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Ultra-Low Emissions Low-Swirl Burner

How it works


Press releases

Technical descriptions


Relevant links


Publications on LSB technology

Combustion research publications using LSB

The heart of the low-swirl burner is a vane-swirler that has two flow passages. The reactants (fuel/air mixture) flow through the openings of the center channel and the gaps between the surrounding swirl vanes. This design creates the low-swirl flow which supports a stable lifted or floating flame that is the characteristic feature of the low-swirl combustion method (see picture below).  


Low-swirl combustion (LSC) is an aerodynamic flame stabilization method discovered in 1991at the Lawrence Berkeley National Laboratory. Originally intended as as a burner for basic research, its turbulent flowfield and flame properties have been investigated by laser diagnostic. The analysis showed that the basic LSC principle is fundamentally different than the conventional high-swirl combustion method and defies many established notions on turbulent flame properties and burner engineering concepts.

Due to its exceptional capability to burn ultra lean flames that emit very low levels of oxides of nitrogen (NOx), the novel low-swirl combustion method has evolved into a simple and robust ultra-low emissions combustion technology for burners in industrial heaters and in gas turbines for electricity production.

Low-swirl combustion can be implemented in two ways. The original approach uses small jets to create the swirling motion. The low-swirl burner (LSB) developed for heaters and gas turbines uses a patented vane-swirler (right).

Basic knowledge gained from the laboratory studies has provided the scientific underpinning to develop an analytical model, scaling laws and engineering guidelines for adapting the LSB to various combustion devices. The LSB remains a mainstay for fundamental research and a growing number of researchers world-wide are using the LSB to investigate the fundamental turbulent flame processes.

  The low-swirl burner is scalable. The smallest (left) in development for residential furnaces is 1 inch in diameter. The largest LSB in development for industrial heating by Maxon (right) is 28 inches in diameter. Regardless of the burner sizes, their performances in terms of turndown, efficiency, and pollutant emissions are the same. Residential appliances contribute to a significant amount of air pollution in the urban areas. These commodity products cannot afford the sophisticated and expensive pollution reduction technologies developed for the larger commercial and industrial systems. A simple and truly affordable ultra-low emission burner such as the LSB will help these appliances to meet the very stringent emissions standard being implemented in large metropolitan areas world-wide.
  NOx emissions from a low-swirl fuel-injector assembly (LSI) developed for Solar Turbines' T70 gas turbine are about 2.5 times less than the levels emitted by the company's current SoLoNOx injector (HSI) that uses the conventional high-swirl burner concept. The NOx emissions data were obtained with single injector tests at partial and full load conditions. A set of LSI was also tested in an engine and produced equally encouraging results. (Figures shows NOx data reported in Johnson et. al., 2005)

Technology Transfer

The US Department of Energy (DOE) Office of Science's Laboratory Technology Research program supported the first commercial development of the LSB. The small 2" diameter burner was engineered for residential natural gas water heaters of 50 KBtu/hr (15kW) and lowered the NOx emissions from well over 120 ppm to below 10 ppm without compromising the efficiency. A significant finding was that the small LSB has a very high turndown (i.e. ratio of the maximum to minimum power outputs). It is capable of operating at up to 2 MMBtu/hr (600 kW). The exceptionally high 40:1 turndown is unmatched by other low-NOx burner technologies and show that the LSB is scalable to the larger sizes for industrial processes.

Under the support of California Institute of Energy Efficiency and DOE's Industrial Technology Program at the Office Energy Efficiency and Renewable Energy, the LSB was scaled to the large sizes and demonstrated successfully in natural gas furnaces and boilers. Maxon Corp. of Muncie IN introduce the first commercial LSB product in 2003. Currently, the company has two lines of LSB products from 300 KBtu/hr (90kW) to 120 MMBtu/hr (35 MW) all emitting less than 9 ppm NOx (corrected to 3 %O2) while delivering exceptionally high performance of 10-15:1 turndown.

