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National Aeronautics and Space Administration
www.nasa.govASM 2016 1
SciTech 201654th AIAA Aerospace Sciences Meeting
San Diego, CAJanuary 4-8, 2016
Impact of an Exhaust Throat on Semi-Idealized Rotating Detonation Engine Performance
Daniel E. PaxsonNASA Glenn Research Center
Cleveland, Ohio
https://ntrs.nasa.gov/search.jsp?R=20160010267 2020-04-18T20:22:35+00:00Z
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Outline
• Background• Problem Statement• Problem Analysis• Accommodation Strategy• Results• Concluding Remarks
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BackgroundRotating Detonation Engines (RDE’s) represent an IntriguingApproach to Detonative Pressure Gain Combustion (PGC)
• 1000+ Hz. cycle frequency• No ‘spark’ required• No lossy DDT devices• Compact
PGC: A periodic process, in a fixed volume, whereby gas expansion by heat release is constrained, causing a rise in stagnation pressure and allowing work extraction by expansion to the initial pressure.
Source: Schwer, AIAA 2011-581
Tem
pera
tureP
ress
ure
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Problem Statement
• Semi-Ideal Means− Mil. Spec. engine inlet− Combustor (RDE) inlet is lossless− Combustor inlet has no reverse flow (i.e. perfect valve)− Engine exit nozzle is lossless (i.e. perfectly expanded)− Adiabatic− Inviscid− Premixed− Retains fundamental entropy sources associated with RDE’s
Consider a Semi-Ideal, Ram-Based, Stoichiometric Hydrogen Fueled RDE at 37,000 ft., Flying at Mach 1.37
(Note-Flight conditions are illustrative only)
RDEInlet Nozzle
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Problem Statement
• RDE 21% above RJ• RDE 22% below PDE
RDEInlet Nozzle
2000
2500
3000
3500
4000
4500
5000
RDE Ramjet PDE
Net
Spe
cific
Impu
lse,
sec
.
Alg
ebra
ic M
odel
CFD
Mod
el
Alg
ebra
icM
odel
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Problem Statement
• 7-10% Disparity
3500
4000
4500
5000
RDE RDE w/KEu
RDE w/KEu-Sm
PDE NU PDE
Net
Spe
cific
Impu
lse,
sec
.
Alg
ebra
ic M
odel
CFD Kin
etic
Ene
rgy
Min
orE
ntro
py
Non
-uni
form
Exi
t Flo
w
Contours of Entropy Relative to Algebraic PDE
Entro
py
Where’s All This Blue Coming From and What Can Be Done About it?
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Problem AnalysisPrimary Analysis Tool
Quasi-2-Dimensional Euler Solver With Sources• Source Terms Model:
• Chemical Reaction• Friction (not used here)• Heat Transfer (not used here)
• 2 Species Reaction (reactant or product)• Simplified Finite Rate Reaction• High Resolution Numerical Scheme• Coarse Numerical Grid (< 10,000 cells)• Adopts Detonation Frame of Reference
• Time derivatives ultimately vanish and solution is steady• Robust Boundary Conditions
• Sub or supersonic exhaust flow• Optional isentropic exhaust throat• Forward or reverse inlet flow with choking possible (not used here)• Physics based inlet loss model from typical restriction (mostly not used here)
• Runs on a laptop• Approximately 20 sec. per wave revolution
• Validated• Compares well with other semi-idealized numerical results• Compares well with experimental results
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Problem AnalysisEffects of Fill Mach Number On 1D PDE
• Algebraic 1D PDE Results Assumed Low Fill Mach Number
• As Fill Mach Increases Post Detonation Entropy Increases and Specific Impulse Decreases
• As Fill Mach Increases PredetonationPressure Drops
• Detonation Does Not Recover Pressure
• So Post-Detonation CJ Pressure Drops
• Less Availability for Thrust Production
4300
4400
4500
4600
4700
4800
4900
5000
7.8
7.9
8.0
8.1
8.2
8.3
8.4
8.5
0.0 0.5 1.0
I ansp
i, se
c.
Entro
py
Fill Mach Number
entropyI_anspientropyIanspi
8
10
12
14
16
18
20
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.5 1.0
p CJ/P
man
p pre
det/P
man
Fill Mach Number
P1/PT1Pcj/PT1ppredet/Pman
pCJ/Pman
16
11162126313641
0.0 0.5 1.0
p/p*
x/L
CJ
Analytical Results for 1D PDE’s Say Fill Mach is the Culprit
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Problem AnalysisEffects of Fill Mach Number On RDE
• Fill Mach Number Tricky to Define• Using axial Mach number just prior to
detonation• Axial Mach Number Is High• Post-Detonation Entropy Is High• Fill Mach and Entropy Follow Same Relationship as 1D PDE
CFD Results for RDE’s Suggest Fill Mach is Indeed the Culprit
7.8
7.9
8.0
8.1
8.2
8.3
8.4
8.5
0.0 0.5 1.0
Entro
py
Fill Mach Number
1D CJRDERDE mass-averaged
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Accommodation StrategyAdd an Exit Throat
• Rate of Exhaust Affects Rate of Fill• Well established from PDE efforts
• Lower Rate of Fill Yields Higher Pre-detonation Pressure, Higher Post-Detonation Pressure, Lower Entropy, Higher Specific Impulse
It Works! 9.4% Specific Impulse Increase
Aexit/Aannulus=0.75
Log(
pres
sure
)Te
mpe
ratu
re
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Accommodation StrategyMore Restriction!
• Throat Sends Strong Waves Upstream
• Waves Affect Inflow• Inflow Changes Affect Detonation Structure
• Detonation Changes Generate Additional Spurious Waves
• Waves Get Reflected• Cascade Established
Unstable Behavior Results
Aexit/Aannulus=0.70
Tem
pera
ture
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Accommodation StrategyInlet Restriction With Loss
• Inlet Restriction Creates Total Pressure Loss…• But Damps Unstable Behavior Allowing Smaller Exit Restrictions…• Ultimately Yielding Net Gain
10.3% Specific Impulse Increase
Aexit/Aannulus=0.70; Ain/Aannulus=0.75
Tem
pera
ture
Log(
pres
sure
)
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Concluding Remarks
• For an idealized, basic RDE configuration, the fill Mach number can be quite high under representative boundary conditions
• Through the same basic mechanism as the PDE, the high fill Mach limits performance as measured by net specific impulse
• The fill Mach can be reduced by adding a throat to the exit, thereby gaining as much as 9% net specific impulse
• Too much exit restriction yields unstable operation• Adding a ‘lossy’ inlet restriction adds stability and allows for
a 10% specific impulse improvement• Experimental validation (or refutation) is justified
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END