Hukam C. Mongia

CFD Best Practices to Predict NOx, CO and Lean Blowout for Combustor Design

An effort was undertaken to identify best practices for CFD analysis of fluid flow in Lean-Direct Injection (LDI) combustors with swirl-venturi swirlers for next-generation LDI design. The National Combustion Code (NCC) was used to perform non-reacting and reacting computations on several LDI injector configurations in a 3×3 element array with different vane angles. Non-reacting and reacting computations were performed with single and multi-element LDI configurations, with a consistent approach to mesh-optimization, spray-modeling and kinetics-modeling with the NCC. Computational predictions of NOx emissions index were compared with experimental data. CFD values of CO were used to assess lean blow-out predictive capability for two different multi-element, LDI configurations and compared to experimental data.Copyright

Engineering Aspects of Complex Gas Turbine Combustion Mixers Part III: 30 OPR

An overview is given on the use of several complex multi-swirler devices in gas turbine combustion and attendant technological advances in emissions, cooling, pattern factor, operability and overall temperature increase across the combustor. The emphasis of this paper is on the initiation of promising lean-direct injection concepts, LDI and how it led to development of several innovative premix- and partially premix-concepts applicable mostly for the 30 overall pressure ratios, OPR gas turbine engines. I INTRODUCTION

A Second Generation Swirl-Venturi Lean Direct Injection Combustion Concept

A low-NOx aircraft gas turbine engine combustion concept was developed and tested. The concept is a second generation swirl-venturi lean direct injection (SV-LDI) concept. LDI is a lean-burn combustion concept in which the fuel is injected directly into the flame zone. Three second generation SV-LDI configurations were developed. All three were based on the baseline 9-point SV-LDI configuration reported previously. These second generation configurations had better low power operability than the baseline 9-point configuration. Two of these second generation configurations were tested in a NASA Glenn Research Center flametube; these two configurations are called the flat dome and 5-recess configurations. Results show that the 5-recess configuration generally had lower NOx emissions than the flat dome configuration. Correlation equations were developed for the flat dome configuration so that the landing-takeoff NOx emissions could be estimated. The flat dome landing-takeoff NOx is estimated to be 87–88% below the CAEP/6 standards, exceeding the ERA project goal of 75% reduction.

N+3 and N+4 Generation Aeropropulsion Engine Combustors: Part 2 — Medium Size Rich-Dome Engines and Lean-Domes

Comprehensive assessment of the medium size rich-dome engines was conducted leading to the following emissions correlations:Display Formula(1)LTO NOx=1.129×OPR1.0899withR2=0.9248Takeoff NOxEI given byDisplay Formula(2)NOxEI=0.0729×OPR1.7197withR2=0.9603Display FormulaCOEIidle=396.42NOxEITakeoff0.814These correlations may be compared with the following for the CFM56 Tech Insertion:Display FormulaTakeoff NOxEICFM_TI=0.0744×OPR1.7151Display FormulaIdle COEICFM_TI=396.42Takeoff NOxEI0.814Display FormulaIdle HCEICFM_TI=0.1609×Idle COEI-3.1959TALON II takeoff NOxEI data are reproduced well by:Display FormulaNOxEITALON II=0.0167×OPR2.1403TALON II gives 10% lower NOx at 26 OPR and its NOx is comparable with the CFM_TI at 34 OPR.The CFM DAC technology is competitive with LEC’s for the low rated thrust engines. However, interaction between the two domes leads to early quenching with resultant higher idle COEI plateau. On the other hand, the 40 OPR lean DAC gave 25% higher NOx than LEC. Moreover, lean DAC (Gen-1) impacted fuel burn adversely making its likelihood to continue as product discouraging.The second generation lean dome technology initially kicked off under NASA sponsorship with significantly larger funding support from the CFMI and GE Aviation (GEA) led to successful introduction of TAPS into products (GEnx-1B and Gen-2B) with potential applications in other future GEA engines.Copyright

Engineering Aspects of Complex Gas Turbine Combustion Mixers Part IV: Swirl cup

An overview is given on the use of several complex multi-swirler devices in gas turbine combustion and attendant technological advances in emissions, cooling, pattern factor, operability and overall temperature increase across the combustor. The emphasis of this paper is on the development and innovative applications of twin-concentric richand lean-direct injection mixers, their use in the dual-annular combustors, and finally making its extension to premixed mixers for the Dry Low Emissions, DLE twin and triple annular combustors.