Subsequent LSB developments targeted gas turbines that utilize natural gas for generating electricity in utility power plants, and in industrial and manufacturing sites. DOE's Distributed Energy Resources at the Office of Electricity supported LBNL and Solar Turbines of San Diego, CA to adapt the LSB to small gas turbines of 5 to 7 MW electricity output. These gas turbines operate at pressure of up to 15 atmospheres and temperature exceeding 400 C. The first challenge was to proof that the LSC method is operable at these conditions. The second is to engineer the LSB to operating in a gas turbine system. Since a vane-swirler is a standard component of the high-swirl burner used in gas turbines, a decision was made to fast track the development by converting Solar Turbines' current high-swirl burner to operate in the low-swirl mode. The conversion was successful and produced a low-swirl fuel injector (LSI) that is a retrofit of the current SoLoNOx high swirl fuel injector (HSI). The LSI helps reduce the NOx emissions to below 5 ppm (corrected to 15 % O2) (see chart on the right). The team was honored by a RD100 Award in 2007.

The LSB has also been adapted to smaller microturbines of 100kW electricity output. The development was supported by California Energy Commission (CEC) as part of a Combined-Heat-and-Power project led by CMC Engineering of Sunnyvale CA. The microturbine is an integral component of a boiler system to produce electricity for powering the air blower of a large boiler burner. CEC is also supporting a new development and demonstration of a microturbine that operates on digester waste gas from a water treatment plant in Oakland CA.

With increasing concern regarding the impact of energy use on global climate change, fuel-flexibility has become a critical requirement of next-generation gas turbines. The US Department of Energy's Office of Fossil Energy is supporting the research to extend the LSB technology to a variety of fuels including syngas and hydrogen for utility size turbines of over 200 MW electricity output. This is one of manyprojects in a large effort to develop Near-Zero Emissions Integrated Gasification Combined Cycle (IGCC) Coal Power Plants. For the hydrogen fueled gas turbines in these power plants, DOE sets a very aggressive goal of less than 2 ppm NOx operation on syngas and up to 90% pure hydrogen at 2500-2600F (1371 - 1426 C) turbine firing temperatures. Burning of hydrogen in a gas turbine presents significant technical and engineering challenges because of the high reactivity of hydrogen, its fast flame speed, and the propensity of the H2/air mixture to auto-ignite and explode. Many conventional approaches may not work without diluting the fuel/air mixture with inert gases or exhaust gas NOx cleanup with catalysts.

The development of LSB for H2 is guided by the scientific principles of the LSC method. An analytical model has been applied to understand the changes in the LSB flame when switching from natural gas to hydrogen. Preliminary laboratory tests results are very encouraging and show that the basic LSB design accepts fuels from pure natural gas to over 90% H2 at simulated gas turbine conditions while meeting the < 2 ppm NOx target. Another important finding is that the model predicts the changes in the LSB flames with fuel contents to confirm its validity as a basis for scaling and system adaptation. However, significant amount of R&D effort is still required to make LSB a reality in hydrogen fueled gas turbines. It will require a gas turbine manufacturer to lead the development and address the operational and safety issues. The basic knowledge on LSC will be an advantage to help the gas turbine manufacturers to overcome the challenges associated with the very energetic hydrogen flames.

The low-swirl burner remains cool to the touch because the lifted flame does not heat up its body. The lifted flame was thought to be highly undesirable because in other burners it signifies unstable flame behaviors. But the unique divergent flowfield of the LSB (shown below) allows the lifted flame to self-adjust and remain robust at the very lean conditions where NOx emissions are at their minimum.  
The flowlines through the LSB flame shows that there is no flow reversal (i.e. flow recirculation) in the vicinity of the flame. The flowlines spread-out below the flame to indicate flow divergence. Color contours in the background show that the disruptive turbulent shear-stresses are insignificant near the flame. The velocity data was obtained by a method called Particle Image Velocity on a lean methane air flame similar to the one shown in the picture above. The studies of laboratory flamse provided the scientific background to develop a top-order model and scaling laws for adaptation of LSB to different combustion systems  

How It Works

Lean premixed combustion is the foundation for the technologies in almost all modern low-emissions combustion equipment. It burns gaseous fuels mixed thoroughly with an amount of air that exceeds the quantity needed to consume the fuel (i.e. excessive air combustion or fuel lean combustion). This is an approach that deliberately weakens the flame to inhibit NOx formation by lower the flame temperature. Weakening the flame has many undesirable consequences including high carbon monoxide (CO) emissions, incomplete combustion, and most significantly, flame instability that can trigger severe pressure oscillations to damage or cripple the engines and equipment. LSB can harness these undesirable side effects to take fully advantage of lean premixed combustion.

