Gas Turbine Internal Air Systems Research: Economic and Environmental Impact
Submitting InstitutionUniversity of Sussex
Unit of AssessmentGeneral Engineering
Summary Impact TypeEconomic
Research Subject Area(s)
Engineering: Mechanical Engineering, Interdisciplinary Engineering
Summary of the impact
There have been both direct and indirect contributions to cost savings,
reduced fuel consumption and reduced CO2emissions through
Sussex research into gas turbine engine technology. Rolls-Royce and GE
Aviation have benefited from experimental measurements that have allowed
improvements to internal air systems flow modelling. This has led to
savings in engine testing of approximately £10M over the period;
indirectly it has also led to substantial economic benefits through
reduced costs for engine manufacturers and their airline clients, and to
improved design of internal cooling and sealing systems, which has direct
impact on reduced fuel consumption and emissions.
A gas turbine's efficiency improves with increased turbine inlet
temperatures; temperatures sufficiently high, ~1800oC, as to
melt the engine. Hence, air-driven component cooling and sealing are
essential. The focus of this study is the improvement of cooling and
sealing flows to critical components.
The behaviour of the complex geometry and material mix comprising an
aero-engine is affected by the passage of the gas and associated
compression and ignition processes.
The main gas path flow is separated from the internal air system by the
peripheral surface, shroud, of the compressor drum. The internal air
system provides cooling to the rotating blades and stationary nozzle guide
vanes; and aerodynamically seals the gap between rotating and stationary
Once used, the cooling and sealing air is ejected into the mainstream
flow which can have a detrimental effect on overall thermodynamic
It is essential that results from simulations are experimentally
validated; thus making acceptable simulated design changes. The wealth of
expertise developed over thirty years, combined with strategic alliances
with Rolls-Royce (1996-2009), and GE Aviation (2011-) has enabled
significant research, both experimental and theoretical, to be undertaken
on the internal air systems of gas turbine engines.
Fully Coupled CFD Model
There is a need to understand the effect of gas property changes on
physical engine components. Prior to this work, circa 1995, two separate
computer simulation codes were required: one to model the gas flow
behaviour (velocities, temperatures, heat transfer by convection); a
second to model engine component behaviour (displacement, stress,
temperature, heat transfer by conduction).
The output from one code formed the input to the other. Transferring the
data required significant manual intervention to compensate for the lack
of coupling; for example for relative displacements, such as the gap
between a rotating and stationary component, affected by thermal
expansion, and directly linked through gas flow and component surface
temperature, each calculated in different codes.
Research, initiated by Rolls-Royce, led to the seamless coupling of the
two codes [R1], now used extensively within Rolls-Royce for engine design,
as predicting surface temperatures inside an engine is crucial in
forecasting thermally-induced stress and displacements; component life;
and cooling and sealing air temperatures.
Measurement and Validation
High precision rigs and measuring techniques have been developed to
provide insight and validation data.
The interaction of the internal air system flow with the compressor discs
and drive shaft, and methods to enhance the heat transfer from compressor
discs were studied experimentally providing:
- measurements and correlations of heat transfer from the disc, drive
cone and peripheral surfaces of a typical high pressure compressor
- measurements and correlations of the discharge coefficients from
transfer holes in a rotating drive shaft [R3];
- measurements of the time constants of the discs under engine
acceleration and deceleration transient conditions [R4].
A two-stage, high-pressure turbine test rig was used to investigate the
flow and heat transfer within cavities adjacent to the main annulus
section, leading to:
- measurements of sealing effectiveness over a range of cooling flows
for different configurations of cooling hole design [R5];
- quantification, by CO2 tracer gas, of ingestion,
reingestion and interstage seal flows [R6].
References to the research
Authors who were staff at the time the research was carried out are
indicated thus: CA Long. All other authors, with the exception of Childs
in [R6], are (or were) employees of Rolls-Royce plc, Derby.
R1 Illingworth, J.B., Hills, N.J., Barnes, C.J.
