Internal Losses and Flow Behavior of a Turbofan Stage at Windmill

Abstract

The flow through a high-bypass ratio fan stage during engine-out conditions is investigated, with the objective of quantifying the internal losses when the rotor is at “windmill”. An analysis of altitude test data at various simulated flight Mach numbers shows that the fan rotational speed scales with the engine mass flow rate. Making use of the known values of the nozzle coefficients, we deduce the stagnation pressure loss of the fan stage, which rises significantly as the mass flow rate increases. In order to better understand this behavior, numerical simulations of the fan stage were carried out. The predicted losses agree well with the test data, and it is found that the bulk of the stagnation pressure loss occurs in the stator. A detailed examination of the flow field reveals that the relative flow leaves the rotor at very nearly the metal angle. Moreover, the rotational speed of the fan is such that the inboard sections of the fan blade add work to the flow, while the outboard sections extract work from it. The overall work is essentially zero so that the absolute swirl angle at the rotor exit is small, causing the stator to operate at a severely negative incidence. A gross separation ensues and the resulting blockage of the stator passage accelerates the flow to high Mach numbers. The highly separated flow in the vane, together with the mixing of the large wakes behind it are responsible for the high losses in the vane. Based on the simulation results for the flow behavior, a simple physical model to estimate the windmill speed of the rotor is developed and is found to be in good agreement with the test data. The utility of this model is that it enables the development of a procedure to predict the internal drag at engine-out conditions, which is discussed.

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

“…For the same tested engine in windmilling conditions, García Rosa et al (2015) state that stagnation pressure losses are encountered in the turbine-like mode zone along the fan stage, that is, for h/h max > 0.4. More specifically, the massive flow separations that develop on stator blades, around 60% span, widely contribute to losses, which is consistent with the conclusions of Prasad and Lord (2010). Finally all the profiles presented in Figure 3 show a fairly good self-similarity with respect to the range of tested inlet windmilling operating points.…”

“…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.…”

“…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. 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).…”

Numerical Simulation of a Mixed-flow Compressor under Windmill Conditions

“…The single windmill conditions of the S-engine mixed-flow compressor were simulated in Section 4.2. Figure 24 shows the distribution of relative Mach numbers around the rotor of an axial compressor from the previous study 2) and our S-engine mixed-flow compressor under single windmill conditions. As can be seen, flow separation on the pressure side is apparent both for the axial and mixed-flow compressor at a span of 80%, while there is no separation at a span of 40%.…”

“…Prasad et al 2) studied a turbofan stage under windmill conditions and revealed that there is almost no influence and that a large total pressure loss occurs at the pressure side of both the rotor and the stator. Gill et al 3) investigated a multistage compressor under windmill conditions both by an experiment and a numerical simulation.…”