VIbration Analysis methodology for FUel MAnifolds of lean burn engines

Executive Summary:

The development of the lean burn engine is a significant step for the European aircraft engine industry to ensure a continuous reduction in fuel consumption and operational cost and stay competitive in an expanding global market. One of the fundamental design features of lean burn combustion is the use of staged combustion technology where a lean burning main zone and rich burning pilot are used to create lower pollutant emissions without compromise to system operability. Amongst other things this creates the need for a more complex fuel delivery pipework system to control the fuel distribution to the different combustion zones, leading to increased complexity, space constraint and an increased requirement for mechanical integrity to avoid reliability reduction. The lean burn programme under Clean Sky – Sage 6 has made significant contributions towards a flying demonstrator, but one major problem that must be addressed to make the lean burn system a reality is the correct prediction of the vibration behaviour of the fuel manifold system. A higher risk of rumble in the engine, and a larger amount of pipes for the staged fuel delivery increase the risk of HCF in such a system, and without a detailed analysis the failure risk of the engine may be too high.

The accurate prediction of the dynamic response of a fuel manifold assembly is a challenging task, since it consists of a wide range of components, including straight and bend pipes, different types of brackets that link to the casing, clips that hold the pipes in place, inserts to connect the pipes together, and sometimes damping devices to reduce the vibration response. Modelling each component in itself is already difficult, but once the system is assembled the prediction of the dynamic response becomes very challenging. The assembly of the components adds a large amount of joints to the system, introducing potential slip in the contact areas and a resulting nonlinear dynamic behaviour. During assembly misalignment can be introduced into the system which in turn can lead to pre-stresses in the pipes, which can affect the dynamic response. The presence of fuel will also impact the dynamic behaviour which will need to be considered in an analysis.

The general objective of VIAFUMA is the development of a fully validated modelling approach to predict the dynamic response of a lean burn fuel manifold system. Based on low and high order linear and nonlinear dynamic modelling the developed methodology will allow feasibility studies of the design during the early design development and enable detailed analysis during the later design stages. A bottom-up approach has been used to develop the modelling approach. Starting with the analysis and testing of the basic components of the fuel manifold system more and more complicated assemblies were gradually analysed and tested, to build confidence in the proposed modelling approach. Research in three main areas was conducted: (i) a low order modelling approach, based on shell and beam models and implicit nonlinear elements allowed fast feasibility studies of the system; (ii) a detailed nonlinear analysis approach using 3D finite element models was introduced for accurate prediction of the response amplitudes of sensitive components; (iii) an extensive test programme was carried out to provide validation data at low and operational vibration levels.

All the objectives of VIAFUMA could be delivered during the project, leading to a low order nonlinear modelling approach for the fuel manifold and high order models to investigate particular features in the dynamic response of the system. This report summarises all the technical achievements of VIAFUMA and presents the developed dynamic modelling strategy for pipe networks.

Project Context and Objectives:

