Inlets
Inlets
The inlet interchanges the organized kinetic and random thermal energies of the gas in
an essentially adiabatic process. The perfect (no-loss) inlet would thus correspond to an
isentropic process. The primary purpose of the inlet is to bring the air required by the
engine from free stream conditions to the conditions required at the entrance of the fan
or compressor with minimum total pressure loss. The fan or compressor works best
with a uniform flow of air at a Mach number of about 0.5
The Requirements of the inlets:
high total pressure ratio πd,
controllable flow matching of requirements,
good uniformity of flow,
Low installation drag,
Good starting and stability,
Low signatures (acoustic, radar, etc.),
Minimum weight and cost while meeting life and reliability goals.
A list of the major design variables for the inlet and nacelle includes the following:
Inlet total pressure ratio and drag at cruise
Engine location on wing or fuselage (avoidance of foreign-object damage,
inlet flow up wash and downwash, exhaust gas re ingestion, ground clearance)
Aircraft attitude envelope (angle of attack, yaw angle, cross-wind takeoff)
Inlet total pressure ratio and distortion levels required for engine operation
Engine-out wind milling airflow and drag (nacelle and engine)
Integration of diffuser and fan flow path contour
Integration of external nacelle contour with thrust reverser and accessories
Flow field interaction between nacelle, pylon, and wing
Noise suppression
requirements. Design considerations:
The airflow entering the compressor or fan must have low Mach number, in the range
0.4 to 0.7, Part of this deceleration occurs upstream of the inlet entrance plane.
The inlet must be designed to prevent boundary layer separation, even when the axis
of the intake is not perfectly aligned with the streamline direction far upstream of the
inlet.
It is important that the stagnation pressure loss in the inlet be small.
It is even more important that the flow velocity and direction leaving the inlet be
uniform, since distortions in the velocity profile at the compressor inlet can severely
upset the compressor aerodynamics and may lead to failure of the blades due to
vibrations.
Design of inlets that must operate efficiently in both supersonic and subsonic flight
poses special problems;
,Inlets
Subsonic Inlets:
Internal flow and Stall in subsonic inlet sand Boundary Layer Separation
Depending on the flight speed and the mass flow demanded by the engine, the inlet may have
to operate with a wide range of incident stream conditions. The Figure shows the streamline
patterns for two typical subsonic conditions and the corresponding thermodynamic path of an
“average” fluid particle.
During level cruise the streamline pattern may include some deceleration of the entering
fluid External to the inlet plane and hence low mass flow rate [Fig. a]. During low-speed
high-thrust operation (e.g., during takeoff and climb), the same engine will demand more
mass flow and the streamline pattern may resemble Fig. b, which illustrates external
acceleration of the stream near the inlet. .
For given air velocities external acceleration raises the inlet velocity and lowers the inlet
pressure, thereby increasing the internal pressure rise across the diffuser. If this pressure
increase is too large, the diffuser may stall because of boundary layer separation; stalling
usually reduces the stagnation pressure of the stream as a whole.
Conversely, external deceleration requires less internal pressure rise and hence a less severe
loading of the boundary layer.
Therefore, the inlet area is often chosen so as to minimize external acceleration during
takeoff, with the result that external deceleration occurs during level-cruise operation. Under
these conditions the “upstream capture area” Aa is less than the inlet area A1, and some flow
is “spilled over” the inlet, accelerating as it passes over the outer surface.
In the actual engine inlet, separation can take place in any of the three zones shown in Fig. 2.
Separation of the external flow in zone 1 may result from local high velocities andsubsequent
, Inlets
deceleration over the outer surface. Separation on the internal surfaces may take place in either
zone 2 or zone 3, depending on the geometry of the duct and the operating conditions.
Zone 3 may be the scene of quite large adverse pressure gradients since the f low accelerates
around the nose of the center body, then decelerates as the curvature decreases
Major features of external flow near a subsonic inlet
Figure shows a typical streamline pattern for large external deceleration. In flowing over the
lip of the inlet, the external flow is accelerated to high velocity, much as the flow is
accelerated over the suction surface of an airfoil. This high velocity and the accompanying
low pressure can adversely affect the boundary layer flow in two ways:
For entirely subsonic flow, the low-pressure region must be followed by a region of rising
pressure in which the boundary layer may separate. Hence one might expect a limiting low
pressure Pmin or, equivalently, a maximum local velocity Umax, beyond which boundary layer
separation can be expected downstream.
For higher flight velocities (or higher local accelerations), partially supersonic flow can
occur. Local supersonic regions usually end abruptly in a shock, and the shock-wall
intersection may cause boundary layer separation. One might expect a limiting local Mach
number that should not be exceeded.
