Combustor technology: present and future

The need for future combustor technology to meet environmental constraints

Air traffic is rapidly increasing today and projections estimate it will have doubled by 2020 (4,1% average growth per year until 2020 estimated at CAEP6–2004). Against this background, it is essential to consider the environmental impacts of aviation to ensure in advance that such a rate of development is sustainable. The health of populations in relation to air quality and the risk of climate change linked to the effects of greenhouse gases are two major concerns. In Europe, greater stringency in air quality directives is to be foreseen. Europe has ratified the Kyoto protocol encouraging reductions in greenhouse gas emissions, despite the withdrawal of the US. For these reasons, aviation emissions have to be significantly reduced, as must all other emissions linked to human activities (anthropic emissions). The report of the European group of personalities “European Aeronautics: a vision for 2020” and the related Strategic Research Agenda (SRA – first release issued in November 2002) of the Advisory Council for Aeronautics Research in Europe (ACARE) have set reduction targets of 50% in CO2 and of 80% in NOx for 2020.

Whereas the reduction in CO2 will mainly be achieved by improvements in engine efficiency and aircraft performance characteristics, NOx and others species such as CO, UHC and particulate emissions can be significantly reduced only by focusing on combustor technology, in particular by introducing new concepts for injection systems. Efforts of this kind are supported in a cluster of European programs focusing on Low Emission Combustor Technology. In coherence with results achieved in previous projects like LOPOCOTEP, or activities supported in the INTELLECT DM project, the proposed Specific Targeted Research TLC Project will go further by:

  • investigating more revolutionary technology to meet the challenging ACARE objectives
  • improving physical knowledge and calibrating CFD tools
  • developing high-quality non-intrusive measurement techniques

The following diagram summarizes the environmental challenges listed in ACARE and which will be kept in mind throughout the TLC project:

.<> Environmental Challenges for Aviation (Source: ACARE/SRA Volume 2)

Low emission combustor technology: State of the art

In recent decades, continuous improvements in technology have substantially reduced all the emissions from modern aircraft. This is clearly the case for CO2 (with reductions of more than 40% in fuel burn per kg km in 40 years), for CO and UHC. These levels of reduction have been achieved by working both on engine and aircraft technology. Progress in NOx emissions has been also demonstrated in terms of DP/F00, which is the parameter used by the ICAO engine regulation. The diagram below highlights this constant progress and shows the various engine technology generations that have been gradually developed in order to meet or anticipate new regulation standards (the recently defined CAEP6 limit should be applied to new certified engines in 2008).

.<> ICAO NOx regulation / generations of in production engines / current R&D programs (Source: derived from ICAO database and CAEP6 results – 2004)

The latest Single Annular Combustor (SAC) generation in service combines the General Electric LEC combustor (already in service on CF6 engines & CF34), the Rolls-Royce Phase 5 combustor (already in service on Trent engines) and the P&W Talon II concept (already in service on PW4000 engines). Technology with similar emission performance levels has been developed by Snecma Moteurs for future SM146 engine which should be certified in 2007. All these concepts are based on combustion close to stoichiometry (slightly rich or slightly lean in average in the primary zone of the combustor, depending on the technology). The Double Annular Combustor (DAC) family of the CFM56 (-5B and –7B), is the only technology of its kind in service, enabling even greater levels of NOx reduction, of up to almost 50% of the current CAEP2 regulatory limit. By favoring sufficient lean combustion at high power (take-off), the concept also improves particulate emissions in term of smoke number (the parameter used by the regulation). The combustor is designed by General Electric. Radial staging with a pilot dome and a main dome provides greater freedom for optimizing the combustor at different regimes. In particular, the main dome operates slightly lean (under the stoichiometry) and is optimised for maximum thrust at take-off conditions.

.<> CFM56 5B DAC combustor

Nevertheless, this DAC combustor slightly deteriorates CO and UHC emission levels in comparison with a SAC, particularly at low regime. In addition, the weight and complexity of the system are increased. New single annular technology with fuel staging has been therefore explored by US company General Electric with the TAPS (Twin Annular Premix System – full annular rig tests already performed in 2000), a concept already tested on CFM56 engines. The idea is to combine the two domes into one with fuel staging using two fuel manifolds. Emission performance characteristics already show this approach to be highly promising, making it possible to reduce all types of emissions compared with a DAC. However CO and UHC emissions remain higher than on a conventional SAC engine. Operability constraints are also still to be checked.

