Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at take-off conditions. A previously-developed flamelet-based approach is used to model turbulence-combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream towards the injector as already described in other studies. This LES, however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at takeoff conditions. A previously developed flamelet-based approach is used to model turbulence–combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilizes in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream toward the injector as already described in other studies. This large eddy simulation (LES), however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream, and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at take-off conditions. A previously-developed flamelet-based approach is used to model turbulence-combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream towards the injector as already described in other studies. This LES, however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

Numerical investigation of a coupled blow-off/flashback process in a high-pressure lean-burn combustor

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at takeoff conditions. A previously developed flamelet-based approach is used to model turbulence–combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilizes in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream toward the injector as already described in other studies. This large eddy simulation (LES), however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream, and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.</p

Large-eddy simulation of magnetohydrodynamics and heat transfer in annular pipe liquid metal flow

Turbulent structures in a concentric annular pipe within a uniform transverse magnetic field are examined for a liquid metal flow. Large-eddy simulations are performed to study the effect of magnetic field on turbulence suppression and heat transfer within this geometry. At the characteristic Prandtl number of liquid metals, the smallest scales based on temperature fluctuations are much larger than those of the velocity, which allows to resolve all the temperature scales with sufficient accuracy. The calculations are run at Reynolds number 8900 for three different Hartmann numbers, Ha = 40, 60, 120. The comparison with available direct numerical simulation data shows encouraging agreement. The main findings of this work show a circumferential dependency of the flow characteristics on the local orientation of the magnetic field, with increased anisotropy observed at all Hartmann numbers studied. Anisotropic effects of the magnetic field are predominant for Ha = 60 and Ha = 120 causing turbulence to deviate from its conventional state. At these Hartmann numbers, a partial redistribution of the turbulent kinetic energy from the axial and radial components to the azimuthal component is observed. This effect, observed here for the first time, appears to be related to the appearance of coexisting quasi-2D and 3D turbulence states. Moreover, large skin friction increments are also observed at Ha = 60 and Ha = 120, while coherent structures stretching and streak suppression are found for all three Hartmann numbers.</p

Large-eddy simulations of magnetohydrodynamics and heat transfer in annular pipe liquid metal flow

Turbulent structures in a concentric annular pipe within a uniform transverse magnetic field are examined for a liquid metal flow. Large-eddy simulations are performed to study the effect of magnetic field on turbulence suppression and heat transfer within this geometry. At the characteristic Prandtl number of liquid metals, the smallest scales based on temperature fluctuations are much larger than those of the velocity, which allows to resolve all the temperature scales with sufficient accuracy. The calculations are run at Reynolds number 8900 for three different Hartmann numbers, Ha = 40, 60, 120. The comparison with available direct numerical simulation data shows encouraging agreement. The main findings of this work show a circumferential dependency of the flow characteristics on the local orientation of the magnetic field, with increased anisotropy observed at all Hartmann numbers studied. Anisotropic effects of the magnetic field are predominant for Ha = 60 and Ha = 120 causing turbulence to deviate from its conventional state. At these Hartmann numbers, a partial redistribution of the turbulent kinetic energy from the axial and radial components to the azimuthal component is observed. This effect, observed here for the first time, appears to be related to the appearance of coexisting quasi-2D and 3D turbulence states. Moreover, large skin friction increments are also observed at Ha = 60 and Ha = 120, while coherent structures stretching and streak suppression are found for all three Hartmann numbers.</p

Numerical study of liquid metal magnetoconvection and heat transfer in an electrically conductive square duct

