Development and Application of a Method for Gas-phase Temperature Measurements in Particle-laden Flows
Date
2022
Authors
Lewis, Elliott William
Editors
Advisors
Nathan, Graham
Alwahabi, Zeyad
Lau, Timothy (University of South Australia)
Alwahabi, Zeyad
Lau, Timothy (University of South Australia)
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Thesis
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Abstract
Suspensions of particles in a carrier flow of gas are utilised in, or being
developed for, several high-temperature industrial processes. These include
for material transformations in calciners and kilns, as fuel in particulate
burners and as the medium for radiation absorption in concentrated solar
thermal receivers. The efficiency, stability, and emissions from such systems
is strongly dependent on the temperature distribution of both the particle
and fluid phases, each of which can be highly variable both spatially and
temporally. While these systems are widely utilised, there is still a lack of
fundamental understanding of the heat transfer processes due to the complexity
of turbulent particle-laden flows with a high particle volume fraction.
Therefore, this work aims to provide insight into these processes for future
optimisation of non-isothermal particle-based systems. This is performed by
adapting and applying the technique of laser induced fluorescence (LIF) to
measure the gas-phase temperature in a particle-laden flow that is heated
using high-flux radiation.
This thesis presents the first demonstration of LIF in the densely loaded
conditions present in particle-laden flows relevant to industrial application,
with the potential for strong optical interference from elastic scattering of
radiation from the excitation laser by particles. The two-colour method for
thermometry, with toluene as the fluorescent tracer, was used to provide
spatially resolved measurements from a < 1 mm thick planar cross-section
of the flow. The particle distribution was measured simultaneously with
the temperature by imaging the laser light scattered by particles (particle
nephelometry). The accuracy and precision of the two-colour LIF method
was assessed for a series of particle materials and diameters, including
materials that luminesce following absorption of the excitation laser light. The
results show that optical filters effectively suppress the detection of elastically
scattered light, with other sources of measurement uncertainty including
particle luminescence, laser attenuation, and signal trapping identified and
assessed. The systematic error in the measurement from these combined sources was shown to increase with local particle loading, but be independent
of particle diameter.
The two-colour LIF and particle nephelometry methods were applied to
simultaneously measure the gas-phase temperature and particle distributions
in a particle-laden flow heated using high-flux radiation, evaluated for systematically
varied series of particle diameter, particle volumetric loading, and
heating power. The measurements were recorded in a particle-laden jet flow
issuing from a long, straight pipe with well-defined inlet and co-flow conditions,
with the particles heated using an axisymmetric, well-characterised
infra-red radiative source generating a beam with a peak flux of up to 42.8
MW/m2 on the axis. The resulting gas-phase temperature profile increased
monotonically with distance down-stream from the start of the heating region,
at up to 2,200 ◦C/m on the jet centreline. Additionally, attenuation of
the heating beam was shown to lead to an asymmetric temperature profile in
the jet flow.
The rate of increase of the gas temperature was shown to be directly
proportional to both the heating flux and the time-averaged particle volumetric
loading, within the range of conditions investigated. The temperature
decreased significantly with an increase in particle diameter, due to the
dependence of radiative and convective heat transfer processes to different
exponents of the diameter. The experimental results for the temperature rise
on the jet centreline were shown to match the trends from a simplified analytical
model. Importantly, the model also predicts that the particle temperature
is significantly greater than the gas, from the heating region to the edge of the
measurement region investigated. The asymmetry of the flow temperature
due to attenuation of the heating beam is also shown to increase with an
increase in the particle loading and a decrease in the particle diameter (i.e.,
an increase in the total cross-sectional area of particles in the flow).
The instantaneous distributions of both the gas-phase temperature and
particle locations were demonstrated to be highly non-uniform in the radiatively
heated particle-laden flow. The particle distributions were analysed
using Voronoi diagrams to determine the locations of particle clusters. Void
regions (i.e., with no nearby particles) were also identified. The gas-phase
temperature around particles was shown to be dependent on the local particle
loading, with the measured temperature inside of clusters also greater than
that outside of clusters. Localised regions of relatively high or low temperature
compared to their surroundings were also identified from the instantaneous
images, with these regions shown to remain coherent to the downstream
edge of the measurement region. The high temperature regions are
shown to be typically associated with regions of high local particle-loading, while regions with low temperature are shown to be in the void regions or
with a low particle loading. These results suggest that the structures in the
flow are long-lived with a sufficient particle-gas temperature difference, both
within the heating region and in the near-field downstream, for convection
between the particles and gas to influence the gas-phase temperature field
more significantly than entrainment, mixing, and convection within the gas
flows.
School/Discipline
School of Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2022
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