IRJET- A Review of Testing of Multi Cylinder S.I. Petrol Engine
Defense Presentation
1. Simulation of Ejector Nozzle in a Low-Bypass
Turbofan Engine Using NPSS
By
Hatim Soeb Rangwala
Aerospace Engineering Graduate Student
Aerodynamics Research Center
University of Texas at Arlington
Supervising Professor : Dr. Donald Wilson
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2. Roadmap
1. Introduction
i. Motivation
ii. Goals
iii. Ejector Nozzle
iv. NPSS
2. Simulation Setup
i. Assumptions
ii. NPSS Model Setup
iii. On-Design Conditions
iv. Off-Design Conditions
v. Working of NPSS
3. Results
i. Parametric Optimization
ii. Comparative Analysis
4. Conclusion
5. Further Work
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3. Motivation
• Conventional propulsive nozzles at off-design
are inefficient.
• Ejector thrust augmentors give superior off-
design performance.
• Research code for simulation is unavailable to
academia.
3
4. Goals
• Design an ejector-cycle low-bypass turbofan
engine using NPSS.
• Conduct parametric optimization study of
engine bypass ratios.
• Compare optimized ejector-cycle model with
other conventional gas turbine engine cycles at
design and off-design conditions.
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6. Numerical Propulsion System Simulation (NPSS)
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• Object-oriented framework for gas-turbine cycle
analyses.
• Engine components are present as predefined
elements that are linked to form engine models.
• Allows ejector nozzles to be designed as three
individual components.[2]
[2] Porter, J. L., and R. A. Squyers, "A Summary/Overview of Ejector Augmentor Theory and Performance: Phase II (Technical Report) Volume I – Technical Discussion," Vought
Corporation Advanced Technology Center, ATC–R–91100/9CR–47A, Dallas, TX, September 1979
7. Roadmap
1. Introduction
i. Motivation
ii. Goals
iii. Ejector Nozzle
iv. NPSS
2. Simulation Setup
i. Assumptions
ii. NPSS Model Setup
iii. On-Design Conditions
iv. Off-Design Conditions
v. Working of NPSS
3. Results
i. Parametric Optimization
ii. Comparative Analysis
4. Conclusion
5. Further Work
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8. Simulation Model
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Assumptions
• Constant-Area Mixer
• Mixing chamber is long enough for uniform velocity
distribution and thermodynamic profiles at the exit of
mixer.
• Nozzle always fully expands the flow to ambient
conditions.
9. Simulation Model
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NPSS Model Setup
Ejector
Bypass
Splitter
Mixer
Afterb
urner
Con-
Div
Nozzle
Mixer
Con-
Div
Nozzle
Ejector
Bypass
Duct
Ambient
Bypass Flow
(from Bypass
duct)
Core Flow
(from Turbine)
10. Simulation Model
10
On-Design Conditions
Altitude 0 ft
Flight Mach Number 0
Required Trust 30000 lbf
Turbine Inlet Temperature 3600 R
Primary Nozzle Inlet Temperature 4000 R
Overall Compressor Pressure Ratio 36
Fan Pressure Ratio 5
HP Compressor Ratio 7.2
12. Simulation Model
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Working of NPSS
• At Design-Point
– Gas turbine engine cycle equations are used for calculations.
– Components are sized based on constraints.
– Throat area of the nozzles, physical area of the mixer and size of
the turbomachinery is calculated.
• At Off-Design
– Turbomachinery performance is obtained from maps.
– Nozzle parameters are fixed (except for exit area).
13. Roadmap
1. Introduction
i. Motivation
ii. Goals
iii. Ejector Nozzle
iv. NPSS
2. Simulation Setup
i. Assumptions
ii. NPSS Model Setup
iii. On-Design Conditions
iv. Off-Design Conditions
v. Working of NPSS
3. Results
i. Parametric Optimization
ii. Comparative Analysis
4. Conclusion
5. Further Work
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23. Results
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Comparative Analysis Setup
• Ejector-cycle turbofan is compared with three other conventionally
configured engine cycle.
• Each engine configuration is simulated at design-point and off-
design conditions.
• Performance characteristics of each engine is compared and
analyzed.
