The auto-ignition of a pulsed methane jet issuing into a coaxial laminar coflow of hot exhaust gas from a lean premixed hydrogen/air flat flame has been studied experimentally by means of high-speed optical diagnostics. The downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame and the auto-ignition time were determined by OH* chemiluminescence (CL) measurements. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution and increased sensitivity. In order to three-dimensionally reconstruct the ignition kernel location in space and only include ignition kernels in the PLIF processing that first occurred inside the laser light sheet, broadband luminosity imaging from a viewing angle perpendicular to the PLIF and CL measurements was performed. The coflow temperature was varied in the temperature range between 1566 K and 1810 K in order to study the influence of the coflow temperature on the autoignition height, auto-ignition time, and on the lift-off height of the steady lifted jet flame. In addition, detailed model simulations were performed to study the influence of temperature and strain on auto-ignition in reacting mixing layers.
The auto-ignition of a pulsed methane jet issuing into a laminar coflow of hot exhaust products of a lean premixed hydrogen/air flat flame was examined using high-speed laser and optical measurement techniques with frame rates of 5 kHz or more. OH* chemiluminescence was used to determine the downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution. Simultaneous imaging of broadband luminosity from a viewing angle perpendicular to the OH* chemiluminescence was applied to three-dimensionally reconstruct the ignition kernel location in space and to determine whether the first occurrence of the kernel was within or beyond the PLIF laser sheet. The development and expansion of the jet was characterized by high-speed Schlieren imaging. Statistics have been compiled for both the ignition time as well as the downstream location of the first auto-ignition kernel and the stabilization height of the steady-state lifted jet flame. From the PLIF images it was found that auto-ignition tended to occur at the interface between bulges of the inflowing jet and the coflow. For steady-state conditions, auto-ignition kernels were observed frequently below the flame base, emphasizing that the lifted jet flame is stabilized by auto-ignition.
A liquid fuel combustor based on the FLOX® burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. The operability limits of the burner were mapped out for full load conditions at 3.5 bars by varying global lambda (λG) from 1.25–2.00 and bulk jet velocity (vBulk) from 80–140 m/s. Exhaust gas measurements show NOx and CO levels below 5 and 10 ppm respectively (corrected for reference 15% O2) at λG = 1.89.
Optical and laser diagnostic measurement techniques have been employed to characterize the spray flames. The flames at stable burner operation points (BOPs) show a predominantly jet like flame shape irrespective of λG and vBulk. Droplets in the size range 2–40 μm have been measured close to the nozzle exit plane. Velocities conditioned on the droplet size show large droplets d > 15 μm transitioning from negative slip velocity at the exit plane to positive slip velocity at downstream location. The positive slip velocities and slow evaporation of large droplets lead to droplets travelling further into the combustion chamber and hence resulting in long flames. A comprehensive data set for the spray characteristic of the new liquid FLOX® burner is made available.
A liquid fuel combustor based on the FLOX ® gas turbine burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. Spray characteristics were measured using droplet mie scattering and phase Doppler interferometry in flames of a stable burner operation point (BOP) at a pressure, preheat temperature, global lambda (G), and jet velocity of 3.5 bars, 300 °C, 1.45 and 120 m/s respectively. The experimental results showed long flames with deep penetration of the spray into the combustion chamber. A comprehensive data set of the spray characteristic with well-defined boundary condition was made available for CFD simulations. The CFD simulation of the two-phase flow was performed by coupling the DLR liquid phase simulation code SPRAYSIM with the commercial CFD-code ANSYS CFX-16.1. The comparison of axial and radial velocity profiles between simulation and experiment clearly showed that the turbulence model used in the numerical simulation was unable to predict the measured turbulence appropriately. The calculated and measured spray behavior in the combustion chamber showed satisfying agreement. The observed differences were mainly due to the simple 1-step global combustion model, which predicted an early onset of the heat release. The simulation showed that even though a large portion of the evaporation happened already inside the nozzle, the remaining spray droplets penetrate deep into the combustion chamber.
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