Lean Direct Injection Research Articles

Characterization of Swirl-Venturi Lean Direct Injection Designs for Aviation Gas Turbine Combustion

Injector geometry, physical mixing, chemical processes, and engine cycle conditions together govern performance, operability, and emission characteristics of aviation gas turbine combustion systems. The present investigation explores swirl-venturi lean direct injection combustor fundamentals, characterizing the influence of key geometric injector parameters on reacting flow physics and emission production trends. In this computational study, a design space exploration was performed using a parameterized swirl-venturi lean direct injector model. From the parametric geometry, 20 three-element lean direct injection combustor sectors were produced and simulated using steady-state Reynolds-averaged Navier–Stokes reacting computations. Species concentrations were solved directly using a reduced 18-step reaction mechanism for Jet A. Turbulence closure was obtained using a nonlinear model. Results demonstrate sensitivities of the geometric perturbations on axially averaged flowfield responses. Output variables include axial velocity, turbulent kinetic energy, static temperature, fuel patternation, and minor species mass fractions. Significant trends have been reduced to surrogate model approximations, intended to guide future injector design trade studies and advance aviation gas turbine combustion research.

Distribution of Two-Stage Direct Injection CNG-Air Mixture near Lean Limit Using CFD

Previous work shows that gas-jet ignition with two-stage injection technique is effective to extend lean combustible ranges of CNG engines. In this report, the robustness of the gas-jet ignition with two-stage injection method was investigated purposely to improve the performance of a lean burn direct injection CNG engine. The experiment was conducted using an engine at speed of 900 rpm, fuel-injection-pressure of 3MPa, equivalence ratio at 0.8, and ignition timing at top dead center. The effect of first injection timing on the test engine performance and exhaust emission was analyzed. First injection timings near the gas-jet ignition produced unstable combustion with occurrence of misfires except at a timing which produced distinctively good combustion with low HC and CO emissions. Computational fluid dynamics was used to provide hindsight of the fuel-air mixture distribution that might be the cause of misfires occurrence at certain injection timings.

Multiscalar Analyses of High-Pressure Swirl-Stabilized Combustion via Single-Shot Dual-SBG Raman Spectroscopy

We report an experimental study and thermochemical analysis of high-pressure swirl-stabilized combustion utilizing subframe burst gating (SBG) Raman spectroscopy. SBG Raman spectroscopy is a novel diagnostic technique that provides increased accuracy of quantitative scalar measurements in a single-shot pointwise manner. A recent modification of our original system allows parallel detection of both Stokes and anti-Stokes spectral components (hence the term dual SBG). We begin by briefly describing the experimental construction of a Raman calibration matrix, which allows us to reduce spectral cross-talk in the measurements. Next we describe the application of dual-SBG Raman spectroscopy to simultaneous single-shot measurement of temperature and species mass fractions in a turbulent flame stabilized over a lean-direct-injection (LDI) burner using gaseous methane fuel at elevated pressure of 17 atm. Our discussion includes the practical challenges of Raman spectroscopy in a pressurized combustion rig. Statistical analyses of the single-shot thermochemical data provide insights into the nature of the partial-premixing process and its impact on the subsequent combustion process.

Large eddy simulations for radiation-spray coupling for a lean direct injector combustor

Large Eddy Simulations (LESs) for a lean-direct injection (LDI) combustor are performed and compared with experimental data. The LDI emissions characteristics, and radiation-spray coupling effect on the predictions are analyzed. The flamelet progress variable approach is employed for chemistry tabulation coupled with a stochastic secondary breakup model. Good comparisons are shown with the experimental data mean and root mean square for both the gas phase and spray droplets profiles. The effect of combustion is found to change the shape and structure of the central recirculation zone to be more compact in length but larger in diameter in the transverse direction. In-addition the results show that the gas phase radiation alters the spray dynamics by changing the local gas-phase temperature distribution. This impacts the spray evaporation rate, the local mixture fraction, and consequently the combustion heat released rate and the predicted emissions. The simulation with no radiation modeling shows over prediction in the temperature distribution, pollutants emissions, higher fuel evaporation rate, and narrower range of droplet size distribution with lower number density for the smaller size particles. The current study suggests that, even for low pressure systems, radiation modeling can be important for accurate emissions prediction.

