A novel approach to combustion has the potential to transform the future design of gas appliances. Burners in typical appliances burn a partially premixed flame, meaning that the fuel jet entrains some but not all of the combustion air into the nozzle. A partially premixed flame burns in two stages: In the first stage the entrained air is consumed, and in the second stage leftover fuel is consumed as it is mixed with ambient air. While partially premixed flames are robust and have a very high turndown, they produce high NOx emissions because the first stage of flame combustion burns at a high temperature.

In 1991, Robert K. Cheng, senior scientist at Lawrence Berkeley National Laboratory, Berkeley, Calif., developed the low-swirl combustion method while conducting research for the Department of Energy, studying interactions between turbulent fluid motions and premixed combustion.

Low-swirl combustion burns what is called a lean premixed flame. In this combustion mode, an excessive amount of air is used in the oxidation of fuel, creating a flame that burns at a lower temperature than that created through partially premixed combustion. While the lean premixed flame method resulted in ultra-low NOx emissions, the problem was that it created a flame that was relatively weak and difficult to stabilize, a result of the excess air used in combustion. Flame instability has the potential to actuate extreme pressure oscillations, which can damage equipment. Furthermore, the flames produced high CO emissions, as well as incomplete combustion.

The low-swirl burner (LSB) overcomes the instability inherent to lean premixed flames, adding robustness and commercial viability. (image: Lawrence Berkeley National Laboratory)

This led Cheng to develop the low-swirl burner (LSB), which overcomes the instability inherent to lean premixed flames, adding robustness and commercial viability to the method. The LSB harnesses the dynamic features of lean premixed flames by utilizing an aerodynamic method that exploits their self-propelling nature. Unlike the conventional lean premixed burner that uses a flame holder or high swirl to anchor the flame, the LSB generates what Cheng calls divergent flow, by virtue of a modified vane swirler, which allows the flame to self propel freely.

Divergent flow spreads out and, thus, slows down the LSB’s flow of fuel and air. While conventional high-swirl combustion uses a recirculating region, in which the back-flow of hot combustion product ignites fresh reactant at swirl intensities well above the vortex breakdown threshold, the LSB divergent flow method takes a counterintuitive, opposite approach. The LSB operates at a swirl intensity well below the vortex breakdown threshold, producing a non-recirculating flow. The flame is self propelled and burns without being influenced by the turbulent shear stresses associated with flow recirculation.

The flame of the LSB hovers far above its source. While such a phenomenon is typically associated as a precursor to flame blow off by combustion engineers, Cheng says that with the LSB this is not the case. With the LSB, the speed of the fuel and air balances the flame speed. And when the flow velocity is increased, increasing the power output of the flame, the structure of divergent flow is unaffected, while turbulence in the flow gives feedback for the flame to burn faster.

Advantages

The LSB burner technology is scalable across a wide range of gas-fired applications. The smaller, 2-in. burner at left could be used for a residential gas furnace. The larger one might be used for industrial heating applications. (image: Lawrence Berkeley National Laboratory)

The LSB can accommodate flame temperature requirements below 2,600 DegF as well as a variety of fuels; for instance, natural gas or propane in the case of appliances. It also eliminates flame flashback, a major risk in premixed burners. Because the speed of the fuel/air mixture is deliberately set to be higher than the flame speed, flashback is prevented, says Cheng.

Like all other burners, the LSB produces flames that are close to 99.9 percent efficient, while at the same time producing low CO emissions and substantially less NOx emissions, a chemical that is in part responsible for smog in urban areas, as well as ozone presence in the lower atmosphere. Depending on the application, LSBs emit 10 times to 100 times less NOx than conventional burners.

Because it requires a high degree of air and fuel pressure, the LSB enables a wide turndown range. For example, a 2-in. LSB designed to operate from 50,000 Btu/hr to 2 MMBtu/hr—a turndown ratio of 40:1—can actually operate at as low as 30,000 Btu/hr to about 2.2 MMBtu/hr—a 60:1 turndown ratio. Cheng says that the requirements for commercial and residential gas appliances are much lower: a 5.1 to 5.2 turndown ratio. In addition, turndown is unaffected by LSB size; for example, there are LSBs 28 in. in diameter.

The LSB’s inherent flame modulation ability should be an area of interest to designers of residential gas appliances because many of these devices lack modulation altogether, being on-off binary systems. Having such flame modulation available for a residential furnace, for example, would enable a more consistent home temperature because the system would not have to keep switching on and off, which results in inconsistent ambient temperature.

Also, the LSB does not require high-precision machining or low-tolerance parts for manufacture. Cheng says that LSBs are routinely made in the lab from hand-cut sheet metal and sometimes even plastics. Furthermore, because the flame does not contact the burner, it is not subject to thermal stresses, eliminating burner wear.

In the field

Like all other burners, the LSB produces flames that are close to 99.9 percent efficient, while at the same time producing low CO emissions and substantially less NOx emissions. (image: Lawrence Berkeley National Laboratory)

The LSB has thus far been incorporated into a commercial application, namely industrial burners used to dry or cure product at factories. In addition, the technology has been demonstrated as viable in water-tube and tube-fired boilers. Potential applications also exist in the commercial foodservice arena as well, says Cheng. Water-heater boosters for commercial dishwashers is one such application. There also has been interest among other industries, including the gas turbine and residential appliance industries.

In terms of residential appliances, Cheng says that the technology is especially promising among gas furnaces, water heaters, and spa heaters, saying that it will help such products meet California emission standards without compromising cost or efficiency. As of yet, there is not an LSB-enabled residential appliance commercially available, though laboratory testing has been performed using pool heaters and furnaces. Within the residential appliance industry, Cheng says an impediment in the adoption of LSB technology is the need for an electric fan for operation. Lean pre-mixed burners cannot entrain all the air needed for combustion, so the use of an electric fan is imperative for operation.

The disassembled components of a low-swirl burner. The brass component at the right is the modified van swirler, which generates the divergent flow that allows the flame to self propel freely. (image: Lawrence Berkeley National Laboratory)

Cheng says that LSB technology is best suited for appliances that function under an induced, or forced, draft design, such as some furnaces and water heaters, for example.

Cheng concedes that LSB technology is more appropriate for high-speed, high-power applications, but feels strongly that the technology is adaptable within the residential appliance industry. A problem, says Cheng, is that it is very difficult to design LSB-enabled appliances for residential use because a home, for example, is not a very well controlled environment, which has the potential to compromise appliance performance.

Cheng does go on to say, however, that there has been interest among many manufacturers of residential appliances in the LSB technology.

For more information, visit www.lbl.gov