Study of air intake configuration

Presented by Chirag.D.soni

Airplanes may kill you but they ain't likely to hurt you.— 'Satchel' (Leroy Robert) Paige, baseball player, circa 1959.

Overview

What is air intake :introduction.

Need of air intake system.

Air intake design requirements.

Intake configurations

I.Subsonic air intake operation and characteristics

II.Supersonic air intake operation and characteristics

What is air intake?

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• 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 • 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

• 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

Subsonic air intake configurations

• Plenum Intakes

Bifurcated intakes

• Subsonic podded nacelle inlet

• Pitot type subsonic intakes

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

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

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

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

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

Various types of supersonic inlets

Flow conditions over wedge and coneIn 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 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

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.

The action of diffusing i.e. the deceleration of flow and build up of pressure is accomplished in two steps:

1. The supersonic flow is (abruptly) decelerated, through the normal shock , to subsonic velocity with an accompanying abrupt increase in static pressure;

2. 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

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)

Axisymmetric oblique-shock diffuser (Lockheed SR-71)

Supersonic air intake case studies F-16 intake characteristics

1.The F-16 intake is of fixed-geometry type, without moveable parts a decision made fairly in design process to save costs. 2.What is remarkable about the inlet is its positioning fairly well under the fuselage a solution resulting from the requirements of aircraft 3.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). 4.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. 5.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. 6.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). 7.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

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