Gregor C. Gebel

An experimental investigation of kerosene droplet breakup by laser-induced blast waves

The work presented in this paper intends to deepen our understanding of the mechanisms involved in the spark ignition of liquid fuel sprays. An experimental study is presented regarding the ignition of monodisperse droplet chains of Jet A-1 aviation kerosene in a generic model combustor under well-defined boundary Conditions. Breakdowns created by focused laser radiation were used as ignition sparks. They featured rapid spatial expansion, resulting in the formation of spherical blast waves in the surrounding air. The focus of this study lay on the effect of the blast waves on the fuel droplets. Blast wave trajectories were investigated by Schlieren imaging. Their interaction with kerosene droplets was observed with a high speed camera via a long distance microscope; the droplets were visualized by laser-induced Mie scattering, Droplets within a distance of 10 mm from the breakdown position were deformed and disintegrated by the aerodynamic forces of the postshock flow field. Different breakup modes were observed, depending on the distance from the breakdown position: Catastrophic breakup was observed at a 5 mm distance, resonant breakup was observed at a 10 mm distance. Breakup by blast waves from ignition sparks is expected to be a crucial mechanism for spray ignition because it supports evaporation. Weber number calculations revealed that the breakup modes observed under lab conditions will also appear in aviation gas turbines at high altitude relight conditions.

Investigation of GTL-Like Jet Fuel Composition on GT Engine Altitude Ignition and Combustion Performance: Part II—Detailed Diagnostics

The ignition and combustion performance of different synthetic paraffinic kerosenes (SPKs) under simulated altitude relight conditions were investigated at the altitude relight test rig at the Rolls-Royce Strategic Research Centre in Derby. The conditions corresponded to a low stratospheric flight altitude between 25,000 and 30,000 feet. The combustor under test was a twin-sector representation of an advanced gas turbine combustor and fuel injector. Five different SPKs and Jet A-1 were tested at different mass flow rates of air and fuel, and at two different sub-atmospheric air pressures and temperatures. The fuel temperature was kept approximately constant. Simultaneous high-speed imaging of the OH* and CH* chemiluminescence, and of the broadband luminosity was used to visualize both the transient flame initiation phenomena and the combustion behavior of the steady burning flames. In addition, flame luminosity spectra were recorded with a spectrometer to obtain spectrally resolved information concerning the different chemiluminescence bands and the soot luminosity. These investigations were performed in conjunction with the comparative evaluation of the ignition and stability regimes of the five SPKs, which is the subject of a separate complementary paper [1]. We found that the observed flame initiation phenomena, the overall combustion behavior and the different ratios of the chemiluminescence from the OH*, CH* and C2 * radicals were not strongly dependent on the fuels investigated. But, the SPK flames showed for all combustor operating conditions significantly lower soot luminosities than the corresponding Jet A-1 flames, indicating a potential benefit of the SPK fuels.Copyright

Validation of an Ignition and Flame Propagation Model for Multiphase Flows

This paper presents a numerical investigation of a generic lab scale combustor with focus on the ignition characteristics. The test case has been examined thoroughly in a comprehensive measurement campaign to provide a detailed set of boundary conditions and a profound data base of results. The experimental setup comprises five parallel-aligned mono-disperse droplet chains which are ignited, using a focused laser beam. One aspect of the experimental study is the ignitability with respect to the imposed boundary conditions. The second covers the growth and the propagation of the flame after the establishment of an initial kernel. The outcome of the numerical simulations is compared to the experimental results which allows an in-depth assessment of the employed numerical models. The chemistry and, thus, the flame propagation behavior is captured by a turbulent flame speed closure approach with an adaptation to render the model suitable to multiphase flows. For the dispersed phase a Lagrangian particle tracking scheme is employed in combination with a continuous thermodynamics fuel model for the evaporation. The overall good agreement demonstrates the capability of a multiphase flow CFD solver in the field of ignition modeling.Copyright

Ignition and Flame Propagation along Planar Monodisperse Droplet Streams

An experimental and numerical study is presented concerning the ignition of a fuel spray under well defined conditions. The development of a flame kernel that follows the generation of a plasma by a focused laser pulse and the subsequent flame propagation along five co-planar monodisperse streams of fuel droplets is investigated. High-speed video of the broadband luminosity and simultaneous fuel- and OH-PLIF provide qualitative and quantitative experimental data. Numerical simulations have shown that the focused laser pulse not only provides an energy/radical source for the ignition but is also responsible for transforming the droplets present in a small region into fuel vapor. Actually, without the quasi-instantaneous phase change there would not be enough vapor fuel to sustain the flame kernel under these time scales and surrounding conditions. The simulations are performed using an Eulerian-Lagrangian turbulent spray combustion code. Detailed chemistry is used to compute the laminar flame speed under relevant conditions, and a turbulent flame closure model (TFC) is then adopted for turbulence-chemistry interaction.

