Ch. Eigenbrod

DETAILED NUMERICAL SIMULATIONS OF THE MULTISTAGE SELF- IGNITION PROCESS OF n-HEPTANE ISOLATED DROPLETS AND THEIR VERIFICATION BY COMPARISON WITH MICROGRAVITY EXPERIMENTS

Detailed understanding of the basic physical and chemical processes of self-ignition phenomena for technical fuel sprays is required for many technical combustion applications. Because single droplets as the basic elements of fuel sprays allow the study of fundamental ignition behavior, this work focuses on the multistage self-ignition behavior of Φ 0.7 mm single n -heptane droplets in air. The investigated ambient conditions are temperatures from 580 to 1000 K and pressures from 0.3 to 1 MPa. A substantial physical and chemical model is developed for the detailed numerical simulation. The chemical reaction mechanism consists of a 62-step kinetic model with special consideration of the low-temperature reaction branch. The program was validated by comparison with results from microgravity experiments at Drop Tower Bremen, which allow clarification of the ignition process free from natural convection. Cool flame and hot flame appearances were obtained from non-intrusive interferometric measurement in a well-tested experimental setup. The calculated ignition delays resulting from temperature and temperature gradient criteria, respectively, were compared with these experimental results. Furthermore, the cool flame temperature was measured with a K-type thermocouple of Φ 25 μ m at the place of its appearance and was also compared with the numerical simulations. A quantitative good agreement for first and total ignition delays as well as for the cool flame temperature could be achieved. With this detailed numerical model, the multistage ignition behavior was analyzed, and the ignition criteria employed in interferometric measurement, which are temperature and temperature gradient, respectively, were confirmed.

Detailed numerical simulations for the multi-stage self-ignition process of n-decane single droplets with complex chemistry

The knowledge of droplet self-ignition processes as the basic element of spray ignition is necessary for many combustion applications. Detailed numerical analysis for the self-ignition process of n-decane single droplets showing the multistage self-ignition behavior of kerosene, are carried out for this study. A substantial chemical reaction model for n-decane with 603 reactions including 67 species is implemented in a former validated detailed one dimensional numerical simulation model for droplet ignition. This reaction mechanism pays special attention on the low temperature reaction path and the balance between high temperature and low temperature reactions. In the compared experiments the staged ignition process is detected by observing the temperature gradient with a Michelson interferometer, which is the numerical tracer for cool flame and hot flame appearance as well. The comparison between the results from the simulations and the experiments under microgravity conditions carried out at Drop Tower Bremen shows good agreement. Furthermore the numerical observation of important species like OH or formaldehyde is possible due to the implementation of detailed chemistry. This gives hints for other experimental methods to detect the multi-stage self-ignition behavior like formaldehyde-PLIF, which is meanwhile an approved tracer in the experiments, so that there is vice versa a further possibility to validate the present numerical model.

UV laser diagnostic system for combustion research under microgravity at drop tower Bremen

This paper describes a UV laser diagnostic system by means of which laser spectroscopic experiments were performed under microgravity conditions in a ground-based drop tower for the first time. A tunable, narrow bandwidth excimer laser is positioned at the top of the drop tower. The laser beam enters a falling drop capsule containing a specially adapted burner or combustion chamber. By the use of laser induced fluorescence spectroscopy measurements of 2D concentration and temperature profiles can be performed. Solutions of selected experimental problems such as laser beam collimation over a distance of more than 120 m, compensation of capsule drift, signal detection, and data acquisition (250 frames/s, 4.7 s measuring period), are discussed in detail. First measurements of laser induced predissociation fluorescence of OH radicals in a methanol flame under microgravity conditions are presented.

Characteriation of spherical hydrocarbon fuel flames: Laser diagnosis of the chemical structure through the oh radical

For the first time, spherical diffusion flames of n -decane, n -heptane, and methanol established around fuel-drenched porous spheres were investigated by applying the two-dimensional laser-induced predissociation fluorescence method to measure the OH radical under microgravity conditions. Focus of this work was laid on the development of a UV-laser diagnostic system attached to the Bremen drop tower for the characterization of combustion under buoyant-free conditions and on its applicability to the diffusion zone of hydrocarbon droplet flames. The experiments carried out on the previously mentioned fuels showed capabilities and limitations of this method applied on liquid fuels. While it is relatively straightforward to investigate the chemical structure of methanol droplet flames because of the absence of Mie scattering soot particles and of nonresonant fluorescing species, it is difficult for larger hydrocarbons such as n -hepatane and n -decane. It is considered that with the development of a luminous zone, strong broadband absorption by fuel vapor and intermediates larger than methane in connection with high laser pulserepetion rates may cause interactions of the input energy with the chemical kinetics of the combustion process.

