Development of simultaneous hyperspectral coherent Raman imaging for advancing reduced emission combustion technology

Abstract

Overall aim
and key objectives Advances in optical imaging techniques over the past decades
have revolutionized our ability to study chemically reactive flows encountered
in air-breathing combustion systems. Emerging technology for unravelling clean-
and efficient heat release is needed for advancing new reduced emission
technology, and is on the central agenda for a wide variety of energy
production- and transport industry. Combustion of fossil fuels remains our
largest source of energy production in the world, and global concerns regarding
energy security, environmental pollution, and anthropogenic climate change have
motivated a large body of research devoted to the experimental measurement and
numerical simulation of combustion systems. Clean combustion engineering is the
search for improved efficiency by means of strengthen the systems fuel-economy
and lowering the emission of NOx, particulates, CO and unburned hydrocarbons
(incomplete combustion). New reduced emission technology, greatly rely upon the
ability to control the heat release and the exhaust produced by the exothermic reactions
between the fuel and the oxidizer in the chemically reactive flow. For the
engineering system design, it exist a significant need to inform on the
flame-physics involved based on direct observation of the combustion reaction
progress and interaction, which is a demanding task for any measurement
technique. Chemically reactive flows are inherently multiscale, fully
characterized in three-dimensional space and evolving on rapid time-scales. The
combustion environment imposes a significant challenge for diagnostics, where
it needs to be collected complete information ideally with correlated-field
multi-parameter measurement capabilities, exhibiting high spatial and temporal
resolution and provided within a snap-shot to freeze the fast dynamics involved.
Concurrent detection of major- and minor molecular species (multiplexing) and
determining the three most important scalars; the temperature, the flow-field,
and the mixture fraction, is vitally important in studies of the reactive flow.
The temperature marks the evolution of heat release and energy transfer, while
species concentration gradients provide critical information on mixing and
chemical reaction. Optical imaging techniques have the advantage of being
non-invasive, which means that the studied process is not significantly
perturbed by the measurement technique, and allowing for the acquisition of
statistics in-situ. Spectroscopy offer intrinsic chemical specificity, in that
different classes of molecules have specific spectral signatures serving as
unique fingerprints for their identification. Laser-based diagnostics may in
general provide measurements with exceptionally high spatial- and temporal
resolution, which is important in producing reliable and accurate experimental
data. Coherent anti-Stokes Raman spectroscopy (CARS) is one such versatile technique,
which has had a profound impact on a wide variety of fields. It was pioneered
in composition- and temperature measurements almost 40 years ago, and is
referred to as authoritative with the level of accuracy and precision it may
provide. A limitation still, has been its main applicability as a single
point-measurement technique, where the experimenter needs to raster-scan the
measurement samples assembling the spatial image. Because many complex systems
can be fully characterized in multidimensional space, there is a large
motivation for the advancement of multidimensional CARS imaging techniques.