P. Monkhouse

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

Two-dimensional visualization of the flame front in an internal combustion engine by laser-induced fluorescence of OH radicals

Two-dimensional laser-induced fluorescence (2D-LIF) imaging of OH radicals, excited at 308 nm, has been employed to visualize the flame front in an internal combustion engine burning air/propane mixtures. Light sheet thicknesses down to 70 μm have been attained for excitation. Hydroxyl radicals were detected up to pressures of 7.5 bar at engine speeds of 500 rpm. An upper limit of 300 μm for the flame front thickness was obtained from line intensity profiles.

Picosecond fluorescence lifetime measurement of the OH radical in an atmospheric pressure flame

Abstract Direct measurements are reported of the OH (A 2Σ+, ν′ = 0→X2, ν″ =0) fluorescence lifetime in a laminar, premixed atmospheric methane-air flame, using a picosecond dye laser system based on the distributed feedback principle. Such measurements are needed for the determination of absolute species concentrations by laser-induced fluorescence. An average value for the lifetime of 1.97±0.26 ns is obtained, irrespective of vertical position in the flame and of equivalence ratio in the range φ = 0.77 to 1.43.

On-line diagnostic methods for metal species in industrial process gas

An overview is presented of the present status of on-line measuring methods for analysis of metal species, particularly alkali and heavy metals, in process gas. The motivation for developing such techniques is both environmental and economic, therefore a summary of the chief issues of concern is given first. Two broad groups of techniques are reviewed: optical methods (absorption, spontaneous emission and induced fluorescence) and ionisation methods, including mass spectrometry. Many of the more recent techniques involve the use of laser technology. The main features of the methods are presented first together with a description of the problems encountered in quantifying measured data. An intermediate section then briefly describes sampling methods. Finally, those measuring techniques that have proved to be amenable to realistic process conditions are considered again in the applications section. Their progress and capabilities to date are illustrated by a representative selection of results obtained in a variety of combustion and incineration facilities.

Flame front imaging in an internal-combustion engine simulator by laser-induced fluorescence of acetaldehyde.

Acetaldehyde has been used as a fluorescent dopant for two-dimensional imaging of the flame front in an internalcombustion-engine simulator. The molecule was excited with a XeCl-laser-light sheet at 308 nm, and broadband fluorescence centered at 400 nm was detected. In this way, the flame front could be marked by mapping regions of unburned gas. Also, the intake process into the engine could be followed.

Laser in situ monitoring of combustion processes

Several examples of laser in situ monitoring of combustion processes are presented. Using a frequency modulated (13)CO(2) waveguide laser, in situ concentrations of NH(3) down to 1 ppm were measured at temperatures up to 600 degrees C in waste incinerators and power or chemical plants. Following ignition of CH(3)OH-O(2) mixtures by a TEA CO(2) laser, gas temperature profiles were measured using rapid scanning tunable diode laser spectroscopy of CO molecules. In laminar CH(4)-air counterflow diffusion flames at atmospheric pressure absolute concentrations, temperatures, and collisional lifetimes of OH radicals were determined by 2-D and picosecond LIF and absorption spectroscopy. Two-dimensional LIF and Mie scattering were used to observe fuel injection and combustion in a diesel engine.

Investigation of flame structure and burning behaviour in an IC engine simulator by 2D-LIF of OH radicals

Turbulent combustion of propane/air mixtures in an internal combustion engine simulator has been studied by 2D-LIF of OH radicals formed in the combustion process. A laser light sheet of thickness 75 μm at 308 nm was used for excitation of OH and the fluorescence imaged onto an image-intensified CCD-camera. From the large number of images recorded, information on the burning behaviour of various flame structures could be obtained. In particular, flame extinction was clearly observed for lean (λ=1.5) mixtures.

DI Diesel Engine Combustion Visualized by Combined Laser Techniques

In this work we demonstrate that the progress of the combustion cycle in a four-cylinder (in-line) 1.9 1 direct injection Diesel engine can be studied effectively using different laser visualization techniques. Direct optical access to the piston bowl was facilitated by inserting quartz windows in one of the pistons. The flow field at the time of injection was characterized by seeding the flow and illuminating the piston bowl with a laser light sheet. Fuel spray development, auto-ignition and flame propagation in a Diesel cycle were followed by laser shadowgraphy and high speed cinematography while simultaneous laser induced fluorescence (LIF) and Mie scattering images were taken to distinguish the fuel distribution in the liquid and vapor phase. In addition, two dimensional distributions of OH and NO, formed during n-heptane/air combustion in the same engine, were recorded in the pressure range 5 to 50 bar by LIF following narrowband excitation using tunable excimer lasers. Finally, further work, designed to obtain quantitative images and hence data for comparison with model calculations, is outlined.

Time-resolved LIF of OH(A 2Σ+, ν′=1 and ν′=0) in atmospheric-pressure flames using picosecond excitation

Abstract Time-resolved fluorescence measurements are reported for the OH radical in premixed, laminar atmospheric-pressure flames, following excitation of the ν′=1 and ν′=0 levels of the A 2Σ+ state. OH radicals were excited using a picosecond dye laser system based on the distributed feedback principle. Fluorescence was detected with a gated streak camera coupled to a CCD camera and temporal analyser. For ν′=0 excitation, a very weak dependence on lifetime was observed with initially excited rotational level N′ in the range N′=1 to 15. In the case of ν′=1 excitation, the lifetime increased by up to 15% in the range N′=5 to 14. The stoichiometry range 0.8–1.26 was reinvestigated; with the improved precision of the present data, a 13% decrease in lifetime could be observed over this range.

Dependence of alkali emissions in PFB combustion on coal composition

Abstract The dependence of alkali release in fluidised bed combustion on fuel composition was studied using excimer laser fragmentation fluorescence (ELIF), an in-situ method that determines gas-phase alkali species with sub-ppb sensitivity. The PFB-reactor at DMT was operated under combustion conditions and ELIF-signals were detected in the flue gas after the cyclone. Within each experiment, the coal composition was varied systematically by doping the coal with known amounts of additives. The chlorine and sodium contents were increased separately, by respectively doping methylene chloride into the air feed of the fluidised bed or sodium acetate into the coal. Sodium chloride was used as dopant in some runs, but this raised both components simultaneously. The addition of chlorine alone leads to overproportional increases in the alkali concentration whereas that of sodium acetate lead to relatively moderate increases. Thus the amount of chlorine in the coal and in the FB reactor is decisive for the level of alkali emitted. In contrast, the presence of clay minerals e.g. kaolin in the coal or the fluidised bed is found to strongly suppress alkali release, especially that of potassium species.