The Pratt & Whitney TALON X Low Emissions Combustor: Revolutionary Results with Evolutionary Technology

Transcription

1 45th AIAA Aerospace Sciences Meeting and Exhibit 8-11 January 2007, Reno, Nevada AIAA The Pratt & Whitney TALON X Low Emissions Combustor: Revolutionary Results with Evolutionary Technology Randal G. McKinney *, Domingo Sepulveda, William Sowa, and Albert K. Cheung Pratt & Whitney, East Hartford, CT, Pratt & Whitney has developed a family of reduced NOx combustors for aircraft gas turbine engines using the rich, quench, lean approach. This paper describes an extension of this family to an advanced concept called TALON X that has demonstrated the potential for very low NOx levels in development testing. The requirements for a combustor design are described and the key technologies investigated including a uniform rich primary zone, optimized quench, advanced cooling, and residence time reduction are discussed. Results demonstrating the potential of TALON X to produce NOx levels 70% below CAEP2 levels are presented. I. Introduction VER the past decade, Pratt & Whitney has developed the TALON ** family of low NOx combustors for Ocommercial aircraft gas turbine engines. Each combustor in this family has achieved progressively lower levels of NOx while still employing the proven, simple, and safe rich quench lean (RQL) combustor design approach. P&W is committed to provide its customers with robust, highly maintainable, low-cost combustor components that provide significant margin to emissions requirements over the lifetime of the product. To this end, the RQL design has been fully evaluated for its potential to meet customer and community needs for low emissions in near term and long term (beyond 2015) products. The newest member of this family, TALON X, has produced NOx levels which improve upon current TALON II by 25% in rig and engine tests of early prototype concepts, indicating the potential to achieve NOx levels up to 70% below CAEP2 regulations in a future product. Additional product concerns of safety, fuel burn/co 2 emissions reduction targets; operability/reliability and customer economics are also well addressed using TALON X. II. Combustion System Requirements & Design Process The inviolate requirement for aircraft system level safety along with the need for gaseous, smoke, CO2 and community noise reductions combined with airline economics requirements can impose competing constraints on combustor design. The interrelationships between these requirements including emissions, pressure loss, stability, operability, altitude relight, exit temperature quality, cost, and weight are a key part of the evaluations that were done to understand the viability and desirability of RQL for future applications. The iterative nature of the commercial combustion system design process can impede the satisfaction of emissions targets as these constraints are addressed in the design. These impediments were considered for TALON X, with favorable results in the final evaluation as it relates to product readiness. III. Combustor Design Strategies In the pursuit of reduced combustor emissions, a number of combustor design strategies are being pursued, including the traditional RQL approach employed in the TALON family and the Lean Staged (or Lean-Direct Injection) concept. The RQL approach has been in use for many years in gas turbine combustors. It is a reliable, * Senior Fellow Combustion Aerothermal, 400 Main Street, AIAA Member Manager, Environmental Regulatory Affairs Emissions, 400 Main Street, AIAA Member F119 Combustor CIPT Manager, 400 Main Street Staff Engineer Combustor Technology, 400 Main Street ** Technology for Advanced Low NOx 1 Copyright 2007 by the, Inc. All rights reserved.

2 low cost approach with many advantages in meeting the full range of combustion system requirements including NOx emissions targets. A description of the fundamentals of RQL combustor design can be found in Ref. 1. Staged combustor designs, such as the Axially Staged Combustor 2, offer additional flexibility to reduce NOx emissions by accommodating lean operation at selected engine power levels. These Lean Staged systems have the disadvantage of increased cost, weight and complexity along with the potential for combustion instabilities and higher emissions of Carbon Monoxide and Unburned Hydrocarbons due to quenching. Moreover the introduction of these systems often requires consideration of multiple system level impacts on hardware and control software. P&W has partnered with NASA in several programs, such as HSR, AST and UEET, to investigate advanced combustor designs as illustrated in Fig LTO nox (g/kn - Percent of CAEP2) HSCT RQL AST TALON III LDI UEET TALON III, X, LDI ICAO Regulation NASA Program & P&W Focus 40 Jan-84 Jan-88 Jan-92 Jan-96 Jan-00 Jan-04 Jan-08 Jan-12 Date Figure 1. NASA / P&W program history These programs established the feasibility of NOx reduction technologies and helped guide the development of NOx stringency regulations. The HSR program established best-to-date RQL performance levels in idealized rig testing that approached Lean, Premixed, Prevaporized NOx emissions, however this potential needed to be translated into viable product designs. The AST program provided the opportunity to investigate and directly compare the RQL approach in viable product designs versus Lean Staged. The outcome of this work 3 led to the selection of RQL for further investigation. The RQL concept was further refined in the UEET program, resulting in technologies that enabled introduction of increasingly lower NOx combustors into the P&W fleet of engines while meeting mandated stringency levels for other pollutants. In addition, the Lean Staged concept was further investigated during the UEET program, where it was determined that the increased complexity in design that results from implementation of Lean Staging in a production engine was not required in light of the potential of RQL. 2

