Patton M. Allison

The effect of fuel composition on swirling kerosene flames

© 2017 by Jennifer Sidey. In this work, the effect of kerosene composition on bluff-body, swirl stabilised spray flames is investigated using flame visualisation techniques. Two kerosene types were investigated: A conventional Jet-A blend, referred to as A2, and an alternative Gevo ATJ kerosene, referred to as C1. Flame photographs, OH* chemiluminescence images, simultaneous OH planar laser induced fluorescence (PLIF) and Mie scattering, and fuel PLIF images all show differences in flame appearance and structure as A2-air and C1-air flames approach extinction. A2-air flames appear to produce more soot than C1-air flames, as expected noting the composition of A2 includes a large content of alkylbenzenes and cycloaromatics, typically associated with an increased sooting tendency in hydrocarbon fuels. A2 fuel drops also appear to fluoresce substantially more than C1 in the OH-PLIF filtering scheme, which is also likely due to the aromatic and ring structure content of the fuel. Additional differences between the kerosene flames include flame brush characteristics, regions above the bluffbody containing fuel liquid and vapour, and flame attachment behaviour. C1-air flames show signs of extinction, marked by the PDF of lift-off height from the base of the combustor, at lower bulk air velocities and higher equivalence ratios than A2-air flames. This indicates that, at the conditions studied here, C1-air flames may be more easily extinguished than A2-air flames. This observation is supported by lean blow-off data presented at a range of fuel flow rate conditions. These findings, particularly relating to lean blow-off, are particularly important to consider during the development of fuel flexible lean gas turbine combustors.

Investigation of dimethyl ether combustion instabilities in a partially - Premixed gas turbine model combustor using high-speed laser diagnostics

© 2014, American Institute of Aeronautics and Astronautics Inc. All rights reserved. Combustion instabilities in gas turbine engines often give rise to acoustic resonances. These resonances occur as manifestations of different acoustic modes, of which a single or multiple modes may be present. In this work, the acoustic behavior of a gas turbine model combustor, developed at DLR Stuttgart by W. Meier et al., was investigated using dimethyl ether (DME). The equivalence ratio and air mass flow rate were systematically varied. The results did not correspond to any one instability mechanism. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of mechanisms, including a Helmholtz-type resonance from the plenum and convective-acoustic effects. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed formaldehyde planar laser-induced fluorescence was applied to study the motion of flame surfaces in response to the pressure oscillations of the instability. Flame shape has been correlated with instability strength and presence. The flame surface density and surface area fluctuated at the acoustic frequency and displayed motions correlated with the precessing vortex core (PVC) rotation. In non-resonating flames, the behavior of the formaldehyde structure and marked flame surfaces were dominated by the PVC motion, but the degree of surface area fluctuations was reduced compared to unstable flames. Results show that the frequency of the combustion instability varies with several operational conditions, including gas velocity, equivalence ratio, and convective time delays.

Fundamental Aspects of Jet Ignition for Natural Gas Engines

© 2017 SAE International. Large-bore natural gas engines may use pre-chamber ignition. Despite extensive research in engine environments, the exact nature of the jet, as it exits the pre-chamber orifice, is not thoroughly understood and this leads to uncertainty in the design of such systems. In this work, a specially-designed rig comprising a quartz pre-chamber fit with an orifice and a turbulent flowing mixture outside the pre-chamber was used to study the pre-chamber flame, the jet, and the subsequent premixed flame initiation mechanism by OH* and CH* chemiluminescence. Ethylene and methane were used. The experimental results are supplemented by LES and 0D modelling, providing insights into the mass flow rate evolution at the orifice and into the nature of the fluid there. Both LES and experiment suggest that for large orifice diameters, the flow that exits the orifice is composed of a column of hot products surrounded by an annulus of unburnt pre-chamber fluid. At the interface between these layers, a cylindrical reaction zone is formed that propagates in the main chamber in the axial direction assisted by convection in the jet, but with limited propagation in the cross-stream direction. For small orifice diameters, this cylinder is too thin, and the stretch rates are too high, for a vigorous reaction zone to escape the pre-chamber, making the subsequent ignition more difficult. The methane jet flame is much weaker than the one from ethylene, consistent with the lower flame speed of methane that suggests curvature-induced quenching at the nozzle and by turbulent stretch further downstream. The velocity of the jet is too high for the ambient turbulence to influence the jet, although the latter will affect the probability of initiating the main premixed flame. The experimental and modelling results are consistent with ongoing Direct Numerical Simulations at ETH Zurich.

Quantitative fuel vapor/air mixing imaging in droplet/gas regions of an evaporating spray flow using filtered Rayleigh scattering.

This Letter demonstrates the application of filtered Rayleigh scattering (FRS) for quantitative two-dimensional fuel vapor/air mixing measurements in an evaporating hydrocarbon fuel spray flow. Using the FRS approach, gas-phase measurements are made in the presence of liquid-phase droplets without interference. Effective suppression of the liquid-phase droplet scattering using FRS is enabled by the high spectral purity of the current Nd:YAG laser system. Simultaneous Mie-scattering imaging is used to visualize the droplet field and illustrate the droplet loading under which the FRS imaging is applied in the current spray flows. The initial quantification of the FRS imaging is based on calibration measurements from a flow cell of known fuel vapor/air mixtures, while future work targets the utilization of a Rayleigh-Brillouin spectral model for quantification of the FRS signals.

Method to Measure Flame Index in a Partially-Premixed Gas Turbine Combustor

This paper describes a new method to image the flame index in turbulent, partially-premixed flames. The flame index indicates the locations where premixed flames and where non-premixed flames exist. This information is needed to improve the modeling of gas turbine combustors. Using simultaneous acetone and NO2 Planar Laser-Induced Fluorescence (PLIF), flame index was measured in a gas turbine model combustor. The fluorescence linearity and saturation characteristics of acetone and NO2 with respect to volume fraction and laser spectral irradiance also were studied.