CFD Study of NOx Emissions in a Model Commercial Aircraft Engine Combustor

Air worthiness requirements of the aircraft engine emission bring new challenges to the combustor research and design. With the motivation to design high performance and clean combustor, computational fluid dynamics (CFD) is utilized as the powerful design approach. In this paper, Reynolds averaged Navier-Stokes (RANS) equations of reactive two-phase flow in an experimental low emission combustor is performed. The numerical approach uses an implicit compressible gas solver together with a Lagrangian liquid-phase tracking method and the extended coherent flamelet model for turbulence-combustion interaction. The NOx formation is modeled by the concept of post-processing, which resolves the NOx transport equation with the assumption of frozen temperature distribution. Both turbulence-combustion interaction model and NOx formation model are firstly evaluated by the comparison of experimental data published in open literature of a lean direct injection (LDI) combustor. The test rig studied in this paper is called low emission stirred swirl (LESS) combustor, which is a two-stage model combustor, fueled with liquid kerosene (RP-3) and designed by Beihang University (BUAA). The main stage of LESS combustor employs the principle of lean prevaporized and premixed (LPP) concept to reduce pollutant, and the pilot stage depends on a diffusion flame for flame stabilization. Detailed numerical results including species distribution, turbulence performance and burning performance are qualitatively and quantitatively evaluated. Numerical prediction of NOx emission shows a good agreement with test data at both idle condition and full power condition of LESS combustor. Preliminary results of the flame structure are shown in this paper. The flame stabilization mechanism and NOx reduction effort are also discussed with in-depth analysis.

Investigation of Flow Characteristics in Lean Direct Injection Combustors

Investigation of Flow Characteristics in Lean Direct Injection Combustors

Investigation of Flow Characteristics in Lean Direct Injection Combustors

Investigation of Flow Characteristics in Lean Direct Injection Combustors

High intensity colorless distributed combustion for ultra low emissions and enhanced performance

Combustion characteristics of colorless distributed combustion have been investigated for application to gas turbine combustors. Very high intensity distributed combustion has been shown for application to stationary gas turbine engines. Various configurations examined have revealed reverse cross-flow mode to be more favorable for desirable combustion characteristics. The reverse-cross flow geometry is further investigated experimentally at range thermal intensities from 53 to 85 MW/m 3 atm with specific focus on exhaust emissions, radical emission, global flame photographs and flowfield using novel but simplified geometry for easy transition to applications in gas turbine engine applications. The high combustion intensity demonstrated here is higher than that used in present stationary gas turbine engines. Numerical simulations are also performed and compared with the experiments for the new design configuration under non-reacting conditions. Ultra low NO x emissions are achieved for both the novel premixed (1 ppm) and non-premixed (4 ppm) combustion modes reported here. Carbon monoxide levels of about 30 ppm are achieved in both novel premixed and non-premixed modes of combustion with a pressure drop of less than 5% across the combustor at the favorable condition. Almost no visible flame color in the reaction zones are observed for both novel premixed and non-premixed modes with volume distributed combustion so that this mode is termed as colorless distributed combustion. This mode of volume distributed colorless combustion is dramatically different than that used in contemporary gas turbine combustion operating under lean premixed, lean direct injection or rich burn, quick quench lean burn gas turbine combustion.

Investigation of low emission combustors using hydrogen lean direct injection

One of the key technology challenges for the use of hydrogen in gas turbine engines is the performance of the combustion system, in particular the fuel injectors. Tests were conducted to measure the nitrogen oxide (NOx) emissions and combustion performance at inlet conditions of 588 to 811 K, 0.4 to 1.4 MPa, and equivalence ratios up to 0.48. All the injectors were based on Lean Direct Injection (LDI) technology with multiple injection points and quick mixing. One challenge to hydrogen-based premixing combustion systems is flashback since hydrogen has a reaction rate over 7 times that of Jet-A.

CFD Approach to the Research and Design of Low Emission Commercial Aircraft Engine Combustor

The foundation of AVIC Commercial Aircraft Engine Company (ACAE) in Shanghai triggers the hot upsurge of the civil aircraft engine development and study in China. Air worthiness requirements of the engine emission brings the new challenge to the combustor research and design. With the motivation of design high performance and clean combustor, CFD is utilized as the powerful design approach. In this paper, Reynolds averaged resolve of Navier-Stokes (RANS) equations of reaction two-phase flow in an experimental low emission combustor are performed. The numerical approach uses an implicit compressible gas solver together with a Lagrangian liquid-phase tracking method and the extended coherent flamelet model for turbulence-combustion interaction. The NOx formation is modeled by the concept of post-processing, which resolves the NOx transport equation with the assumption of frozen temperature distribution. Both turbulence-combustion interaction model and NOx formation model are firstly evaluated by the comparison of experimental data published in open literatures of a lean direct injection (LDI) combustor. The test rig studied in this paper is called Low Emission Stirred Swirl (LESS) combustor, which is a two stage model combustor, fueled with liquid RP-3 kerosene and designed by Beijing University of Aeronautics and Astronautics (BUAA). The main stage of LESS combustor employs the principle of lean prevaporized and premixed (LPP) concept to reduce pollutant, and the pilot stage depends on a diffusion flame for flame stabilization. Detailed numerical results including of species distribution, turbulence performance and burning performance are qualitatively and quantitatively evaluated. Numerical prediction of NOx emission shows a good agreement with test data both at idle condition and full power condition of LESS combustor. Preliminary results of the flame structure are shown in this paper. The flame stabilization mechanism and NOx reduction effort are also discussed with in-depth analysis.

