Michel Cazalens

Combustion Instability Problems Analysis for High-Pressure Jet Engine Cores

The design of a clean combustion technology based on lean combustion principles will have to face combustion instability. This oscillation is often discovered late in engine development when, unfortunately, only a few degrees of freedom still exist to solve the problem. Individual component test rigs are usually not useful in detecting combustion instability at an early stage because they do not have the same acoustic boundary conditions as the full engine. An example of this unsteady activity phenomenon observed during the operation of a high-pressure core is presented and analyzed. To support the investigation, two numerical tools have been extensively used. First, the experimental measurement of unsteady pressure and the results of a multidimensional acoustic code are used to confirm that the frequency variations of the observed modes within the operating domain of the high-pressure core are due to the excitation of the first and second azimuthal combustor modes. The impact of acoustic boundary conditions for the combustor exhaust is shown to control the appearance and mode transition of this unsteady activity. Second, the three-dimensional reacting and nonreacting large eddy simulations for the complete combustor and for the injection system cup alone suggest that the aerodynamic instability of the flow passing through the cup could be the noise source exciting the azimuthal acoustic modes of the chamber. Based on these results, the air system (cup) was redesigned to suppress this aerodynamic instability, and experimental combustion tests confirm that the new system is free of combustion instability.

Outlet-Boundary-Condition Influence for Large Eddy Simulation of Combustion Instabilities in Gas Turbines

Large eddy simulations of combustion require more precise boundary conditions than classical Reynolds-averaged methods. This study shows how the reacting flow within a gas turbine combustion chamber can be influenced by the description of the downstream boundary. Large eddy simulation calculations are performed on a combustion chamber terminated by a high-pressure stator containing vanes in which the flow is usually choked and submitted to strong rotation effects. High-pressure stators are present in all real gas turbines, but they are often not computed and are simply replaced by a constant-pressure-outlet condition. This study compares a large eddy simulation calculation in which the high-pressure stator is replaced by a constant-pressure-outlet surface and a large eddy simulation in which the high-pressure stator is included in the computational domain and explicitly computed. The comparison of the flow in the chamber in both large eddy simulations reveals that the presence of the high-pressure stator modifies the mean flow in the second part of the chamber but does not affect the primary combustion zone. The unsteady field, on the other hand, is strongly affected by the high-pressure stator. Results demonstrate that the high-pressure stator should be included in realistic large eddy simulations of combustion chambers for gas turbines.

Flame Stabilization by Hot Products Gases Recirculation in a Trapped Vortex Combustor

New regulations regarding NOx emissions are forcing manufacturers to develop advanced research and technology strategies. Ultra-lean combustion is considered as an attractive solution; however, it generally produces combustion instabilities in swirl-stabilized burners. This work provides experimental results for a new burner technology based on two concepts: the trapped vortex combustor (TVC) and the ultra-compact combustor (UCC). Methane/air flame stabilization was achieved by generating hot product recirculation, with a rich pilot flame located in an annular cavity, and by flame holders located in the main flow slightly upstream of the cavity. In addition, azimuthal gyration could be added to the main flow to reproduce the suppression of the last diffuser stage, which increased the velocity and modified the mixing between the cavity and the mainstream due to centrifugal forces. The combustor characterization was performed by coupling several optical diagnostics, pollutant emissions, and pressure measurements (for both cold and reactive conditions) at atmospheric pressure. An understanding of the combustion dynamics was achieved through phase averaged PIV/CH* images. The analysis highlighted the importance of the stabilization process of a double vortex structure inside the cavity and the presence of reactive gas close to the upstream cavity wall. These conditions were improved by a high cavity equivalence ratio and a high main airflow rate. The addition of swirl considerably increased the flame stability.Copyright