Acoustic Behavior of a Partially-Premixed Gas Turbine Model Combustor

Combustion instabilities in gas turbine engines often give rise to acoustic resonances. These resonances occur as manifestations of different acoustic modes, of which a single or multiple modes may be present. In this work, the acoustic behavior of a model gas turbine combustor, developed at DLR Stuttgart by Meier, was investigated using both syn(thetic) gas and standard hydrocarbon fuels. Syngas displayed significantly different behavior than hydrocarbon fuels, even when the laminar flame speeds of the fuels were matched. The following operating parameters were systematically varied: equivalence ratio, air mass flow rate, burner temperature, flame speed, and fuel type. The results did not correspond to any one known instability mechanism. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of mechanisms. These include organ tone resonances, Helmholtz-type instability, and convective-acoustic effects. Another possible factor is the fluid mechanical switching observed between the two swirler air passages, due to blockage caused by flame position and shape. Flame characteristics such as anchoring and liftoff height appear to play a major role in the determination of instability strength. As hydrogen content in syngas is increased, a lifted flame eventually anchors, resulting in a drastic decrease in the acoustic amplitude associated with non-resonating flames. Thus, increasing the flame speed causes two competing effects to occur. Propane acoustic data are collapsed using scaling parameters that indicate that the burner temperature and the air mass flow effects behave independently.

Transient behavior of kerosene flames in a bluff-body stabilized swirl combustor

© 2017 by Patton M. Allison. Transient behavior of kerosene spray flames in a bluff-body, swirl stabilized combustor has been investigated with regards to lean blowout and thermoacoustic response to applied acoustic forcing, in order to study differences between several types of liquid kerosene fuels. The responses of a standard Jet-A blend (A2), Gevo-ATJ kerosene (C1), and a decane/trimethlybenzene blend (C5), to forced perturbations at 160 Hz and 40Hz, are evaluated through 5 kHz imaging of OH* chemiluminescence and pressure measurements. Flame transfer functions, which describe the coupling between the heat release and velocity fields within the combustor, depict large differences in the thermoacoustic behavior of each fuel. The transient lean blowout behavior is observed with high speed OH* imaging and some differences between flame structure and the temporal response at blowout for each fuel are observed. Local fuel distributions have been acquired through UV-Fuel PLIF as the flame approached lean blowout, showing a different fuel vapour pattern among the various fuels. The data highlight important differences between the studied kerosenes and can assist computational modelling efforts.

Low frequency combustion instabilities imaged in a gas turbine combustor flame tube

An “equivalence ratio oscillation” was studied that produced a strong low frequency combustion instability (at 80 and 160 Hz) when a commercial lean premixed prevaporized (LPP) fuel injector was operated using Jet-A fuel at realistic conditions. The elevated pressures, temperatures, air flow rates and overall equivalence ratios were close to conditions at engine idle. However the fraction of fuel diverted to the various injector ports was sufficiently off-design to create an instability. High speed movies showed that the flame base violently moves upstream and downstream at 80 Hz, and that flashback plays a role in the rapid upstream movements. Flame shape also changes at 80 Hz from a flat flame to a more elongated flame, which affects the heat release pattern and causes oscillations in the air flow rate. A regime diagram is reported which contains the boundary that marks the onset of the instability. Other higher frequency instabilities were also present and were due to longitudinal and azimuthal organ tones, but their magnitudes were more than ten times smaller than the pressure fluctuations caused by the equivalence ratio oscillation.

Response of flames with different degrees of premixedness to acoustic oscillations

ABSTRACT The response of three flames with different degrees of premixedness (fully premixed, non-premixed with radial, and non-premixed with axial fuel injection) to acoustic oscillations is studied experimentally. The flames were imaged using OH* chemiluminescence and OH planar laser-induced fluorescence at 5 kHz. In addition to a flame kinematics analysis, the amplitude dependence of the transfer function was calculated. The dominant spatial structures of the heat release and their periodicity were examined using the proper orthogonal decomposition (POD) method. The Non-Premixed system with Radial fuel injection (NPR) showed the highest response to acoustic forcing, followed by the fully premixed and the Non-Premixed system with Axial fuel injection (NPA). In addition, the response of the non-premixed system with radial fuel injection was greater than that of the fully premixed system for various bulk velocities , global equivalence ratios , forcing amplitudes , and forcing frequencies . In the fully premixed system, the heat release modulation was mainly through flame surface area modulation, while in the NPR system, both the flame area and the equivalence ratio modulations were found to be important mechanisms of the heat release oscillations. About 70% of the energy of the total fluctuations in the NPR case was contained in the first four POD modes, a percentage that decreased with overall equivalence ratio, but only this dropped to about 40% for the NPA flame. The frequency spectra of the coefficients of the POD modes exhibited peaks at the forcing frequency, with increasing broadband contributions in higher modes and for the NPA flame.

Lean Blowoff Scaling of Swirling, Bluff-Body Stabilized Spray Flames

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Lean blowoff behavior of spray flames in a bluff-body, swirl stabilized combustor has been investigated with regards to stability and compositional effects in order to study differences between several types of standard liquid alkanes and kerosene fuels. The blowoff limits of standard Jet-A, Gevo-ATJ kerosene, n-octane, iso-octane, heptane, and a decane/trimethlybenzene blend, are studied in collaboration with the National Jet Fuel Combustion Program in order to evaluate predicted correlations of the derived cetane number with the blowoff equivalence ratio. Scaling with the blowout Damkohler number show consistent trends across kerosene fuels indicating that the flame speed may play an important role with regards to blowoff timescales.