Intake and Nozzle Design
Engine mass-flow demand varies significantly, as shown in Figure 10.23. To size the intake area, the reference cross-section of the incoming airmass-stream tube is taken at the maximum cruise condition, as shown in Figure 10.23a, when it has a cross-sectional area almost equal to that of the highlighted area (i. e., ATO = A1; see Section 10.8.1 for nomenclature). The ratio of mass flow rates (MFR) relative to the reference condition (i. e., airmass flow at maximum cruise) is a measure of the spread the intake would encounter. At the maximum cruise condition, MFR = 1 as a result of ATO = A1. At the typical cruise condition, the intake-airmass demand is lower (MFR < 1, as shown in Figure 10.23b). At the maximum takeoff rating (MFR > 1), the intake airmass-flow demand is high; the streamlined patterns are shown in Figure 10.23c. Variations in the intake airmass-flow demand are significant.
If the takeoff airmass flow demand is high enough, then a blow-in door – which closes automatically when demand drops off – can be provided. Figure 10.23d shows a typical flow pattern at incidence at high demand, when an automatic blow-in door may be necessary. At idle, the engine continues running with little thrust generation (MFR ^ 1). At an inoperative condition, the rotor continues windmilling to minimize drag. If a rotor seizes due to mechanical failure, there is a considerable drag increase.
Currently, the engine (i. e., fan) face should not exceed Mach 0.5 to avoid degradation due to compressibility effects. At a fan-face Mach number above 0.5, the relative velocity at the fan-tip region approaches sonic speed due to the high blade – rotational speed.
The purpose of the intake is to provide engine airmass-flow demand as smoothly as possible – there should be no flow distortion at the compressor face due to separation and/or flow asymmetry. The nacelle intake-lip cross-section is designed using logic similar to the aerofoil LE cross-section – that is, the flow should not separate within the flight envelope.