• The engine cycles are:
1. Ejector-Cycle Turbofan
2. Unmixed-flow Turbofan
3. Mixed-flow Turbofan
4. Turbojet
31. Roadmap
1. Introduction
i. Motivation
ii. Goals
iii. Ejector Nozzle
iv. NPSS
2. Simulation Setup
i. Assumptions
ii. NPSS Model Setup
iii. On-Design Conditions
iv. Off-Design Conditions
v. Working of NPSS
3. Results
i. Parametric Optimization
ii. Comparative Analysis
4. Conclusion
5. Further Work
31
32. Conclusion
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• An ejector-cycle thrust augmented low-bypass turbofan
engine was modeled using NPSS.
• Parametric Optimization of bypass ratios was conducted.
• Performance of the developed model was compared
with conventional engine cycles and studied.
• The research model developed provides a base for
future work in ejector-cycle analyses.
33. Further Work
33
• Independent NPSS element of an ejector nozzle can be
developed.
• Experimental and computational analyses can be
conducted for obtaining mixing parameters.
• Transient analysis can be conducted to replicate change
in throttle settings, take-off and landing. Acoustic
modelling can also be conducted.
• Development of a unified theory of working of ejector-
cycle.
Roadmap (Introduction)
This is an index of my research work that I’ll be covering today.
The following section discusses the motivation that lead to the research we well as the goals of this study.
The ejector nozzle as well as the simulation software, NPSS will also be described
Motivation
From the thrust equation, we know that there is a pressure flux that varies with ambient conditions leading to poor off-design performance.
This has led to the development of thrust augmenting techniques that have been researched since World War 2. (Plug Nozzle, Spike Nozzle, Ejector Nozzle being some of them)
Research code developed for ejectors is mostly proprietary to the industry and as a result, academia cannot simulate ejectors.
Goals
To develop a low-bypass turbofan-engine NPSS model that can replicate an ejector-cycle.
As pressure-ratio optimization study has already been conducted, a parametric optimization of bypass ratios will be performed.
Due to the absence of experimental data to validate the results, the ejector cycle will be compared with 3 other conventionally configured gas turbine engine cycles.
Ejector Nozzle
Consists of three components: Primary Nozzle, Constant-Area Mixer and Secondary Nozzle.
From the turbine exhaust (or afterburner, if present), core flow (which is denoted by red lines) is expanded to the same static pressure as bypass flow through the primary nozzle (Via the movement of the flaps)
In the constant-area mixer, energy transfer takes place through viscous mixing process.
The hotter and faster core flow exchanges internal energy and momentum to the bypass flow.
The mixed flow is expanded to ambient pressure through the secondary nozzle.
There is an increase in kinetic energy at the mixer exit plane that provides the thrust augmentation.
NPSS
Numerical Propulsion System Simulation software (or NPSS) is used in this study.
It is an object oriented framework that allows the modeling and designing of gas-turbine engine cycles.
Multiple analyses can be conducted within a very short span of time.
The engine components are written as coded Elements which are linked to each other by fluid ports and flow stations.
Individual elements allow the simulation of the ejector nozzle by defining a primary conv-divg nozzle followed by a mixer and then a secondary conv-divg nozzle.
Roadmap (Simulation Setup)
Having introduced the research, the in-depth details of the simulation model will be elaborated in this section.
Simulation Model:
Assumptions
The first assumption is about the mixing process. Mixing can happen in two processes: Constant-Area mixing & Constant-Pressure mixing. Studies have shown that a constant-area mixer provides better performance than a constant-pressure mixer. Hence a constant-area process for the mixer is assumed.
The length of the mixing chamber is assumed to be long enough for complete mixing, resulting in the flow having a uniform velocity distribution and thermodynamic profiles at the mixer exit plane. This assumption is due to the lack of experimental data that can provide mixing loss parameters accounting for mixing chamber length.
The secondary nozzle has a variable exit area (in terms of flaps) which allow the flow to expand completely.
Simulation Model:
NPSS Model Setup
The NPSS model is a conventional open Brayton cycle for gas turbine engines with few changes for the ejector nozzle.
The bypass flow is split to the afterburner and the ejector mixer.
