Optical diagnostic techniques in combustion

Advanced optical diagnostics are developed and used in order to investigate combustion phenomena. Laser spectroscopy techniques are employed for temperature measurements and species concentration determination in combustion systems. Particular interest is devoted to the determination of polycyclic aromatic hydrocarbons (PAH’s) and soot in diffusion and rich premixed flames, aerosol analysis and pollutants determination at the exhaust of combustion systems and in the environment. In the laboratory the following techniques are normally utilized:

• spontaneous emission (SE) and chemiluminescence
• laser absorption (LA)
• laser induced fluorescence (LIF) and planar LIF (PLIF)
• multi-angular laser scattering from particulate (fig. 1)
• laser induced incandescence (LII)
• laser-induced beakdown spectroscopy (LIBS)
• FT-IR spectroscopy
• Rayleigh scattering (RS)
• Spontaneous Raman Scattering (SRS)

A brief description of some of the optical techniques that can be utilized for combustion diagnostics can be found in the Tutorials in the attachment sections.

An example of the differences and possibilities of the techniques is given by the comparison of fig. 2 and 3. Here two images, in false colors, of the OH radical are compared. They were obtained in the same experimental condition on a Bunsen flame. The image of the spontaneous emission of OH* excited by chemiluminescence and is mainly concentrated along the reaction zone. The PLIF image shows that the actual concentration of OH is mainly found in the post-reaction zone with maximum at the flame borders where a secondary reaction zone with the surrounding air is present.

Another example of the possibilities of the LIF technique is shown in fig. 4 where fluorescence spectra taken along the axis of a propane diffusion flame are reported. The excitation wavelength was the 4th harmonic of a pulsed Nd:YAG laser at 266 nm. The sharp peaks are due to soot scattering. Low in the flame the LIF band is concentrated mainly around 350 nm and is attributed to small PAH’s. High in the flame where soot scattering becomes significant, the spectrum is broader and show the presence of PAH’s of high molecular mass.

The Laser-induced Incandescence (LII) technique is employed for soot concentration and sizing in flames and in the environment. By exploiting this technique a portable instrument has been realized. Fig. 5 shows the instrument during a measurement campaign on a fluidized bed combustion system at Istituto Ricerche sulla Combustione (IRC) of CNR.

Given the importance of detecting polycyclic aromatic compounds both in flames and in the environment an electronic Atlas of the fluorescence spectra of PAH’s, as reported in literature, has been realized. The Atlas can be downloaded in the Attachments. A sample page of the Atlas is shown in fig. 6. From the analysis of the collected data it is possible to correlate the spectral position of the emission band with the molecular weight of the compounds. Fig. 7 shows that the higher the molecular weight the more red shifted is the fluorescence spectrum. These data are helpful for the interpretation of experimental data such the ones reported in fig. 4.

In our laboratory, different techniques are applied to study the basic aspects of combustion processes. Laminar and turbulent flames, premixed, rich-premixed and diffusion flames, formation and detection of pollutants, soot and polycyclic aromatic hydrocarbons formation are our major research interests.

Current research activities concern:

• Laser diagnostics for soot detection
• Development and application of the LII technique for the detection of fine carbonaceous particulate in the environment
• Polycyclic aromatic hydrocarbons detection with Laser-Induced Fluorescence techniques
• Nanoparticle and pollutants detection with FT-IR techniques
• Influence of hydrogen addition on soot formation
• Development and characterization of microcombustors, both catalytic and fluidodynamic
• Application of a dielectric barrier discharge (DBD) plasma per methane reforming

Recently the scientific interest of the group was extended to the study of nanoparticle formation processes in combustion systems. 