A numerical investigation of the ignition characteristics of a spray flame under atmospheric conditions

This study presents a numerical investigation of the characteristics of a transient spray ame ignited by laser-induced breakdown. The simulations are carried out under atmospheric conditions within well de ned boundary conditions. The multiphase set-up comprises a kerosene blend as the liquid fuel provided by an air assisted nozzle with droplet velocities up to 40 m/s and an air coow with velocities ranging from 1.0 m/s to 4.5 m/s. An initial ame kernel is created by a laser-induced breakdown at the spray cone edge. Onsetting vaporization and further propagation of the ame, largely on the outskirt of the ame cone surface lead to a growing and convected self-sustaining ame. Results concerning the ame growth and the ame center position, representing the major general characteristics of the instationary combustion process are provided. As experimental data is available for the same set-up and boundary conditions, the simulation outcome is further compared with regard to its qualitative and quantitative agreement. The study shows that the employed numerical tools for predicting transient combustion are of very satisfying quality and can subsequently be used for more complex scenarios to evaluate questions concerning ame kernel development and igniter location.

An analytical approach for the design of carbon-fibre-reinforced polymer laminates with minimised thermal expansion

The potential of carbon-fibre-reinforced polymers (CFRP) for the design of structures with extremely small coefficients of thermal expansion (CTE) is commonly known. Such structures play an important role in space applications, like earth observation satellites and space telescopes. But they also have a high potential for earthbound applications, for example, for precise mechanical and optical measurement devices. This paper demonstrates that orthotropic and quasi-isotropic laminates are most suitable for the design of laminates with minimised CTE. The derivation of a shortcut equation to the tensor calculations of the classical lamination theory is presented. Its purpose is to assist engineers in the selection of suitable carbon fibres and matrix polymers for the design of a laminate with high thermomechanical stability. The application of the equation is discussed and demonstrated on examples with practical relevance.

Laser-Induced Blast Waves in Air and Their Effect on Monodisperse Droplet Chains of Ethanol and Kerosene

Spark ignition of fuel sprays is a complex process involving multiple mechanisms which are related to the fields of plasma physics, chemical kinetics, heat transfer, two-phase flows, evaporation and gas dynamics. Moreover, the requirements on reliability, efficiency and low emission increase continuously, driving improvements in combustor design.

An Experimental Investigation of Kerosene Droplet Breakup by Laser-Induced Blast Waves

The work presented in this paper intends to deepen our understanding of the mechanisms involved in the spark ignition of liquid fuel sprays. An experimental study is presented regarding the ignition of monodisperse droplet chains of Jet A-1 aviation kerosene in a generic model combustor under well-defined boundary conditions. Breakdowns created by focused laser radiation were used as ignition sparks. They featured rapid spatial expansion, resulting in the formation of spherical blast waves in the surrounding air. The focus of this study lay on the effect of the blast waves on the fuel droplets. Blast wave trajectories were investigated by Schlieren imaging. Their interaction with kerosene droplets was observed by a high-speed camera via a long distance microscope; the droplets were visualized by laser-induced Mie scattering. Droplets within a distance of ten millimetres from the breakdown position were disintegrated by the aerodynamic forces of the post-shock flow field. Different breakup modes were observed, depending on the distance from the breakdown position.Copyright

GTL fuels and their effects on aircraft aas turbine altitude ignition – detailed diagnostics

Abstract There has been growing interest for alternative fuels in aviation in the past three years. An alternative fuel can be defined by the triplet ‘feedstock-process-fuel’. Presently, the triplet with one of the highest technology readiness levels for gas turbine (GT) applications involves using natural gas in a Fischer-Tropsch thermo-chemical process to derive synthetic paraffinic kerosene (SPK). Generic Fischer-Tropsch fuels were approved in September 2009 for use as 50% blends with Jet A-1. Gas-to-Liquid (GTL), as a final product of the aforementioned triplet meets those specifications and is in a deployment stage, with one plant currently under construction in Qatar. The product of a Fischer-Tropsch process, such as GTL, is feedstock agnostic. As such, its composition can be tailored to meet or exceed some of the current specifications for jet fuel. Investigating the potential benefits of composition changes in GTL-like jet fuel defines the general scope of our research program. This article presents the results of tests conducted on the Rolls-Royce plc TRL3 sub-atmospheric altitude ignition facility in Derby, UK. The test campaign aimed at investigating the impact of the carbon number distribution (narrow/wide cut), the iso- to normal-paraffin ratio and the total cyclic paraffin content characterizing the surrogate GTL-like fuel composition on the ignition and combustion performance of a single sector advanced GT combustor and fuel injector under simulated altitude conditions. The detailed diagnostics consisted of simultaneous high-speed imaging of hydroxyl (OH*) and methylidyne (CH*) chemiluminescence and broadband luminescence measurements of the ignition process. By observing the processes in the visible and the UV simultaneously, it was possible to distinguish between radiation (originating from e.g. soot) and the chemiluminescent emissions from the OH* and CH* radicals. These are markers for chemical activity in the different regions of the combustor and therefore provide information concerning the temporal and spatial development of the flame kernel.