1. Axisymetric Inlet Design for Combined-Cycle Engines Jesse R
Axisymetric Inlet Design for Combined-Cycle Engines Jesse R. Colville, Ryan P. Starkey, and Mark J. Lewis University of Maryland, College Park, Maryland
Presented by: Andrew MacKrell
AME – Intermediate Thermodynamics
University of Notre Dame
April 30, 2008

2. Abstract
Examined a new strategy for designing inlets for Turbine-based combined-cycle (TBCC) engines
Starting characteristics are examined compared with of the Kantrowitz limit
Widened shoulder centerbody and variable cone with reextension designs have the ability to remain started into the Mach 6-7 range

3. TBCC Engines
Low speed Turbojets merged with high speed Ramjets and Scramjets
Challenge: Integration of the TBCC cycle modes into a single system
Each requires unique flow properties to operate properly
Inlet must provide efficient compression for all components across a wide Mach spectrum
A combined cycle is characteristic of a power producing engine or plant that employs more than one thermodynamic cycle. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). The remaining heat from combustion is generally wasted. Combining two or more "cycles" such as the Brayton cycle and Rankine cycle results in improved overall efficiency.
Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion.
Ramjets and scramjets essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air.
A scramjet requires supersonic airflow through the engine, thus, similar to a ramjet, scramjets have a minimum functional speed.
A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic.

4. Inlet Performance
Imposes a significant constraint on overall operation of engine
Needs to:
diffuse required amount of air and maximize pressure recovery while minimizing shockwave
Supply air with tolerable flow distortions
Minimize amount of added external drag
Minimize added mass
Provide self-starting capability
The issue of inlet starting is highly critical because inlet for TBCC engines must function properly during transition from one cycle mode to another.
Basically the ability to remain operational without external assistence

5. Study Model & Assumptions
SR-71 Blackbird engine inlet
fastest manned air breathing aircraft
Constant Ratio of Specific Heats
Level, Steady Flight
Incoming air is uniform
One-D Isentropic Flow
Air is perfect gas
Steady flight - zero angle of attack
Uniform - in reality, the flow will be perturbed depending on the placement of the inlet on the aircraft. The flow entering the inlet is distributed by the conical shock coming off the nose of the vehicle.

6. Calculations and Measurements
Internal Area Ratio that produces sonic flow at the throat:
Measured Experimentally
Self-start benchmark: Kantrowitz Limit a
Isentropic Contraction Limit: A
Internal area ratio – measured experimentally.
For a specific inlet geometry and set of freestream conditions…
Preliminary estimates of the internal contraction ratio that will self-start can be obtained from the Kantrowitz limit. This limit is determined by assuming a normal shock at the eginning of the internal contraction and calculating the one-dimentional, isentropic, internal area ratio that will produce sonic flow at the throat
M2 is mass averaged Mach number
lambda is the ratio of specific heats - function of tem, pressure, etc.

7. Inlet Modifications Variable Cowl Leading Edge
Variable Cone Centerbody
Reextended Spike
Widened Shoulder Centerbody
Variable Cone With Reextension

8. Results
Variable Cowl Leading Edge and Variable Cone Centerbody modifications exhibited sub standard starting characteristics
Variable Cone with Reextension and widened shoulder centerbody showed promise
- couldn’t achieve Mach self start over mach 4
- up to Mach 7

9. Conclusions
Complex variable geometry is needed to design inlets for high speed aircrafts
Numerical simulation was not entirely accurate
Mechanical complexity associated with the complex variable geometry is the main obstacle that would need to be overcome to realize these design schemes
that can accommodate speed ranges from the low supersonic regime into the hypersonic corridor
More detailed flow analysis including viscous and boundary layer control is needed to quantify the flowfield properties and self-starting characteristics