The general objective of VIAFUMA is the development of a new and fully validated approach to predict the dynamic response of a lean burn fuel manifold system. Exploiting basic and detailed linear and nonlinear dynamic modelling approaches will allow feasibility studies of the design during the early development and enable detailed analysis and optimisation during the later design stages. The immediate advantages lie in a general increase of life of the fuel manifold system, but the developed strategies will also help to reduce the weight of upcoming designs, leading to a lighter and more efficient jet engine.
VIAFUMA consisted of two Work Packages (WP), WP1 concerned with experimental work, and WP2 focussed on the analytic modelling approach.
2.1 WP1: Experimental
The overall objective of WP1 was to identify the linear and nonlinear dynamic behaviour of the basic, sub, and full assembly of the fuel manifold, and to provide reliable validation data for the newly developed analysis techniques from WP2.
Initially an experimental characterisation of basic pipe assembly (Task 3) was planned to:
• Develop specific test setup for each component of pipework assembly that isolates its vibration behaviour from the support structure. Straight and bend pipes will be investigated together with different types of mounts, inserts, and clips.
• Conduct low level impact hammer and shaker tests for each component, extracting operating deflection shapes with the help of a Scanning Laser Doppler Vibrometer (SLDV).
• Identify from frequency response the stiffness, damping and nonlinear behaviour of each component.
• Investigate the most basic assembly of a straight and bend pipe with the minimum amount of mounts, clips and inserts.
• Design, build and test a setup with a short length of pipe that can be pressurised with oil at different levels to characterise the mass loading and damping properties of the liquid inside the pipe.
• Introduce known misalignment in the assembly to achieve controlled pre-stressing of the pipework and evaluate the effect on the dynamic response.
This was followed by planned operational level testing of the fuel pipe assembly in Task 5:
• A sub-assembly will be tested consisting of several bend pipes, different mounting systems, inserts, and damping elements. Empty and pressurised pipework will be used
• A flat rigid and a flexible curved substructure will be used to determine the influence of impedance on the measured dynamic response
• The tests will be conducted with the High Amplitude Dynamic Excitation System (HADES) at Imperial College London. Operational level vibrations with sine sweep, random and wave form replication inputs will be used
• Non contact SLDV measurement techniques will be used in combination with strain gauges to determine the response of the system during testing
• The dynamic behaviour of the system will be measured, the main sources of nonlinearity identified, and the influence of pressurised pipework on the response investigated
• Misalignment will be introduced to the pipe assembly to generate a known pre-stress in the system, and its influence on the linear and nonlinear response investigated
• Several reassembles of the setup will be tested to understand the variability in the measurement results and provide averaged values for the model validation
The experimental work was to be completed by operational level testing of the full casing assembly in Task 8, including:
• A simplified full casing assembly will be manufactured and tested on the HADES shaker system to reach realistic excitation levels in the required frequency range.
• The linear and nonlinear dynamic response will be monitored with a SLDV to obtain full field operating deflection shapes of the cylindrical structure.

2.2 WP2: Analytical
The overall objective of this task was the development of low and high order models for the prediction of the nonlinear dynamic response of a fuel manifold, including all its components.
This included an initial state of the art review on current pipe work modelling (Task 1) and then focused on the development of detailed models for the basic pipe-mount assembly (Task 4)
• Development of detailed three dimensional solid linear finite element models for each pipework component.
• Validation of the developed linear models against available test data from Task 3
• Nonlinear response predictions of the basic pipework assembly with the in house multiharmonic balance solver FOrced Response SuitE (FORSE)
• Inclusion of fuel effect in to the finite element model by distributed mass loading
• Analysis of pre-stress due to misalignment in the assembly and predictions of its effect on the linear and nonlinear dynamic response of the assembled system.
• Validation of the developed nonlinear models against available test data from Task 3
The knowledge obtained during the basic modelling was then to be used for the development of a high order modelling approach for fuel pipes on the casing (Task 6)
• A detailed solid finite element model of the rigid and flexible subassembly with and without pressurised fuel will be created and its linear dynamic response predicted.
• Based on the linear model of the subassembly a nonlinear dynamic analysis in FORSE will be conducted to predict the forced response of the pipework.
• The sub assembly models will be validated against the obtained test data from Task 5.
• A detailed full assembly model will be generated and its linear and nonlinear dynamic response predicted. This will involve large scale linear and nonlinear modelling with several 1000 nonlinear elements, for which new modelling techniques will be required.
• A comparison between the rigid and flexible support will be used to identify the influence of impedance of the substructure on the test results and to understand how obtained test data can be used to validate the full-assembly model.
As an additional approach to model the fuel manifold response a low order modelling approach for fuel manifolds would be developed (Task 7):
• A low fidelity model of each component, based on a mixture of shell and beam elements will be created to allow a fast computation of the dynamic results. These models will be validated against test and detailed FE data.
• Based on the identified main source of nonlinearity in the assembly, it is proposed to develop a new implicit nonlinear element for FORSE that allows a quick and reliable prediction of the pipework response. The element will require typical characteristics of the components as input, such as amplitude dependent damping and stiffness.
• The pre-stress in the low fidelity models will either be included by modifying the linear modelling approach, or adding a stiffness term during the nonlinear computation of the response.
• The assembled low fidelity model will be validated against the measured data obtained from Task 5 and 8.
Given the novelty of the set task, with a very limited understanding of the fuel manifold dynamic response, the objectives as stated in the Annex A of VIAFUMA, were adjusted to include the obtained knowledge and ensure the most effective way to analyse large scale fuel pipe networks.