Mixed convection of an electrically conductive fluid in a square duct with imposed transverse magnetic field is studied using Large Eddy Simulation (LES) paradigms. The duct walls are electrically conductive, with the wall conductivity parameter cw ranging from 0 to 0.5. The Reynolds number is Re = 5602 and the Prandtl number is Pr = 0.0238. The focus of the study is on flows at Hartmann numbers Ha ⩽ 125, Richardson numbers Ri ⩽ 10 and two different thermal boundary conditions are considered: four wall uniform heat fluxes and one-sided heating (fixed wall temperature). The results show that the transition from laminar to turbulent flow depends not only on the ratio Ri/Ha, but also on cw and on the local thermal boundary conditions. In the turbulent regime with one-sided heating, the turbulent heat fluxes play an important role in the total heat transfer, in contrast with the typical behaviours of liquid metals. Moreover, the turbulent and thermal structures are highly dependent on the thermal boundary conditions, which completely alter the flow structure. It is also found that at cw ≥ 0.01 the turbulent heat fluxes decrease.</p

Supplementary information files for "Numerical study of liquid metal magnetoconvection and heat transfer in an electrically conductive square duct"

Supplementary information files for article "Numerical study of liquid metal magnetoconvection and heat transfer in an electrically conductive square duct"The Reynolds number is Re = 5602 and the Prandtl number is Pr = 0.0238. The focus of the study is on flows at Hartmann numbers Ha ⩽ 125, Richardson numbers Ri ⩽ 10 and two different thermal boundary conditions are considered: four wall uniform heat fluxes and one-sided heating (fixed wall temperature). The results show that the transition from laminar to turbulent flow depends not only on the ratio Ri/Ha, but also on cw and on the local thermal boundary conditions. In the turbulent regime with one-sided heating, the turbulent heat fluxes play an important role in the total heat transfer, in contrast with the typical behaviours of liquid metals. Moreover, the turbulent and thermal structures are highly dependent on the thermal boundary conditions, which completely alter the flow structure. It is also found that at cw ≥ 0.01 the turbulent heat fluxes decrease.©The Authors, CC BY 4.0</p

Effect of compressor unsteady wakes on a gas turbine combustor flow

In gas turbines, combustor inlets are characterised by significant levels of unsteady circumferential distortion due to compressor wakes and secondary flows, together with additional radial non-uniformity induced by the adverse pressure gradients in the pre-diffuser. This can cause non-uniform velocity distributions across the fuel injector, although the exact interaction mechanism, and the effects it has on the downstream air-fuel mixing, is not fully understood. This paper investigates the flow in an a single sector of a fully featured isothermal rig comprising of compression and combustion systems, exploiting the synchronous coupling of a compressible unsteady RANS simulation with a low-Mach LES. Validation against five-hole probe measurements shows that the coupled approach can correctly predict distortion onset and development, with no solution discontinuity at the coupling interface, and is able to preserve unsteady information. The coupled prediction is then compared against a standalone combustor simulation carried out using a circumferentially uniform inlet profile, showing that the additional turbulence from the wakes interacts with the injector, reducing the coherence of the precessing vortex core and potentially affecting the air-fuel mixing characteristics

Effect of compressor unsteady wakes on a gas turbine combustor flow

In gas turbines, combustor inlets are characterized by significant levels of unsteady circumferential distortion due to compressor wakes and secondary flows, together with additional radial nonuniformity induced by the adverse pressure gradients in the prediffuser. This can cause nonuniform velocity distributions across the fuel injector, although the exact interaction mechanism, and the effects it has on the downstream air–fuel mixing, is not fully understood. This paper investigates the flow in an a single sector of a fully featured isothermal rig comprising of compression and combustion systems, exploiting the synchronous coupling of a compressible unsteady RANS simulation with a low-Mach large eddy simulation (LES). Validation against five-hole probe measurements shows that the coupled approach can correctly predict distortion onset and development, with no solution discontinuity at the coupling interface, and is able to preserve unsteady information. The coupled prediction is then compared against a standalone combustor simulation carried out using a circumferentially uniform inlet profile, showing that the additional turbulence from the wakes interacts with the injector, reducing the coherence of the precessing vortex core (PVC) and potentially affecting the air–fuel mixing characteristics.</p