The non-linear optical diagnostic techniques, previously developed to monitor on-line gas-phase reactions of interest in material processing and environmental protection, are currently applied to specific fields related to the control of atmospheric pollutant formation and release in combustion reactions.
Combustion diagnostics require single-shot space and time resolved determination of:
In our laboratory the following techniques are currently applied:
which are based on the use of pulsed high power tunable laser sources to perform either Coherent Scattering or fluorescence spectroscopies upon multiple-photon excitation schemes. Acronyms indicate respectively Coherent AntiStokes Raman Scattering (CARS), Resonantly Enhanced CARS (RECARS), and Laser Induced Grating Spectroscopies (LIGS). LIGS techniques utilize either three resonant laser beams at the same wavelength (UV, visible) in Degenerate Four Wave Mixing (DFWM) schemes or two resonant beams (IR, visible) and an off-resonance probe visible laser in Laser Induced Thermal Grating Spectroscopy.
CARS and RECARS
Hydrocarbon/air combustion has been investigated at atmospheric pressure. Since it is usually assumed that the NOx formation is related with the combustion temperature, broadband CARS technique has been used to monitor on N2 single shot space resolved temperatures in small premixed laboratory flames, as shown in the Picture below.
Picture of the planar CARS geometry with the laser beams sampling a laboratory flame generated by a Bunsen burner. The two intense green beams focused on the flame are mixed with a red Stoke beam (from the dye laser) and with the blue AntiStoke beam (the generated signal) respectively.
The broadband CARS set-up, previously built in order to monitor at low resolution different molecules in chemical and photochemical reactors and operated in collinear geometry, has been up-graded to perform single shot flame temperature determinations. In order to achieve a good threedimensional space resolution and to avoid interference for the cold N2 present in the surroundings, a planar geometry has been adopted after splitting in two parts the green pump beam and refocussing the pumps and the Stokes beams on an interaction volume smallest than 1 mm3. Temperature data have been obtained from the fit of the vibrorotational envelope of the N2 high resolution CARS spectra measured at different flame positions and heights.
The development of software for data analysis with conventional spectroscopic methods and with the use of Neural Network algorithms has been achieved for CARS temperature measurements on N2 and is in progress for the other techniques.
In monitoring the combustion, hydrocarbon (CH4, C2H2 and i-C4H10) decomposition and soot (C2) formation have also been investigated as a function of the hydrocarbon/air ratio in collinear geometry by CARS and RECARS, respectively.
LIGS: DFWM and LITGS
In order to detect directly the NOx formation at the primary steps of the combustion a dedicated set-up has been built, suitable both to LIF (Laser Induced Fluorescence) and LIGS diagnostics. This set-up has been used for UV-LIF NO detection either after UV single-photon or visible two-photon excitation. NO2 has been detected also by using visible single-photon LIF.
In order to improve detection limits and to facilitate the NOx signal extraction in the hostile combustion environment, LIGS techniques have been developed, which involve the same absorption band used for LIF in NO and NO2. In particular visible DFWM has been settled to detect NO2 while UV-DFWM has been performed to detect NO. The visible Backscattered DFWM set-up used for NO2 detection is shown in the Picture below.
Picture of the Optical line in B-DFWM in NO2. The four counterpropagating blue beams (3 lasers and the signal beam), crossing in the calibration cell, can be distinguished.
NO molecule and OH radical have been determined on small undoped laboratory flames by performing space resolved concentration and temperature measurements. DFWM data have been taken in Forward BOXCARS geometry. Detection of 30 ppm of NO has been achieved in a C2H2/air lean flame after resonant UV excitation near 226nm. In the same flame by F-DFWM, OH concentration and temperature measurements are currently in progress upon excitation near 282 nm. The optical paths followed by the three UV laser beams focussed in the probed volume are shown in the Picture below.
Picture of the optical line during UV F-DFWM in BOXCARS geometry for NO detection in a flame. The small laboratory burner is shown in the inset.
Different LIGS techniques have been used for reactants or products during combustion. Resonant excitation has been selected either in the visible or in the IR, visible detection has been performed in both cases in order to allow for image detection. LITGS Signals, enhanced for the formation of pressure induced thermal gratings, have been examined in order to establish the detection limit of different gaseous species at atmospheric pressure of higher.
Visible LITGS signal have been generated in NO2, upon blue excitation on different blue bands peaked 452 nm and 472 nm. The radiation scattered from a non-resonant pulsed beam of a longer wavelength (green or red) has been detected at proper time delay.
The time evolution of IR-LITGS signals, generated in different hydrocarbons and pollutants with strong vibrations in the 10 µm region, has been followed by recording the visible scattered signal from a c.w. He-Ne red laser. This technique is especially suitable for imaging. Time integrated, single shot images of the unburnt hydrocarbon excited in IR-LITGS have been obtained by using a gated CCD camera to monitor a small laboratory flame. A typical distribution is shown in the picture below.
Unburnt C2H4 in a C2H4/air flame at atmospheric pressure monitored by IR-LITGS. IR laser beams are tuned to a C2H4 resonance, the red HeNe probe, giving rise to the scattered signal beam, is intersecting the LISG grating at Bragg angle.