(2005) `3D fluid — Solid Heat Transfer Coupling of an Aero Engine
Pre-Swirl System', Proceedings of the ASME Turbo Expo, 3 PART A,
R2 Long, C.A., and Childs, P.R.N. (2007) `Shroud
Heat Transfer Measurements Inside a Heated Rotating Multiple Cavity with
Axial Throughflow', International Journal of Heat and Fluid Flow,
Vol. 28(6), 1405-1417, DOI:10.1016/j.ijheatfluidflow.2007.04.009
R3 Alexiou, A., Hills, N.J., Long, C.A., Turner,
A.B., L., Wong, L-S., and Millward, J.A. (2000) `Discharge
Coefficients for Flow Through Holes Normal to a Rotating Shaft', International
Journal of Heat and Fluid Flow, Vol. 21(6), 701-709,
R4 Atkins, N.R. (2013) `Investigation of a Radial-Inflow
Bleed as a Potential for Compressor Clearance Control', Proceedings
of ASME Turbo Expo 2013, Volume 3A: Heat Transfer, Paper No. GT2013-95768,
R5 Dixon, J.A., Valencia, A.G. Coren, D.D., Eastwood,
D., and Long. C.A. (2012) `Main Annulus Gas Path Interactions —
Turbine Stator Well Heat Transfer', Proceedings of ASME Turbo Expo
2012, Paper No. GT2012-68588. Also in ASME Journal of Turbomachinery
Vol. 136 (2), DOI: 10.1115/1.4023622
R6 Eastwood, D., Coren, D.D., Long, C.A, Atkins,
N.R. Childs, P.R.N., Scanlon, T.J., and Guijarro-Valencia, A.
(2012) `Experimental Investigation of Turbine Stator Well Rim Seal,
Reingestion and Interstage Seal Flows Using Gas Concentration Techniques
and Displacement Measurements', ASME Journal of Engineering for Gas
Turbines and Power, Vol. 134(8), pp 082501-1-082501-9,
Outputs R2, R4 and R6 best indicate the quality of the underpinning
• A Alexiou at Sussex 1995 - 2002, Research Fellow, now Senior
Researcher at the Laboratory of Thermal Turbomachines, National Technical
• NR Atkins at Sussex 2007 - 2009, Lecturer, now lecturer at the
Whittle Lab, University of Cambridge
• PRN Childs at Sussex 1987-2008, Professor, now Professor in
Engineering Design at Imperial College
• DD Coren at Sussex 2002 - 2010, Research Fellow, now Senior
Lecturer, School of Computing, Engineering and Mathematics, University of
• D Eastwood at Sussex 2006 - 2010, Graduate Assistant and PhD
Student, now Development Engineer for PTL, Shoreham by Sea, West Sussex
• NJ Hills at Sussex 1998 - 2005, Senior Research Fellow, now
Professor of Computational Engineering, University of Surrey
• JB Illingworth at Sussex 2001 - 2005, Tutor and PhD student now
Powertrain Integration Engineer for Ford Motor Company, Essex
• V Kanjirakkad at Sussex 2011 — present, Lecturer and Early
Career Researcher working on GE projects
• CA Long at Sussex, 1982 — present, Reader since 2001 at
University of Sussex
• AB Turner at Sussex 1983 - 2009, Professor
• LS Wong at Sussex 1998 - 2002, Research Officer
• H Xia at Sussex 2011 — present, Lecturer and Early Career
Details of the impact
Improved understanding of internal air system flows and heat transfer has
reduced engine development time and testing costs; and indirectly
contributed to improvements in production and operating costs, component
life and weight, efficiency, and CO2 emissions.
The use of computational models in place of engine tests has allowed
Rolls-Royce to save an estimated £10M on engine testing and contributed to
the improvement of engine design models in a range of areas [C1].
Fully Coupled CFD Model
The computational fluid dynamics model was coupled to the Rolls-Royce
in-house thermo-mechanical analysis program SC03 in 2006; and has a
capability which is still at least four years ahead of that available
commercially [C1]. The fully coupled model is used extensively by
Rolls-Royce and has been applied effectively to many engine projects
including the V2500, Trent 700, 900 and 1000 engines.
