The design of combustor systems for small turbines is much more than simply a scale down of designs that have been shown to be operative in large turbine engines. Regardless of combustor size, there is a minimum residence time for fuel and oxidant within the combustor necessary to effect sufficiently complete combustion to generate the gases of combustion required to drive a turbine wheel. Given the dynamics of gas flow in and out of a combustor to a turbine wheel, it will be readily appreciated that as the size of the combustor is decreased, if conventional techniques were employed, combustion would only be starting, if occurring at all, by the time the fuel oxidant mixture was exiting the combustor outlet.
Moreover, in small combustors, which necessarily provided with small fuel injectors and consequently having small fuel flow at each injector, it is difficult to provide the needed fine fuel atomization utilizing conventional techniques. This is due to the fact that small scale effects increase viscus losses resulting in a deterioration in fuel atomization at the injector. In addition, the small fuel metering orifices associated with such small fuel injectors tend to promote premature fuel spray deterioration due to orifice fouling which in turn can cause early engine failure due to gas temperature maldistributions. Conventional injector design techniques are ordinarily complex and costly. Consequently, when employed for reduced scale design for use in small combustors, it results in a very high additional cost.
Recognizing these difficulties, in recent years there has been a trend towards combustor systems employing so-called "sidewinding." Combustors used in sidewinding systems are annular combustors. Unlike conventional combustors, the path of travel for fuel and oxidant in the flame zone as well as the products of combustion is primarily in the circumferential direction around the annular combustor. The axial flow component of fuel, oxidant and products of combustion is minimized. This arrangement maximizes the time available for combustion within a given small volume and also permits a significant reduction in the number of fuel injectors without a resultant undesirably high turbine inlet temperature maldistribution as would be obtained using more conventional design techniques if the number of injectors is reduced. Maximizing the time available for mixing and combustion while minimizing the number of fuel injectors is most advantageous from cost and efficiency standpoints, particularly when accomplished in small turbines.
In recent proposals for annular combustors operating on the sidewinding technique, it is typical to find a series of oxidant blast tubes circumferentially spaced about the combustor and normally located in a radially outer wall thereof. One end of each tube is open to the interior of the combustor while the opposite end is opened to the space between the radially outer wall of the combustor and the outer combustor case. As is known, this space is typically charged with compressed oxidant from the compressor associated with a gas turbine engine or from a storage place such as a pressure bottle containing the oxidant. These tubes are directed tangentially into the annular combustion space of the combustor. For fuel injection purposes, fuel injection tubes have typically been mounted within the oxidant blast tubes. As a consequence, fuel atomization of fuel injected from the tubes may be achieved as the fuel is injected toward the combustion space in an associated oxidant blast tube as the oxidant passing through the oxidant blast tube provides oxidant blast atomization. While this works well for its intended purpose, as smaller and smaller combustors are designed, because the diameter of the oxidant blast tubes becomes commensurately reduced, it becomes increasingly difficult to locate the fuel injection tubes inside the oxidant blast tubes. Furthermore, space available for so called "start" injectors, which are typically pressure atomization injectors used only when the turbine is in a starting mode, and which are not always effective at high altitudes, is commensurately reduced.
It can be shown that the combustion process in a turbine engine requires a total time which is the sum of the times required for a) fuel evaporation, b) fuel-oxidant mixing, and c) fuel-oxidant reaction. The mixing of fuel and oxidant is speeded up in direct proportion to the reduction in scale and therefore does not pose a new problem in a small scale combustor. The fuel-oxidant reaction time can, for the most part, be assumed to be infinitely fast as compared to the time required for fuel-oxidant mixing and fuel evaporation and thus is negligible as far as small scale is concerned.
However, for a fixed fuel droplet size, the time for fuel evaporation is also fixed, independent of combustor size. As a consequence, as the combustor scale is reduced, an increasing proportion of the combustor volume must be devoted to fuel evaporation. Hence, the portion of total combustor volume devoted to fuel evaporation increases as scale is reduced.
In addition, the flame performance as defined by combustion efficiency and flame stability has been found by experience to suffer as a consequence. However, the problem of fuel evaporation may be avoided as scale is reduced by reducing the fuel droplet size. As is well known, the time required for fuel evaporation is more or less inversely proportional to the square of fuel droplet size. Using conventional fuel injection design methods, the fuel droplet size tends to increase with reduced scale, primarily due to viscus loss effects. Consequently, novel design techniques are needed to improve the fuel atomization while avoiding the cost and reliability problems usually associated with small size fuel injectors.
The present invention is directed to overcoming one or more of the above problems.