RENSSELAER POLYTECHNIC INSTITUTE
Analysis And Simulation On A
Passive Ejector For Improving
Performance Of An
Overexpanded Nozzle For A Gas
Turbine Engine
9/30/2014
Jonathan M. Jause
Executive Summary
While fixed CD nozzles have been used for many years, the need for variable geometry
nozzles has arisen because of widely varying mission cycles for aircraft. Several options
are available to achieve a variable nozzle including kinematics that control both throat
and exit areas, converging flaps, and ejectors. All of the options still have to resolve the
fundamental problems associated with CD nozzles, specifically, overexpansion and
underexpansion. These phenomena cause adverse performance characteristics in the form
of decreased thrust and increased drag. The worst of these two phenomena is
overexpansion. A potential solution for recovering lost performance in the nozzle is to
introduce a second stream in the divergent section of the nozzle to fill the remaining area
of the nozzle left open due to overexpansion. This will be the topic of research proposed.
Introduction
Most supersonic aircraft use afterburning for supersonic flight. But because they are also
required to operate without afterburners the need for at least a two-position nozzle arises.
This can be achieved in many different ways including the use of eyelid segments that
close over a larger nozzle, or a nozzle the has actuation which allows for opening and
closing of the throat, A8, and exit, A9, areas. Another feasible option and the one that
will be the topic of research is the ejector. But first, design and overall considerations for
nozzles must be discussed.
Convergent-Divergent, CD, nozzles are used for supersonic aircraft because they can
produce exit Mach numbers greater than 1. While it is necessary for the nozzle to perform
ideally for optimal thrust, these nozzles can have overexpansion or underexpansion of the
flow relative to the ambient conditions. In the case of underexpansion, the nozzle exit
pressure is greater than the ambient pressure and, thus, a series alternating expansion fans
to oblique shocks are formed in order for the exit pressure to be reduced to the ambient
pressure. While this has adverse affects on performance it mostly comes in the form of
loss of potential thrust. Similarly, overexpansion has a series of shocks emitting from the
engine but in reverse order. Since exit pressure is less than ambient pressure, oblique
shocks form at the exit and alternate to expansion fans until the exhaust gases completely
mix and dissipate into the ambient air. Overexpansion has greater performance losses due
to the increased drag associated with separation. Ejector nozzles follow a similar
construction in that they form an effective convergent-divergent nozzle. However, they
accomplish this a different way.
Ejector nozzles are used in many supersonic aircraft because they can vary the throat area
by use a secondary stream. A typical ejector nozzle design is shown in Figure 1. Primary
air from the core of the engine flows through a convergent nozzle while a secondary
airflow of higher pressure flows over the primary stream. The pressure provided by this
secondary air is what controls the mass flow through the secondary nozzle thus
controlling the nozzle area available to the primary airflow and, therefore, the nozzle
expansion ratio.
Figure 1. Schematic of 2 types of ejector nozzles in which the secondary airflow is
used to vary the expansion ratio of the nozzle.
If this nozzle expansion ratio is not ideal, the ejector nozzle will face similar
performance affects to that of the CD nozzle through over and under expansion.
Problem Description
Underexpansion and overexpansion are on opposite sides of the spectrum when it comes
to design options for improving performance of the nozzle. Increasing A9 will reduce the
likelihood of underexpansion and regain the lost thrust potential. However, this will
increase the risk of overexpansion and, hence, drag. Likewise, a reduction in A9 will
reduce the risk of overexpansion while increasing the risk of underexpansion. An ejector
nozzle is a potential option for improving the CD performance through a wider flight
envelope. However, one requirement for the secondary airflow is to have a higher
pressure than the primary air. The source of this high pressure air is the challenge of
conventional ejectors. Most often, it is taken from the inlet where high static pressures are
located as aircraft speed increases. This, however, requires increased inlet area, which in
most cases of low observable requirements, is not an option. Another possible source
could be bleeding air from one of the compressor stages although this will have a direct
impact to engine performance and efficiency.
It is the goal of this project to analytically and numerically propose potential solutions
and trade studies as to the geometry and performance of a nozzle that reduces the impacts
of overexpansion through the use of a passive ejector nozzle. The proposed design will
consist of a 2 stream flow in which the primary stream flows through a nozzle and the
secondary stream flows into the divergent section of the primary stream at a certain
location. The location of this stream will depend on the required static pressure in order
to flow the second stream and the difference between the ideal nozzle expansion pressure
ratio and the overexpanded pressure ratio.
Figure 2. Schematic of a passive ejector nozzle.
