Calibrated Low-Order Transient Thermal and Flow Models for Robust Test Facility Design

Abstract

This paper describes the high temperature upgrade of the Engine Component Aero Thermal (ECAT) facility, an established engine-parts facility at the University of Oxford. The facility is used for high technology readiness level research and development, new technology demonstration and for component validation. The current facility has a modular working section which houses a full annulus of engine nozzle guide vanes (typically HP NGVs from a large civil jet-engine, but reconfigurable for smaller core sizes) which can be run at engine conditions of Reynolds number, Mach number and coolant-to-mainstream pressure ratio. The facility has a combustor simulator, which is used for combustor-turbine interaction studies with either rich or lean-burn temperature, swirl and turbulence profiles. The ECAT facility is capable of highly accurate measurements of capacity, aerodynamic loss, and metal effectiveness. The facility can operate in either semi-transient blow-down mode (typically used for capacity characteristics) or steady-state regulated mode (typically used for aerodynamic traversing). For metal effectiveness measurements, a coolant-to-mainstream temperature ratio range of 1.00-1.28 can be achieved by heating the mainstream with two 1 MW heaters. Although ECAT can be run at very close to engine conditions, the limitation on temperature ratio capability leads to imperfectly matched specific heat capacity flux ratio and compressibility effects (ratio of recovery temperature ratio to coolant-to-mainstream temperature ratio). The development described in this paper (foreseen as a requirement when ECAT was developed) addresses this scaling mismatch. This paper is about the design and optimisation of increased temperature capability for the ECAT facility, a system which will increase the mainstream inlet temperature to 600 K (327 °C), allowing coolant-to-mainstream temperature ratio to be matched to engine conditions. This is desirable as it will allow direct validation of temperature ratio scaling methods in addition to providing closer engine similarity. This is a critical development for engine designers as it allows higher technology readiness level testing to be achieved than has previously been possible, providing a test bed in which all important non-dimensional parameters for aero-thermal behaviour can be exactly matched to engine conditions. The aim is reduced engine development times, by providing earlier and higher fidelity testing than has previously been possible. As the technology matures it is possible to foresee that engine development testing (as opposed to Pass-Off testing) may be avoided with a test vehicle of this type. To accurately predict the operating conditions of the facility, a low order transient thermal model was developed in which the air delivery system and working section are modelled as a series of distributed thermal masses. Nusselt number correlations were used to calculate convective heat transfer to and from the fluid in the pipes and working section. The correlation was tuned and validated with accurate and extensive experimental results taken from test campaigns conducted in the existing facility. This modelling exercise informed a number of high-level facility design decisions to be taken, and will provide an accurate estimate of the running conditions of the facility. We present detailed results from the low-order modelling, and discuss the key design decisions. We also present a discussion of challenges in the mechanical design of the working section, which is complicated by transient thermal stress induced in the working section components during start-up of the facility. This analysis is benchmarked with directly measured boundary conditions from the existing working section, scaled appropriately to upgraded facility conditions. The staged development of the ECAT test-bed allows robust component analysis during the design phase. The high-temperature core for the ECAT test-bed has unusually high TRL capacity for a research organisation, and it is expected that the development and underlying methodology will be of interest to both engine designers and the research community. The facility will contribute to accelerated development time of novel engine technology in addition to further enabling fundamental research to be carried out engine representative environments.