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In any application subsonic transport or supersonic fighter the air intake is essentially a fluid flow duct whose task is to process the airflow in a way that ensures the engines functions properly to generate thrust Need of air intake in an aircraft : Need of air intake in an aircraft A widely used method to increase the thrust generated by the aircraft engine is to increase the air flow rate in the air intake by using auxiliary air intake systems. The air flow enters the intake and is required to reach the engine face with optimum levels of total pressure and flow uniformity hence need of an air intake system. Deceleration of airflow at high flight mach numbers or aerodynamic compression with help of air intake. Air intake design requirements : Air intake design requirements The air intake requires enormous effort properly to control airflow to the engine. The intake must be designed to provide the appropriate amount of airflow required by the engine. Furthermore this flow when leaving the intake section to enter the compressor should be uniform stable and of high quality. Good air intake design is therefore a prerequisite if installed engine performance is to come close to performance figures obtained at the static test bench. The engine intake must be a low drag, light weight construction ,that is carefully and exactly manufactures. These above conditions must be met not only during all phases of flight but also on the ground with the aircraft at rest and the engine demand maximum, thrust prior to take off INTAKE CONFIGURATIONS : INTAKE CONFIGURATIONS Broadly the intake configurations may be classified as Piston engine intakes Turbo propeller intakes Jet Engine Intakes: Subsonic Jet Engine Intakes: Supersonic Turboprop engine air intake : Turboprop engine air intake Subsonic air intake configurations : Subsonic air intake configurations Plenum Intakes Bifurcated intakes : Bifurcated intakes Slide 10: Subsonic podded nacelle inlet Pitot type subsonic intakes NACA submerged inlet in a euro fighter : NACA submerged inlet in a euro fighter The NACA submerged type intake is not very efficient for use with propulsion installations. However, they are frequently used as intakes of auxiliary systems (auxiliary power unit, heating and avionics bay cooling) as seen in Fig above Intake flow field : Intake flow field Determination of size of stream tube : Determination of size of stream tube Cross section A0, of the stream tube well ahead of the intake is determined by the engine mass flow rate, the size of the stream tube may simply be determined by applying the continuity considerations. Continuity requires mass flow rate m. at any cross section within the stream tube to be the same, which is hence a constant. Mass flow rate at cross-section A0, in particular ,exactly equals mass flow rate at the compressor face A=2=,which itself reflects engine mass flow .hence: m.0=m.2 Mass conservation may be expressed for the a particular flow path station (upstream infinity) and 2(compressor face) as follows Station 0(upstream infinity) m.0=p0v0A0 Station 2 (compressor face): m.2=p2v2A2 Therefore cross section of the stream tube at upstream infinity will result as simple expression: A0= (p2/p0)*(v2/v0)*A2 If air density is assumed not to change within the stream tube between the stations 0 and 2 ,then stream tube cross-section A0 depends only on aircraft flight speed v0 , because air stream velocity at compressor face is determined by the compressor ,with compressor cross section A2 a constant by design Deceleration of airflow at high flight mach numbers or aerodynamic compression with help of air intake : Deceleration of airflow at high flight mach numbers or aerodynamic compression with help of air intake We know that for an air breathing engine to function correctly compression of air is a prerequisite. Aerodynamic compression occurs in flow ducts whose cross-sectional area gradually increases in stream wise direction. A duct with the ability to retard the flow and convert energy into pressure energy is termed as diffuser. At sufficiently high mach numbers, for instance at cruising flight, airflow approaching the engine will be faster then would be tolerable for the compressor. Due to the diffuser action of air intake which is deceleration of the air flow and a buildup of pressure, airstream velocity will be adapted to the need of the compressor as seen in fig 3.2a. Additionally, due to the rise in pressure, a considerable benefit to the engine cycles results so that less mechanical energy is required for compression. Pressure recovery and nose suction formation : Pressure recovery and nose suction formation Air intake characteristics of Lockheed C-141 strlifter military transport : Air intake characteristics of Lockheed C-141 strlifter military transport The intake is particularly noteworthy because of its short duct, denotes as ‘zero-length inlet’ by Lockheed, which enabled a light weight constructions of high aerodynamic performance (fig 3.3). Due to its small radius, the intake lip is relatively sharp-edged which made necessary a secondary