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The Application of CFD to Underpin Material Selection for Gas Turbine Rotors
The rotor assemblies of modern gas turbine engines operate in a particularly harsh environment, in which they are subjected to a combination of very large centrifugal forces and elevated temperatures. In order to endure these conditions, exotic rotor materials have been developed to sustain the imposed loading and complex air systems have been incorporated into engine designs to seal internal cavities and reduce metal temperatures.
In all engine designs a delicate economic
balance must be struck between the incorporation of
high temperature materials - which add significantly
to unit costs - and the extent to which cooling air
is drawn off from the various compressor stages. In
the latter case air drawn off from the compressor stages
makes no direct contribution to engine thrust and is
therefore manifest as an increase in specific fuel consumption
leading to higher operational costs.
In the present work computational fluid
dynamics (CFD) techniques have been used to simulate
the air system flow within a gas turbine rotor-stator
cavity. Various rotor cooling arrangements are considered
in order to use the available cooling air to best effect.
The results obtained clearly demonstrate the potential
for CFD to be applied to achieve the optimal economic
balance between the conflicting requirements to minimise
engine component and operation costs.
Automated Fluid-Structure Interaction Analysis
An automated Fluid-Structure Interaction (FSI) analysis procedure has been developed at ATK Thiokol Propulsion that couples computational fluid dynamics (CFD) and structural finite element (FE) analysis to solve FSI problems. The procedure externally couples a steady-state CFD analysis using Fluent® and a structural FE analysis using ABAQUS®. Pressure results from the CFD solution are interpolated and applied as pressure boundary conditions on the structural FE model. Displacements from the structural analysis are interpolated and applied to the boundary of the CFD mesh. Iteration between the CFD and the structural analysis continues until a solution is reached. The FSI procedure provides controls to monitor the solution and define termination criteria, as well as manage output. Automatic report generation of the solution is another feature of the FSI procedure. Plans and funding are in place to extend the FSI procedure to include coupling with thermal analysis as well.
The FEM Builder® program provides pre-
and post-processing functions for the FSI procedure,
such as geometry creation, finite element mesh generation,
material property definition, and boundary condition
application. Several of the pre-processing functions
were created exclusively for FSI solutions. The FEM
Builder® program provides interfaces to other finite
element pre/postprocessors and a number of analysis
programs. Scripted access to FEM Builder® program functions
is provided through the FEM Python module. The FEM Python
module functions provide the basis of the FSI procedure.
The FEM Builder® FSI procedure is applied to the analysis
of a fictitious solid rocket motor. The problem of bore
choking is examined in order to demonstrate the capabilities
of the FSI procedure on a problem with potentially large
structural deformations. An overview of the input required
by the FSI procedure to solve this problem is discussed.
Modeling Pulse Tube Internal Flows with CFD
A commercial computational fluid dynamics (CFD) software package is used to model the oscillating flow inside a pulse tube cryocooler. The 2D axi-symmetric simulations demonstrate the time varying temperature field and the heat fluxes at the hot and cold heat exchangers and at the tube sidewall. The only externally imposed boundary conditions are a cyclically varying pressure at the tube inlet and constant temperatures at the external walls of the hot and cold heat exchangers.
Conventional explanations of the mechanism
of pulse tube refrigeration rely on the pressure and
mass flow phase difference in orifice pulse tubes and
heat storage in the tube sidewall for basic pulse tubes.
This work includes CFD animations to reveal the basic
heat and momentum transport mechanisms inside the tube
that result in cooling at one end and heating at the
other. The CFD results also demonstrate the influence
of orifice restriction and tube sidewall storage on
net refrigeration. The animations and resulting heat
transfer data allow insight into the heat and momentum
transport processes inside pulse tubes and are beneficial
to guiding pulse tube design.
Numerical Modeling of Unsteady Thermofluid Dynamics in Cryogenic Systems
Unsteady thermofluid dynamic phenomenons are common in cryogenic systems. They include pressurization and blowdown of cryogenic tanks, sudden opening or closing of valves in long pipeline, chilldown of cryogenic transfer line and rocket engines prior to ignition. Development of accurate, robust and economic numerical model is a critical need for design and operation of such systems. This paper describes the progress we have made at Marshall Space Flight Center in recent years to develop this capability using a general purpose flow network code, Generalized Fluid System Simulation Program (GFSSP).
Enabling Technologies for High Fidelity Simulations involving Fluids and Thermal Analysis
With the advent and rapid development of high performance computing and communication (HPCC) and robust and efficient mathematical/numerical algorithms, computational field simulation (CFS) has rapidly emerged as an essential tool for engineering analysis and design environment. This has fundamentally changed the way underlying principles of science and engineering are applied to research, design, and development. For example, computational fluid dynamics (CFD) and computational mechanics techniques, traditionally used in fluid mechanics and structural mechanics problems involving aerodynamics, hydrodynamics, automotive, and heat and mass transfer applications, are now being applied to electromagnetic, bio-engineering, bio-medical, semi-conductor, atmospheric science, environmental and civil transport, and other physical field problems.
The state-of-the-art (SOA) and the state-of-the-practice
(SOP) of computational simulation and computer-aided
engineering with respect to their influence on engineering
research and applications programs will be presented.
The current barriers are in improving accuracy, throughput,
efficiency and robustness of simulation process with
reducing cost of overall simulation. The Enabling Technology
Laboratory (ETL) at UAB has been established to address
these shortcomings. The progress realized in the development
of these enabling technologies - Mesh generation and
Adaptation, Visualization and Feature Detection, Parallel
Algorithms, Numerical Algebra Tools, and Computer Aided
Geometry Design (CAGD) applicable to disparate time
and length (continuum-micro-nano-atomic) scale simulations
will be presented. The development of several too kits
associated with these enabling technologies will be
described. The perspectives, vision, strategic plan,
and road map associated with the research in multidisciplinary
simulations will be included with the concentration
on fluid and heat transfer. Computational examples and
demonstrations will be presented to demonstrate the
success of the developed enabling technologies.
