LBL Combustion researcher creates a better flame

March 11,1994

By Mike Wooldridge, [email protected]

For years, combustion scientists have known one way to decrease the amount of pollutants released from gas burners: create the burner flame with higher air-to-fuel ratios, allowing for more thorough, cleaner combustion.

Unfortunately, such so-called lean flames are notoriously unstable, and tend to blow out with the slightest cross-current.

Mechanical engineer Robert Cheng, of LBL's Energy and Environment Division, has devised a method to steady such fickle flames with a system that gently swirls the fuel-air mixture as it is released from the burner. The device produces a lean flame that releases much less NOx than conventional burners--fewer than 10 parts per million.

DOE recently slated funding for the technology through the Energy Research Laboratory Technology Transfer program. A Cooperative Research and Development Agreement will be signed soon with Teledyne-Laars in Southern California.

While Teledyne-Laars hopes to develop the device for commercial water heaters, Cheng expects a finished design could also be applied to furnaces, boilers, and other devices that burn natural gas.

"It's very attractive from an industry standpoint since the design increases manufacturing costs very little," he says. "The burner won't require a lot of new, expensive parts."

Cheng originally designed the device not out of a desire to create a green technology, but as a way to do basic research on turbulent combustion.

"Our lab has to have burners that operate over a very wide range of conditions--from very rich flames to very lean flames," he says. "We also want them to be as simple as possible so that we can use lasers to characterize the combustion."

Creating such a multipurpose apparatus required mastering the rather complicated fluid dynamics of flames.

A burning flame involves essentially two competing rates: The flow of the fuel-air mixture being released from the burner device, and a burning rate intrinsic to that particular fuel-air mixture--the flame speed. Both phenomena typically work opposite one another, the fuel-air flow coming out of the burner and the flame traveling down into the fuel-air source.

If the flow rate of a burner is greater than the flame speed, an ignited flame will shoot straight up and blow itself out. On the other hand, if the flow rate is less than the flame speed, the flame will shoot back down--the flashback effect. Creating an equilibrium between flow rate and flame speed in lean burners has been a challenge, and researchers have only succeeded in stabilizing very small lean flames with elaborate lab equipment.

To design his ultra-lean burner, Cheng put to work a rather elementary physics principle. The principle states that a swirling current of air will gradually expand by centrifugal force, its air speed decreasing as it expands. Such an expanding mixture of fuel and air coming from a burner will at some point reach a state where the flow speed and flame speed are balanced. At that point, Cheng discovered, a lean flame will stabilize.

Cheng created the weak swirl system by adding a pair of air injectors near the rim where the fuel-air mixture escapes the burner cylinder. He attached the injectors parallel and off center to one another, so they gently stirred the escaping mixture.

Such an apparatus can create a light blue, almost invisible flame, with a fuel content of only 6 percent. Because of the unique flow generated by the swirling current, the burner flame is disk-shaped. Such a disk-shaped flame has added attractiveness for applications such as boilers, Cheng says, since it spreads its heat over a wider surface area.

Manufacturers have generally lowered combustion emissions with expensive procedures such as catalytic processing, stage combustion, and flue gas recirculation. Such procedures have been too expensive to apply to small heating systems such as water heaters and furnaces.

Cheng's device is a step up from such procedures in that it uses a clean, lean-burning flame in the first place, rather than taking steps to purify dirty, fuel-rich flames.

"People have been able to stabilize lean flames in the past, but never at a power output for practical use," Cheng says. "We've finally done it."