High-speed video reveals the complexity of fluid instabilities leading to atomization–the breakup of a liquid sheet into droplets. A thin annular liquid sheet is sandwiched between concentric air streams. As the velocity of the air on either side of the liquid sheet varies, shear forces cause the sheet to develop waves that result in mushroom-like shapes that break down into ligaments and droplets. Quick breakup into droplets is important in many applications, most notably combustion, where the size and dispersal of fuel droplets affects the efficiency of an engine. (Video credit: D. Duke, D. Honnery, and J. Soria)
Tag: combustion
Fireball in Slow Motion
The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter. It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines. Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion. The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.
The Olympic Torch
[original media no longer available]
Today marks the beginning of the 2012 Olympic Games in London. In the opening ceremony, the Olympic flame will complete its journey from Olympia to London, having been carried by some 8,000 torch bearers. Modern Olympic torches are expected to withstand wind, rain, snow, and human error to keep the flame alive and are specially designed and tested for these conditions. Each individual torch is fueled by a mixture of propane and butane stored as a pressurized liquid. The liquid fuel travels through a series of evaporation coils around the burner before combustion. Each torch carries sufficient fuel to burn about fourteen minutes. In addition to computer simulation, the 2012 Olympic torch design was tested in BMW’s Environmental Wind Tunnel to ensure a visible, stable flame for orientations within 45 degrees of vertical in conditions ranging from -5 degrees to 40 degrees Celsius, rain, snow, 35 mph winds, and 50 mph wind gusts. For more on the current torch and previous designs, see How Stuff Works, E&T, and the BBC.
FYFD is celebrating the Olympics by featuring the role of fluid dynamics in sports starting Monday. If you have any burning questions, feel free to ask or email!
Reader Question: Fire as a Fluid?
Reader David L asks:
I understand that fire is a form of energy rather than a fluid in the physical/tangible sense. However, is it possible for fire to exhibit fluid-like behaviours to a certain extent.
In other words, could the dynamic properties of fire be described with pseudo-variables analogical to variables that describe a physical fluid (i.e. viscosity, density, Re, etc.)?
Actually, combustion is a major topic of research among fluid dynamicists. Since the part of fire that we identify as visible flame is a reacting mixture of gas and some solid particles, it moves according to the same equations of motion as any other gas. However, when studying combustion thermodynamical equations and chemical reactions must also be tracked in addition to mass and momentum, which makes modeling fire very difficult. Combustion plays a major role in internal flows like those in car, jet, and rocket engines. (Photo credit: master.blitzy)
The Invisible Forces Behind a Lighter
This high-speed schlieren video reveals the ignition of a butane lighter. The schlieren optical technique exaggerates differences in refractive index caused by density variations, enabling experimentalists to see thermal eddies, shock waves, and other phenomena invisible to the naked eye. Here a jet of butane shoots upward from the lighter as a valve is released. Then the spark from the lighter ignites the butane gas near the bottom of the jet. A flame front the propagates outward and upward, completing the lighting process. (submitted by @Mark_K_Quinn)
Atomizing Jets
The breakup of impinging jets into droplets (also called atomization) and the subsequent dynamics of those droplets are important in applications like jet and rocket engines where the mixing of liquid fuel with oxygen is necessary for efficient combustion. This video showcases recent efforts in high fidelity numerical simulation and modeling of such flows. The complexity of the problem requires clever ways of reducing the computational efforts required. One such method uses adaptotive meshing to concentrate grid points in areas where variables are changing quickly while leaving the grid sparse in areas of less interest. Because the flow is constantly evolving, the mesh must be able to adapt as the simulation steps forward in time. Even so, such calculations typically require supercomputers to complete. (Video credit: X. Chen et al)
Testing Flames in Space
In microgravity, flames behave very differently than on earth due to a lack of buoyant forces. On earth, a flame can continue burning because, as the warm air around it rises, cooler air gets entrained, drawing fresh oxygen to the flame. In microgravity, both the heat from the flame and the oxygen it needs to burn move only by molecular diffusion, the random motion of molecules, or the background environmental flow (air circulation on the ISS, for example). This video shows a test of the Flame Extinguishment Experiment (FLEX) currently flying onboard the ISS. A fuel droplet is ignited, burns in a symmetric sphere and then eventually extinguishes either due to a lack of fuel or a lack of oxygen. Check out this NASA press release for more, including great quotes like this:
“As a Princeton undergrad, I saw in a graduate course the conservation equations of combustion and realized that those equations were complex enough to occupy me for the rest of my life; they contained so much interesting physics.” – Forman Williams
Sewer Combustion
Enjoy a little high-speed video of combustion (the safe way!) this Thanksgiving holiday. For non-U.S. folks, have a great Thursday!
Diesel Ignition
In a diesel engine, ignition of the injected fuel occurs due to the heat caused by the compression of the fuel/air mixture. (In petrol/gasoline engines, spark plugs are used for ignition.) The subsequent expansion of gases drives the pistons of the engine downward, creating mechanical energy. This high-speed video shows the in-cylinder combustion within a diesel engine. Note the symmetry and vorticity of the flow.
Mapping Flames
Combustion remains a fascinating and only partially understood phenomenon. Here scientists work to map a flame in three dimensions using high-speed cameras and digital reconstruction. (submitted by Chi M)