Inlets
The inlet interchanges the organized kinetic and random thermal energies of the gas in
an essentially adiabatic process. The perfect (no-loss) inlet would thus correspond to an
isentropic process. The primary purpose of the inlet is to bring the air required by the
engine from free stream conditions to the conditions required at the entrance of the fan
or compressor with minimum total pressure loss. The fan or compressor works best
with a uniform flow of air at a Mach number of about 0.5
The Requirements of the inlets:
high total pressure ratio πd,
controllable flow matching of requirements,
good uniformity of flow,
Low installation drag,
Good starting and stability,
Low signatures (acoustic, radar, etc.),
Minimum weight and cost while meeting life and reliability goals.
A list of the major design variables for the inlet and nacelle includes the following:
Inlet total pressure ratio and drag at cruise
Engine location on wing or fuselage (avoidance of foreign-object damage,
inlet flow up wash and downwash, exhaust gas re ingestion, ground clearance)
Aircraft attitude envelope (angle of attack, yaw angle, cross-wind takeoff)
Inlet total pressure ratio and distortion levels required for engine operation
Engine-out wind milling airflow and drag (nacelle and engine)
Integration of diffuser and fan flow path contour
Integration of external nacelle contour with thrust reverser and accessories
Flow field interaction between nacelle, pylon, and wing
Noise suppression
requirements. Design considerations:
The airflow entering the compressor or fan must have low Mach number, in the range
0.4 to 0.7, Part of this deceleration occurs upstream of the inlet entrance plane.
The inlet must be designed to prevent boundary layer separation, even when the axis
of the intake is not perfectly aligned with the streamline direction far upstream of the
inlet.
It is important that the stagnation pressure loss in the inlet be small.
It is even more important that the flow velocity and direction leaving the inlet be
uniform, since distortions in the velocity profile at the compressor inlet can severely
upset the compressor aerodynamics and may lead to failure of the blades due to
vibrations.
Design of inlets that must operate efficiently in both supersonic and subsonic flight
poses special problems;
,Inlets
Subsonic Inlets:
Internal flow and Stall in subsonic inlet sand Boundary Layer Separation
Depending on the flight speed and the mass flow demanded by the engine, the inlet may have
to operate with a wide range of incident stream conditions. The Figure shows the streamline
patterns for two typical subsonic conditions and the corresponding thermodynamic path of an
“average” fluid particle.
During level cruise the streamline pattern may include some deceleration of the entering
fluid External to the inlet plane and hence low mass flow rate [Fig. a]. During low-speed
high-thrust operation (e.g., during takeoff and climb), the same engine will demand more
mass flow and the streamline pattern may resemble Fig. b, which illustrates external
acceleration of the stream near the inlet. .
For given air velocities external acceleration raises the inlet velocity and lowers the inlet
pressure, thereby increasing the internal pressure rise across the diffuser. If this pressure
increase is too large, the diffuser may stall because of boundary layer separation; stalling
usually reduces the stagnation pressure of the stream as a whole.
Conversely, external deceleration requires less internal pressure rise and hence a less severe
loading of the boundary layer.
Therefore, the inlet area is often chosen so as to minimize external acceleration during
takeoff, with the result that external deceleration occurs during level-cruise operation. Under
these conditions the “upstream capture area” Aa is less than the inlet area A1, and some flow
is “spilled over” the inlet, accelerating as it passes over the outer surface.
In the actual engine inlet, separation can take place in any of the three zones shown in Fig. 2.
Separation of the external flow in zone 1 may result from local high velocities andsubsequent
, Inlets
deceleration over the outer surface. Separation on the internal surfaces may take place in either
zone 2 or zone 3, depending on the geometry of the duct and the operating conditions.
Zone 3 may be the scene of quite large adverse pressure gradients since the f low accelerates
around the nose of the center body, then decelerates as the curvature decreases
Major features of external flow near a subsonic inlet
Figure shows a typical streamline pattern for large external deceleration. In flowing over the
lip of the inlet, the external flow is accelerated to high velocity, much as the flow is
accelerated over the suction surface of an airfoil. This high velocity and the accompanying
low pressure can adversely affect the boundary layer flow in two ways:
For entirely subsonic flow, the low-pressure region must be followed by a region of rising
pressure in which the boundary layer may separate. Hence one might expect a limiting low
pressure Pmin or, equivalently, a maximum local velocity Umax, beyond which boundary layer
separation can be expected downstream.
For higher flight velocities (or higher local accelerations), partially supersonic flow can
occur. Local supersonic regions usually end abruptly in a shock, and the shock-wall
intersection may cause boundary layer separation. One might expect a limiting local Mach
number that should not be exceeded.
