Turbomach, A Division of Solar Turbines International, An Operating Group of International Harvester, manufactures the small 10kW Gemini gas turbine. The very small size of the combustor precluded the use of conventional gas turbine combustor design techniques. A novel solution was arrived at based primarily on an amalgam of design practices used in furnaces and reciprocating engines. Use was made of the centrifugal force effects of swirl flow (Rayleigh Instability Criteria) both as a method of fuel evaporation and as a method of control of the rate of flame propagation. Substantial advantage can be obtained by this design technique provided that a fine and accurately located fuel spray is achieved. Various applications of this method of combustor design are outlined with emphasis on aerodynamics and fuel atomization and volatility rather than reaction rate criteria as the dominant influences.

Experiments were performed in laboratory-and full-scale combustors to test the feasibility of meeting proposed EPA emission standards. It was found that by uniformly mixing gaseous fuel and primary zone air prior to combustion and burning fuel leanly (equivalence ratio <1.0), it was possible to meet the proposed emission standards in an industrial gas turbine. The characteristic narrow range of flame stability obtained with lean premix combustion necessitated the use of fuel staging or variable geometry to handle the operational range of the engine. Fuel staging was selected for its relative simplicity. Consequently, EPA proposed emission standards were met only over a narrow range covering the engine operation at and near the design point. Experiments on small scale models of various sizes operated with gaseous and liquid fuels showed that, contrary to expectation, NOx production from a lean premix combustion system is independent of the system pressure in the pressure range investigated (1 atm to 16 atm). The desirability of high combustor inlet temperature and pressure for premixing was indicated. Despite the complexities of premixing fuel and air, such a combustion system, in addition to meeting the proposed emission standards, offers advantages such as easing of combustor wall cooling problems, improved combustor exit temperature distribution, and freedom from exhaust and primary zone smoke.

An axisymmetric annular vortex combustor was tested atmospherically to determine the feasibility of meeting the 1977 proposed EPA emission standards for stationary gas turbines. Flame stabilization was achieved solely by swirl induced recirculation without the jet assisted recirculation of conventional practice. Buoyancy forces are used to enhance or diminish turbulent mixing. It was found that by uniformly mixing gaseous fuel and air prior to combustion and burning fuel lean it was possible to meet the proposed emission standards. The resultant narrow range of flame stability necessitated fuel staging for light-off and off-design operation. Fuel staging was achieved by stratified charge combustion (diffusion flame). Changes in combustor geometry were made and extensive traversing of the flame zone was carried out to measure concentrations of chemical species.

The Radial Engine Division of Solar Turbines International, an Operating Group of International Harvester, under contract to the U.S. Army Mobility Equipment Research & Development Command, developed and qualified a 10 kW gas turbine generator set. The very small size of the gas turbine created problems and, in the combustor, novel solutions were necessary. Differing types of fuel injectors, combustion chambers, and flame stabilizing methods were investigated. The arrangement chosen had a rotating cup fuel injector, in a can combustor, with conventional swirl flame stabilization but was devoid of the usual jet stirred recirculation. The use of centrifugal force to control combustion conferred substantial benefit (Rayleigh Instability Criteria). Three types of combustion processes were identified: stratified and unstratified charge (diffusion flames) and pre-mix. Emphasis is placed on five nondimensional groups (Richardson, Bagnold, Damko¨hler, Mach, and Reynolds numbers) for the better control of these combustion processes.

In recent years, the role of oxides of nitrogen (NOX) within the overall production mechanism of photochemical smog has been recognized, if not completely understood, and in consequence, these oxides are currently considered as serious, immediate, environmental pollutants.