Parametric Design of Injectors for LDI-3 Combustors

Application of a partially calibrated National Combustion Code (NCC) for providing guidance in the design of the 3 generation of the Lean-Direct Injection (LDI) multi-element combustion configuration (LDI-3) is summarized. NCC was used to perform non-reacting and two-phase reacting flow computations on several LDI-3 injector configurations in a single-element and a five-element injector array. All computations were performed with a consistent approach for mesh-generation, turbulence, spray simulations, ignition and chemical kinetics-modeling. Both qualitative and quantitative assessment of the computed flowfield characteristics of the several design options led to selection of an optimal injector LDI3 design that met all the requirements including effective area, aerodynamics and fuel-air mixing criteria. Computed LDI-3 emissions (namely, NOx, CO and UHC) will be compared with the prior generation LDI2 combustor experimental data at relevant engine cycle conditions.

N+3 and N+4 Generation Aeropropulsion Engine Combustors: Part 1 — Large Engines’ Emissions

A comprehensive assessment of emissions characteristics of the 1st, N and N+1 generation rich-dome combustion products has been done to identify the lowest emissions products. Focus of this paper is on the large rich-dome engines with its potential application for the (N+3) and (N+4) mixers with inspirational target takeoff NOxEI of 5 at 55 OPR.A total of ten engine models of the 1st generation were selected in addition to eight recently certified large engines. After evaluating several choices for conducting comparative assessment, the following three expressions were proposed for average takeoff NOxEI, idle COEI and HCEI entitlements, respectively:Display FormulaNOxEIL=0.0288×OPR1.991Display FormulaIdle COEIL=815.36Takeoff NOxEIL1.159Display FormulaIdle HCEIL=0.15×Idle COEIL-2.0In regard to application of the rich-dome technology to the (N+2) cycle based (N+3) mixers, the author tentatively gives it low probability of success barring success story stemming from Lee et al. [2012].Copyright

CFD Based Design of a Filming Injector for N+3 Combustors

An effort was undertaken to perform CFD analysis of fluid flow in Lean-Direct Injection (LDI) combustors with axial swirl-venturi elements for next-generation LDI-3 combustor design. The National Combustion Code (NCC) was used to perform non-reacting and two-phase reacting flow computations for a newly-designed pre-filming type fuel injector LDI-3 injector, in a single-injector and a five-injector array configuration. All computations were performed with a consistent approach of mesh-optimization, spray-modeling, ignition and kinetics-modeling. Computational predictions of the aerodynamics of the single-injector were used to arrive at an optimized main-injector design that meets effective area and fuel-air mixing criteria. Emissions (EINOx) characteristics were predicted for a medium-power engine cycle condition, and will be compared with data when it is made available from experimental measurements. The use of a PDF-like turbulence-chemistry interaction model with NCCs Time-Filtered Navier-Stokes (TFNS) solver is shown to produce a significant impact on the CFD results, when compared with a laminar-chemistry TFNS approach for the five-injector computations.