Core flow that is mixed with part of the bypass flow before the afterburner, is expanded in a conv-divg nozzle.
The bypass air in the ejector duct is mixed with the core flow in the constant area mixer.
The mixed flow is expanded to ambient pressure through a secondary conv-divg nozzle.
Simulation Model:
On-Design Conditions
These are the on-design conditions chosen for the study.
Most engine data is given for SLS conditions. Hence the design point of the engine is taken as SLS.
In industries, while designing an engine, a thrust requirement is first set. That is why in this research, I have kept a thrust requirement as a constraint on the engine design. This allows for a fairer comparison of engine performance.
The turbine inlet and nozzle inlet temperatures are set due to material constraints (taken from Mattingly’s level 4 component technologies). Efficiencies and pressure losses are also taken from Mattingly.
The pressure ratios are typical values found in modern military jet engines (taken from Kerrebrock).
Simulation Model:
Off-Design Conditions
For off-design analysis, three sections of a flight envelope of a typical military fighter aircraft are chosen.
They are Supercruise, Dash and Subsonic Cruise.
Supercruise is supersonic flight with the afterburner OFF.
Dash is supersonic flight with the afterburner ON.
Subsonic cruise is subsonic flight with afterburner OFF.
Simulation Model:
Working of NPSS
At Design-point, NPSS performs the following operation:
The components are sized according to the constraints.
The throat area of the nozzles, physical area of the mixer and the size of the turbomachinery is fixed.
At Off-Design conditions,
NPSS uses the sized engine model from design-point and runs the analysis with the temperature constraints as the upper limits. The thrust constraint is removed (Obviously!).
Performance of the turbomachinery is computed using inbuilt Maps.
Exit-area of the nozzle is varied to provide complete expansion of flow for the secondary nozzle.
Roadmap (Results)
The results from the study are explained in this section.
The fan bypass ratio (FBP) is the fraction of core flow that goes into the bypass duct. Ejector bypass ratio (EBP) is the fraction of the duct flow that goes into the ejector mixer.
Bypass ratios are varied from 0.1 to 0.9.
Each dot in the following graphs denotes an individual engine that has been sized with the particular bypass ratios for a thrust requirement of 30000 lbf.
Results:
Parametric Optimization
The mass flow rate increases with increases in bypass ratios. Increase in mass flow rate results in a bigger engine.
Results:
Parametric Optimization
As thrust output is kept constant, the specific thrust can be used as a comparison tool.
A decrease in specific thrust results in a larger engine.
This can be seen at higher bypass ratios (dark green), where the mass flow rate was higher leading to a lesser specific thrust.
Hence low fan bypass ratio is an advantage with regard to engine size.
Results:
Parametric Optimization
Specific Fuel consumption is the rate of fuel per thrust produced.
Intuitively, higher fan bypass ratios would produce lower SFC as there is less mass flow rate going through the core.
But in this analysis, the mass flow rate is not kept constant: only the thrust requirement is constant.
Hence the engine still has to produce 30000 lbf regardless of the mass flow rate.
Therefore the lowest SFC is the one with the lowest fan bypass ratio. The explanation as to how this occurs will appear in the following slides.
Results:
Parametric Optimization
As discussed in the previous slide, high fan bypass ratios produce low fuel flow rate values for the combustion chamber.
We can see that by the FBP=0.9 (dark green) and FBP=0.1 (light blue) lines.
Results:
Parametric Optimization
But due to the thrust requirement of the engine, the afterburner has to burn more fuel to achieve the requirement.
This causes the fuel flow rate for high fan bypass ratio engine to spike up, while the low fan bypass ratio engine has low afterburner fuel flow rate.
Results:
Parametric Optimization
Adding the combustion chamber and afterburner fuel flow rates, the total fuel flow rate for the FBP=0.9 (dark green) is higher than FBP=0.1 (light blue) engines.
This proves the original conclusion that low FBP = low SFC for a given thrust requirement.
Results:
Parametric Optimization
The nozzle is the largest component of the engine (in terms of diameter). By having a smaller nozzle exit area, the engine is smaller in size, lighter in weight and does not need a large and heavy support structure.
