Simulation-expanded
Henrik Runnemalm, Director Research and Technology
GKN Aerospace Engine Systems
In the aero-engine business, small gains in efficiency can mean major gains
on the bottom line. GKN Aerospace Engine Systems in Sweden is using
simulation techniques to wrest the maximum performance from the
powerplants for which it provides components.
At its heart, a jet engine operates on a remarkably simple principle.
Air is drawn in, compressed, mixed with fuel and ignited, with the hot
expanding gases producing thrust. (A process memorably and succinctly
described by one wag many years ago as ‘Suck – Squeeze – Bang – Blow’.)
However, wringing the greatest possible performance out of such a
powerplant is anything but simple. In fact, it is one of the most complex
undertakings in manufacturing industry.
Every year, millions of man-hours and billions of dollars are poured into
improving turbofans; the price of fuel and the vast amounts of Jet A-1
consumed by the world’s airlines mean that even a 1% improvement in fuelburn is regarded as a worthwhile advance. (And in recent years, the
concomitant reduction in the emission of greenhouse gases has also become a
major factor in successfully selling engines.)
Get the formula right in making an engine more efficient and huge sales can be
forthcoming. Both Boeing and Airbus have backlogs of several thousand
aircraft on their order books, with new models of the fast-selling Boeing 737
and Airbus A320 families due to appear in the next few years and extending
the production runs of the types well into the 2020s. If your engine is a few per
cent more efficient than your competitor’s, major orders are likely to flow your
way.
And, with the air transportation sector predicted to grow at 3 - 4 % annually for
the foreseeable future, it’s a market that will bring successful engine
manufacturers (and their suppliers, such as GKN Aerospace) that phenomenon
much beloved by company CFOs, a reliable, long-term revenue stream.
Since the 1960s, new airliners have shown steady improvements in reduction
of fuel-burn, emissions and noise. In recent years this positive performance
trend has started to flatten, requiring more and more effort to maintain
progress.
Simulation is an increasingly important tool being used by GKN Aerospace in
this constant process of refining the performance of jet engines. By using
simulation as a critical path in developing jet engine components timescales –
and thus costs – can be trimmed, to allow the best possible use of personnel
and machinery in delivering products to a customer.
Allied with this, the consistently high price of fuel means that the requirement
for lighter, and thus more fuel-efficient, aircraft is greater than ever;
lightweight technology has a key role to play in cutting fuel-burn and
simulation is increasingly important in developing lighter components.
Fan Static Structure for High By Pass Ratio Engine
That simulation process not only allows the development of components but
components that can be certified to meet all the necessary regulatory
requirements and that are producible with predictable results.
For example, GKN Aerospace develops load-carrying structures for several
engine families produced by the ‘Big Three’ powerplant manufacturers – Pratt
& Whitney, General Electric and Rolls-Royce. Simulation helps set the
requirements for temperature capabilities for the material to be used in those
structures.
Similarly, when dealing with airflow requirements through the engine,
simulation helps engineers at GKN Aerospace to optimize the aerodynamic
shape of components and make them as efficient as ever possible. The desired
thrust should be produced with minimum total pressure loss.
Making the greatest use of simulation, says Henrik Runnemalm, director of
research and technology at GKN Aerospace , means running design and
manufacturing simulation in a closed loop process. Design simulation involves
the creation of the proposed components ‘virtually’; manufacturing simulation
tools are used to predict optimised factory logistics, machine tool and robot
movements, component deformations and specific manufacturing process
physics.
Original Version
New Version
How simulation in all disciplines (CAD-CFD-FEM) is used by GKN Aerospace to
find an optimal component
Results from the manufacturing simulations are then fed back into the design
process. “This ‘loop’ is what we’re trying to build up here,” he says. “You don’t
want to find it’s not possible to produce a part, or that it’s creating too high
stress levels which could force the part in service time to be reduced.” Finding
a balance between the technical ideal, producibility and cost is vital.
“The key here is that the design capability simulation needs to be totally
integrated with the manufacturing side, which also has its part to play in
simulating what’s happening with the ‘lifing’ of the product” – that is, the
length of time a component can survive in the engine before being removed
for maintenance or replacement.
“When we design a part that is fabricated by welding, then the stresses and
deformation created during manufacturing are actually part of the lifing,” says
Runnemalm. Simulation of these manufacturing stresses can reliably predict
the effect of manufacturing processes such as welding on a product. Welding,
for example, can result in unwanted deformation and stresses within the
material.
“If you’re trying to design an apple and don’t include all the manufacturing
stresses you tend to end up with a pear,” is how he describes it.
In producing any new engine component, manufacturing simulations link
design and manufacturing during product development and act as a tool for
designers and manufacturing engineers to evaluate different concepts or
manufacturing processes.
He divides the design aspect of the product development process into three
stages – concept design, preliminary design and detailed design. Similarly, he
divides the manufacturing part of product development into three sections
that track their design counterparts – inventory of known methods,
preliminary preparation and detailed preparation.
Simulation is used to help with some of the most basic aspects of
development, such as aerodynamic performance, strength and vibration
dynamics, before heading into manufacturing territory through simulation of
processes such as machining or heat treatment.
Stresses on components being welded can be reduced by careful sequencing of
individual welds. Such sequencing does not always follow the pattern that
might be expected.
Once that sequence has been determined, simulation is also used to
programme the welding robot to perform the necessary manoeuvres to follow
that sequence.
On a legacy product, such as the turbine exhaust case (TEC) of the Pratt &
Whitney PW2000, which powers the Boeing 757, the aim is to optimise existing
processes, such as the weld sequence, says Runnemalm.
