Formaldehyde Laser-induced Fluorescence Research Articles

The effects of injector size on the dynamics and instabilities of lean premixed swirling flame

The external- and self-excited premixed swirling flames are experimentally studied to understand the thermoacoustic instabilities under different injector sizes. We use high-speed CH* chemiluminescence, formaldehyde laser-induced fluorescence and particle image velocimetry to measure the dynamic characteristics of the flame surface and flow field. Reducing the size of the injector from 20 mm to 16 mm changes the thermoacoustic system's stability. The flame transfer function shows that reducing the size of the injector significantly reduces the phase delay, enhances the response amplitude, and annihilates the local gain extrema. The low-order network model analysis is performed, and it is found that the phase variation caused by changing injector size is the main reason for the induced thermoacoustic instability. The pixel-by-pixel decomposition of CH* images is used to extract the weighted gain and phase maps of local heat release oscillation. The swirling flame with a small injector size of 16 mm has a short time delay and a slow phase velocity, which results in strong in-phase interference of the local heat release oscillators and enhanced gains. Reducing the size of the injector strengthens the inner and outer shear layers, thereby suppressing the heat release fluctuation of the flame base. The enhanced vorticity supply of the shear layer increases the size and strength of the outer vortex ring, resulting in strong heat release fluctuations at the flame tip, which is responsible for its large unadulterated fluctuation intensity. These changes in the flow field change the relative heat release intensity and fluctuation amplitude between the flame base and tip. The out-of-phase interference among multiple vortices causes weak flame response at high frequencies.

Kinetic effects of n-propylbenzene on n-dodecane counterflow nonpremixed cool flames

The development of alternative drop-in fuels and fuel blends with petroleum-derived fuels necessitate an understanding of the kinetic interactions between aromatics and alkanes for the development of advanced low-temperature combustion engines. In the present study, the role of aromatic chemistry on n-alkane low-temperature chemistry is investigated by using nitrogen-diluted nonpremixed cool flames of n-dodecane/n-propylbenzene blends in an atmospheric counterflow burner. Effects of aromatic addition on n-alkane reactivity are examined by fixing the concentration of n-dodecane while varying the ratio of nitrogen/n-propylbenzene. The cool flame structure is probed using formaldehyde laser-induced fluorescence and thermocouple thermometry, and measurements of the cool flame and hot flame extinction limits are obtained. Fundamental differences between the response of cool flame and hot flame heat release and radical production to stretch are also discussed. The results show that, while the specific heat release rate characterizes the burning intensity of a hot flame, the same is not true for a cool flame due to the competition between reactant leakage and negative temperature coefficient reactivity. It is also found that, contrary to hot flames, in cool flames the addition of n-propylbenzene promotes n-dodecane cool flame extinction. Kinetic analysis reveals that aromatics act as a sink of hydroxyl and hydroperoxyl radicals in cool flames by forming stable oxygenated aromatics due to the low flame temperatures. As a result, the low-temperature reactivity of n-dodecane is significantly inhibited by the addition of aromatics, even relative to inert substitution. The present experimental results are valuable for comprehensive chemical kinetic model development and validation.

Ignition delay times of single kerosene droplets based on formaldehyde LIF detection

Single droplet ignition experiments were performed with GTL kerosene, Jet A-1, and Exxsol D80 (reduced aromatics) in the temperature range from 550 to 900K in air at 3bar. The laser-induced fluorescence (LIF) intensities of liquid GTL kerosene and Exxsol D80 are on a lower level than that of Jet A-1. Formaldehyde LIF and background emission were measured with pulse-to-pulse wavelength switching at closely spaced excitation wavelengths (353.373 and 353.386nm). Typically, formaldehyde LIF and background emission were observed simultaneously. However, in some droplet ignition sequences background emission was observed first followed by formaldehyde LIF after a time delay. Therefore, in the case of kerosene background subtraction has to be performed carefully to derive correct ignition delay times based on formaldehyde LIF (cool flame). A pulsating combustion was observed in several kerosene droplet ignition experiments. The cool flame ignition was found to appear significantly earlier for the synthetic GTL kerosene compared to Exxsol D80 and conventional Jet A-1 kerosenes. For GTL kerosene we found a cool flame ignition delay time of 624ms, while corresponding values for Exxsol D80 and Jet A-1 amount to 1467 and 1699ms, respectively. The differences for the starting time of hot ignition (around 1050ms) and ignition temperatures (near 700K) are comparatively small under the conditions of our experiments.

Formaldehyde LIF detection with background subtraction around single igniting GTL diesel droplets

Formaldehyde is an important combustion intermediate, which is often used for the analysis of turbulent flames, internal combustion engines or droplet/spray ignition processes. Due to its high concentration in hydrocarbon fuel combustion, laser induced fluorescence (LIF) of formaldehyde can easily be excited (e.g. 339, 343, 353 and 355nm) and measured (360–550nm). However, it is often superimposed by background emission from other combustion intermediates as, e.g. polycyclic aromatic hydrocarbons (PAHs).We present formaldehyde LIF excitation spectra obtained with a XeF excimer laser in combination with a spectrometer and an ICCD camera in dependence of pressure (50mbar to 20bar). For formaldehyde LIF and background excitation, we selected the wavelengths 353.373 and 353.386nm, respectively. LIF measurements at the background wavelength contain a rising formaldehyde contribution with increasing pressure. We present a novel method for the calculation of the real background. This method is applied to the ignition of single gas-to-liquid (GTL) diesel droplets. However, the method can also be applied to other fuels. In the case of a GTL diesel ignition experiment (628K, 3bar) the real background amounted to only 30% of the emission which was measured at the background excitation wavelength (353.386nm).

Single-shot imaging of formaldehyde in hydrocarbon flames by XeF excimer laser-induced fluorescence

A novel way to excite laser-induced fluorescence (LIF) of formaldehyde was applied to measure LIF of hydrocarbon flames (diethyl ether, ethene, n-heptane, and kerosene). LIF excitation spectra of formal-dehyde in a reference cell and of flames were obtained by tuning a spectrally narrowed XeF excimer laser (Δλ=4pm, 30 mJ pulse energy) between 353.1 and 353.6 nm and integrating the emission between 360 and 550 nm. The LIF excitation spectra of the flames showed a contribution from formaldehyde and a broadband background from, for example, polycyclic aromatic hydrocarbons (PAHs). Exploiting the high excitation efficiency and pulse energy of the narrowband XeF excimer laser, single laser shots were sufficient to measure two-dimensional distributions of formaldehyde LIF (light sheet technique [3×3 cm2], intensified charge-coupled device [CCD] camera). Fast wavelength switching between on-and off-resonant excitation of formaldehyde was successfully applied to subtract background emission in the case of diethyl ether, n-heptane, and ethene flames. Furthermore, it was shown that the images of formaldehyde LIF complement the measurements of OH LIF. OH is only found in higher-temperature flame regions.