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Development of simulation methods and tools to predict the idle and sub-idle behavior of future large Very High Bypass Ratio geared civil turbofan engines.

Periodic Reporting for period 1 - PROTEUS (Development of simulation methods and tools to predict the idle and sub-idle behavior of future large Very High Bypass Ratio geared civil turbofan engines.)

Reporting period: 2018-02-05 to 2019-08-04

Clean Sky 2 identifies the Very High Bypass Ratio (VHBR) geared turbofan engines amongst the main candidates to reduce aviation’s impact on environmental pollution for near-future large civil aircraft applications. Innovative technologies for VHBR geared turbofans are expected to improve fuel efficiency by more than 25% with a view towards achieving the ACARE Flightpath 2050 targets. Despite the potential benefits of the new VHBR geared engine architectures, specific challenges and limitations have been identified in terms of the performance at the edges of the operating envelope, such as idle and sub-idle operation.

The project PROTEUS (PeRformance & Operability of Turbofan Engines Under Sub-idle) focuses on the research and development of tools and methods capable of predicting the idle and sub-idle performance and operability of VHBR geared turbofan engines. This will be achieved through detailed component aerodynamic performance characterization of validated high-order methods, which will be subsequently reduced to 0D, and integrated for rapid whole-engine analysis. Through a better understanding of the idle and sub-idle performance of the next generation of large turbofans, project PROTEUS directly contributes towards enabling the Topic Manager, Rolls-Royce plc, and the European aviation industry meet Flightpath 2050 targets. The work will also enable sub-idle considerations to be included in the engine’s preliminary design process and aid in certification.
PROTEUS capitalises on several in-house methods, previously developed and also currently under development by the Consortium partners, for idle and sub-idle component performance analysis and representation. The Consortium comprises three Universities: Cranfield University (CU) in the UK (Consortium Lead), University of Cambridge (UCAM) in the UK and the Karlsruhe Institute of Technology (KIT) in Germany. (See Figure 1).

The overall objectives set to achieve the aim of this work are described as follows:

1.Improve understanding on axial-flow compressor operation under idle and sub-idle conditions and develop methods to predict sub-idle performance including the effects of bleed valves and variable geometry, heat soakage and stall drop-in during cranking and light-up.
2.Understand staged lean-burn combustor operability within a VHBR geared turbofan at sub-idle conditions and the effect of spray pattern and heat release on the combustor performance.
3.Understand the idle and sub-idle performance of the LP system by analysing fan-intake interactions, installation effects from the wing and aircraft on the exit pressure field, and turbine performance.
4.Integration of methods and tools for component characterisation in NPSS to calculate whole-engine performance at sub-idle and idle conditions.
Work performed from the beginning of the project to the end of the period covered

WP2. Axial-flow compressor 3D CFD simulations were performed at idle and sub-idle speeds, including locked rotor and windmilling conditions to numerically obtain sub-idle compressor maps. Various RANS steady-state simulations were conducted for two modern VHBR turbofan compressors, where sub-idle characteristics were obtained. Time-averaged unsteady RANS SST SAS simulations were necessary to simulate windmill-relight conditions, obtain characteristics and provide boundary conditions to the UCAM partners for the completion of Milestone 4. 2D CFD stage simulations were also performed on a range of blade geometries with different combinations of stagger, solidity and incidence to expand and confirm sub-idle loss and deviation correlations. These correlations were incorporated into low-order methods (LOM) and specifically in one mean-line (1D) and two through-flow codes (2D and quasi-3D respectively). Additionally, the first insights into the effect of bleed valves, variable-geometry, stall drop-out during light-up, and heat soakage on sub-idle performance were obtained.

WP3. The contribution of KIT focuses on the study of the atomization at sub-idle conditions by means of a 2D Smoothed Particle Hydrodynamics (SPH) method. A 2D SPH air-blast atomiser model simulation was performed to provide spray starting conditions for the staged lean-burn combustion and data for a primary atomization model. This work supported the achievement of Milestone 3 to produce the spray pattern prediction using 2D SPH. The contribution of UCAM focused on the calibration of a low-order ignition model (SPINTHIR) for kerosene combustion and its application to a preliminary gas turbine geometry based on a LES cold-flow solution for starting conditions. This effort supported the successful completion of Milestone 4 to produce CFD results for the unignited flow and low-order modelling of the ignition process.

WP4. The impact of the aircraft’s geometry in terms of the wing and nacelle was studied to analyse the effect on the bypass nozzle exit static pressure field. In this regard, nozzle characteristic maps for idle descent cases were mapped using tool-sets developed within CU. 3-D CFD simulations along descent were carried out for an uninstalled engine and compared to an installed aircraft configuration, where nozzle exhaust metrics were obtained. Furthermore, a novel method for evaluating the velocity coefficients for the individual nozzle streams was also developed.

WP5. The NPSS whole-engine performance model for a 3-spool engine was provided by the Topic Manager. Testing, adaptation and modification of the NPSS 3-spool whole-engine model for sub-idle characteristics was performed. The R-R NPSS platform was adopted within the University Technology Centre (UTC) at CU and it now exists for the integration of improved component models. Preliminary whole-engine model runs were conducted for windmill relight conditions, demonstrating the capabilities of the tool to simulate engine start.

In the first period of the PROTEUS project, the following major achievements were obtained with direct impact and benefit to the Topic Manager:
• A set of CFD-based sub-idle loss and deviation correlations were generated and delivered to the Topic Manager.
• A map generator for sub-idle characteristics (SIMAP) was developed by CU and delivered to the Topic Manager. This tool is currently in use by the R-R compressor department.
• The previously developed SPINTHIR tool by UCAM was improved to account for fuel fluctuations. The updated version of the code will be delivered to the Topic Manager as an earlier version of the code is currently in use by the R-R combustion department.
• The 2D SPH tool developed by KIT was calibrated for planar air-blast atomiser physics to represent film thickness and the evolution of droplets and ligaments.
• Two potential inventions were submitted to the Topic Manager for consideration of patenting:
-Shape-shifting combustion diffuser that responds to temperature.
-A new sub-idle map generation approach.

Expected Results

A whole-engine model will be created to simulate idle and sub-idle conditions for ground-start and windmill-relight operations. The new toolset will allow to meet the today's safety requirements for in-flight relight and windmilling drag. During the engine design consideration, the new methods will allow to increase the performance gain for start-up, and sizing of the front-end and starter.