3 PW4084 PW4098 TALON I PW6000 TALON II Figure 2. Existng TALON combustor family For the existing TALON family of combustors shown in Fig. 2, NOx reduction has been achieved by improving four fundamental characteristics of the RQL concept. Figure 3 illustrates these focus areas: A uniform rich primary zone Optimized quench mixing Advanced cooling technologies Reduced combustor residence time Uniform Rich Primary Zone Driven by Injector Design Optimized Quench Mixing Using Tailored Single Row of Holes Reduced Combustor Residence Time Via Minimized Volume Figure 3. TALON X combustor focus areas 3

4 The resulting emissions characteristics for the existing, in service TALON I and TALON II combustors is compared to legacy P&W designs in Fig. 4. The development process for these designs used a range of analysis tools 4 as well as subscale rigs, full annular rigs, and engine tests. The TALON X approach is a significant extension of the TALON family to further reduce NOx by continuing to improve these characteristics LTO nox (g/kn - Characteristic) V2500 PW4084 PW6000 TALON II PW4168 PW4168 TALON II PW4158 PW4158 TALON II PW4098 TALON I LTO CO (g/kn - Characteristic) Figure 4. LTO NOx versus CO for the existing TALON family A. Uniform, Rich Primary Zone Design To minimize NOx in the primary zone, it is essential that liquid fuel is vaporized and mixed very quickly so as to create a uniformly mixed rich zone which has excellent stability characteristics and is sufficiently rich to minimize NOx production without producing unacceptable levels of smoke. This uniformly-rich front-end is achieved by developing fuel injection systems that produce fine, well-distributed sprays that are well-matched to the swirler flow field. The TALON X injector is of the high shear style 5 shown in Fig. 5. This system creates a fine spray by distributing the fuel over a large-diameter filming surface, resulting in a thin film that leads to very small fuel droplets. These small droplets evaporate quickly, and track the swirler air flow easily, enabling excellent fuel/air mixing within a short distance of the swirler exit. The spray cone angle and primary zone aerodynamics can be modified by changing the swirl angles of the two swirling air passages and the air flow split between them. The TALON X injector and swirler combination was optimized to provide the required level of mixing and the combustor mechanical design was also improved to control the relative positions of the injector and swirler 5 at operating temperatures and pressures. 4

5 Figure 5. High Shear swirler Difference between ideal and achieved is dependent on mixing rate NO x (EI) Average Achieved Ideal Mixing Combustor Exit Combustor Front End Equivalence Ratio Figure 6. High mixing rate is critical to achieving RQL potential B. Optimized Quench Mixing The second focus area for TALON X development was improved quench zone mixing. As shown in Fig. 6, it is critical that rapid mixing is achieved to complete combustion of the rich mixture exiting the primary zone with 5

6 minimum NOx production. TALON X uses an optimized single row of mixing holes developed using a combination of experimental and analytical tools. Jet-in-cross flow mixing experiments using Carbon Dioxide introduced at the fuel injectors and gas sampling instrumentation were used to optimize the hole locations, combustor height and hole sizes 6. The resulting hole distribution is a single row pattern similar to the TALON II, but with circumferential location and size variation optimized for the TALON X application to improve both exit-plane mixedness and quench-zone mixing. The former is a prime driver for pattern factor reduction, whereas the latter is crucial to NOx reduction. In addition the data from the experiment was used to develop and validate improved sub-models for the ALLSTAR CFD code of Reference 2. This code was then utilized to run a matrix of combustor configurations at appropriate operating conditions to evaluate the performance of the combustor and ensure that it meets the other requirements such as combustor exit temperature quality. The candidate configurations from the CFD analysis were then tested in a full annular rig to verify and further optimize the design. C. Advanced Cooling Technology A key supporting technology for quench mixing is combustor liner cooling design as it determines the total amount of air available for the mixing process. The TALON X combustor utilizes an improved version of the P&W FLOATWALL configuration which provides high levels of cooling efficiency combined with improved materials and thermal barrier coatings. The resulting cooling air budget for TALON X is 20% of combustor exit flow compared to 33% for the base PW4084 combustor. D. Reduced Combustor Residence Time The fourth focus area for TALON X was reduced combustor residence time, especially in the quench zone. This is accomplished via optimization of the combustor total volume as well as the distribution of combustor area as a function of length. The reduced cooling air demand discussed above also contributes to a reduction in effective residence time. The resulting residence times for the TALON X prototype and the resulting emissions index of NOx at take-off power conditions are compared to the existing TALON family in Fig Normalized residence Time SLTO NOX (EI) 0.00 PW4084 PW4098 TALON IPW6000 TALON II TALON X 0.0 Figure 7. TALON residence time reduction 6

