Christian Eigenbrod

Two Stage Ignition of n-Heptane Isolated Droplets

ABSTRACT Experimental and theoretical studies on the spontaneous ignition process of isolated fuel droplets were carried out. Time dependent temperature fields around the igniting droplets were observed by interferometry so that two step temperature rise can be detected. Some experiments are performed under microgravity to obtain reference data. Induction times are examined as a function of ambient temperature. As a result, a zero temperature coefficient region is found, which is equivalent to the NTC (negative temperature coefficient) region for the ignition of premixed gas. A numerical model is developed applying a simplified chemical reaction model that includes the low and the high temperature reactions. The model is able to reproduce the two step temperature rise and the roles of the two kinds of reactions on the ignition process up to the establishment of a diffusion flame around the droplet are examined.

Spontaneous ignition of liquid droplets from a view of non-homogeneous mixture formation and transient chemical reactions

Spontaneous ignition of isolated single fuel droplets in air is investigated. Special attention is given to the transient behavior of chemical reactions. Single-stage and two-stage ignition or cool flame behavior is investigated. A fuel droplet of n -heptane, n -dodecane, or iso -octane is suddenly exposed in a high-temperature, high-pressure air to ignite. The succeeding process is observed by a Michelson interferometer that visualizes the instantaneous temperature field around a droplet. Experiments are done with air of 500–1100 K in temperature and 0.1–2.0 MPa in pressure. Ignition regions are mapped on a temperature-pressure plane and the roles of the low-and high-temperature branches of chemical reactions are described. Induction times and their temperature or pressure dependence are analyzed. The diameter dependence of the induction times shows influences of physical process only on first induction time. Pressure dependence of the second induction time indicates that the dominating factor for the induction time is the nature of a cool flame during the first stage. Mixture conditions at the occurrence of a cool flame and the relation between mixture condition and second induction times are examined. Fuel and temperature dependence of second induction time and TC (zero-temperature coefficient) behavior of total induction time are explained by the dependence of the mixture condition on those parameters through cool flame temperature.

Effects of dilution by aromatic hydrocarbons on staged ignition behavior of n-decane droplets

Spontaneous ignition of isolated two-component fuel droplets has been experimentally studied. The components were n-decane (ND)/1-methylnaphthalene (MN), or ND/1,2,4-trimethylbenzene (TMB). Both are n-alkane/aromatic mixtures, and therefore are candidates for model fuels of multicomponent commercial fuels refined from crude petroleum. ND showed two-stage ignition behavior, while the other two fuels (aromatic hydrocarbons) did not and were less reactive. A suspended droplet was suddenly brought to a high temperature for ignition. Observation by a Michelson interferometer detected cool-flame appearance as well as hot-flame appearance. The ambient gas was air, and the droplet diameter was 0.7 mm. The experimental conditions that were varied were ambient temperature, Ta, (500–1000K), ambient pressure, Pa, (0.1–2.0 MPa), and the initial mole fraction of ND in the liquid phase, Z (0–1). In the case Z≠1, Pa was fixed to 0.3 MPa. Ignition delays for cool-flame and hot-flame appearance (t1 and ttot) and the difference between them (t2) were measured, and ignition types (no ignition, cool-flame only, singlestage ignition, and two-stage ignition) were classified on a “Z-Ta” map. It is common to ND/MN and ND/TMB that two-stage ignition region is narrowed and finally vanishes as Z decreases, and that t1, t2, and ttot all increase monotonically as Z decreases. However, TMB was affected more than MN because of its higher volatility. (Normal boiling point: ND 447.3 K, MN 517.9 K, TMB 442.5 K) Numerical simulation with a fully transient one-dimensional model was employed to help the interpretation.

