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Isaac G. Boxx, Cherian A. Idicheria and Noel T. Clemens
The turbulent jet-flame in a crossflow (JFICF) is a flowfield of theoretical and applied importance. It serves as a simplified model for more complex combustion systems such as fuel-injectors, oil-well flares and even rocket control thrusters. Accordingly, this flowfield has been the focus of a great deal of research, both theoretical and experimental. Studies have shown the trajectory of a JFICF scales as a power-law of its momentum-flux ratio,
(where
= crossflow density and 
= crossflow velocity) per the relation
(where d is the jet-exit diameter, x is aligned with the jet-exit direction, z with the crossflow direction). This power-law scaling is clearly appropriate for momentum dominated JFICF, but it has not been established conclusively that it applies to buoyancy-influenced flames also.
Buoyancy has long been known to affect the large-scale structure of turbulent-flames and jet-flames in a crossflow. Study of this effect is severely complicated by the difficulty of isolating buoyancy from competing effects in the system. Buoyancy is hard to study because it’s so hard to get rid of. One particularly useful method for studying the effect though, is to compare flames in normal gravity to similar ones in a microgravity environment, such as can be produced in a drop-tower. A drop tower allows one to effectively “turn off” buoyancy by cancelling out the effect of gravity. That’s what we do at the University of Texas Drop Tower Facility (UT-DTF).
In the past decade planar laser diagnostics (in particular particle image velocimetry and planar laser-induced fluorescence) have become an integral tool in the study of JFICF. Applying these diagnostics in microgravity, and in particular in a drop-tower, is quite challenging. Difficulties specific to a droptower include size, power and weight constraints, equipment survivability, short run times and attaining reliable flow seeding. Although PIV has been successfully demonstrated in microgravity, including on flights aboard parabolic trajectory aircraft and in drop-towers, its potential has yet to be fully realized. At the UT-DTF we are working to develop these diagnostics and use them to study turbulent JFICF.
Drop Rig
The experiment is packaged in a self-contained drop-rig. Volumetric constraints necessitate a
very compact design.
Low-gravity experiments are conducted in the self-contained drop-rig shown above. The rig is housed in an aluminium frame provided by the NASA-GRC 2.2 Second Drop Tower, which measures just 0.965m long by 0.406m wide and stands 0.914m tall. The drop rig contains several subsystems, including a blow-through jet-flame in crossflow facility, pressurized gas and electrical systems, a computer control and data acquisition system and elements of the PLS/PIV system such as the camera and sheet-forming optics. Fully loaded, the drop rig has a mass of 117kgs. The rig is placed in an aerodynamic drag-shield during an experiment drop to protect is from wind-resistance, which could affect the quality of its weightless environment.
Luminosity Imaging
r = 10, Red=4930, low-gravity, Dt= 4.26ms
r=10, Red = 4930, normal-gravity, Dt= 4.26ms
Shown above are a series of images taken of a highly-luminous ethylene-fueled jet-flame in a crossflow in normal and low-gravity. Aside from their difference in buoyancy, their characteristics are identical. One can immediately see how dramatically buoyancy effects the large-scale structure of these flames. In normal gravity, the shear-layer vortices further separated from one another and they tend to convect upwards more quickly. The normal gravity flame also appears to burn brighter than its low-gravity counterpart. An analysis of the mean flame luminosity has also shown that while normal-gravity JFICF depart from their power-law scaling behavior near their flame tips, their low-gravity counterparts do not.
PLMS Flow-Visualization
Shown above is the flow-facility we use in the drop-rig we built at the UT-DTF. Air is blown in from fans on either side and goes through a flow-conditioning device before entering the test section. A jet of fuel is injected from an orifice in one wall and is blown upwards by the crossflow. This is the jet-in-crossflow. A laser sheet is then painted in through a window opposite the jet-exit and scattering from the particles we previously seeded into the fuel jet are imaged by high-speed camera. To get the best resolution we can, we focus in on only one part of the flame at a time. A typical sequence of images from the experiment is shown below.
PLMS Imaging of a low-gravity hydrogen JFICF (r = 7, Red = 900), seen here speeded up by a factor
of ten. PLMS allows us to track the jet-fluid as it moves into and mixes with the crossflow.
PLMS Imaging of the same JFICF in normal-gravity. Note the more stretched and disorderly
nature of the flow.
We conduct frame-by-frame inspections of the PLMS image sequences like the ones above to look for differences between normal- and low-gravity JFICF. For example, in low-gravity sequence above, the large scale structures appear to roll up and convect out of the field of view in a fairly regular manner. In normal gravity however, the same structures appear far more non-uniform, stretched and wrinkled. A similar observation has been made in non-premixed jet flames by Idicheria et al. (2004) as they used PLMS imaging and showed more regular or coherent structures under low-gravity conditions. Thus it appears that buoyancy has the effect of disrupting the hydrodynamic instability of the jet flames and hence causing the turbulence to be more disorganized.
Cinematographic PIV
We use cinematographic PIV to study the moment-to-moment velocity, vorticity and shear of our JFICF in normal and low-gravity. Shown below are a series of images taken in low-gravity. The images were taken at 6000fps, so only every seventh image of the original sequence is displayed here. By using cinematographic PIV we were able to track how the vortex seen in the images rolled up and distorted as it moved up through the field of view of the camera. This diagnostic is now being systematically applied to study both the mean and moment-to-moment velocity and vorticity fields of the flow.
Original Image Sequence. Hydrogen JFICF. (r = 7, Red=900), Imaged at 6000fps,
speeded up here by a factor of 7
Velocity vectors derived from the images above
Vorticity fields corresponding to the images above. Note how the vertical structures do not appear to
Exactly overlap the structures seen in the original images.
Publications:
- Kilohertz PIV/PLMS of Low-Gravity Turbulent Flames in a Drop
Tower, I.G. Boxx, C.A. Idicheria and N.T. Clemens, 12th International Symposium on Applications
of Laser Techniques to Fluid Mechanics. Lisbon,
- An exploratory study of the effects of buoyancy on turbulent nonpremixed ethylene jet flames in crossflow. I. G. Boxx, C. A. Idicheria and N. T. Clemens, AIAA-2003-1151, AIAA 41st Aerospace Sciences Meeting, 2003.
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