Propulsion & Energy Conversion Laboratory

A premixed flame in turbulence is stretched, wrinkled and folded at a multitude of length and time scales. We know, the flame surface is continuously annihilated by reactant island formation followed by flame collision. Then to have a non-vanishing turbulent flame, flame surface must be continuously generated to make up for the annihilation. How does it happen?

We address: from where and how do the complex topology and physico-chemical state of a fully developed turbulent premixed flame generate and evolve in time by analyzing the motion of flame particles. Flame particles are points that co-move with reactive isoscalar surfaces which are representative of turbulent premixed flames. Direct Numerical Simulation (DNS) of H2-air turbulent premixed flame with detailed chemistry is combined with a computational methodology called the Backward Flame Particle Tracking (BFPT) algorithm. Uniform distribution of flame particles that entirely span isotherms at time tf are tracked backward to an earlier time ti (ti<tf). On backtracking, the once uniformly distributed flame particles form multiple clusters in the leading locations of the corresponding isotherms. Since Zeldovich, such leading locations or leading points have remained an enigmatic concept in combustion literature inducing strong hypotheses without concrete proofs on their role. The critical observation that entire flame surface evolves from multiple clusters of leading points at earlier time allows a Finite Strain Theoretic description of the turbulent flame in terms of these points. Stretching is initiated by flame propagation along the direction of maximum curvature. Using Finite Strain Theory, we observe that at the flame surface around the leading points, the direction of minimum curvature gets preferentially aligned with the most extensive direction of the left Cauchy-Green strain-rate tensor and vorticity. These two stretching mechanisms cause the leading regions to become finite sized surfaces, several of which subsequently join together generating the complete surface at tf.

A relationship is developed between the turbulent flame speed at time tf and the flame displacement speed and flow-flame properties like stretch-rate of the leading points from ti. Finally, we have used two distinct sets of flame particles: Set-G and Set-D, which generate and destroy the flame surface, respectively. Using the observations that the flame particles in these sets follow a modified Batchelor’s pair dispersion law, we have identified an important length-scale known as the Gibson scale. Curved flame propagation dominates dispersion of flame particles upto Gibson scale, while turbulence dominates dispersion of flame particles beyond this scale.

References

• Dave HL, Mohan A, Chaudhuri S. Genesis and evolution of premixed flames in turbulence. Combust Flame 2018;196:386-99.

Flame Particle Tracking (FPT) is a computational diagnostic with a novel algorithm that we developed for turbulent premixed flames based on the surface point concept introduced by Pope SB, Evolution of surfaces in turbulence, Int. Journal of Engg. Science 1985; 26 445-69. For other papers on FPT see below.

References

• Uranakara HA, Chaudhuri S, Lakshmisha KN. On the extinction of igniting kernels in near-isotropic turbulence. Proc Combust Inst 2017;36(2):1793-800.
• Uranakara HA, Chaudhuri S, Dave HL, Arias PG, Im HG. A flame particle tracking analysis of turbulence-chemistry interaction in hydrogen-air premixed flames. Combust Flame 2016;163:220-40.
• Chaudhuri S. Pair dispersion of turbulent premixed flame elements. Phys Rev E Stat Nonlinear Soft Matter Phys 2015;91(2).
• Chaudhuri S. Life of flame particles embedded in premixed flames interacting with near isotropic turbulence. Proc Combust Inst 2015;35(2):1305-12.

Local propagation characteristics of a turbulent premixed flame is described by the local flame displacement speed. In this work, we assessed the veracity of models for density-weighted local flame displacement speed of turbulent premixed flames. We have shown that a combination of two models, one for the weakly stretched laminar flame state and another derived for a configuration where a curved laminar flame interacts with itself to annihilate, can describe most local realizations of a turbulent premixed flame. To that end, we performed direct numerical simulations of a reactive mixture of hydrogen–air at atmospheric pressure using a detailed chemical reaction mechanism and analysed the dataset with recently developed flame particle tracking techniques. Forward tracking a large number of flame particles from the generating locations of the corresponding flame surfaces (given by backward tracking) to the corresponding annihilating locations, creates a manifold of local states that can represent nearly all possible states realizable for the turbulent premixed flame under consideration. With all the states of the flame accessible over time, we first assessed the applicability of the two-parameter Markstein length based flame speed model. It was found that the model prediction was reasonably accurate for a significant part of the flame particles’ lifetime, for turbulent premixed flames with Karlovitz number O(10) . However, during the final stage of annihilation of the flame particles in the negatively curved trailing regions, the local structure of the flame no longer resembled a standard premixed flame, even qualitatively. A new interaction model for the flame displacement speed, during these final stages of annihilation of the flame elements, was derived.

References

• Dave, H. L., Chaudhuri S. Evolution of Local Flame Displacement Speeds in Turbulence, Journal of Fluid Mechanics; 2020, 884, A46: 1-47.

It is one of the biggest challenges in high-speed air-breathing propulsion engines.

To address the unresolved question on the mechanism of flame stabilization and blowoff in high-speed aero engines utilizing combustion in high speed flows, we have conceptualized, designed and developed a direct connect combustion facility, ab-initio. This facility has recently been successfully tested and has been integrated with the optically accessible, kerosene-fueled combustor. Soon, we will launch an ambitious experimental campaign to optically diagnose the flow-flame interaction mechanisms that lead to flame blowoff in supersonic flows.