Project Results:

The VIAFUMA project was designed to provide detailed understanding of the fuel manifold vibration response and to introduce validated analysis tools to predict such behaviour. The starting point for the project was a basic understanding of the pipe work behaviour, and for this reason the project was designed with a strong experimental focus to provide the required understanding. Once a good understanding was obtained, appropriate modelling techniques were developed to replicate the behaviour and provide predictive tools for future designs.

The experimental work within VIAFUMA highlighted a strong nonlinear dynamic response of the pipe work and very large uncertainty in the response, due to the presence of a multitude of joints in the assembly. A large number of low and operational level tests were conducted on each individual basic component, subassemblies of the pipe work, and a full casing assemblies, in order to identify the crucial components in the dynamic response, their nonlinear interaction, and their behaviour on the global system. The pipe clips were identified as the main source of nonlinearity in the system, with all other components, such as pipes, brackets and connectors showing a linear behaviour. The interpretation of the resulting modal responses of the system proved to be very challenging so that new measurement technology was developed (high speed camera and digital image correlation techniques) to identify the dynamic behaviour of the system.

Throughout the experimental campaign a large variability in the test results was observed, which was attributed to the rather poorly defined contact conditions of the pipe-clip interface and large variability in the alignment of the clips with the rest of the assembly. The many testing campaigns of VIAFUMA let to the availability of a large data set, allowing an accurate identification of the linear and nonlinear parameters that define the dynamic response of a fuel manifold, and providing high quality validation data for the nonlinear modelling.

The developed modelling approach for the fuel manifold was mainly based on the experimental findings in the project, since at the outset of the project it was not clear what kind of behaviour had to be replicated, and how much detail was required to capture such behaviour. Initial testing revealed the need of nonlinear dynamic modelling, and highlighted large uncertainty in the experimental response data. This suggested a low order nonlinear modelling approach, which could cope more easily with the unknown sources of variability, instead of highly detailed nonlinear models which would struggle with the required input accuracy. The final modelling strategy for VIAFUMA was a combination of low order and high order modelling, where the low order models are used to predict the global nonlinear pipe work response, and detailed high order models were employed to investigate special features, such as the fuel effect or misalignments of the pipes.

A low order modelling approach was developed, based on a linear representation of the pipe work, the connectors and the brackets, and an implicit nonlinear dynamic model for the clips. Nonlinear identification techniques were used in combination with specially designed test setups to provide input data for the implicit nonlinear models. The models where than validated against operational level test data ensuring their predictive accuracy. The low order approach allowed fast and robust processing of the nonlinear dynamic response within the observed experimental uncertainty, satisfying the objective of an accurate prediction tool without the need of computationally expensive detailed nonlinear models.

Detailed linear and nonlinear finite element modelling was used when the low order modelling approach did not provide reliable accuracy, highlighting a minor effect of pressurised pipes and pipe misalignment on the dynamic response, and indicating the strong mass loading effect due to the fuel.

3.3 Conclusions

The main objectives of VIAFUMA were to obtain a detailed understanding of the dynamic behaviour of complicated pipe assemblies, and develop an appropriate modelling approach to model the observed behaviour.