Methodology/Approach
Based upon previous methods and research conducted for similar projects (see references
1-3), the research will be conducted in a two-step approach. First, a series of baseline
tests will be conducted on a typical CD nozzle looking at ideal, overexpanded, and
underexpanded solutions. The results from these tests will then be compared to the 1D
hand calculations for CFD calibration and correlation. The purpose of this is to validate
the computational fluid dynamics modeling, meshing, boundary conditions, etc in order
to establish confidence in the analysis tools. Once confidence has been achieved in the
analysis tools, the second step will consist of conducting the passive ejector nozzle series
of tests. A brief sensitivity study will be run testing different configurations of passive
ejector geometries from which the best configuration will be used in a more detailed
analysis of thrust and drag comparisons for ideal, overexpanded, and underexpanded
cases. Variables in the sensitivity study will include location of the ejector in the
divergent section of the nozzle, area of the ejector, and angle of the ejector.
The suite of computational tools to be used for this analysis will be ANSYS Workbench
version 14.5 and Fluent. Should these tools not adequately produce results, another
possible option for conducting the analysis is COMSOL. This, however, should not be
the case since ANSYS and Fluent are reliable and readily accessible tools.
Several outcomes are expected from this analysis and simulation.
1. Correlation of conventional CD nozzle with ideal 1D analysis.
2. Developed equations for ideal 1D analysis of proposed design config.
3. Coefficient of Thrust vs varying ejector locations
4. Trade studies between ejector location and required secondary stream static
pressure
Milestones/Deadlines
Based on the expected outcomes for this portion of the project, a schedule and expected
deadlines have been established to meet the required deadline. This schedule is presented
below.
Deliverable
Due Date
Project Proposal
9/12/2014
Baseline Analysis for Correlation
10/3/2014
9/12/2014
Layout basic nozzle configuration
1D solution for CD nozzle
Model, mesh, set up bc's and run analysis
Post-process
Correlate to 1D ideal solutions
Passive Ejector Nozzle Analysis
Develop 1D solution for passive ejector nozzle
Develop model
Refine meshing
Set up boundary conditions
Run analysis
Iterate for different locations of ejector
Post-process
Final Draft
Preliminary Final Report
Final Report
9/15/2014
9/17/2014
9/18/2014
9/24/2014
10/3/2014
10/31/2014
10/6/2014
10/6/2014
10/8/2014
10/10/2014
10/15/2014
10/16/2014
10/20/2014
10/24/2014
10/31/2014
11/7/2014
11/28/2014
12/12/2014
Conclusions
While fixed CD nozzles have been used for many years, the need for variable geometry
nozzles has arisen because of widely varying mission cycles for aircraft. Several options
are available to achieve a variable nozzle including kinematics that control both throat
and exit areas, converging flaps, and ejectors. All of the options still have to resolve the
fundamental problems associated with CD nozzles, specifically, overexpansion and
underexpansion. These phenomena cause adverse performance characteristics in the form
of decreased thrust and increased drag. The worst of these two phenomena is
overexpansion. A potential solution for recovering lost performance in the nozzle is to
introduce a second stream in the divergent section of the nozzle to fill the remaining area
of the nozzle left open due to overexpansion.
The research conducted in this project will provide detailed analysis of the geometry and
configuration of the passive ejector nozzle to recover performance. Trade studies will be
conducted on the location of the ejector in the nozzle and angle of the ejector as well as
the required pressure at that location and each of their performance impacts. It is the
intent of this project to establish generic trades on ejector location and sizing in order to
be used in actual nozzle design.
References
(1) Gamble, E., Terrell, D., DeFrancesco, R. “Nozzle Selection and Design Criteria.”
American Insitute of Aeronautics and Astronautics 2004
(2) Elements of Propulsion Gas Turbines and Rockets, J.D. Mattingly, 2006
(3) Aircraft Engines and Gas Turbines, Jack Kerrebrock
(4) Aerospace Propulsion Systems, Thomas A. Ward, 2010
(5) Aerodynamics Aeronautics and Flight Mechanics, 2nd Edition, Barnes W.
McCormick, 1995
(6) Chong, D. “Experimental and Numerical Analysis of Supersonic Air Ejector.”
Applied Energy Journal 130 (2014) pgs 679-684.
(7) Subramanian, G. “Comparison of Numerical and Experimental Investigations
of Jet Ejector With Blower.” International Journal of Thermal Sciences 84
(2014) pgs 134-142.
(8) Rao, S.M.V., “Novel Supersonic Nozzles For Mixing Enhancement in
Supersonic Ejectors.” Applied Thermal Engineering Journal 71 (2014) pgs
62-71.