.<> GE TAPS Combustor (Source: ICAO emission workshop – Neuilly, Sept. 2002)

Rolls-Royce plc is also developing a similar approach with the ANTLE technology. Preliminary satisfactory results were also demonstrated during 2003 in ANTLE technology platform and LOPOCOTEP program. In both the TAPS and ANTLE technology, the injection system is of the “LP” type, which means that principally for high power the mixture is lean and premixed (LP) before reaching the reaction zone, but the fuel is not yet fully pre-vaporized.

.<> ANTLE Combustor with LP injection system (Source: ICAO emission workshop – Neuilly, Sept. 2002)

Another alternative has been adopted with the CLEAN combustor approach developed by Snecma Moteurs, MTU and Avio. In this case, an axially-staged combustor is used, and the main dome is designed to achieve the best possible lean premixed pre-vaporized combustion thanks to a so-called “LPP” injection system. The use of a DAC makes it easier to meet operability constraints (stability, altitude relight) and in parallel makes it possible to improve the optimisation of the main dome injection system to achieve lean combustion. If premixing and pre-vaporizing are satisfactorily accomplished, one can expect greater levels of NOx reductions (and particulate reductions at the same time) than those demonstrated with the TAPS combustor, resulting in a reduction of more than 60% in relation to the CAEP2 limit. Once again, however, CO and UHC levels increase slightly in relation to the SAC design, combustor mass and complexity are higher, and the cooling issue is more difficult to solve.

This is the main reason why all European engine manufacturers are currently looking at fuel staging on a single annular combustor. This trend was already observed in the FP5 LOPOCOTEP program (RRD and SNM activities), and Rolls Royce is currently exploring the area with the ANTLE technological platform.

.<> CLEAN Axially-staged Combustor with main dome equipped with LPP injection system

Specific results obtained in recent European low emission combustor technology programs

Various FP4 and FP5 European technological programs have been addressing the lean combustion target. This continuous effort is justified by the fact that many engine conditions have to be covered (small engines with reduced OPR to large engines with OPR up to 40), many potential designs and concepts are possible (combustor architecture: SAC or DAC; injection system concept: LPP, LP, LDI, multi-points…), various specific difficulties have to be solved (auto-ignition, flashback, instabilities, lean extinction limit). It should be stressed also that the maturity achieved by each industrial is not necessary the same and that the NOx reduction ambition may also vary.

The last main programs dedicated to low emission combustor technology developments by most of the European engine manufacturers are LowNOx III (FP4) and LOPOCOTEP. Their special focus was on injection systems of LPP type. LowNOx III part 1 (FP4) was completed in 2001, and part 2 was reoriented both by Snecma Moteurs and Rolls-Royce plc in support of the respective CLEAN and ANTLE technological platforms described above. LOPOCOTEP (FP5) ends in 2005 when TLC started and satisfactory progress was validated at the mid-term. The objective of both programs was to reduce NOx emissions during the LTO cycle with a target reduction of 60% in relation to CAEP2 in LowNOx and of 80% in relation to CAEP2 in the LOPOCOTEP program.

LowNOx III investigated LPP injection system designs covering every level of engine compression ratio, from OPR=15 for small engines to OPR=40 for large engines. Except for small engines, all the manufacturers were working on axially or radially-staged concepts. The main conclusion of the project as observed in the final reports was that LPP technology was easier to develop for smaller OPR engines. The NOx reduction targets were met in some cases (small engines: –50% NOx reduction compared to conventional technology; medium engines –60% in relation to CAEP2). Unexpected difficulties arose in the case of large engines at high pressure (severe instabilities) but partial results on the LPP modules were nevertheless encouraging. In fact, a higher OPR not only increases the risk of auto-ignition but seems to generate a propensity for instability in well premixed systems.

The work was pursued in the LOPOCOTEP program. All the industrial partners have pursued the improvement of the performances: emissions level, stability limit, operating range conditions. In addition they may have focused on specific points: imperative size reduction for Turbomeca in collaboration with Avio, high pressure multi-sector test of the ANTLE injection system of LDI type by Rolls-Royce plc, first investigation of LPP injection systems applied to single annular concepts (thanks to fuel staging) by Snecma and Rolls-Royce Deutschland.

.<> LPP Injection system scheme – fuel spray & mixing

.<> CLEAN: LPP Injection System (SNM) (injection system of the main dome)

.<> LOPOCOTEP: LP (P) 4 injection. System (RRD) (Multi-plain jet lean module with centrally integrated pilot burner)

LowNOx III/1: LP injection system (RR) (LP centre body stabilized injector used for HP tests and large engines)

.<> LOPOCOTEP: LPP injection system (Turbomeca)