Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - TUCLA (Towards a deepened understanding of combustion processes using advanced laser diagnostics)

Teaser

Combustion is of utmost societal/industrial importance since it accounts for a large part of energy supply and utilization world-wide. However, combustion also significantly contributes to creation of green-house gases, such as CO2, and emissions from combustion processes...

Summary

Combustion is of utmost societal/industrial importance since it accounts for a large part of energy supply and utilization world-wide. However, combustion also significantly contributes to creation of green-house gases, such as CO2, and emissions from combustion processes constitute a major part of today\'s air pollutants, e.g. >90% of NOx and >50% of SOx originate from combustion devices. Detailed understanding of the complex processes in combustion, to achieve efficient fuel utilization as well as abatement of harmful environmental pollutants, poses outstanding scientific challenges. In order to adress these challenges, crucial information can be obtained by non-intrusive laser-diagnostic techniques with high spatial and temporal resolution for measurements of key parameters such as species concentrations and temperatures. With the overall objective develop and apply such techniques for further understanding of combustion processes, the research of the TUCLA project, arranged in five work packages (WP), includes

Development of new diagnostic techniques.

Concepts based on structured illumination (WP1) add a new dimension to present diagnostics based on temporal, intensity and spectral properties. They allow for multi-scalar measurements and efficient suppression of background light. In WP2, ultrafast femto/picosecond lasers are employed for investigating the diagnostic applicability of filamentation, new aspects of non-linear techniques and diagnostic aspects of photodissociation phenomena.

Phenomenological combustion studies using advanced laser diagnostics.

A very important aspect of the project is to use the developed and available diagnostic techniques to assure experimental data in extremely challenging environments and together with modelling experts enhance the understanding of combustion phenomena. Studies are carried out on three different topics:

- Flame structures in laminar flames at high pressure as well as turbulent flames at atmospheric/high pressure (WP3).
- Biomass gasification, where complex fuels require new techniques to measure nitrogen, alkali, chlorine and sulphur compounds, as well as for measurements inside fuel particles (WP4).
- Combustion enhancement by electric activation, which can be introduced to handle flame oscillations and instabilities (WP5).

Work performed

Within Work Package 1 (WP1) - Structured Illumination - work has been performed towards high-speed imaging. It has been demonstrated how structured illumination can be used to acquire a multitude of images using only a single detector. The method is based on superimposing a structural code onto the illumination to encrypt a single event, which is then deciphered in a post-processing step. This new imaging concept, referred to as Frequency Recognition Algorithm for Multiple Exposures (FRAME), opens up for new diagnostic opportunities, where we have successfully demonstrated (1) instantaneous three-dimensional imaging and (2) record-breaking high-speed videography. In the former experiments, the formaldehyde distribution in a turbulent flame was measured in four planes simultaneously using the FRAME concept and the data was computationally assembled for 3D. In the latter work, the FRAME concept was used together with a femtosecond laser from which a sequence of four pulses was created – each pulse illuminating the sample. A camera detected an accumulated image of the responses induced by each pulse, and thanks to the FRAME concept, individual images can be separated in the data post-processing. We demonstrated the ability to record videos of one-time events up to a frame rate corresponding to 5 trillion frames per second, i.e. with 200 femtosecond temporal resolution, by capturing the propagation of a laser pulse.

A number of novel diagnostic concepts have been developed in WP2 by exploiting ultrashort (pico- and femtosecond) laser pulses. The development has been focused on four different areas:
a) photofragmentation laser-induced fluorescence (PFLIF) for detection and imaging of non-fluorescing species without photochemical interferences and with improved species selectivity,
b) two-photon laser-induced fluorescence (TPLIF) for detection and imaging of primarily atoms in reacting flows,
c) single-ended detection of species based on backward lasing induced by two-photon pumping and
d) ultrafast (fs) videography as outlined above.

A summary of obtained results are:
a) PFLIF with ps-laser pulses allows for detection of hydrogen peroxides in flames with virtually no photochemical interferences in the product gases. However, for hydroperoxyl radical (HO2) detection, it was found that a photochemical interference in the flame reaction zone arises due to the increased probability for multi-photon dissociation with ps-laser pulses.
b) TPLIF based on fs-laser pulses has been successfully demonstrated for detection and imaging of hydrogen atoms and carbon monoxide (CO) in methane/air flames. The results are very promising, indicating that hydrogen atoms, a highly reactive intermediate species in combustion chemistry, can be imaged without photolytical interference. Due to the strong signals obtained, the recorded CO data allow quantitative validation of theoretical models.
c) Promising results on backward lasing have been obtained in flame through two-photon pumping of hydrogen atoms. Time-resolved detection of the backward lasing signal allows for measurements with a spatial resolution of about 1 mm.

In addition to the work in WP2, diagnostic development includes methods for temperature measurement, mid-infrared Laser-Induced Thermal Grating Spectroscopy (LITGs) and two-line atomic fluorescence, of which the latter allows for imaging of temperature distributions. Moreover, laser-induced fluorescence has been developed for detection of the amdiogen (NH2) radical, a key species in fuel-nitrogen conversion chemistry as well as NOx reduction processes.

Diagnostic techniques have been employed for studies of combustion and gasification in WP3-4. Laminar flames, with well-defined flow conditions, allow for detailed studies of chemistry with development and validation of kinetic mechanisms. Such investigations have been carried out for fuel-nitrogen conversion in ammonia-hydrocarbon combustion. Investigations have also inclu

Final results

- The progress in diagnostic development (WP1-2) with structured illumination as well as utilization of ultrashort laser pulses are significant achievements that open up for new diagnostic possibilities, have gained strong international recognition and impact in the research community, and also contribute to interdisciplinary knowledge transfer. In particular, the invented structured illumination FRAME concept got extensive attention and publicity.
- Laser-based measurements have provided quantitative data for validation of combustion models.
- New insights have been gained in combustion under highly turbulent conditions with distributed chemical reactions for which previous knowledge and understanding is limited.
- The establishment of strong research on electrical activation (WP5) has introduced the research group into a new field with studies of plasma phenomena. New insights on microwave enhancement of combustion have been gained.