7 IV. Results A prototype TALON X configuration has completed both annular rig and engine evaluation with excellent results. Figure 8 adds the results of the TALON X testing to the NOx versus CO plot of Fig. 4, showing a significant improvement in both for incorporation of the combustor into the new P&W Geared Turbo Fan engine configuration. Annular rig test results also show that TALON X meets the other key performance parameters noted above. Combustion efficiency data at cruise conditions confirm that there has been no negative impact on this characteristic and exit temperature data shows that the combustor successfully met this requirement. Engine testing for combustor operability, including starting and lean blow out also met development requirements. As shown in Fig. 9, these results show the capability to achieve approximately 30% of the CAEP2 standard, and further improvement is likely as development continues LTO nox (g/kn - Characteristic) V2500 PW4084 PW6000 TALON II PW4168 PW4168 TALON II PW4158 PW4158 TALON II PW4098 TALON I GTF TALON X LTO CO (g/kn - Characteristic) Figure 8. LTO NOx versus CO including TALON X V. Research Needs There are several areas of research that would improve the ability to design and model RQL combustors. New instrumentation capability and experiments to measure reacting flow in a rich primary zone would aid development of modeling tools as would measurements of reacting jet-in-cross flow mixing with a rich incoming mixture. Also, development and validation of soot, NOx, and CO production and consumption models for incorporation into CFD codes would be beneficial. VI. Conclusions & Future Work The TALON X is a promising concept with very good results in development to date. It offers a simple, reliable combustion system that will meet all requirements at low cost. The next phases of TALON X development focus on targeted application requirements in products under consideration by P&W. 7

8 Acknowledgments The authors would like to thank the NASA Glenn Research Center for their financial support and technical assistance LTO nox (g/kn - Percent of CAEP2) RQL TALON II TALON III, X, LDI 0 HSCT AST UEET Jan-84 Jan-88 Jan-92 Jan-96 Jan-00 Jan-04 Jan-08 Jan-12 ICAO Regulation V2500 PW4158 PW4168 PW4084 PW4098 TALON I PW4158 TALON II PW4168 TALON II PW6000 TALON II TALON X in GTF P&W Focus NASA Program Figure 9. TALON X Emissions Level Compared to CAEP 2 References 1 G. J. Sturgess, R. McKinney, and S. Morford, Modification of Combustor Stoichiometry Distribution for Reduced NOx Emission from Aircraft Engines, Journal of Engineering for Gas Turbines and Power, Vol. 115, No 3. July I. Segalman, R. G. McKinney, G. J. Sturgess, and L. M. Huang; Reduction of NOx by Fuel Staging in Gas Turbine Engines A Commitment to the Future ; Presented at an AGARD Meeting on Fuels and Combustion Technology for Advanced Aircraft Engines, May A. Cheung, R. G, McKinney, and S. Syed Overcoming Barriers to Ultra Low Emissions"; ISABE ; Presented at the 16th International Association of Air Breathing Engines Symposium T. Snyder, J. Stewart, M. Stoner, and R. McKinney Application of an Advanced CFD-Based Analysis System to the PW6000 Combustor to Optimize Exit Temperature Distribution - Part II: Comparison of Predictions to Full Annular Rig Test Data ;; 2001-GT-0064; J. M. Cohen and T. J. Rosfjord Influences on the Sprays Formed by High-Shear Fuel Nozzle/Swirler Assemblies ; Journal of Propulsion and Power, Vol. 9, no 1. January NASA AST Program Progress Report, November