Dual-pump coherent anti-Stokes Raman scattering system for temperature and species measurements in an optically accessible high-pressure gas turbine combustor facility

The development and implementation of a dual-pump coherent anti-Stokes Raman scattering (DP-CARS) system employing two optical sub-systems to measure temperature and major species concentrations at multiple locations in the flame zone of a high-pressure, liquid-fueled gas turbine combustor are discussed. An optically accessible gas turbine combustor facility (GTCF) was utilized to perform these experiments. A window assembly has been designed, fabricated, and assembled in the GTCF to allow optical access from three directions using a pair of thin and thick fused silica windows on each side. A lean direct injection (LDI) device consisting of an array of nine integrated air swirlers and fuel injectors was operated using Jet-A fuel at inlet air temperatures up to 725 K and combustor pressures up to 1.03 MPa. The DP-CARS system was used to measure temperature and CO2/N2 concentration ratio on single laser shots. An injection-seeded optical parametric oscillator (OPO) was used as a narrowband pump laser source in order to potentially reduce shot-to-shot fluctuations in the CARS data. Large prisms mounted on computer-controlled translation stages were used to direct the CARS beams either into the main leg optical system for measurements in the GTCF or to a reference leg optical system for measurements of the non-resonant spectrum and for alignment of the CARS system. The spatial maps of temperature and major species concentrations were obtained in high-pressure LDI flames by translating the CARS probe volume in the axial and vertical directions inside the combustor rig without loss of optical alignment.

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Three-dimensional n-heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.

NO x Reduction by Air-Side Versus Fuel-Side Dilution in Hydrogen Diffusion Flame Combustors

Lean-direct-injection (LDI) combustion is being considered at the National Energy Technology Laboratory as a means to attain low NOx emissions in a high-hydrogen gas turbine combustor. Integrated gasification combined cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent CO2 separation. The IGCC’s air separation unit produces a volume of N2 roughly equivalent to the volume of H2 in the gasifier product stream, which can be used to help reduce peak flame temperatures and NOx in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and it has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on NOx emissions. From a peak flame temperature perspective, greater NOx reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower NOx production potential for fuel-side dilution due to its smaller flame size, whereas air dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal NOx generation. For more complex staged-air LDI combustor designs, the dilution of the primary combustion air at fuel-rich conditions can result in the full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for NOx reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with the dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and NOx emissions. Strategies for accurate computational modeling of LDI combustors’ stability characteristics are also discussed.

Flame Spectra of a Turbulent Liquid-Fueled Swirl-Stabilized Lean-Direct Injection Combustor

Flame spectra within the UV/visible-light range are measured for a turbulent, liquid-fueled, swirl-stabilized, lean-direct injection combustor. The flame spectra are quite similar to those of lean premixed combustion, which can be attributed to the small droplet size and fast fuel/air mixing. Broadband background emissions around 431 nm, which mainly consist of CO * 2 chemiluminescence, are found to be self-similar with wavelength for both stable and unstable combustion. Chemiluminescence from OH * , CH * , and CO * 2 is found to be a nonlinear function of the airflow rate, the equivalence ratio, preheat temperature, and pressure. Procedures for the determination of the instantaneous heat release rate and equivalence ratios along the flame front are developed. For both combustion instability and forced flame responses, the prediction errors are within 2.0 % in the mean heat release rate, the mean air consumption rate, and the mean equivalence ratio. The assumption that chemiluminescence is proportional to the instantaneous heat releaser rate is generally invalid. Probably the proportionality is valid only in the weakly turbulent or wrinkled flamelet region in the absence of equivalence ratio variations and strong acoustic oscillations.

