4.3 Molecules Investigated
4.3.2 Formaldehyde
Formaldehyde is another important intermediate in the hydrocarbon combustion and has experienced growing interest in the recent years as an important molecu-lar species to monitor combustion events. This interest is due to several reasons, on the one handCH2O is formed by oxygen attack on primary fuel radicals (e.g.
CH3) and, therefore is indicative as a center for the occurrence of oxidation re-action. It is destroyed by the hydrogen abstraction resulting in the production of HCO, another intermediate species that correlates best with the peak reaction rate in methane-air combustion [56].
In combustion, LIF measurement of native HCHO in the laminar Wolfhard-Parker slot burner flame was first reported by Harrington and Smyth [59]. This was followed by several studies for the detection of formaldehyde molecule (HCHO) using point and planar measurement techniques in combustion flames, both ex-perimentally [60]-[66] and numerically [55],[67].
Also, formaldehyde is chemically very active species and spectroscopically ac-cessed with ease in the near-UV spectral region using existing laser sources. At excitation wavelengths shorter than 290 nm, the dissociation phenomenon be-comes so fast that the fluorescence is undetectable. Because of pre-dissociation, the lowest energy vibronically allows one-photon transition. A˜− X4˜ 10, is the strongest fluorescence band. This band is well suited for diagnostics applications, extending from about 352 to 357 nm in the absorption spectrum [see figure 4.4]
[68]. Some literature have also accessed the electronic band at 339 nm, which can be easily accessible from state-of-the-art tunable laser system.
Figure 4.4: Emission spectrum of formaldehyde CH20 taken from literature.
Two-dimensional imaging of HCHO in an spark-ignition engine has been re-ported by B¨auerle et al. [60] and Graf et al. [63]. In both cases, excitation via band ˜A1A2 −X˜1A1 (near 353 nm) was chosen. Other excitation wavelength at 338 nm [30], or at 370 nm [64] were also used for various application in flames.
These transition was accessed with a tunable dye laser pumped either by a XeCl excimer or an Nd:Yag pulsed laser. Not many literature have reported an ex-citation wavelength of 355nm, due to its weak exex-citation scheme. Even though the laser light at this wavelength is easily accessible using a non-linear crys-tal that generates 3rd harmonic from an Nd:Yag laser. Recently, Fayoux et al.
have measured planar laser induced fluorescence of formaldehyde molecule in a counter-flow premixed laminar flame by exiting it using laser light at wavelength of 355 nm [57]. We have also chosen this 355 nm weak excitation scheme for formaldehyde detection in the turbulent flames.
The emission spectrum of the formaldehyde molecule was determined in a lam-inar flame and compared with the available literature spectrum. The measured spectrum was found to match well with the literature spectrum. The details of the burner configuration, flame parameter and the location of measurement is provided in the next section. The emission spectrum from the flame was regis-tered using point laser induced fluorescence and the signal was detected using a detected that was attached to the spectrometer.
Figure 4.5: Schematic figure of EKT standard burner.
4.3.2.1 Burner Configuration and Flame Measured
Investigation and confirmation for the formaldehyde molecule was done in a laminar methane-air premixed flame from EKT standard burner. As can be seen from the figure 4.2, most of the carbon in the flame chemistry reduce through single C species and not through C2 species at the stoichiometric mixture [25].
Also, the approximation that the simultaneous measurement of HCHO and OH molecule compares well with the local temperature gradient, is valid between λB = 1.0 to λB = 1.2 [55]. Therefore, flame with an λB = 1.2 was chosen for testing purpose.
A schematic of EKT standard burner with nozzle diameter of 11 mm, is de-picted in the figure 4.5. A mixture of fuel and air flows through the center tube of 500 mm in length and the fluid exits at the nozzle end of 11 mm in diameter.
The flame chosen was a laminar flame with Re=500, φ= 0.88. The mass flow of surrounding co-flow (air) was 4800 lt/hr. The locations above the burner, where
the formaldehyde spectrum was taken using the point laser induced fluorescence technique are shown in the figure 4.6.
Figure 4.6: Location of point measurement for the formaldehyde spectrum in the standard burner, h is the height in mm and r is the radial distance from the center burner axis. h3r0 indicates center point at height 3 mm above the nozzle.
4.3.2.2 Spectrum Analysis
Figure 5.11 shows the result from the spectrum measurement of the formalde-hyde molecule excited through a Nd:Yag laser beam at an excitation wavelength of 355 nm in a methane/air laminar premixed flame. A spectrum between 405-515 nm was captured through a two-dimensional charge coupled intensifier located at the exit of the spectrometer. The details on the experimental set-up and the equipment used for spectrum measurements are provided in chapter 5. The formaldehyde spectrum is reproduced very well in comparison with the litera-ture spectrum shown in figure 4.4. The resultant profile is the result from the accumulation of 1500 single-shot spectrums acquired by the detector (ICCD).
Figure 4.7: Measured spectrum from the formaldehyde in Methane/Air premixed flame at the locations given in figure [4.6]. Comparison of measured spectrum (lower) is shown with the literature spectrum (upper).
This chapter outlines the experimental methods utilized for the application of laser induced fluorescence (both point and planar technique) in the turbulent flame diagnostics. A detailed description is provided on the experimental set-up and the instrumentation required for the successful completion of experimental measurement.