Until now, premixed burners have used the high-swirl flame stabilization method, which evolved from the non-premixed combustion technology found in older and more polluting combustion systems. This traditional method uses a recirculating region (i.e. back flow or reversed flow) to trap and retain a portion of the hot combustion products to ignite fresh reactants. Generations of combustion engineers have been trained to design burners with swirl intensities well above the vortex breakdown threshold to ensure strong recirculation. Combustion researchers are still developing theories and computational methods on high-swirl flames to predict the strengths and size of the recirculation zone to support the engineering designs.

The LSB adopts the opposite approach by operating at swirl intensity well below the vortex breakdown threshold. It produces a non-recirculating flow characterized by a flow divergence region where the axial flow velocity decays linearly with the distance away from the burner exit. The lean premixed turbulent flame self-propels and burns its most natural state without being influenced or restrained by the turbulent shear stresses associated with flow recircluation. The operating principle exploits the most fundamental property of premixed combustion - the premixed flame behaves as a "propagating wave" that moves through and consumes the reactants at a flame speed controlled by mixture composition, and turbulence intensity. The divergence rate of the LSB can be aerodynamically "tuned" to accommodate the turbulent flame speed. And turbulence intensity provides the feedback for the flame to burn faster and slower with load change. This theory is fundamentally different than the flame-holding approach of the traditional high-swirl method.

The LSB show in the photograph at the top of this web page has a simple design that features an annulus vane swirler surrounding a cylindrical center channel. The center channel allows a portion of the reactants to pass without being swirled. The centrifugal forces of the swirling flow acting on the un-swirling center core create flow divergence downstream of the exit. In the figure shown to the left, the divergent nature of the flowfield in the nearfield region at x < 10 mm is illustrated by streamlines that spread outward above x = 0. The rate of flow divergence, i.e. the spreading rate, is a LSB design parameter proportional to the ratio of the unswirled and the swirled flows. Flow divergence creates a flowfield where the axial velocity decays linearly with increasing x. When the velocity at the exit is maintained higher than the turbulent flame speed, ST, the flame rides on this velocity "down-ramp" and self propels at the position where the local flow velocity is equal and opposite to ST.

   Eq. 1

The divergence rate is adjustable via the parameters that define the LSI swirl number in Eq. 1. Here a is the vane angle, R = Rc/Ri, where Rc and Ri are the corresponding radii of the center-channel and the burner, and m= mc/msrepresents the flow-split between the unswirled and the swirled flow passages where mc and ms are respectively the mass fluxes through the center-channel and the swirl annulus. The presence of m in Eq 1 distinguishes it from the swirl number definition for high-swirl burners. When m = 0, i.e. no flow through the center, Eq 1 reduces to the swirl number definition for high-swirl burner. m in a LSB can be controlled by placing a perforated plate over the open center-channel to create aerodynamic drag. m is then the ratio of the drag coefficients (or pressure drops) for the perforated plate and the swirl vanes. A convenient means to vary the LSB swirl number, S, is by changing the blockage ratios or hole sizes of the perforated plate.

The engineering guideline for the LSB is specified in terms of a range of swirl number (0.4 < S < 0.55), and swirler recess (2 < Li/Ri < 3). Typically, two of the three parameters in Eq 1 are fixed by the swirler geometry. To meet the design criteria, perforated screens with blockages of 30 to 60% are used to render S within the design range. To a casual observer, the LSB has a striking resemblance to the high-swirl burner. The key and fundamental difference is the high-swirl burner has only one flow passage, and its solid centerbody promotes flow recirculation in its wake. When the centerbody is removed and replaced by an open channel, the unswirled flow in the center of the LSB prevents vortex breakdown to inhibit recirculation.