LEMCOTEC: Improving the Core-Engine Thermal Efficiency

This paper describes the research carried out in the European Commission co-funded project LEMCOTEC (Low Emission Core Engine Technology), which is aiming at a significant increase of the engine overall pressure ratio. The technical work is split in four technical sub-projects on ultra-high pressure ratio compressors, lean combustion and fuel injection, structures and thermal management and engine performance assessment. The technology will be developed at subsystem and component level and validated in test rigs up to TRL5. The developed technologies will be assessed using three generic study engines (i.e. regional turbofan, mid-size open rotor, and large turbofan) representing about 90% of the expected future commercial aero-engine market. Two additional study engines from the previous NEWAC project will be used for comparison. These are based on intercooled and intercooled-recuperated future engine concepts.The compressor work is targeting efficiency, stability margin and flow capacity by improved aerodynamic design. High-pressure and intermediate-pressure compressors are addressed. The mechanical and thermo-mechanical functions, including the variable-stator-systems, will be improved. Axial-centrifugal compressors with impeller and centrifugal diffuser are under investigation too.Three lean burn fuel injection systems are developed to match the technology to the corresponding engine pressure levels. These are the PERM (Partially Evaporating Rapid Mixing), the MSFI (Multiple Staged Fuel Injection) and the advanced LDI (Lean Direct Injection) combustion systems. The air flow and combustion systems are investigated. The fuel control systems are adapted to the requirements of the ultra-high pressure engines with lean fuel injection. Combustor-turbine interaction will be investigated. A fuel system analysis will be performed using CFD methods.Improved structural design and thermal management is required to reduce the losses and to reduce component weight. The application of new materials and manufacturing processes, including welding and casting aspects, will be investigated. The aim is to reduce the cooling air requirements and improve turbine aerodynamics to support the high-pressure engine cycles.The final objective is to have innovative ultra-high pressure-ratio core-engine technologies successfully validated at subsystem and component level. Increasing the thermal efficiency of the engine cycles relative to year 2000 in-service engines with OPR of up to 70 (at max. condition) is an enabler and key lever of the core-engine technologies to achieve and even exceed the ACARE 2020 targets on CO2, NOx and other pollutant emissions:• 20 to 30 % CO2 reduction at the engine level, exceeding both, the ACARE 15 to 20% CO2 reduction target for the engine and subsequently the overall 50% committed CO2 and the fuel burn reduction target on system level (including the contributions from operations and airframe improvements),• 65 to 70 % NOx reduction at the engine level (CAEP/2) to attain and exceed the ACARE objective of 80% overall NOx reduction (including the contributions from both, operational efficiency and airframe improvement), reduction of other emissions (CO, UHC and smoke/particulates) and• Reduction of the propulsion system weight (engine including nacelle without pylon).Copyright

Analysis of the Flame Structure in a Trapped Vortex Combustor Using Low and High-Speed OH-PLIF

Operating with lean combustion has led to more efficient “Low-NOx” burners but has also brought several technological issues. The burner design geometry is among the most important element as it controls, in a general way, the whole combustion process, the pollutant emissions and the flame stability. Investigation of new geometry concepts associating lean combustion is still under development, and new solutions have to meet the future pollutant regulations. This paper reports the experimental investigation of an innovative staged lean premixed burner. The retained annular geometry follows the Trapped Vortex Combustor concept (TVC) which operates with a two stage combustion chamber: a main lean flame (1) is stabilized by passing past a vortex shape rich-pilot flame (2) located within a cavity. This concept, presented in GT2012-68451 and GT2013-94704, seems to be promising but exhibits combustion instabilities in certain cases, then leading to undesirable level of pollutant emissions and could possibly conduct to serious material damages. No precise information have been reported in the literature about the chain of reasons leading to such an operation. The aim of this paper is to have insights about the main parameters controlling the combustion in this geometry. The flame structure dynamics is examined and compared for two specific operating conditions, producing an acoustically self-excited and a stable burner. Low and high-speed OH-PLIF laser diagnostics (up to 10 kHz) are used to have access to the flame curvature and to time-resolved events. Results show that the cavity jets location can lead to flow-field oscillations and a non-constant flame’s heat release. The associated flame structure, naturally influenced by turbulence is also affected by hot gases thermal expansion. Achieving a good and rapid mixing at the interface between the cavity and the main channel leads to a stable flame.Copyright

Computational Methodology for Carbon Monoxide Emission for Aeroengine Combustor Design

Design of lean combustion technologies will have to face the problem of the increasing difficulty to control carbon monoxide emission. To help the designer, a 91 species and 1329 elementary reactions chemical mechanism for Jet Al kerosene is tabulated and coupled with the turbulent flow through a presumed probability density function approach. The capacities of the methodology for finite rate chemistry prediction are assessed by the computation of several operating conditions of an experimental combustor. The first case corresponds to high-power operating conditions and a good agreement is achieved between the predicted and measured carbon monoxide emissions and temperature profile at the combustor outlet. The second case corresponding to low-power operating conditions for which finite rate chemistry effects are important is focused on the study of carbon monoxide emissions variations with respect to the global fuel air ratio of the combustor, while keeping constant the inlet air mass flow rate and the airflow split. A detailed discussion is provided to carefully check the coherence of the obtained results, then the full relevance of this tabulated approach regarding the prediction of finite rate chemistry effects for turbulent combustion in aeronautical combustors with liquid injection is concluded.

