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Content archived on 2024-04-16

M I D C O M I Multidimensional imaging diagnostics for combustion


Develop multidimensional diagnostics and in particular those based on the imaging of Laser Induced Fluorescence from a laser sheet (2D-LIF). This programme targeted at the application of 2D-LIF to pollutant formation in non-premixed combustion and sooty flames, such as the Diesel engine, gas turbine or furnace burner and to identify new measurement opportunities made possible by laser imaging, including velocimetry and thermometry.
Application of multidimensional laser diagnostics imaging to the measurement of species distribution and physical properties in simple combustion systems was demonstrated. 2-dimensional or planar laser induced fluorescence technique (2D-LIF) was developed for fuel, hydroxyl radical and polycyclic aromatic hydrocarbons measurements in sooty flames and was demonstrated for use in 2-phase fuel sprays. The latter was aimed at defining further the role for these diagnostic methods in nonpremixed combustion such as the diesel engine, gas turbine or furnace burners. Work was also done on velocimetry, thermometry and novel methods for coherent imaging. Supporting activities, such as the measurement of fluorescence quenching rates for differing chemical components and for realistic fuels were completed.

A method has been developed for imaging parent fuel fraction in nonpremixed gas fuelled flames, based on fluorescence from acetaldehyde. The method requires a high level of seeding in turbulent flames to overcome competing signals from polycyclic aromatic hydrocarbon (PAH) fluorescence and laser induced incandescence (LII). Photolytically generated carbon(2) following multiphoton absorption by fuel molecules was found to be a problem for short wavelength excitation around 248 nm but it was not significant for excitation at 308 nm.

The parent fuel fraction measurements were all performed with a xenon chloride Excimer laser operating at a wavelength of 308 nm and with laser fluences in the laser sheet of circa 10 to 30 MW cm{-2}. Images of parent fuel fraction have been successfully measured in both a laminar Wolfhard-Parker flame and a simple turbulent jet flame; in the latter case both average and single pulse images were recorded.

It is shown that the seed species, acetaldehyde, behaves in a chemically similar fashion to methane, from an analysis of a full kinetic calculation of a 1-dimensional counter flow nonpremixed flame. For undiluted nonpremixed hydrocarbon combustion, the method is best suited to the study of pure aldehyde flames because of the requirement for high seeding levels of aldehyde to dominate competing signals from LII and PAH fluorescence.
Currently, the major problems of non-premixed combustion are: formation of NOx, soot and associated heavy hydrocarbons such as Polycyclic Aromatic Hydrocarbon (PAH). The production of soot and PAH critically depends on the fuel concentration, the temperature field and the presence of oxidants such as O2, O and OH. The two-dimensional LIF offers the possibility to image from a two-dimensional slice in the combustion space the instantaneous distribution of physical properties such as temperature and velocity and key species such as OH, NO, O2, hydrocarbons and soot precursors, e.g.PAH. The 2D-LIF images can be used for the formulation and validation of combustion models and as an input for combustor design. This contract was preceded by the "Turbulent Combustion and Diagnostics Project" (contract EN3E-0088-UK, 1986-1989) in which the development of laser diagnostics for point measurement in high pressure combustion and turbulent flames was carried out and the possibility of imaging complete flow field was rec gnized.

The follow-up of the project, MIDCOM II , is carried out under contract JOUE-CT91-0085.


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United Kingdom Atomic Energy Authority
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OX11 0RA Didcot - Oxfordshire
United Kingdom

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