The TEC requires about 200 welds and, at one point in production of the
engine, problems arose with geometrical tolerances in the engine.
Tolerances between components in a modern turbofan are very tight and
meeting these tolerance criteria can be difficult because of internal stresses
created in the component by processes such as heat treatment. Simulation can
identify the best changes in the production process to improve those
tolerances.
Heat treatment simulation is used to predict material, thermal and stress
distribution of a component
In the case of the TEC, several welding sequence concepts were investigated to
meet these tolerances. Welding simulations showed that residual stresses
could be lowered by using a different welding sequence. Moreover,
simulations also concluded that to avoid problems with tolerances, a predeformation should be given to the product before welding.
On the General Electric GEnx, the Dreamliner engine (Boeing 787) that was
created from a ‘blank sheet of paper’ design, and for which GKN Aerospace
manufactures the turbine rear frame, “we use the tool to say ‘We need a weld
in this position because it’s creating less stress.’”
Turbine structure is defined using advanced simulation capability both from a
design and a manufacturing perspective
Simulation can also be used as an investigative tool and allows investigation of
welding sequences that had previously been too complex and costly to
explore.
But does simulation always provide the correct answers? “That’s really the key
of GKN Aerospace’s capability,” says Runnemalm. “We’ve been working really
hard proving that our simulation tools are giving us the right answer, so we can
trust them.”
Simulation can bring its own problems. In the virtual world, edges of
components can be designed to be infinitely sharp. In practice, however, there
has to be a balance between that aerodynamic ideal and the limitations of the
manufacturing process.
It is essential that the methods used to shape engine components to handle
the airflow through the engine have been validated with experiments. GKN
Aerospace often participates in European research projects and in-house
validation efforts to ensure that the simulation results are correct.
Testing facility at Chalmers University of Technology. Used for research and
validation of turbine outlet guide vanes designed by GKN Aerospace
Design guidelines and experience is still necessary and sometimes simulation
results from even the most powerful computers cannot be trusted – client’s
caution.
CFD analysis of a turbine duct with turning struts. Designed by GKN
Aerospace in the EU project Dream
“Some of our customers don’t want to share their knowledge,” acknowledges
Dr Jonas Larsson, Aerodynamics and Computational Fluid Dynamics specialist.
Intellectual property, especially at the frontiers of modern technology, is
extremely valuable and OEMs are understandably cautious about allowing it to
leave their control.
For those customers that are hesitant over divulging their IP, GKN Aerospace
has the capability to deliver solutions to the design problem that can actually
improve on the client’s original design, says Larsson.
“Once, I was asked directly by a customer, when we were starting work on a
turbine exhaust case: ‘If we let you do this, how can we be sure that our
aerodynamic competence won’t end up in a competitor’s engine?’
“My response was that we had to show we could do this on our own, that our
design was at least as good, if not better, than our customer could do
themselves.
“In another example, we were working on an intermediate compressor case.
Our customer did the basic aerodynamic design and we then started doing the
structural design around it.”
At one point, analysis showed risk of cracking at the trailing edge of the
structure and the customer called GKN Aerospace - Engine Systems Sweden –
in its previous guise of Volvo Aero – for help. “The customer was initially
reluctant to give us the go-ahead to conduct detailed work on their design, but
eventually relented and a solution was found by using simulation to move a
stress point away from the problem area,” notes Larsson.
In fact, the Swedish solution even enhanced aerodynamic performance by
around 8%, which was an added bonus appreciated by the engine-maker.
“That’s a typical problem where we’re involved with support from our own
structural people and designers,” says Larsson.
When an engine manufacturer is prepared to delegate authority to design
areas of a powerplant, GKN Aerospace has the experience and simulation tools
required to take on the job.
For example, Pratt & Whitney has given GKN Aerospace full aerodynamic
design responsibility for the turbine exit case of its PurePower series engines,
which are due to power a new generation of narrow-body jets – notably
Airbus’s A320neo, Bombardier’s CSeries and the Mitsubishi Regional Jet.
“We do the full design and the customer reviews it,” says Larsson.
Simulation was also used to help redesign a turbine exhaust case (TEC) on a
modern wide-body airliner: “The manufacturer had a part that was too heavy
and didn’t fulfil the design requirements. We changed a few things on the
aerodynamic design to accommodate a new structural design. We helped
them redesign it and got a TEC that was much lighter.”
Simulation is also being used to find new ways of minimising pressure loss.
Cast surfaces, for example, are slightly rough and this will cause unnecessary
losses. By using simulation to find out where this is important one can learn
where the castings should be polished or replaced with smoother sheet metal
parts, further cutting the pressure loss as the air flows through the engine.
“It’s also essential that we can demonstrate that the new methods we’re
developing for smoother surfaces have been validated. It’s very important to
show to the customer that what we predict with simulation is also reality,”
notes Larsson.
To support its work, GKN Aerospace works with a group of universities and
other institutions in fields such as aero performance, solid mechanics and
material characterisation and model building. Some partners carry out
specialised manufacturing-related work – for example, how a manufacturing
process is described to a computer in terms that it can understand and act
upon.
Today, more than ever, time is money. Companies have to focus on getting it
right first time, both when producing a new product, or modifying an existing
one to work more efficiently.
Decreasing costs, time and risk by increasing the information available about a
product and its manufacturing processes will help a company achieve a better
market position and improve its competitiveness.
This article first appeared in the 2013 Engine Yearbook (www.mro-network.com)