Selection of published papers

• F. Cignoli, S. De Iiuliis, G, Zizak, Soot load versus aromatic concentration in diesel oil premixed flames, Fuel 80, 945-955 (2001)
• F. Cignoli, S. De Iuliis, V. Manta, G. Zizak, Two-dimensional two-wavelength emission technique for soot diagnostics, Appl. Opt. 40, 5370 (2001)
• F. Cignoli, S. De Iuliis, G. Zizak, A webcam as a light probe beam profiler, Appl. Spectrosc., 58(11), 1372 (2004)
• S. De Iuliis, F. Cignoli, G. Zizak, Two-color laser-induced incandescence (2C.LII) technique for absolute soot volume fraction measurements in flames, Appl. Opt., 44, 7414 (2005)
• S. De Iuliis, F. Migliorini, F. Cignoli, G. Zizak, Peak soot temperature in laser-induced incandescence measurements, Appl. Phys. B 83, 397-402 (2006)
• F. Migliorini, S. De Iuliis, F. Cignoli, G. Zizak, Absorption correction of two-color laser-induced incandescence signals for soot volume fraction measurements, Appl. Opt., 45, 7706-7711 (2006)
• S. De Iuliis, F. Migliorini, F. Cignoli, G. Zizak, 2D soot volume fraction imaging in an ethylene diffusion flame by two-color laser-induced incandescence (2D-LII) technique and comparison with results from other optical diagnostics, Proc. Combustion Institute 31, 869-879 (2007)
• F. Migliorini, S. De Iuliis, F. Cignoli, G. Zizak, How “flat” is the rich premixed flame produced by your McKenna burner?, Comb. Flame 153, 384-393 (2008)
• S. De Iuliis, N. Chaumeix, M. Idir, C.-E. Paillard, Scattering/extinction measurements of soot formation in a shock tube, Exp. Thermal Fluid Science, 32, 1354-1362 (2008)
• F. Migliorini, S. De Iuliis, S. Maffi, F. Cignoli, G. Zizak, Investigation on the influence of soot size on prompt LII signals in flames, Appl. Phys. B 96, 637-643 (2009)
• De Iuliis, S. Maffi, F. Cignoli, G. Zizak, Three-angle scattering/extinction versus TEM measurements on soot in premixed ethylene/air flame, Appl. Phys. B 102, 891-903 (2011)

Contracts

The research activities are performer within the frame work of:

• Commessa “Advanced diagnostics for innovative materials and combustion systems” of Dipartimento Energia e Trasporti del CNR
• IEA (International Energy Agency) Implementing Agreement on Energy Conservation and Emission Reduction in Combustion” in the collaborative task “Nanoparticle Diagnostics”

Active Contracts

• Accordo di programma MSE/CNR “Attività di RS di interesse generale del Sistema Elettrico Nazionale”, gruppo tematico “Carbone Pulito –CO2 capture”, progetto: “Diagnostiche ottiche per la misura del particolato e di microinquinanti”
• PRIN 2008EZ2NL4 “Microcombustori in pressione per applicazioni a microturbine e/o propulsione”

In the past the research activities have been financed by several national and international contracts. A brief list of the past contract includes:

• FIRB RBAU01JEJ3 “Caratterizzazione ottica e morfologica del particolato carbonioso in fiamme di idrocarburi”
• ODAR07230 (2007) CESI Ricerca- CNR-IENI “Studio per la caratterizzazione e messa a punto della tecnica spettroscopica in emissione per la misura della temperature ingresso turbina
• Sovvenzione globale INGENIO (2007) progetto “Misura del particolato con tecnica laser”
• FISR (2005-2008) “Matrici di micro combustori a idrogeno”
• INTAS-05-1000005-7664 “Hydrogen production and safety promotion by innovative processes”
• INTAS-03-514736 “Kinetics and mechanism of ignition/combustion initiated by electronically excited oxygen”
• INTAS-00-00556 “Combustion by electronically excited oxygen”
• JOF3-CT97-0030 “Low aromatic threshold”
• CHRX-CT94-0623 “Gravity dependant phenomena in combustion”
• JOU2-CT93-0379 “Fuel formulation”
• JOUE-CT91-0085 “Midcom I and II”
• EN3E-0088-UK “Turbulent Combustion”

Contacts

• Dr. Silvana De Iuliis, phone +39 02 66173 297
• Dr. Silvia Maffi, phone +39 02 66173 303
• Dr. Francesca Migliorini, phone +39 02 66173 384
• Enio Fantin, phone +39 02 66173 221
• Dr. Giorgio Zizak (associate), phone +39 02 66173 304