The most distinct characteristic of the LSB is a detached flame that is lifted above its exit. This feature is quite unnerving to engineers who consider lifted flames to be inherently unstable because they learned from combustion texts that flame detachment from the flame holder is a prelude to combustion instability and flame out. The LSB countered this notion by demonstrating that a change from recirculating to non-recirculating flow offer stable operation over a wider range of conditions including those that achieve near zero emissions levels of < 2 ppm NOx and CO. This exceptional performance is due to a coupling of the self-similar feature of the divergent flowfield and a linear correlation of the flame speed with turbulence intensity as expressed in a top-order analytical model. As flow velocity changes with load (i.e. power output), the structure of the divergent flow remains unchanged due to self-similarity but turbulence level increases and decreases accordingly. The flame "rides" the divergent flow and burns faster or slower in synchronous with the flow velocity because turbulence gives the critical feedback to the flame. The net effect is the flame remaining stationary regardless of the variation in the flow velocity. This is a unique auto-adjusting mechanism that allows the LSB to support very lean flames that emit very low concentrations of pollutants.

The self-similar flowfield of the LSB also accommodates the changes in flame properties associated with the use of different gaseous fuels. Due to the variation in chemical properties, some fuels burn faster than others. Hydrogen is among the fastest burning fuels and the premixed hydrogen flames respond more readily to turbulence than natural gas flames. This is expressed by the value of the turbulent flame speed correlation parameter for hydrogen being two times higher than the values for methane or natural gas flames. According to the top-order model, a change in the value of the turbulent flame correlation parameter implies a change in the flame position. Recent laboratory studies of LSB flames with methane and hydrogen flames at standard atmospheric and gas turbine conditions have provided the data to support the modeling prediction. These results shows that the flame/flowfield couping processes described by the model is valid at gas turbine conditions. Such knowledge is important for future development of the LSB technology for the hydrogen turbines in near-zero emissions coal power plants.

The LSB technology is available for license for gas turbines and certain other fields of use.

For information, go to



1991 Discovery of low-swirl flame stabilization principle

1994 Development of vnae-LSB

1996 Demonstrated LSB in water heaters

1999 Scaled LSB from 15kW to 600 kW

2000 Demonstrated LSB at gas turbine conditions and achieved < 2 ppm NOx

2003 First commercialize LSB

2005 LSB tested in gas turbine

2007 LSB tested with syngases and pure hydrogen at gas turbine conditions

Press Releases

  1. Low-swirl combustion wins a 2007 R&D 100 award link
  2. Low-swirl combustion fires up in hydeogen for power generation link
  3. From the Lab to the Market Place link

General Technical Description

  1. Overview of low-swirl combustion technology Download
  2. Analytical model for low-swirl combustion Download


  1. ICEPAG Presentation, Jan. 30, 2008 Download
  2. EPRI workshop on hydrgen combustion March. 2007 Download
  3. LBNL/DOE-FE/EPRI Webcast, Nov. 8, 2006 Link
  4. DOE-DER Peer Review Presentation, December 2005 Download
  5. Seminar presented at NETL November 2004 Download
  6. Paper Presentation at AFRC/JFRC Symposium, October 2004 Download
  7. High Significance Energy R&D Lecture given to Laboratory Energy R&D Working Group (LERDWG), Washington DC, April 16, 2004 Download
  8. Presentation to American Boiler Manufacturer Association, January 2004 Download

Relevant Links

  1. Lawrence Berkeley National Laboratory, Technology Transfer Department page on LSB
  2. From the Lab to the Market Place, 10 years later, Environmental Energy Technologies Division, LBNL.
  3. International Workshop on Premixed Turbulent Flames
  4. Laboratory Configurations for Premixed Turbulent Flames
  5. Maxon M-PAKT burner and SLS burners utilize low-swirl combustion technology
  6. LBNL CCSE 3D time dependent simulation of low-swirl burner
  7. Science beat article July 2003
  8. DOE EERE Industrial Combustion Program flier on low-swirl burner