A large range of low and operational level tests was conducted, on individual components, sub-assemblies, and on a full assembly, leading to the identification of a strongly nonlinear response driven by the clip element, and a relatively linear behaviour of all the other components. Extremely high modal density was observed on the assembled structure, leading to the development of a high speed camera and digital image correlation technique to track the vibration of individual components. Large observed response variations after reassembly, could be attributed to the alignment and orientation of the clips after a detailed study of the experimental setup. Based on these findings a test setup was developed that allowed a reliable extraction of the input parameters for the modelling approach, and provided good quality validation data for the system.

The experimental understanding of the dynamic behaviour of a pipe network, led to the development of modelling approach to predict its dynamic response. A combination of detailed finite element analysis and low order nonlinear dynamic modelling was employed to reach this objective. Detailed models where thereby used to investigate particular effects of interest and determine their impact on the dynamic response, while a novel low order modelling approach was used to capture the global dynamic response. It could be shown that fuel pressure and misalignment in the pipes only play a minor role, but features such as the geometry of the pipes, the mass loading effect, and the nonlinear components in the assembly must be included in the modelling approach to achieve accurate results. Based on the findings a low order modelling method was developed and validated, which is able to capture the dynamic response within the uncertainty of the experimental data, and which provides a much better understanding of the fuel manifold vibration response.

Potential Impact:

The need for the work in VIAFUMA emerged from the Clean Sky project SAGE 6 for the development of a lean burn fuel system. Lean burn combustor systems are a key technology to reduce NOx emissions of future aircraft engine generations. They use high overall pressure ratios and turbine entry temperatures to substantially reduced pollutant emissions compared to current technology and support the future long term technology goals of the ICAO Committee on Aviation Environmental Protection (CAEP) as well as the future visions laid out in ACARE 2020 and Flightpath 2050.

One particular challenge for lean burn systems is the large number of pipes required, which can significantly increase the likelihood of high cycle fatigue failure due to vibration during operation. VIAFUMA was designed to provide the required inside into the nonlinear vibration behaviour of the pipes and supply predictive capability to model the dynamic response of fuel manifolds accurately in advance.

VIAFUMA has led to a significant increase in understanding of the pipe work vibration for industry, already having helped with the analysis of an existing systems, and providing predictive tools for the design of the upcoming lean burn systems. The outcome of the project has helped to shape design practice and provided confidence in the design approach. It has already been used by industry to analysis the vibration behaviour of a Trent 1000 aero engine fuel manifold, providing invaluable information on the vibration behaviour of the fuel supply pipes. For future engine generations, and in particular for the lean burn engines, the outcome of VIAFUMA will allow the optimisation of the fuel manifold system during the development program of the engine thereby reducing weight, minimising the need for engine test data, reducing wear in the components, extending the fatigue life, and lowering the risk of failure of the system. This will support the introduction of lean burn systems and lead to lower cost during the development process of the engine, by eliminating potentially expensive re-designs.

The better understanding of the vibration response of fuel manifolds, provided by VIAFUMA, will ensure an increased reliability and operational safety of the structure. The avoidance of vibration induced fatigue and failure will allow longer maintenance intervals on the engines, leading to lower operational costs for the airlines, and less risk of an engine shut down, reducing the risk of an engine failure and thereby increasing passenger safety. Design optimisation of the fuel manifold system with regards to vibration response will support the introduction of lean burn systems, and also lead to reduced weight, with only the minimum number of clips and brackets present in the setup. This will lead to more efficient engines, which on one hand will further reduce the operational cost for the airlines due to a reduced fuel consumption, and on the other hand lead to a reduction in CO2 and NOx emissions over the lifetime of the engine.

The VIAFUMA team has published on reviewed journal publication and attended six conferences and workshops to disseminate information to the scientific community. In addition, reports, regular link calls and face to face meetings were used to share the outcome of the project with the industrial costumer, to inform their design process as the program progressed.

Principle Investigator:

Dr. Christoph Schwingshackl

Room 559, City and Guilds Building

Imperial College

Exhibition Road

London SW7 2AZ

Email: c.schwingshackl@imperial.ac.uk