By applying laser-based diagnostics (PIV, PLIF, Rayleigh scattering) and high-speed imaging in lab scale setups and in a prototypical afterburner combustor, we proposed and validated new mechanisms of lean blowoff of bluff body stabilized turbulent premixed flames in unvitiated, vitiated, thermo-acoustically stable as well as in unstable subsonic conditions, at the University of Connecticut. While probably the most important contributions of laser diagnostics in combustion have been towards speciation measurements, these works serve as examples on how the diagnostics could be used to solve complex, outstanding problems like blowoff, pervasive in propulsion systems, by connecting them to the very fundamental principles.

References

• Chaudhuri S, Kostka S, Renfro MW, Cetegen BM. Blowoff dynamics of bluff body stabilized turbulent premixed flames. Combust Flame 2010;157(4):790-802.
• Chaudhuri S, Kostka S, Tuttle SG, Renfro MW, Cetegen BM. Blowoff mechanism of two dimensional bluff-body stabilized turbulent premixed flames in a prototypical combustor. Combust Flame 2011;158(7):1358-71.
• Tuttle SG, Chaudhuri S, Kostka Jr. S, Kopp-Vaughan KM, Jensen TR, Cetegen BM, Renfro MW. Time-resolved blowoff transition measurements for two-dimensional bluff body-stabilized flames in vitiated flow. Combust Flame 2012;159(1):291-305.
• Chaudhuri S, Kostka S, Renfro MW, Cetegen BM. Blowoff mechanism of harmonically forced bluff body stabilized turbulent premixed flames. Combust Flame 2012;159(2):638-40.
• Tuttle SG, Chaudhuri S, Kopp-Vaughan KM, Jensen TR, Cetegen BM, Renfro MW, Cohen JM. Lean blowoff behavior of asymmetrically-fueled bluff body-stabilized flamesLean blowoff behavior of asymmetrically-fueled bluff body-stabilized flames. Combust Flame 2013;160(9):1677-92.

Interaction of swirl flows: non-reacting or reacting is a common yet poorly understood phenomenon in gas turbine engines.

In the above four flows, the axial velocity of the jet through the central swirler is kept fixed while the flow conditions of its’ neighbors are varied. Can flow condition of the neighbors alter the flow structure through the central swirler?

Top left figure (a) shows the streamlines when the central nozzle (with 45 degree swirler) is open and side nozzles are blocked. The top right figure (b) shows the streamlines when the straight jets are in the side swirlers and swirling flow from the 45 degree siwrler emerges from the center. The bottom left figure (c) shows the flows from 30, 45, 30 degree swirler.  Finally, the bottom right figure (d) shows flows with all identical 45, 45, 45 degree swirler configuration. Clearly, a bubble type breakdown is formed in the last two cases which are not formed in the first two. Therefore, the bubble type breakdown can be induced by the neighboring swirlers by confining the jet from the central swirler, without varying its inlet boundary condition.

References

• Vishwanath RB, Tilak PM, Chaudhuri S. An experimental study of interacting swirl flows in a model gas turbine combustor. Exp Fluids 2018;59(3).
• Singh J, Vishwanath RB, Chaudhuri S, Sujith RI. Network structure of turbulent premixed flames. Chaos 2017;27(4).

Using theory and experiments we obtained a new scaling for turbulent flame speed, for expanding flames in turbulence. The scaling was found to be fuel independent as long  as Lewis number was greater than unity for the corresponding fuel-air mixtures. Numerous experiments were performed in a constant pressure, fan-stirred combustion chamber, at Princeton University.  The scaling relationship that we discovered in 2011 had the following feature: the rate of change of average radius of the turbulent expanding flame is proportional to the average radius to the power half. This implied that the turbulent flame accelerates on average:  the larger the flame the faster it propagates ! This scaling has recently been validated by independent studies and data from different groups around the world.

References:

• Chaudhuri S, Akkerman V, Law CK. Spectral formulation of turbulent flame speed and its modification by hydrodynamic instability. Physical Review E. 2011;84. 2011
• Chaudhuri S, Wu F, Zhu D, Law CK. Flame speed and self-similar propagation of expanding turbulent premixed flames. Physical Review Letters. 2012;108. 2012
• Chaudhuri S, Wu F, Law CK. Scaling turbulent flame speed for expanding flames with Markstein diffusion considerations. Physical Review E. 2013;88. 2013

Stacking pure solvent droplets on a solid substrate is apparently impossible in the absence of an external force as the second droplet will invariably spill over the first leading to a large wetted area. However, the unique feature that emerges during the drying of a nanoparticle laden droplet is the progressively enlarging thin solid film along the evaporating sessile droplet liquid periphery. This solid interface: the edge of which we shall refer to as the agglomeration front comprises of a thin layer of nanoparticle assembly and can support a carefully dispensed second droplet thereby allowing droplet stacking. It will be shown that the growth of this agglomeration front can also be effectively controlled by the dispensing time difference and the nanoparticle concentration in the two droplets. So far, we are commonly aware of material stacking in solid phase. Our research demonstrates stacking in the liquid phase and control over the thin solid interface growth.

References:

• Kabi P, Basu S, Sanyal A, Chaudhuri S. Precision Stacking of Nanoparticle Laden Sessile Droplets to Control Solute Deposit Morphology. Applied Physics Letters. 2015;106:6. 2015
• Kabi P, Chaudhuri S, Basu S. Controlling nanofluid droplet drying towards multi-scale surface patterning using a novel wall-less confinement architecture. Langmuir. 2016;42. 2016
• P. Kabi, S. Chaudhuri, S. Basu. Micro To Nanoscale Engineering of Surface Precipitates Using Reconfigurable Contact Lines. Langmuir. Volume 34, (2018), 2109–2120. 2018