A low emission combustor for a 100kW ceramic gas turbine, which is intended to meet Japanese emission standards for gasoline passenger cars, has been designed and subjected to initial performance tests. A prevaporization-premixing combustion system was chosen as the most suitable system for the combustor. The detailed combustor design, including the use of ceramic components and fuel injectors, was pursued taking into account the allowable engine dimensions for vehicle installation. In the initial performance tests conducted at a combustor inlet temperature of 773K, a low NOx level was obtained that satisfied the steady state target at this temperature level.

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The lean ignition limit, the lean blowout limit and the exhaust emission characteristics of spray combustion have been investigated experimentally using a pre-chamber type vortex combustor developed for a 300KW large-bus gas turbine engine. It has been verified that these depend on the spray characteristics of the fuel injector and the air flow pattern or the distribution of air in the chamber. Ignition succeeds through three processes. The first step is the formation of a flame kernel near the sparking ignitor, the second step is the propagation of the flame kernel into a flame holding region, and the last step is the formation of a rotating flame in that region. The lean blowout limit of the rotating flame depends on the air flow pattern in the pre-chamber when the air temperature in the combustor inlet is under 470K, while a constant fuel-air ratio of less than 0.001 is maintained at 470K and above. With no or a little secondary air, the NOx emission index does not increase in proportion to the fuel-air ratio, because both the gas temperature and residence time decrease due to the radiative heat loss caused by soot formation and reduction of a recirculation region in the main-chamber. These phenomena were evaluated with 3 dimensional numerical simulations taking account of spray combustion, soot formation, the extended-Zeldovich thermal NO formation and radiative heat loss.