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

Abstract

A detailed study of the air flow through the fan stage of a high-bypass, geared turbofan in windmilling conditions is proposed, to address the key performance issues of this severe case of off-design operation. Experiments are conducted in the turbofan test rig of ISAE, specifically suited to reproduce windmilling operation in an ambient ground setup. The engine is equipped with conventional measurements and radial profiles of flow quantities are measured using directional five-hole probes to characterize the flow across the fan stage and derive windmilling performance parameters. These results bring experimental evidence of the findings of the literature that both the fan rotor and stator operate under severe off-design angle-of-attack, leading to flow separation and stagnation pressure loss. The fan rotor operates in a mixed fashion: spanwise, the inner sections of the rotor blades add work to the flow while the outer sections extract work and generate a pressure loss. The overall work is negative, revealing the resistive loads on the fan, caused by the bearing friction and work exchange in the different components of the fan shaft. The parametric study shows that the fan rotational speed is proportional to the mass flow rate, but the fan rotor inlet and outlet relative flow angles, as well as the fan load profile, remain constant, for different values of mass flow rate. Estimations of engine bypass ratio have been done, yielding values higher than six times the design value. The comprehensive database that was built will allow the validation of 3D Reynolds-averaged Navier–Stokes (RANS) simulations to provide a better understanding of the internal losses in windmilling conditions.

Experimental analysis of the unsteady, turbilent flow through the fan stage of a high-bypass turbofan in windmilling conditions

“…Available detailed experimental and numerical studies, conducted on a compressor cascade (Zachos et al, 2011) and on a contemporary high-bypass ratio turbofan in an altitude test cell (Prasad and Lord, 2010) indicate that, in windmilling operation, the fan stage operates under severe off-design (negative) angle-of-attack conditions, leading to the development of flow separation on both rotor and stator. These observations were confirmed by a previous experimental study on a small, geared turbofan, at ground level (García Rosa et al, 2015). These studies agree that the relative flow leaves the fan at nearly the metal angle, and that the bulk of the pressure loss occurs in the stator.…”

“…This effect is more pronounced close to the hub due to the meridional channel geometry (Figure 1). In windmilling, the inner sections of the fan operate in compressor mode, while the outer sections operate in turbine mode (Prasad and Lord, 2010;García Rosa et al, 2015). However, since the inlet flow is purely axial, the absolute velocity increases along the entire span.…”

Validation and flow structure analysis in a turbofan stage at windmill

“…Meridional view of the engine, with nomenclature for the measurement stations. 0.16 at the engine inlet [16]. In the present study the focus is on two windmilling operating points, corresponding to inlet Mach numbers of M 0 = 0.16 and M 0 = 0.08, respectively.…”

“…To reproduce windmilling conditions, a 75 kW centrifugal fan is used to blow air through the engine, up to 6 kg/s mass flow rate and a local Mach number of 0.16 at the engine inlet. 16 In the present study the focus is on two windmilling operating points, corresponding to inlet Mach numbers of M2A=0.16 and M2A=0.08, respectively.…”

“…In this context, the problematic of the present study is to gain more insight into the physics of the flow in a fan stage at windmill. To this end, the proposed approach consists in first validating flow predictions against detailed measurements in a complete modern highbypass-ratio geared turbofan (García Rosa et al [16]). Then, a simplified computational model of a section of the stage is simulated with an unsteady approach, in order to analyze rotor/stator interactions and unsteady separated-flow patterns.…”