Systems Design and Experimental Evaluation of a High-Altitude Relight Test Facility

Abstract

Novel advances in gas turbine combustor technology, led by endeavors into fuel efficiency and demanding environmental regulations, have been fraught with performance and safety concerns. While the majority of low emissions gas turbine engine combustor technology has been necessary for power-generation applications, the push for ultra-low NOx combustion in aircraft jet engines has been ever present. Recent state-of-the-art combustor designs notably tackle historic emissions challenges by operating at fuel-lean conditions, which are characterized by an increase in the amount of air flow sent to the primary combustion zone. While beneficial in reducing NOx emissions, the fuel-lean mechanisms that characterize these combustor designs rely heavily upon high-energy and high-velocity air flows to sufficiently mix and atomize fuel droplets, ultimately leading to flame stability concerns during low-power operation. When operating at high-altitude conditions, these issues are further exacerbated by the presence of low ambient air pressures and temperatures, which can lead to engine flame-out situations and hamper engine relight attempts. To aid academic and commercial research ventures into improving the high-altitude lean blow-out (LBO) and relight performance of modern aero turbine combustor technologies, the High-Altitude Relight Test Facility (HARTF) was designed and constructed at the University of Cincinnati Combustion & Fire Research Laboratory (CFRL). This paper presents an overview of its design and an experimental evaluation of its abilities to facilitate optically-accessible combustion and spray testing for aero engine combustor hardware at simulated high-altitude conditions. Extensive testing of its vacuum and cryogenic air-chilling capabilities was performed with regard to end-user control — the creation and the maintenance of a realistic high-altitude simulation — providing a performance limit reference when utilizing the modularity of the facility to implement different aero turbine combustor hardware. Ignition testing was conducted at challenging high-altitude windmilling conditions with a linearly-arranged five fuel-air swirler array to replicate the implementation of a multi-cup gas turbine combustor sector and to evaluate suitable diagnostic tools for the facility. High-speed imaging, for example, was executed during the ignition process to observe flame kernel generation and propagation throughout the primary, or near-field, combustion zones. In the evaluation performed, the HARTF was found to successfully simulate the atmospheric environments of altitudes ranging from sea level to beyond 10,700 m for the employed combustor sector. Diagnostic methods found compatible with the facility include high-speed flame imaging, combustion emission analysis, laser light sheet spray visualization, phase Doppler particle analysis (PDPA), and high-speed particle image velocimetry (HSPIV). Herein discussed are correlations drawn — linking altitude simulation capability to the size of the implemented combustor hardware — and challenges found — vacuum sealing, low pressure fuel injection, fuel vapor autoignition, and frost formation.