Investigators: Professor Tom Povey

Students: Francesco Ornano

Sponsors: Rolls-Royce Turbines

High-pressure (HP) nozzle guide vane (NGV) endwalls are often characterized by highly three-dimensional (3D) flows. The flow structure depends on the incoming boundary layer state (inlet total pressure profile) and the (static) pressure gradients within the vane passage. In many engine applications, this can lead to strong secondary flows. The prediction and design of optimized endwall film cooling systems is therefore challenging and is a topic of current research interest. Detailed experimental investigations of the film effectiveness distribution on engine-realistic endwalls geometry can be performed at the Large-Scale Platform Cooling Facility by means of infrared (IR) thermography. Adiabatic film effectiveness distributions can be measured using IR cameras, and tests can be performed across a wide range of coolant-to-mainstream momentum-flux and mass flow ratios (MFRs).

The facility is a large-scale, low-speed linear cascade with seven vanes. The facility was designed during the earlier work of Thomas [1]. The aerodynamic profile is based on a modern engine geometry. The vane and endwall geometries include engine realistic features such as the combustor-turbine gap, hub and casing annulus line profiles, and vane fillets. A drawing of the test facility is shown in Figure 1. The facility includes a rich-burn combustor simulator with scaled dilution holes to set high turbulence intensity with representative length scales upstream of the NGV cascade. Removable cassettes at the hub and case allow back-to- back tests of different endwall film cooling designs. An example of the test section is given in Figure 2. The removable cassettes, NGV endwalls, and combustor simulator are manufactured from Rohacell, a structural material with very low thermal conductivity, to allow film effectiveness measurements to be performed using infrared thermography with minimal conduction error. The mainstream inlet flow is heated with an upstream heater mesh. Hub and casing endwall coolant mass flow rates are measured using critical-flow Venturi nozzles. The mainstream and coolant flow temperatures are measured with thermocouples and surface temperature measurements are obtained with foil thermocouples on the endwall surfaces.

Complex interactions between coolant film and vane secondary flows were presented and discussed in [2]. A particular feature of interest is the suppression of secondary flows (and associated improved adiabatic film effectiveness) beyond a critical momentum flux ratio. Jet liftoff effects are also observed and discussed in the context of sensitivity to local momentum flux ratio (an example of the results is given in Figure 3). Full coverage experimental results are also compared to 3D, steady-state computational fluid dynamics (CFD) simulations. The work in [2] provides insights into the effects of momentum flux ratio in establishing similarity between cascade conditions and engine conditions and gives design guidelines for engine designers in relation to minimum endwall cooling momentum flux requirements to suppress endwall secondary flows.

[1] Thomas, M., 2014, “Optimization of Film-Cooling in Axial Turbines,” PhD thesis, University of Oxford, Oxford, UK.

[2] Ornano, F., Povey, T., 2017, “Experimental and Computational Study of the Effect of Momentum-Flux Ratio on High-Pressure Nozzle Guide Vane Endwall Cooling Systems,” J. Turbomach, 139(12):121002-121002-14.

Figure 1: Computer-aided design model and components of the super-scale platform cooling facility: (a) front view and (b) rear view. A—vane and platforms, B—rear cameras, C—front cameras, D—combustor simulator, E—tailboards, F—hub cassette, and G—casing cassette.

Figure 2: (a) NGV cascade viewed from upstream, (b) detail of the casing endwall, (c) casing film cooling pattern, and (d) vane hub and casing cooling hole inclinations angles
Figure 3: Endwall adiabatic film effectiveness contours (upstream view) for 0.40 < I <