  1. Taking away the bluff-body flame stabilize: When the horizontal tangential swirl-air jets at the base of the burner are turned on they produce the divergent flowfield to allow the flame to detached from the bluff-body and freely propagates. Play
  2. Igniting a low-swirl burner can be done by spark (play spark video) or by a pilot (play pilot video)

Publications on Low-Swirl Burner Technology

  1. Cheng, R.K., Turbulent Combustion Properties of Premixed Syngases. Comb. Sci. Tech., 2009. in preess.
  2. Cheng, R.K., D. Littlejohn, P. Strakey, and T. Sidwell, Laboratory Investigations of Low-Swirl Injectors with H2 and CH4 at Gas Turbine Conditions. Proc. Comb. Inst., 2009. 32. Download
  3. Littlejohn, D., R.K. Cheng, D.R. Noble, and T. Lieuwen. Laboratory Investivations of Low-Swirl Injector Operating with Syngases. in ASME Turbo Expo 2008. 2008. Germany: ASME GT2008-
  4. Cheng, R.K. and D. Littlejohn. Effects of Combustor Geometry on the Flowfields and Flame Properties of a Low-Swirl Injector. in Turbo Expo 2008. 2008. Berlin, Germany: ASME GT2008-
  5. Cheng, R.K., D. Littlejohn, W.A. Nazeer, and K.O. Smith, Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Application to Fuel-Flexible Turbines. Journal of Engineering for Gas Turbines and Power, 2008. 130(2): p. 21501-21511.
  6. Cheng, R.K. and D. Littlejohn, Laboratory Study of Premixed H2-Air & H2-N2-Air Flames in a Low-swirl Injector for Ultra-Low Emissions Gas Turbines. Journal of Engineering for Gas Turbines and Power, 2008. 130: p. 31503-31511.
  7. Littlejohn, D. and R.K. Cheng, Fuel Effects on a Low-swirl Injector for Lean Premixed Gas Turbines. Proc. Comb. Inst., 2007. 31(2): p. 3155-3162. Download
  8. Nazeer, W.A., K.O. Smith, P. Sheppard, R.K. Cheng, and D. Littlejohn. Full Scale Testing of a Low Swirl Fuel Injector Concept for Ultra-Low NOx Gas Turbine Combustion Systems. in ASME Turbo Expo 2006: Power for Land, Sean and Air. 2006. Barcelona, Spain: ASME.
  9. Cheng, R.K., Low Swirl Combustion, in DOE Gas Turbine Handbook. 2006. Download
  10. Nazeer, W., Smith, K.O. Shepherd, R.K. Cheng, & D. Littlejohn "Full Scale Testing of a Low Swirl Fuel Injector Concept for Ultra-Low NOx Gas Turbine Combustion Systems " Proceedings of GT 2006, Paper GT2006-90150.
  11. Johnson, M.R., D. Littlejohn, W.A. Nazeer, K.O. Smith, and R.K. Cheng, A Comparison of the Flowfields and Emissions of High-swirl Injectors and Low-swirl Injectors for Lean Premixed Gas Turbines. Proc. Comb. Inst, 2005. 30: p. 2867 - 2874. Download
  12. Littlejohn, D., M.J. Majeski, S. Tonse, C. Castaldini, and R.K. Cheng, Laboratory Investigation of an Untralow NOx Premixed Combustion Concept for Industrial Boilers. Proc. Comb. Inst., 2002. 29: p. 1115 - 1121. Download
  13. Cheng, R.K., D.T. Yegian, M.M. Miyasato, G.S. Samuelsen, R. Pellizzari, P. Loftus, and C. Benson, Scaling and Development of Low-Swirl Burners for Low-Emission Furnaces and Boilers. Proc. Comb. Inst., 2000. 28: p. 1305-1313 . Download

