Gas Dynamics for Design of Intakes P M V Subbarao Professor Mechanical Engineering Department Better Geometrical Solutions to Convert Macroscopic Kinetic Energy to Microscopic KE & vice Versa…..

Geometrical Details of Intakes & Nozzles

Intakes A well designed intake is essential to allow the aircraft to manoeuvre to high angles of attack and sideslip without disrupting flow to the compressor during cruising. The inlet is so important to overall aircraft operation, it is usually designed and tested by the airframe company, not the engine manufacturer. As inlet operation is so important to engine performance, all engine manufacturers also employ inlet aerodynamicists.

Complex designs of Intakes Curved Intakes Submerged Intakes Twin Intake Supersonic Intakes

Design Steps for Intake Only for Supersonic flights: Decide and optimize number of oblique shocks and final downstream conditions of normal shock. For both subsonic & Supersonic flights: Compute all properties at the just upstream of inlet. Subsonic flight: Outer cone affected Ambient air properties and flight Mach number are upstream conditions for inlet. Define upstream inlet conditions as: pi,Ti,Vi …… Cycle calculations demand a mass flow rate.

Exit Conditions for Inlets Achieving the required exit Mach Number. Constraints: Limiting the exit dimensions. Limiting the length of the inlet. Limit on Pressure recovery factor. Exit Conditions for Nozzles Ambient Pressure or Back Pressure Constraints: Limit of Pressure recovery factor.

Step by Step Design of Intakes or Nozzles Select appropriate value of dx. Assume that all properties remain unchanged over the length dx and same as inlet condition. Therefore,

Estimation of Friction Term

Change in Thermodynamic Properties of Air : dx

Total Pressure Recovery Pressure Recovery after dx: Continue till the exit of the intake.

Extra Input Data !!!! Variation of diameter of Intake.(dDh/dx) Variation of Area of Intake (dA/dx). Variation of Mach Number (dM/dx) Final Design is A compromise between size and performance.

Off-design Aerodynamics of Nozzles Climb Ground run High-speed cruise Top speed

Aerodynamic Performance of Subsonic Intakes Subsonic intakes exhibit four different flow regimes or stall characteristics. Non dimensional axial length (length/inlet area) and the total divergence angle of the diffuser are chosen as the primary variables to identify the flow regimes. Fully attached exit flow with steady internal flow Fully attached exit flow with internal unsteady flow Fully developed stall flow Jet Flow

Flow Regimes in Subsonic Intake

Characteristics of Flow Regimens in Subsonic Intake Axial Length/Entrance Area

Quantification of Irreversibilities in Intakes The amount of disruption of the flow is characterized by a numerical inlet distortion index. Different air-framers use different indices. Diffuser Efficiency Ram Efficiency Pressure Recovery Factor. The performance of the intake depends on various geometrical and dynamical parameters, namely, Exit to inlet area ratio, Shape of the inlet, Angle of turn of the limbs, Length of the duct, Inlet Reynolds number, Mach number.

Entropy Generation due to Irreversible Intake Temperature Entropy Diagram for An adiabatic irreversible intake. p0in p0exit T0in pexit Texit T pin  p Tin s Major Irreversibility : Friction

Diffuser Efficiency & Ram Efficiency

Thermodynamic Performance of Intakes

Spillage Drag There is an additional propulsion performance penalty charged against the inlet called spillage drag. Spillage drag, as the name implies, occurs when an inlet "spills" air around the outside instead of conducting the air to the compressor face. The amount of air that goes through the inlet is set by the engine and changes with altitude and throttle setting. The inlet is usually sized to pass the maximum airflow that the engine can ever demand. For all other conditions, the inlet spills the difference between the actual engine airflow and the maximum air demanded. As the air spills over the external cowl lip, the air accelerates and the pressure decreases.

Gas Dynamics of Spillage Flow : Mac = 0.85 @10680m Mach Number Contours Static Pressure Contours Contours of Static pressure to Total Pressure

Spillage Drag Model Spill Drag model is similar to Thrust equation: K: Lip Suction factor