As the nozzle exit area is a function of the mass flow rate, we see that the FBP=0.1 (light blue) curve has the lowest exit area while the FBP=0.9 (dark green) curve has the highest exit area.
Results:
Parametric Optimization
The exit static temperature is a measure of how effectively the kinetic energy has augmented the thrust of the engine. (Hotter air @ exit = lesser kinetic energy of the flow)
At an ejector bypass ratio of 0.5, at all values of fan bypass ratios, the exit static temperature is equal. This is desirable as it provides the most balanced off-design performance.
Results:
Parametric Optimization
To conclude the parametric optimization of the bypass ratios, a fan bypass ratio of 0.1 and an ejector bypass ratio of 0.5 are cchosen as the design point values for the ejector-cycle turbofan.
Results:
Comparative Analysis
Now the optimized ejector-cycle turbofan is used in a comparative analysis with the 3 other engine cycles to validate the results.
They are simulated at SLS, Supercruise, Dash and Subsonic Cruise conditions.
Performance characteristics are compared and analyzed.
The 3 engine cycles are, Unmixed, Mixed and Turbojet.
The Unmixed flow engine has two separate nozzles for the bypass and core flows.
In the mixed flow, all the bypassed air goes into the afterburner, followed by a conv-divg nozzle exhausting to ambient.
The Turbojet does not have bypass and is the original open Brayton cycle engine.
Results:
Comparative Analysis
Due to the constant thrust requirement and thrust augmentation of the ejector cycle, at on-design condition, the ejector-cycle turbofan has the lowest mass flow rate.
The engine being sized for that particular mass flow rate, has correspondingly low mass flow rates across the off-design conditions.
Results:
Comparative Analysis
The gross thrust is the total thrust produced by the engine.
We see that it is higher for ENT for supercruise and lower for dash and subsonic cruise.
Results:
Comparative Analysis
The net thrust of ENT is highest for supercruise and on par for dash and subsonic cruise.
This is because the ejector results in a flow that has higher kinetic energy.
Results:
Comparative Analysis
By comparing the net thrust as %Gross thrust of each engine, it shows the higher percentages of the ejector.
This shows that the smaller ejector engine is more effective at converting the gross thrust to net thrust.
Results:
Comparative Analysis
The extremely low mass flow rate, coupled with the high net thrust of the ejector-cycle turbofan gives superior specific thrust across all off-design conditions.
At supercruise, the specific thrust is 100% better than the other configurations, confirming that ejector-nozzle turbofans are meant for supercruise flights.
Results:
Comparative Analysis
Low mass flow rates also result in reduced nozzle exit area, thereby reducing size of the engine.
Difference is highly apparent at off-design conditions.
Results:
Comparative Analysis
The low mass flow rate for the ejector nozzle coupled with the thrust augmentation, provide an extremely low SFC for the ENT when compared to the other engines.
SFC is 50% lower at supercruise and 30% lower at dash and subsonic cruise.
Roadmap (Conclusion & Further Work)
So now I’d like to conclude the research study and provide information on which further work can be performed.
Conclusion:
Requirement of an ejector-cycle turbofan engine model for research was fulfilled. The model was developed using NPSS.
Bypass ratios were optimized for a given thrust requirement. Low values of fan bypass ratio was found to be optimum for performance.
The performance of the engine at design & off-design conditions was compared with conventionally configured engine cycles. The ejector-cycle turbofan was found to be excellent for supercruise and on-par at dash and subsonic cruise.
The model that was developed can now be used as a base for further work in ejector thrust augmenters.
Further Work:
In NPSS, the current model involves linking three individual components. Instead, an ejector nozzle component can be created that does all the cycle calculations internally.
The mixing chamber was assumed to be long enough for complete mixing. In reality, this is not possible as engine lengths are constrained. There are few parameters that account for losses due to incomplete mixing. The values for these parameters can only be obtained by experimental and computational analysis.
To completely simulate an engine’s performance, even transient parts of the flight envelope like acceleration, deceleration, take-off & landing must be simulated. Acoustic modeling can also be conducted.
Further work can be performed to develop a unified theory of the working of the ejector cycle with the inclusion of inviscid and viscous studies.
