Evaluation of Fuel Distribution in a Gas-Turbine Premixer: Influence of Swirl

Abstract

The typical permissible overspeed limit is about 5-7% of design mechanical speed. Thus fan-corrected speed needs to be reduced if its mechanical speed exceeds the limiting value. It will alter the engine steady-stateperformance,and the optimum system de nition as obtained earlier will also change.To investigate this issue, another optimization study was performed in which the fan mechanical speed was limited to 1.07 times its design speed. It results in W TO and W ENG;SLS that are of the same order of magnitude as at the conventional design point.As design Mach number increases at H D 9:0 km., optimum TR begins to decrease. This is because the increase in design Mach number causes the T 1;DP and, hence, TET DP also to increase, which reduces TR. There is a ight point, which is M D 1:55 in the present case, at which TR equals 1.0. If a higher Mach number, e.g., 1.60, is chosen as the design point, the optimum value of W TO begins to increase because the least value that TR can take is 1.0. Thus as design Mach number increases, TET max occurs at a higher Mach number (or T 1;DP ), and the engine operates at relativelyreduced TET at a large number of ight points, where Mach number (or T 1 ) is lower than that of the design point.Though not investigated,the trends as observed at design altitude of 9.0 km should also hold true at other design altitudes, because any designcombinationof H=M can be translatedinto an equivalent T 1;DP , which then dictates the quality of chosen ight point as the engine designpoint. As a typical example,cycle optimizationresults at H D 6:0 km/M D 1:3 in ISA at DT amb D 0 K (T 1;DP D 333 K) are not signi cantly different in comparison to that at H D 9:0 km/M D 1:5 in ISA at DT amb D 0 K, which also corresponds to T 1;DP D 333 K.To summarize, at a prescribed design altitude there is a lower limit on T 1;DP , below which (despite a lower value of optimum W TO ) the resulting cycle is not practically feasible. There also exists an upper limit on T 1;DP beyond which cycle optimization results in an increased optimum W TO . Between these limits of T 1;DP , cycle optimization, results do not differ signi cantly within themselves and in comparison with that at the conventional design point. Thus instead of searching for an optimum design point, it is suf cient to perform conceptual design system optimization at the conventional engine design point. ConclusionsThe value that T 1;DP takes at an H=M combination dictates its suitability as the engine design point. If minimization of W TO is the criteria for engine cycle optimization, then a ight condition with a low T 1;DP is more suited as the design point because it results in a lower value of optimum W TO . However, it also requires the engine to overspeed continuously for long durations, thereby causing an increased engine weight and reduction in the life of the rotating components. If this overspeeding is restricted to the current design limits of about 7% of design speed, savings in W TO diminish.As T 1;DP increases, there arises a ight point at ...