In some very specific cases [C1] the code has been validated against
engine test data, and used with Certification Authority approval, in
support of life predictions of critical engine components. It is estimated
that the use of the code has prevented four engine tests from having to be
carried out, a direct saving of £8M.
The use of this Certification Authority approved method has increased the
accuracy of component life predictions and increased the operating periods
between recalls; thus Rolls-Royce, as well as their customers, have seen
considerable reductions in cost and disruption. The value is impossible to
calculate, but has been informally estimated at around a billion pounds
over a ten year period.
Measurement and Validation
Correlations of heat transfer data acquired from the experimental work,
since the early 1990s and subsequently built upon, have been adopted by
Rolls-Royce for use in computational models predicting surface
temperature, displacement, stress and critical component life. These have
been applied to the internal rotating surfaces of high pressure compressor
drums, where the use of computational models of the fluid flow, CFD, to
acquire heat transfer data are impractical due to the unsteady nature of
the flow field coupled with buoyancy effects.
The correlations are used extensively in current thermal models of Trent
series engines. The main benefit to Rolls-Royce is more accurate
predictions of the life of critical components, which can be used to
determine service and replacement intervals, resulting in improved
operation with the accompanying financial benefits [C1].
The work on the interaction of the internal air system flow with the
drive shaft provided discharge coefficient data for the holes in the drive
shaft used to transfer the internal air system flow to the turbine stages.
This allowed Rolls-Royce to proceed with the design of the Trent series
contra-rotating turbine section, allowing a 0.5% improvement in Specific
Fuel Consumption (SFC), without having to carry out a £2M engine test.
The study that investigated enhancement of heat transfer from compressor
discs has provided very comprehensive design data for future generations
of actively-cooled engines. At the very least, this would allow a 30%
reduction in the clearance between compressor blades and engine casing at
cruise conditions with a corresponding 0.2% reduction in SFC. An
appreciation of the value of this data can be obtained from the fact that
Rolls-Royce are exploiting the findings and applying the results to
current and new engine designs [C1].
Rolls-Royce has used the turbine rim seal flow and heat transfer data to
validate computational predictions of flow structure and convective heat
transfer for some (Trent series) engine components. The understanding
gained through this research is being used to optimise cooling air
consumption in Trent series engines, and to determine turbine disc
integrity and cyclic life. The results from this work have not allowed
engine tests to be replaced with modelling but it does have the potential
to do so [C2]. A direct impact of the work is a reduction of 0.1% in SFC
of Trent XWB engines now on order, this amounts to an extra passenger on
GE Aviation Impact
A new research partnership was formed with GE Aviation in 2011. This was
motivated by the unique test capabilities and skills at Sussex, as
described in the open literature, [R2, R4-R6]. Work carried out for GE has
already had an effect on processes and productivity improvements leading
to a direct saving of $0.55M (£0.34M) [C2].
The environmental impact of the research described, beyond the economic
benefits to Rolls-Royce and GE, is to contribute towards a 2% reduction in
SFC from reduced turbine cooling flow, improved turbine and compressor
efficiency, and a 2% reduction in CO2 emissions, by the
engines currently in use. Broadly, the benefits of research into gas
turbine technology are indicated by an improvement of 1% in cruise
specific fuel consumption (SFC) on a single aircraft engine being worth
around $100,000 (£62,000) / year, 2013 prices, in reduced fuel costs. The
improvement in SFC is matched by an equal saving in CO2
emissions and a 1% reduction in CO2 is equivalent to a saving
of about 600 tonnes of CO2 per engine / year. To put this in
perspective, the current fleet of British Airways aircraft has over 600
engines, and Virgin Atlantic has about 150.
Sources to corroborate the impact
C1 Chief of Thermal Systems, Rolls-Royce.
C2 Advanced Testing Leader, GE Aviation