Relight Envelope of a Military Gas Turbine Engine: An Experimental Study

Abstract

A twin spool low bypass turbofan engine under development and its combustor in full-scale were tested independently at altitude conditions to establish the relight envelope of the engine. Demonstration of relight capability and defining its boundary are mandatory for military gas turbine engines and for single engine application in particular. The engine was first subjected to windmill to establish its windmilling characteristics. The full engine was then tested for light-off in an altitude test facility simulating windmilling conditions from 4 to 12 km altitude with flight Mach numbers from 0.2 to 1.0. The relight boundary is defined based on the successful light-off points achieved from engine tests. Similar tests were carried out on the full-scale combustion chamber in a stand-alone mode simulating altitude conditions at engine flame-out. The combustor test has defined the light-off and lean blow out limits of the at each point on the relight boundary. The information of fuel-air ratio at light-off and blow-out is very useful in setting the engine fuel schedule for altitude operation and relight. In this paper an attempt is made to highlight various tests carried out on engine and its combustor to define the relight boundary of the engine. The paper also emphasizes the experience of combustor development and associated problems in meeting the relight challenges of military engines. These problems include the necessity of higher fuel-air ratio at high altitudes, the role of additional localized fuel injection through start-up atomizers, and effect of single igniter on relight characteristics. The relight envelope demonstrated by the engine is very satisfactory to meet the first flight requirement where the flight mission generally concentrate in the zone of 0.6 to 0.8 Mach and altitude does not exceed 10 to 12 km. Combustor and atomizer modification is needed to improve relight performance and to shift the boundary to further left.

Validation and flow structure analysis in a turbofan stage at windmill

“…The imposed ram pressure ratios of the windmilling operating points lead to values of flight Mach number comprised between 0.14 and 0.29, which correspond to 41% and 84% of the nominal flight Mach number of the engine. These relative values match well with what is reported in the literature on larger engines, 5,12,17 where the relative values examined go down to 46%.…”

Experimental Analysis of the Global Performance and the Flow Through a High-Bypass Turbofan in Windmilling Conditions

“…The imposed ram pressure ratios lead to values of flight Mach number comprised between 41% and 82% of the nominal flight Mach number of the engine (Table 1), which matches well with what is explored in the literature, on larger engines (see, for example, Refs. [3], [8], and [12]). However, windmilling on the engine under consideration leads to significantly lower values of flow parameter and rotational speeds.…”

“…These parameter values are heavily reliant upon engine architecture and either model predictions or test data are needed at the earliest opportunity. Studies on turbofan windmilling and, more generally, on sub-idle operation, focus on the whole engine by resorting to either Mach number scaling [1], system-level performance modeling [2][3][4][5][6], or experiments [7,8]. To predict the off-design performance, engine models rely on component data (such as compressor maps) which is usually available near the design point, and rarely in severe off-design conditions such as sub-idle or windmilling [2], making it necessary to extrapolate from above-idle data [9].…”

Periodic flow structures in a turbofan fan stage in windmilling

“…Although engine cycle modelling [1][2][3] and performance experiments 4,5 can directly provide information on global engine performance, these methods bring little detail on the local loss mechanisms. Yet a detailed understanding of the flow features inside the critical components is essential for a reliable prediction of the overall performance, especially in severe off-design operating conditions.…”