Simulation of spray–turbulence–flame interactions in a lean direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required. The inflow spray is specified by a Kelvin–Helmholtz (or aerodynamic) breakup model and compared with LES without breakup, where the incoming spray is approximated using measured data just downstream of the injector. Overall, both time-averaged gas and droplet velocity predictions compare well with the measured data. The major impact of breakup is on fuel evaporation in the vicinity of the injector. Further downstream, a broad spectrum of drop sizes are recovered by the breakup simulation and produces spray quality, as in the no-breakup case. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flame-holding mechanism to stabilize the flame. Unsteady features such as the efficient dispersion of the spray by the precessing vortex core (PVC) are well captured. Flame structure analysis using the Takeno flame index shows the presence of a diffusion flame in the central portion, whereas premixed burning mode is observed farther away. Instantaneous thermochemical states of fuel–air mixing and oxidation indicate significant departure from the gaseous diffusion limits, consistent with earlier observations. Additionally, particle–particle and particle–gas correlations are analyzed and discussed.

SCR technology for a lean mixture di gasoline engine

At the Institute for Combustion Engines of the Technical University of Kaiserslautern, a potential analysis and some basic tests for the use of the SCR-Technology at lean mixture direct-injection gasoline engines were conducted based on year-long experience with selective catalytic reduction at the exhaust gas of diesel engines. The main focus was on the aspects of consumption of reducing agent and achievable conversion of nitrogen oxides.

Bonding and Integration of Silicon Carbide Based Materials for Multifunctional Applications

Robust bonding and integration technologies are critically needed for the successful implementation of silicon carbide based components and systems in a wide variety of aerospace and ground based applications. These technologies include bonding of silicon carbide to silicon carbide as well as silicon carbide to metallic systems. A diffusion bonding based approach has been utilized for joining of silicon carbide (SiC) to silicon carbide sub-elements for a micro-electro-mechanical systems lean direct injector (MEMS LDI) application. The objective is to join SiC sub-elements to from a leak-free injector that has complex internal passages for the flow and mixing of fuel and air. A previous bonding approach relied upon silica glass-based interlayers that were non-uniform and not leak free. In the newly developed joining approach, titanium foils and physically vapor deposited titanium coatings were used to form diffusion bonds between SiC materials using hot pressing. Microscopy results show the formation of well adhered diffusion bonds. Initial tests show that the bond strength is much higher than required for the component system. Benefits of the joining technology are fabrication of leak free joints with high temperature and mechanical capability.

Modeling of Internal Reacting Flows and External Static Stall Flows Using RANS and PRNS

The reacting flow in a research lean direct injection (LDI) hydrogen combustor and the static stall of NACA0012 airfoil were simulated using both Reynolds averaged Navier–Stokes (RANS) and partially resolved numerical simulation (PRNS) approaches. The concept and the main features of the PRNS approach are briefly described. The PRNS basic equations are grid independent or grid invariant; the subscale models are a dynamic equation system. We consider PRNS as an engineering tool for the very large eddy simulation of complex turbulent flows. Two CFD codes, NCC and Wind-US, with two different subscale models (i.e. two- and one-transport equation models, respectively) are used in the presented PRNS simulations. Based on the comparisons with available experimental data, the numerical results indicate that the PRNS subscale models seem to be able to capture important large scale turbulent structures and to improve the quality of numerical simulations while keeping a relatively low cost comparable to the unsteady RANS simulations.

Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.

Lean Direct Wall Injection Mode Atomization of Liquid Jets in Swirling Flow

Introduction T HE environmental and energy challenges for gas turbines require new combustion concepts.1 The technique described in this study is called lean direct wall injection (LDWI), and it can be described as injection of a liquid fuel jet from the combustor wall, without premixing and prevaporization, directly into the swirling flow of the main combustor.2,3 Liquid jet atomization is a critical process for LDWI because the fuel is not premixed and prevaporized and the combustion efficiency and NOx emission of this concept depend heavily on the fuel distribution. The behavior of a liquid jet injected transversely into a high-velocity crossflow has been examined in both supersonic and subsonic flows largely through experiment.4−7 However, results from angled injection in the crossflow regime are not directly applicable to the swirling flow regime. It will be necessary to produce uniform and rapid atomization of the fuel jet in LDWI to form a uniform gaseous-phase fuel and air mixture in the practical application. It was the purpose of this investigation to examine the effects of atomization factors on the breakup and atomization processes of liquid jets in swirling flow. In the present study, as the first stage toward understanding the combustion phenomena in a LDWI mode, the hydrodynamic behavior of wall-injected liquid jets in confined cold swirling air flows were investigated.