Central States Section
  The Combustion Institute

Sriram Bharath Hariharan, Michael J. Gollner, Elaine S. Oran (University of Maryland)

Saeid Zare, Sharbanti Roy, Omid Askari (Mississippi State University)

Radi Alsulami, Brye Windell, and Bret Windom (Colorado State University)

Saeid Zare, Omid Askari (Mississippi State University)

Robert Miller, Ajay K. Agrawal (University of Alabama)

“The ‘blue whirl’ is a newly discovered flame structure that naturally evolves from a traditional yellow fire whirl under specific conditions of fuel flow and air circulation. While a traditional yellow fire whirl is known to be sooty, the blue whirl (typically ~8 cm high) is seen to be completely blue, indicating the absence of soot in the flame. This image shows an instance of a yellow fire whirl transitioning to a blue whirl. Such transitions show yellow streaks of soot that travel downwards into the flame, and swirl within the blue conical structure.

This image was captured using a Nikon D7100 at f/5.3 with an exposure time of 3.125 ms. A visible blue ring forms the edge of the blue conical structure, with the yellow soot trace within it. Since the flame is formed using n-heptane poured over a water surface, a mirror reflection of the flame is seen below.”
Sriram Bharath Hariharan, Michael J. Gollner, Elaine S. Oran (University of Maryland)

“In IC engines, deposition of fuel droplets on the engine walls is highly undesirable as it facilitates fuel-rich burning, incomplete combustion and soot formation. When a droplet impacts a liquid surface, it traps a microscopic air-film underneath, which provides a cushioning effect. If the cushioning is strong enough, the droplet can actually bounce from the surface instead of merging with it. The air-film cannot be seen directly, and its miniscule thickness (~1 micron) also cannot be easily measured. In this experiment we shone a white light from the bottom of a wetted glass surface, and observed beautiful interference patterns, just like the rainbow colors on a soap bubble. Different colors represent different thicknesses of the air-film from which the shape of the air-film can be spatially resolved.

Here, we portray the life-cycle of this air cushion upoon droplet impact, with each slice representing a time instant. The impact starts from the 12 o’clock position and time lapses clockwise. The first two slices are when the droplet is approaching the surface and the rest are when the droplet is leaving the surface. Surrounding side-view images show relative droplet locations.”
Xiaoyu Tang, Abhishek Saha, Chung K. Law (Princeton University)

“Otherwise regular and smooth surface of an expanding flame can be converted into irregular and wrinkled morphology either by cellular instability or by turbulence. Under both these situations, wrinkling of flame-front becomes stronger as the flame grows, either by the increase in number of cells or by the effect of larger eddies. While these wrinkled flame-fronts consist of segments with widely varying local curvatures, it is interesting to watch how locally the curvature evolves with time.

If the instantaneous flame edges from various time instances colored by the local curvature are superimposed for expanding wrinkled flames, beautiful patterns emerge. Two of such flames are portrayed here, Top row: Cellularly unstable laminar flame and Bottom row: Turbulent expanding flame, with colors representing negatively (green circle), positively (pink circles) and weakly (blue dots) curved flame segments. Amidst the beautiful plethora of colorful dots, the evolution of local curvature can be visualized by tracing the same color dots.”
Abhishek Saha, Sheng Yang, Chung K. Law (Princeton University)

“A Hele-Shaw cell is used for this laminar flame instability research. Dimensions of entire visible area is 15.75in by 23.62in. The Hele-Shaw cell is first evacuated, and then filled with desired premixed gas mixture using partial pressure method. The gas mixture is ignited at one end of the cell by three ignition sparks. The flame propagates under constant pressure toward the closed end of the cell with ignition end open to the atmosphere. There is no turbulence pre-ignition in the cell. Light emitted from hot products behind flames is filmed in total darkness. The video contains several individual experiments of different gas mixtures and propagation orientations. They are also colored using Adobe Premiere during post process. The conditions for each experiments are:

• C₃H₈-O₂-N₂ mixture with equivalence ratio of 2 and calculated adiabatic flame temperature around 1850K horizontally propagation through a 0.5in narrow gap between two plates.
• H₂-O₂-N₂ flames of equivalence ratio of 2 and calculated adiabatic flame temperature around 1200K downward propagating through a 0.5in narrow gap between two plates.
• H₂-O₂-N₂ flames of equivalence ratio of 2 and calculated adiabatic flame temperature around 1200K horizontally propagating through a 0.5in narrow gap between two plates.
• H₂-O₂-CO₂ flames of equivalence ratio of 0.2 and calculated adiabatic flame temperature around 900K upward propagating through a 0.5in narrow gap between two plates.
• H₂-O₂-CO₂ flames of equivalence ratio of 0.8 and calculated adiabatic flame temperature around 1200K downward propagating through a 0.25in narrow gap between two plates.
• H₂-O₂-N₂ flames of equivalence ratio of 0.8 and calculated adiabatic flame temperature around 1300K horizontally propagating through a 0.5in narrow gap between two plates.
• H₂-O₂-CO₂ flames of equivalence ratio of 0.35 and calculated adiabatic flame temperature around 1050K downward propagating through a 0.5in narrow gap between two plates.

Otherwise regular and smooth surface of an expanding flame can be converted into irregular and wrinkled morphology either by cellular instability or by turbulence. Under both these situations, wrinkling of flame-front becomes stronger as the flame grows, either by the increase in number of cells or by the effect of larger eddies. While these wrinkled flame-fronts consist of segments with widely varying local curvatures, it is interesting to watch how locally the curvature evolves with time.