A Comparison of Three Second-generation Swirl-Venturi Lean Direct Injection Combustor Concepts

Three variations of a low emissions aircraft gas turbine engine combustion concept were developed and tested. The concept is a second generation swirl-venturi lean direct injection (SV-LDI) concept. LDI is a lean-burn combustion concept in which the fuel is injected directly into the flame zone. All three variations were based on the baseline 9- point SV-LDI configuration reported previously. The three second generation SV-LDI variations are called the 5-recess configuration, the flat dome configuration, and the 9- recess configuration. These three configurations were tested in a NASA Glenn Research Center medium pressure flametube. All three second generation variations had better low power operability than the baseline 9-point configuration. All three configurations had low NO(sub x) emissions, with the 5-recess configuration generally having slightly lower NO(x) than the flat dome or 9-recess configurations. Due to the limitations of the flametube that prevented testing at pressures above 20 atm, correlation equations were developed for the at dome and 9-recess configurations so that the landing-takeoff NO(sub x) emissions could be estimated. The flat dome and 9-recess landing-takeoff NO(x) emissions are estimated to be 81-88% below the CAEP/6 standards, exceeding the project goal of 75% reduction.

Future Trends in Commercial Aviation Engines’ Combustion

This article gives an overview of the current rich-dome combustion system design, requirements and challenges; followed by the first alternative to rich-domes that have been successfully introduced as products; the lowest levels of achievable NOx (so called entitlement) as determined from small scale rig testing; summary of recent engine emissions data with “green” alternative fuels; description of the 2nd alternative to rich-dome products that may be of interest to the OEM’s for the N+2 and N+3 generation aviation engines; a brief discussion on the modeling and correlation accuracy expectations from future efforts in this area; the 3rd alternative to rich domes which was shown promising for autoignition times closer to 0.2 ms. The article concludes with a short section on operability and dynamics. Several large low-NOx rich domes’ takeoff NOx emission index is reproduced well by a simple correlation NO x RD −L = 0.0303PR 1.9722 w / R 2 = 0.9906 including Talon II and Trent1000. However, the LTO NOx is correlated well by a similarly good quality curve only for the group of combustors without Trent1000, DP / F 00 = 0.6793PR 1.2241 compared to lower value expression for Trent1000 alone given by DP / F 00 = 0.1292PR 1.6327. Consistent with the NOx stringency pattern set by CAEP4, CAEP6 and CAEP8 and longtime goal for achieving 85 % reduction in takeoff NOx at 30OPR, we propose the long-term LTO regulatory standard of CAEP / 18 = −37.763 + 2π effective December 31, 2033. The combustor inlet temperatures for desired overall pressure ratio at sea-level standard day static condition can be estimated by using T 3,SSS = 317.544OPR 0.2704; 320.1955OPR 0.272, respectively for the N+1 and N+2 generation engines. This along with generally accepted requirements for combustor operability, we have to manage significantly increased range of P3, T3 and fuel/air ratio, viz. T3: 216.67–1084 K; P3: 0.33–60 atm; ΔP: ~ (0.1–1.2) ΔPdesign; and FAR: FARmin - FARmax. The numerical values of the dome design pressure drop (ΔPdesign), minimum and maximum fuel air ratios (FARmin and FARmax) depend upon the combustion system design and its potential applications. We will assume typical values of these variables, respectively, 3–5 %, 0.005–008 and 0.025–0.040 for the aviation engines. The 1st generation of lean dome products met their original objectives of achieving lower NOx within the specified design constraints including cooling technology. They were immediately followed by the 2nd generation lean dome products known popularly as TAPS in GEnx in addition to planned LEAP-X and GE9X. The takeoff NOx of GEnx is given by NOx GEnx = 1.079 × 10-5 OPR 3.971 W / R 2 = 0.991. These products will be able to meet the proposed long-term LTO NOx regulatory standard within the generally accepted design modification and refinement process. TALON-X and recently introduced PW an interesting technologies competition. The 2nd generation lean domes produce an order of magnitude lower exhaust smoke number than the rich domes. For all other design requirements both the lean and rich domes have comparable characteristics. The effect of FT fuel blends on combustion efficiency and NOx is insignificant; but its benefits in regard to particulate emissions are enormous in terms of both the number density and mass emissions due primarily to significantly lower aromatic and sulfur contents. Future CFD and semi-empirical model should be developed with the proposed long term accuracy goal expressed in term of the standard deviation σ goal: 3 % of takeoff NOx, 7.5 % and 15 % respectively of idle CO and HC emission indices.