A model for devolatilization and ignition of an axisymmetric coal particle

A two-dimensional numerical model is developed to predict shape-dependent devolatilization and ignition of single coal particles exposed to radiant heat flux in an ambience at initially constant temperature. Intraparticle temperature gradients and radiation absorption are taken into account as well as the local release of volatiles and therefore the reduction of the raw coal. During pyrolysis, internal pressure gradients occur, which cause a viscous flow of gas through the pores described by Darcys law. The model considers intrinsic char combustion depending on the local oxygen concentration and internal and external volatile oxidation. Ignition is identified either by an increase of gas-phase temperature or the inflection condition of the particle surface temperature for the homogeneous and the heterogeneous ignition mode, respectively. Comparison with a published one-dimensional model, which assumes an isothermal particle, provides reasonable agreement for the ignition delays of particles with diameters of 50–1000 μm inserted into hot air. In contrast to the published data, the calculated results show a different primary ignition phase for particles larger and equal to 300 μm due to intraparticle temperature gradients. Former experiments on coal particle ignition by radiation in cold air under microgravity conditions show different homogeneous ignition delays for spherical, cylindrical, and flat particles. These experimental results are confirmed by the present model: in addition, heterogeneous ignition points are detected in the calculations. Although the homogeneous ignition delays increase in the order of slabs, cylinders, and spheres, which is the order where specific surface decreases, ignition delays can not be sufficiently described in terms of particle-specific surface, especially not for the calculated heterogeneous ignition.

Effects of natural convection on two stage ignition of an n-dodecane droplet

The effects of natural convection on two-stage spontaneous ignition of a fuel droplet were studied experimentally. A suspended n -dodecane droplet of 0.7 mm in its initial diameter was employed. Ignition phenomena were observed by a fine thermocouple (0.025 mm) and by an interferometer so as to detect an invisible cool flame. A direct photographic method with a back light was also used to measure the droplet diameter. Ambient temperature was limited within the range where the cool flame can be observed (500 to around 800 K) and ambient pressure ranged between 0.1 and 1.0 MPa. Experiments were carried out under normal and microgravity conditions, and results were compared. Investigations were done mainly on the induction times at 0.2 and 1.0 MPa. Experiments showed longer first induction times (time to appearance of cool flame) under normal gravity than under microgravity in certain ambient conditions. Significant differences between critical temperatures, below which cool flame or hot flame does not appear, were also observed between the two gravity conditions. The induction times differed up to a factor of 3, and the critical temperature differed about 60 K. For the second stage, slightly increased induction times were observed under normal gravity conditions at 0.2 MPa. Differences observed between both gravity conditions can be explained by the effects of natural convection driven by the down-ward force due to the dense fuel vapor and by the buoyancy due to the cool flame.

On the Performance of Porous Sound Absorbent Material in High Temperature Applications

This paper discusses the use of novel porous sound absorbent ceramic tiles as heat shields in combustion chambers with respect to their sound absorption. For this purpose, a theory describing the bulk properties of a homogeneous porous absorber layer was combined with a transfer matrix approach to account for the temperature gradient within the absorber. By means of a high temperature scenario, the maximum absorption performance and the required microscale properties of the absorber are presented.

Spaceborne Lightcraft Applications – an Experimental Approach

An experimental approach is proposed for the near‐term demonstration of space‐borne laser propulsion. A feasibility study at the ZARM Drop Tower Bremen is planned. The facility provides microgravity conditions within a drop capsule for ∼9 seconds. An excimer laser is used for energy beaming operating at a wavelength of 248 nm with max. 500 mJ pulse energy and a repetition rate of 250 Hz. Within the drop capsule, free flights of a lightcraft are intended to be conducted in air as well as under vacuum conditions. Different propellants are reviewed regarding their features for propulsion with a UV laser. The scalability of previous ground‐based flight experiments is discussed with respect to microgravity conditions and moderate pulse energies. Space logistic and sample return missions are discussed as possible applications.

Diode pumped solid state kilohertz disk laser system for time-resolved combustion diagnostics under microgravity at the drop tower Bremen.