Use of a Single Finite-Element Mesh for a STOP-G Analysis for the LISA Spacecraft
The LISA mission is designed to measure gravity waves using a large, space-based laser interferometry system. It consists of three identical spacecraft flying in an equilateral triangle formation with a distance of five million kilometers between spacecraft. Each spacecraft includes two free-floating proof masses, where the gravitational forces between the spacecraft and the proof masses must be balanced. As such, the proof masses should only change position as a function of external gravitational forces (i.e. a gravity wave). The spacecraft (gravitationally balanced about the proof masses) should move in response to the proof masses, but have no effect on them. Changes in the position of one spacecraft relative to the other two will indicate movement of the proof masses as a result of external gravitational influences.
In order to minimize spacecraft effects
on the proof mass, LISA has very strict thermal requirements,
related to thermal distortions, which could unbalance
the self-gravity characteristics of the design. As such,
a STOP-G (Structural, Thermal, OPtical, self-Gravity)
analysis using a single, inter-disciplinary model is
required to validate any design iteration. Therefore,
it is foreseen to have a continuous feedback analysis
effort requiring numerous individual STOP-G analyses.
A primary goal of these analyses is to minimize error
sources through all phases (including the passing of
data among various disciplines).
To eliminate errors associated with temperature
mapping between a thermal and structural model, it has
been proposed to use a single finite element mesh for
thermal, structural, and self-gravity analyses. While
this will simplify model generation and consistency,
it has drawbacks, since required detail in a thermal
model is often quite different than that required by
a structural model. Two thermal codes were investigated
for their capability to generate temperatures from a
spacecraft-representative, finite-element-model: Thermal
Desktop and TMG. Thermal results were passed back to
the structural analyst for thermal distortion analysis;
thermal distortions were in turn passed to the self-gravity
and optics analysts for further evaluation.
This paper presents lessons learned from
this effort, using a single mesh for all disciplines
in the STOP-G analysis. It should be noted that the
goal of this effort was to test the process and results
from these runs are in no way indicative of expected
results from the LISA design. Through this process,
strengths and weaknesses of each of the thermal codes
were identified and are presented herein.
Heating Analysis of Entry System Break-up Particles for Planetary Protection
Numerous missions for the exploration
and investigation of Mars are presently being proposed
and planned. Each of these missions must assess the
planetary protection requirements that protect solar
system bodies from biological contamination. NASA Policy
Guideline NPG 8020.12B, “Planetary Protection
Provisions for Robotic Extraterrestrial Missions,”
provides the fundamental definitions and requirements
for this policy. The planetary protection sterilization
requirement basically states that all material entering
an extraterrestrial solar system body must reach a specified
temperature for a specific period of time to be considered
sterile. Presently, this requirement is 500°C for
0.5 seconds. If a mission can show through analysis
that debris from a Martian entry will meet or exceed
the thermal planetary protection requirements, then
the cleaning and sterilization efforts prior to launch
may be substantially reduced or eliminated. This can
result in a substantial cost and schedule savings prior
to launch.
The increasing number of future Martian
missions and the requirement to assess the bio-burden
of each mission necessitates the development of an analysis
tool that can be used to quickly and easily evaluate
the Martian planetary protection requirement for small
particle debris. Typically, a rather exhaustive analysis
is performed to evaluate the breakup and burn-up of
the larger items and components on the satellite; however,
smaller components are often assumed to burn-up in the
atmosphere. This may be a reasonable assumption for
Earth entry; however, a recent study of small meteorites
entering the Martian atmosphere suggests that small
particles may not burn-up prior to impacting the surface.
Because of this data it is imperative that small particles
are evaluated during a burn-up/breakup analysis to ensure
they meet the planetary protection requirements and
to add robustness to the final planetary protection
analysis.
The presented analysis strives to provide
quantitative data on the heating rates and subsequent
maximum temperatures achieved by small particles that
enter the Mars atmosphere as a result of an entry system
break-up. The objective is to identify the particle
sizes needed to ensure sterilization for a range of
entry conditions. Of specific interest are the combinations
of conditions (e.g. particle size, initial velocity,
material) that are required for a given particle to
achieve a through-thickness temperature of at least
500°C for a minimum of 0.5 seconds. Conservative
assumptions were implemented in order to simplify and
bound the problem, the primary assumption being that
all particles, regardless of size, were considered to
be perfectly spherical and were analyzed using simplified
aerodynamics. Validation of the trajectory calculations
was performed using the Program to Optimize Simulated
Trajectories (POST). A modified form of the Sutton-Graves
predictor was used to calculate the stagnation point
aeroheating for the spherical particles which takes
into account a hot wall correction Lees’ approximate
aeroheating profile for a body of revolution was used.
Both the stagnation point heat flux calculations and
the approximate aeroheating profile were validated against
a LAURA solution for a selected case. For those particles
which were very small, flow about the spheres at higher
altitudes could not be considered to be in continuum,
and Bird’s analytical solution for free molecular
heating about a sphere had to be implemented. A lumped
capacitance method was used to assess the thermal response
of the particles as a function of trajectory time. Temperature
results were validated and compared against both MSC.Patran
Thermal and CFDesign results.
For the purposes of keeping this paper
short, only the results for one of the 3 materials examined
will be presented. The final product of this effort
is envisioned to be a set of curves that can be used
in a generic sense to provide guidance in particle sterilization
and planetary protection for a multitude of mission
scenarios.
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