in Spray Combustion

A prediction methodology based on Large-Eddy Simulation (LES) has been used to study turbulence-chemistry interactions in spray combustion. The unsteady interactions between spray dispersion and vaporization, fuel-air mixing and heat release has been investigated using a Stochastic Separated Flow model for spray within the LES formulation. The effects of swirl intensity and heat release are investigated here. Results show that the central toroidal recirculation zone (CTRZ), which is a manifestation of the vortex breakdown process, occurs only under high swirl conditions. Under non-reacting condition, droplets tend to concentrate in regions of low vorticity and increase in swirl increases the dispersion of the droplets. Mixing efficiency is enhanced and the size of the corner recirculation zone is decreased with increase in swirl. Increase in swirl also enhances the combustion processes for cases with heat release.Copyright

Combustion Dynamics of Swirling Turbulent Flames

A generalized Large-Eddy Simulation (LES) methodology has been developed to simulate premixed and non-premixed gas-phase and two-phase combustion in complex flows such as those typically encountered in gas-turbine combustors. This formulation allows the study and analysis of the fundamental physics involved in such flows, i.e., vortex/flame interaction, combustion dynamics and stability, fuel-air mixing, droplet vaporization, and other aspects of combustion. Results for swirling premixed combustion undergoing combustion instability and for swirling spray combustion in full-scale gas turbine engines are discussed here. Results show that swirl can stabilize combustion in premixed system and can reduce the magnitude of the amplitude of the pressure oscillation. In two-phase systems, significant modification to the high shear regions due to vaporization of droplets is observed. Droplets are also seen to concentrate in regions of low vorticity and when they vaporize, the gaseous fuel gets entrained into regions of high vorticity. This process plays a major role in fuel-air mixing and combustion processes in two-phase systems.