We describe a specially designed diode pumped solid state laser system based on the disk laser architecture for combustion diagnostics under microgravity (μg) conditions at the drop tower in Bremen. The two-stage oscillator-amplifier-system provides an excellent beam profile (TEM00) at narrowband operation (Δλ < 1 pm) and is tunable from 1018 nm to 1052 nm. The laser repetition rate of up to 4 kHz at pulse durations of 10 ns enables the tracking of processes on a millisecond time scale. Depending on the specific issue it is possible to convert the output radiation up to the fourth harmonic around 257 nm. The very compact laser system is integrated in a slightly modified drop capsule and withstands decelerations of up to 50 g (>11 ms). At first the concept of the two-stage disk laser is briefly explained, followed by a detailed description of the disk laser adaption to the drop tower requirements with special focus on the intended use under μg conditions. In order to demonstrate the capabilities of the capsule laser as a tool for μg combustion diagnostics, we finally present an investigation of the precursor-reactions before the droplet ignition using 2D imaging of the Laser Induced Fluorescence of formaldehyde.

Numerical analysis of the cool flame behavior of igniting n-Heptane droplets

For many technical combustion applications like Lean Prevaporized Premixed (LPP) Systems the knowledge of droplet self-ignition processes as the basic element of spray ignition is necessary. Especially the two-stage ignition behavior due to a cool flame and therefore the Zero Temperature Coefficient Behavior (ZTC) are hereby important. Detailed Numerical Simulations give the opportunity to analyze these processes leading to droplet self-ignition, to get a deeper insight into the existing phenomena. The present study is carried out with a former validated, detailed numerical model including a substantial chemical reaction mechanism for n-Heptane with 437 reactions and 92 species. The Temperature Profiles as a function of time and as a function of radius as well as the profiles of the temperature gradient as a function of time and as a function of radius clarify the characteristics of the ignition process. The mixture fraction due to evaporation and diffusion and therefore the pool on the for the cool flame appearance important species OQ’OOH determines the activity of the low temperature reactions and thus the maximum possible cool flame temperature. The first local temperature rise, which is produced by the low temperature reactions, takes place below ambient temperature at very low fuel / air mixture ratios. It can be found, that the place of the maximum temperature and of the maximum temperature gradient as well, do not coincide during the first stage of ignition. The spatial in-homogeneity of the temperature and species concentration field around the droplet allows for a parallel burning of a cool flame caused by low temperature reactions, even when hot flame ignition has already occurred.

Development of Large-Scale Spacecraft Fire Safety Experiments

The status is presented of a spacecraft fire safety research project that is being developed to reduce the uncertainty and risk in the design of spacecraft fire safety systems by testing at nearly full scale in low-gravity. Future crewed missions are expected to be longer in duration than previous exploration missions outside of low-earth orbit and accordingly, more complex in terms of operations, logistics, and safety. This will increase the challenge of ensuring a fire-safe environment for the crew throughout the mission. Based on our fundamental uncertainty of the behavior of fires in low-gravity, the need for realistic scale testing at reduced gravity has been demonstrated. To address this knowledge gap, the NASA Advanced Exploration Systems Program Office in the Human Exploration and Operations Mission Directorate has established a project with the goal of substantially advancing our understanding of the spacecraft fire safety risk. The activity of this project is supported by an international topical team of fire experts from other space agencies who conduct research that is integrated into the overall experiment design. The large-scale space flight experiment will be conducted in an Orbital Sciences Corporation Cygnus vehicle after it has deberthed from the ISS. Although the experiment will need to meet rigorous safety requirements to ensure the carrier vehicle does not sustain damage, the absence of a crew removes the need for strict containment of combustion products. The tests will be fully automated with the data downlinked at the conclusion of the test before the Cygnus vehicle reenters the atmosphere. Several computer modeling and ground-based experiment efforts will complement the flight experiment effort. The international topical team is collaborating with the NASA team in the definition of the experiment requirements and performing supporting analysis, experimentation and technology development. The status of the overall experiment and the associated international technology development efforts are summarized.