Determination of Criteria for Flame Stability in an Annular Trapped Vortex Combustor

Development of lean premixed (LP) combustion is still a challenge as it results in considerable constraints for the combustor design. Indeed, new combustors using LP combustion are more prone to flashback, blow-off or even thermo-acoustic instabilities. A detailed understanding of mechanisms leading to such extreme conditions is then crucial to reduce pollutant emissions, widen the range of operating conditions, and reduce design time. This paper reports the experimental study of an innovative LP trapped vortex combustor (TVC). The TVC concept uses a recirculating rich flow trapped in a cavity to create a stable flame that continuously ignites a main lean mixture passing above the cavity. This concept gave promising performances but some workers highlighted the existence of combustion instabilities for some operating conditions. Detailed studies have therefore been carried out in order to understand the occurrence of these drastic operating conditions. Results showed that the cavity flow dynamics in conjunction with the location of the interfacial mixing zone (between the cavity and the mainstream) were the driving forces to obtain stable combustion regimes. The goal of this work has been to take advantage of these detailed recommendations to determine stability maps, trends, and dimensionless parameters which could be easily used as early design rules. For this reason, the study introduced a simple and robust criterion, based on the global pressure fluctuation energy. The latter was used to distinguish stable and unstable modes. An aerodynamic momentum flux ratio and a chemical stratification ratio (taken between the cavity and the mainstream) were defined to scale all measurements. Results indicated that the mainstream velocity was critically important to confine the cavity and to prevent combustion instabilities. Remarkably, this trend was verified and even more pronounced for larger cavity powers. In addition, flame stabilization above the cavity resulted in the existence of specific stratification ratios, in order to obtain a soft gradient of gas composition between the rich and lean regions. Finally, a linear relation between the mainstream and cavity velocities became apparent, thereby making possible to simply predict the combustor stability.

Spark Ignition of Confined Swirled Flames: Experimental and Numerical Investigation

A swirl burner was designed to experimentally study the impact of spark location on ignition efficiency and detailed ignition scenarios until flame stabilization or blow-off were established, following experimental observations. Premixed and non-premixed configurations were investigated for the same turbulent flow, in order to evaluate the fuel heterogeneities on ignition efficiency. Attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic PIV) and fuel mole fraction field statistics (obtained by PLIF on acetone). Ignition probability maps were established for all conditions by using laser-induced spark for a constant level of deposited energy. No systematic correlations were observed between local flow properties and ignition probability, which leads to the conclusion that history of the flame kernel inside the combustion chamber, must be taken into account to fully explain the ignition mechanism. From this conclusion, ignition scenarios were built using fast flame visualization and dynamic pressure record. Different steps of the ignition process were identified according to the location of the spark.In order to evaluate ignition probability according to spark location and flow conditions (velocity, turbulence and mixing), we extended the predictive model of Neophytou et al. [1], with some modifications, to examine whether it can be applied to ignition of swirling premixed flames. Flame particles are emitted by the spark and tracked in the flow with a Langevin equation by using non-reactive velocity fields obtained by PIV. Physical criteria are proposed to represent flame particles generation, expansion and extinction. Results indicate a relatively good agreement with the experimental database and the ignition scenarios are also well reproduced.Copyright

Control of the Shear-Layer in the Wake of an Axisymmetrical Airfoil Using a DBD Plasma Actuator

Several studies have shown that a surface Dielectric Barrier Discharge (DBD) may be used as an ElectroHydroDynamic (EHD) actuator. This actuator adds momentum inside the boundary layer close to the wall and could be used for airflow control. In this paper, the actuator has been set up on a small axisymmetrical airfoil and the discharge is used to modify the characteristics of the shear-layer in its wake. Results show that the plasma actuator modifies strongly the airflow around the airfoil for velocities up to 20 m/s.