Application of UV-laser-diagnostics to combustion research under zero-gravity

Abstract The application of modern laser diagnostical measurement techniques for combustion research in earth-based laboratories has brought essential experimental progress. In this paper the development of an UV-laser system is described, which for the first time will allow the application of two dimensional laser spectroscopic measurement techniques for experiments at the drop tower “Bremen”. The laser system will be integrated at the top of the tower, the laser beam follows the falling drop capsule and enters it from above. The drift between capsule and laser beam has to be compensated with an accuracy in the sub-mm range. Described are the laser-, control-, detection- and data acquisition systems, first results of the experimental properties and planned applications for experiments at the drop tower, “Bremen”.

Experimental study of pre-ignition phenomena of self igniting fuel droplets observed by a new diagnostic method

Interaction effects with special attention on staged ignition phenomena between two self igniting n-decane fuel spheres were studied in microgravity conditions and compared to single droplet results. All combustion experiments were done at the 4.7s Drop Tower Bremen and were performed with a fixed droplet size of 1.5mm, varying mid spacings and ambient temperatures and pressures up to 15bar. The work was done to support the development of numerical models for fuel spray auto ignition. A new technique, based upon chemiluminescence was applied to visualize staged ignition phenomena. chemically excited formaldehyde (CH2O*) was used as a excellent natural tracer for low temperature reactions. According to the experimental requirements an intensified camera (ICCD) concept was developed, so that the real weak chemiluminescence signals become detectable with a sufficient quantum of efficiency. For diagnostics evaluation data derived from chemiluminescence monitoring were compared with those from planar laser induced fluorescence (LIF) of formaldehyde excited at 352.2nm by means of an XeF-Excimer Laser.

Digital high-speed camera system for combustion research using UV-laser diagnostic under microgravity at Bremen drop tower

A digital high-speed camera- and recording system for 2D UV- laser spectroscopy was recently completed at Bremen drip tower. At the moment the primary users are the microgravity combustion researchers. The current project studies the reaction zones during the process of combustion. Particularly OH-radicals are detected 2D by using the method of laser induced predissociation fluorescence (LIPF). A pulsed high-energy excimer lasersystem combined with a two- staged intensified CCD-camera allows a repetition rate of 250 images per second, according to the maximum laser pulse repetition. The laser system is integrated at the top of the 110 m high evacuatable drop tube. Motorized mirrors are necessary to achieve a stable beam position within the area of interest during the drop of the experiment-capsule. The duration of 1 drop will be 4.7 seconds. About 1500 images are captured and stored onboard the drop capsule 96 Mbyte RAM image storagesystem. After saving capsule and data, a special PC-based image processing software visualizes the movies and extracts physical information out of the images. Now, after two and a half years of development the system is working operational and capable of high temporal 2D LIPF- measuring of OH, H2O, O2 and CO concentrations and 2D temperature distribution of these species.

Two-dimensional UV-laser diagnostic by high-speed imaging for microgravity combustion research at Bremen drop tower

A UV-laser based non-intrusive, highly specially and temporally resolving diagnostics for experiments under microgravity condition has been accomplished at (he drop tower Bremen recently. By means of an UV-excimer laser beam, that is guided from the top of the tower into the falling drop bus, two dimensional measurements of the concentration field of selected species can be conducted with a framing rate of up to 250 pictures/ sec. The data are beeing acquired by a digital high speed camera, that is equipped with a two staged, gated intensifier. The complete picture sequence of 1500 frames (256 x 256 pixel, 8 bit resolution) is stored real time aboard the capsule in a 96MByte RAM image storage unit. For the first time, in situ data of transient combustion phenomena preceding under conditions without thermal bouancy can be obtained by laser-inducedfluorescence LIF and laser-induced-predissociationfluorescence LIPF. In the current project, the gas phase reaction of burning droplets using different hydrocarbon fuels are beeing investigated by OH-LIPF. The structure of the flame has been identified and the limitations of the technique are discussed.

Application of a digital high-speed camera system for combustion research by using UV laser diagnostics under microgravity at Bremen drop tower

This paper describes a digital high-speed camera- and recording system that will be used primary for combustion research under microgravity ((mu) g) at Bremen drop tower. To study the reactionzones during the process of combustion particularly OH-radicals are detected 2D by using the method of laser induced predissociation fluorescence (LIPF). A pulsed high-energy excimer lasersystem combined with a two- staged intensified CCD-camera allows a repetition rate of 250 images (256 X 256 pixel) per second, according to the maximum laser pulse repetition. The laser system is integrated at the top of the 110 m high evacutable drop tube. Motorized mirrors are necessary to achieve a stable beam position within the area of interest during the drop of the experiment-capsule. The duration of 1 drop will be 4.7 seconds (microgravity conditions). About 1500 images are captured and stored onboard the drop capsule 96 Mbyte RAM image storagesystem. After saving capsule and datas, a special PC-based image processing software visualizes the movies and extracts physical information out of the images. Now, after two and a half years of developments the system is working operational and capable of high temporal 2D LIPF- measuring of OH, H2O, O2, and CO concentrations and 2D temperature distribution of these species.