Asymmetric whirl combustion was investigated as a promising new approach for ultra-low NOx gas turbine combustor design. Asymmetric whirl combustion was shown to have low NOx emissions, an abbreviated flame length, and unusual stability even at overall equivalence ratios below the lean flammability limit. Whirl flames were created by injecting air into a cylindrical combustor with a strong tangential but no axial velocity component. Whirl combustion is substantially different than conventional swirl combustion which injects combustion air with both a tangential and a strong axial velocity component, and produces flames with a markedly different appearance, structure, and method of flame stabilization.

An experimental combustor was constructed to study the fundamental characteristics of whirl combustion in a controlled environment. Symmetrically fueling a whirl combustor, by injecting fuel along the centerline of the combustor, produced a long luminous orange flame which burned with the characteristics of a diffusion flame. However, by simply moving the fuel injection point off the centerline axis, and asymmetrically fueling the whirl combustor, a radically different non-luminous blue flame was produced which was only one-third the length of the symmetric flame, and appeared to react with the characteristics of a premixed flame.

Experiments using asymmetric fuel injection to react methane and air in the atmospheric laboratory scale whirl combustor demonstrated low NO emissions levels coupled with modest CO emissions. NO emissions below 15 PPM were measured with associated CO below 25 PPM at 15% O₂. Similar results were obtained using natural gas. Experiments using propane and ethylene as fuels were observed to soot heavily. Asymmetric whirl combustion using methane was noted to exhibit unusual stability even at overall equivalence ratios below ϕ = 0.1 — significantly below the overall lean flammability limit of ϕ = 0.5 for laminar methane-air flames.

Numerical simulation of asymmetrically-fueled whirl flames predicted that the internal combustor flow field was composed of two zones:​ a high temperature (~2000 K), quiescent, central core which extends over 80% of the combustor width, a cooler, fast flowing, outer zone with temperatures on the order of the adiabatic flame temperature based on the overall mixture ratio, and a distributed reaction zone of approximately one combustor diameter between the two zones. Asymmetric whirl combustion injected fuel directly into the turbulent region between the two zones which promoted rapid mixing and chemical reaction. No reaction was apparent in the central core.

The hot central zone, at a temperature significantly higher than the adiabatic flame temperature based on the overall mixture ratio, was formed by a radial inward counterflow of fuel and air along the fuel injection back plate which reacted in proportions higher than the overall equivalence ratio. The resulting hot combustion products, e.g., CO₂ and H₂O at temperatures of ~2000 K, were then transported into the center region by the same counterflow and were trapped within the low pressure trough at the center of the combustor. The high temperature central zone contributed to the unique stability of this flame by acting as a source of additional enthalpy and radicals for the reaction zone, and holds the potential for lean combustion at equivalence ratios below that achievable by the current generation of lean premixed gas turbine combustors.