Combustion Research Publications Using Low-Swirl Burner

  1. Tachibana, S., Yamashita, J., Zimmer, L., Suzuki, K., and Hayashi, A. K., Dynamic Behavior of a Freely-propagating Turbulent Premixed Flame under Global Streth Rate Oscillations, Proc. Comb. Inst, 2009, 32.
  2. Bonaldo A., and Kelman, J. B., Experimental Characterisation of Swirl Stabilized Annular Stratified Flames, in press Comb. Flame. 2008.
  3. Petersson, P., J. Olofsson, C. Brackman, H. Seyfried, J. Zetterberg, M. Richter, M. Alden, M.A. Linne, R.K. Cheng, A. Nauert, D. Geyer, and A. Dreizler, Simultaneous PIV/PH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl-flame. Applied Optics, 2007. 46(19): p. 3928-3936.
  4. Abbilian, S., and Dunn-Rankin, D., Combustion oscillations of a low-swirl burner induced inside a Rijke tube in 5th U.S. Combusiton Meeting. 2007. San Diego: Western States Section of the Combusiton Institute. Paper H01
  5. Hwang, Y., A. Ratner, and B. Bethel. Chamber Pressure Perturbation Coupling wiht a Swirl-Stabilized Lean Premixed Flame at Elevated Pressures. in 5th U.S. Combusiton Meeting. 2007. San Diego: Western States Section of the Combusiton Institute. Paper E31
  6. Sequera, D. and A.K. Agrawal. Effects of Fuel Composition on Emissions from a Low-swirl Burner. in ASME Turbo Expo 2007. Montreal, Canada. ASME GT2007-28044
  7. Bell, J.B., R.K. Cheng, M.S. Day, and I.G. Shepherd, Numerical simulation of Lewis number effects on lean premixed turbulent flames. Proc. Comb. Inst, 2006. 31.
  8. de Goey, L.P.H., T. Plessing, R.T.E. Hermanns, and N. Peters, Analysis of the flame thickness of turbulent flamelets in the thin reaction zones regime. Proceedings of the Combustion Institute, 2005. 30(1): p. 859-866. Shepherd, I.G., R.K. Cheng, T. Plessing, C. Kortschik, and N. Peters, Premixed Flame Front Structure in Intense Turbulence. Proc. Comb. Institute, 2002(29): p. 1833 - 1840.
  9. Kortschik, C., T. Plessing, and N. Peters, Laser optical investigation of turbulent transport of temperature ahead of the preheat zone in a premixed flame. Combustion and Flame, 2004. 136(1-2): p. 43-50.
  10. Cheng, R.K., I.G. Shepherd, B. Bedat, and L. Talbot, Premixed turbulent flame structures in moderate and intense isotropic turbulence. Combustion Science and Technology, 2002. 174(1): p. 29-59.
  11. Shepherd, I.G. and R.K. Cheng, The burning rate of premixed flames in moderate and intense turbulence. Combustion and Flame, 2001. 127(3): p. 2066-2075.
  12. Plessing, T., C. Kortschik, M.S. Mansour, N. Peters, and R.K. Cheng, Measurement of the Turbulent Burning Velocity and the Structure of Premixed Flames on a Low Swirl Burner. Proc. Comb. Inst., 2000. 28: p. 359-366.
  13. Kostiuk, L.W., I.G. Shepherd, and K.N.C. Bray, Experimental study of premixed turbulent combustion in opposed streams. Part III--spatial structure of flames. Combustion and Flame, 1999. 118(1-2): p. 129-139.
  14. Cheng, R.K., Velocity and Scalar Characteristics of Premixed Turbulent Flames Stabilized By Weak Swirl. Combustion and Flame, 1995. 101(1-2): p. 1-14.
  15. Bedat, B. and R.K. Cheng, Experimental Study of Premixed Flames in Intense Isotropic Turbulence. Combustion and Flame, 1995. 100(3): p. 485-494.
  16. Shepherd, I.G. and L.W. Kostiuk, The burning rate of premixed turbulent flames in divergent flows*1. Combustion and Flame, 1994. 96(4): p. 371-380.
  17. Chan, C.K., K.S. Lau, W.K. Chin, and R.K. Cheng, Freely Propagating Open Premixed Turbulent Flames Stabilized by Swirl. Proc. Comb. Inst., 1992. 24: p. 511-518.

R.K. Cheng, updated , Oct. 14, 2008