intake system that comes into effect at high airflow rates with aircraft static , or at low speed. The slotted inlet embodies 12 sets of outer doors pivoted at the cowl. The door opens against a spring force if a [pressure drop exists between the low static pressure on the engine side of doors relative to that of the external side of the doors. Jet engine supersonic intakes : Jet engine supersonic intakes They are of the following types: 1. Pitot Intake 2. External compression Intake 3. Mixed (or external/internal) compression Intake A Pitot Intake has a number of attractive features, notably low drag and a stable flow characteristic with good flow distribution. Its disadvantage lies in the level of pressure recovery achieved. As shown in Fig 1.6, this type of intake has been used in aircrafts like the Mig 21 Slide 19: Various types of supersonic inlets Flow conditions over wedge and cone : Flow conditions over wedge and cone In the design of supersonic air intakes flow conditions over wedge and cone are of the greatest importance as these are simple geometric bodies and relatively easy to manufacture. Comparison of supersonic flow over cone and wedge : Comparison of supersonic flow over cone and wedge The major advantage of a (supersonic) conical flow is a smaller total pressure loss (when compared to a wedge of the same half-angle), together with the fact that a conical shock sustains lower mach numbers until it becomes detached to form a high loss bow shock. A major disadvantage of conical flows is that it is less tolerant to asymmetric flow conditions which cause distortion to the intake flow. As combat aircraft are frequently required to maneuver at higher angles of attack, the flow inevitably gets asymmetric- hence a performance for the (horizontally arranged) wedge in all modern combat aircraft, despite its reduced efficiency. Intake configuration and operation : Intake configuration and operation Present-day turbine aero engines require subsonic flow at the entry to the compressor, even if the aircraft is flying at supersonic speed. The task of air intake is therefore to decelerate the supersonic external flow to a subsonic speed acceptable to the compressor. As intake discharge mach number are required to be in range of mach 0.4 to 0.7 great care must be exercised when decelerating the flow in order to keep total pressure losses to a minimum . Normal shock diffuser For aircraft operating at a maximum speed equivalent to mach 1.5 a normal shock diffuser is generally sufficient to decelerate the supersonic airflow efficiently to the speed needed by the compressor. Slide 24: The action of diffusing i.e. the deceleration of flow and build up of pressure is accomplished in two steps: The supersonic flow is (abruptly) decelerated, through the normal shock , to subsonic velocity with an accompanying abrupt increase in static pressure; In the diverging (subsonic) duct, where the flow is sill faster then would be acceptable to the compressor, deceleration of the flow continues with pressure increasing further. Oblique shock diffuser intake characteristics : Oblique shock diffuser intake characteristics Examples of use off oblique shock diffusers : Examples of use off oblique shock diffusers Mirage ||| fighter with side mounted oblique-shock diffuser Two dimensional oblique shock diffuser (Northrop F-5 with vertical ramp) Slide 28: Axisymmetric oblique-shock diffuser (Lockheed SR-71) Supersonic air intake case studies : Supersonic air intake case studies F-16 intake characteristics Slide 30: The F-16 intake is of fixed-geometry type, without moveable parts a decision made fairly in design process to save costs. What is remarkable about the inlet is its positioning fairly well under the fuselage a solution resulting from the requirements of aircraft The F-16 was designed to have exceptional maneuverability and this required to operate at high angle of attack. In these considerations the long fuselage fore body performs a shielding function which serves to align the (inclined) axis of intake (fig 3-17a). The intake itself features a short duct which not only contributes to the light weight design of the aircraft, but also minimizes flow distortion ahead of the compressor. Another problem facing the combat aircraft is the hot gas from gun muzzles that may be ingested and cause engine flame out. By placing the gun muzzle above the leading-edge extension or strake, the high temperature gas from the gun will be kept effectively away from the intake before being carried away by the external flow as shown in fig 3-17 below. The intake cowl features a moderately blunt lower lip that transitions into a sharp leading-edge extension or splitter plate on the upper side (close to the fuselage). The splitter plate extends 25cmsahead of the lower cowl lip to isolate the inlet normal shock from the fuselage boundary layer (fig-17b). A short length of splitter plate keeps boundary layer buildup small, so eliminating the need of boundary layer bleed on the splitter. F-14 intake characteristics : F-14 intake characteristics Slide 34: Thank you You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.