If the instantaneous flame edges from various time instances colored by the local curvature are superimposed for expanding wrinkled flames, beautiful patterns emerge. Two of such flames are portrayed here, Top row: Cellularly unstable laminar flame and Bottom row: Turbulent expanding flame, with colors representing negatively (green circle), positively (pink circles) and weakly (blue dots) curved flame segments. Amidst the beautiful plethora of colorful dots, the evolution of local curvature can be visualized by tracing the same color dots.”
Si Shen (University of Southern California)

“The image is a combination of non-premixed, premixed, and solid propellant combustion.”
Sayan Biswas, Aman Satija, Michael Powell, Morgan Ruesch, Steven Son, Robert Lucht, Li Qiao (Purdue University)

“The counterflow flame burner consists of two opposed vertically-oriented concentric burners. The upper burner supplies the fuel jet and the lower burner supplies the oxidizer jet. The two center jet exits are surrounded by a co-flow to shield the flame from the environment resulting in the circular flame shape. To capture the sequenced temporal evolution of the flame, we first ran the methane and air through the upper and lower parts of the counterflow burner respectively before the mixture was ignited. That results in a large rich flame around the upper part of the counterflow burner. After that, we introduced nitrogen as the co-flow gas to shield the flame. As we increased the rate of the co-flow, the inner circular flame started to shape. Finally, we decreased the oxidizer flow rate in order to shrink the flame. The pictures were taken through all the steps.”
Radi Alsulami and Colin Gould (Colorado State University)

1. Before adding monopropellant (inner rectangle)
2. After adding monopropellant (intermediate rectangle)
3. After monopropellant burns out (outer rectangle)

“The pictures were taken for fuel flexibility experiments carried out at the University of California Irvine Combustion Laboratory (UCICL). The pictures show the structure of premixed flames with multiple fuel compositions and air to fuel ratios (only on the fuel lean regime). The fuel compositions used include fuel blends of hydrogen, natural gas and biogas.

The burner presented in the picture uses a porous material to stabilize the reactions. The porous ceramic burner presented in the pictures is the Duratherm™ by ALZETA Corp.”
Andres F. Colorado (University of California Irvine)

“Each eye is actually an image of a burning droplet with its surrounding soot and flame, suspending from a carbon fiber in normal gravity. The fuel used for this experiment was diesel doped with 3% (by weight) of a very heavy polymer, Polybutadiene, to investigate the effect of polymer additives on combustion characteristics. Due to presence of polymer, there is a period during which the droplet undergoes strong bubbling and swelling and takes many irregular shapes. Through sputtering, plumes of gas leave droplet surface and change flame standing and soot shell distribution. The image has been just colored and all the shadows around the eye are natural and represent the location of a very sooty flame around the droplet. For this experiment images were taken at 500 frames per second and the mean droplet diameter in the instant shown is about 0.55 mm.”
Mohsen Ghamari and Albert Ratner (University of Iowa)

“Turbulence-flame interaction is a key to turbulent combustion modeling. Except for statistically one-dimensional planar turbulent flames, mean flow features such as shear layers also influence the interaction. The V-flame is one of flame configurations that emphasize this additional complexity due to flame geometry. A turbulent hydrogen-air V-flame has been simulated using the direct numerical simulation methodology. The image shows volume rendered vortices (cyan) and temperature field (red-yellow). The interaction characteristics change with the streamwise distance (vertical distance from the bottom). Near the flame holder, local flames interact with quite a few vortices parts of which are generated by the mean shear or wake behind the flame holder. The vortices quickly decay to have small effect on reaction zones in the downstream.”
Yuki Minamoto (Sandia National Laboratories) and Mamoru Tanahashi (Tokyo Institute of Technology)

“Tiny soot particles are visible around a free-floating and stationary n-decane droplet in this image at one instant of the droplet burning history. External convective effects are absent. The resulting thermal symmetry leads to soot aggregates forming a spherical porous cloud (the flame, not visible, is about ten times the droplet diameter). This symmetric configuration of trapped soot aggregates belies the complexity of the processes that bring about its existence.”
Yuhao Xu (Cornell), Yu-Cheng Liu (U. Michigan-Flint), Michael Hicks (NASA) and Thomas Avedisian (Cornell)

The top flame shows the ideal, reference case of a stable, smooth flame surface in a quiescent environment at atmospheric pressure.

The middle flame is taken under elevated pressure simulating that within an internal combustion engine. The flame surface is now hydrodynamically unstable (i.e. the Darrieus-Landau instability) and develops cells, causing the flame to propagate and hence burn faster because of the increased flame surface area.

The bottom flame is taken in a highly turbulent environment which simulates another aspect of the engine interior. Now the flame surface is distorted by the multi-scale turbulence, which again leads to an increase of the burning rate. All images were taken at 8000 frame per second, using schlieren photography. Radius of the top flame is 11.4 mm.”
Chung K. Law, Swetaprovo Chaudhuri, Fujia Wu (Princeton University)

The image shows the carbon monoxide (CO) concentration in the gas phase.

The blade corresponds to production of CO by devolatilization of the coal particle, the guard corresponds to homogeneous ignition and the grip corresponds to CO production by char oxidation.”
Babak Goshayeshi, James C. Sutherland (University of Utah)