Insights from a new method providing single-shot, planar measurement of gas-phase temperature in particle-laden flows under high-flux radiation

Published online: 31 March 2021Two-colour laser-induced fluorescence (LIF) of toluene has been demonstrated to provide in situ, spatially resolved, planar measurements of the gas-phase temperature in a particle-laden flow with strong radiative heating at fluxes up to 42.8 MW/m². Toluene was seeded in trace quantities into the gas flow laden with particles of mean diameter 173 μm at a volumetric loading sufficiently high for particle–fluid and particle–particle interactions to be significant. The particle number density was also measured simultaneously using Mie scattering. The two-colour LIF method was found to resolve temperature with a pixelto- pixel standard deviation of 17.8 °C for unheated measurements in this system despite significant attenuation of the probe laser and signal trapping of the fluorescence emissions from the densely loaded particles. Following heating of the particles using high flux radiation, the increase in the gas-phase temperature from convection was found to be spatially non-uniform with highly localised regions of temperature spanning from ambient to 150 °C. This gas-phase heating continued well downstream from the limits of the region with radiative heating, with the time-averaged gas temperature increasing with distance at up to 2,200 °C/m on the jet centreline. The temperature of the flow was non-symmetrical in the direction of the heating beam, because the particles attenuate the radiation through absorption and scattering. The addition of radiation at fluxes up to 42.8 MW/m² did not significantly change the particle number density distribution within the region investigated here.Elliott W. Lewis, Timothy C. W. Lau, Zhiwei Sun, Zeyad T. Alwahabi, Graham J. Natha

Renormalisation of particle distributions in an initially-biased turbulent jet by swirl and radial injection

The distribution of particles in a jet with an initial bias is assessed using planar nephelometry, which is a planar imaging technique using Mie-scattering. Initial biases in particle distributions can have large and significant effects, particularly in solid fuel combustion systems, where inhomogeneities will affect the local and global performance of a flame. In this study, a two-phase jet flow with a controlled initial bias is generated at a Stokes number, based on the large-eddy time-scale, of Sk(D) = 0.39, and various strategies are explored to correct this bias. The investigation examines the usefulness of injecting air at various momentum ratios and through various configurations upstream from the nozzle exit plane. The study examines the differential roles of swirl (tangential) and radial injection of air at various locations from the flow exit. Radial injection of momentum is found to be superior to injection of swirl, consistent with swirl being a magnifier of eccentricities of a flow. Radial injection upstream from the nozzle can achieve a re-homogenised flow to develop prior to the exit, at least for the present Stokes numbers, but can also lead to downstream biases in particle distribution for some conditions. Injecting momentum directly at the nozzle exit does not allow a symmetrical distribution to be achieved, but does have an observed influence on the emerging flow-field.Cristian H. Birzer , Peter A.M. Kalt , Timothy C.W. Lau, Graham J. Natha

The effect of particle size and volumetric loading on the gas temperature distributions in a particle-laden flow heated with high-flux radiation

The instantaneous, spatially resolved gas-phase temperature distribution within a particle-laden flow heated using high-flux radiation has been measured for a series of heating fluxes, particle volumetric loadings and particle diameters using two-colour laser induced fluorescence of toluene. The temperature of the gas downstream from the start of the heating region was found to increase with an increase in heat flux, an increase in particle loading and a decrease in particle diameter. Coherent regions of high and low temperature in the instantaneous flow associated with spatial variations in the particle distribution were identified for all particle diameters investigated. The time-averaged gas-phase temperature on the jet axis was found to increase approximately linearly with distance in the region downstream from the heating beam to the edge of the measurement region investigated, indicating near-constant convective heat transfer due to the large temperature difference between the gas and radiatively heated particles throughout this region. The axial gradient of gas-phase temperature with distance was also calculated using a simplified, one-dimensional heat transfer model. The difference between the model and measurements was, on average, less than 20%, with the magnitude of this difference found to increase with a decrease in particle diameter and an increase in particle loading.Elliott W. Lewis, Timothy C.W. Laua, Zhiwei Suna, Zeyad T. Alwahabi, Graham J. Natha

The effect of instantaneous particle distributions on the gas-phase temperature in an unsteady particle-laden jet heated with high-flux radiation

Detailed simultaneous planar measurements of particle number density and gas-phase temperature were performed in radiatively heated particle-laden jet with average particle volumetric loadings in the range 0.625 − −1.4 × 10−3, which is within the transition region between the two- and four-way coupling regimes, to evaluate the correlation between the local particle volume fraction and temperature for inertial particles with a series of diameter distributions and radiative heating powers. Utilising novel optical measurement and image processing techniques, together with Voronoi analysis, regions of high instantaneous particle number density and localised regions of high/low gas-phase temperatures were identified. The results show that the particle volume fraction measured within the identified ‘hot regions’ was more than 1.5 times greater than the mean value for each case, while within the ‘cold regions’ the particle volume fraction was typically less than the mean. Similarly, the temperature around individual particles was found to increase with an increase in the local particle volume fraction, while the variation in local gas temperature in the vicinity of particles increases with a decrease in particle diameter. Furthermore, the temperature surrounding particles that were determined to be within closely spaced cluster regions (which form in the present measurements due to the random distribution of particles in the flow) was found to be greater than that around particles outside of clusters, even for the same local particle volume fraction (measured in a radius of 0.1 times the jet pipe diameter around each particle). The particle distributions were found to closely match a random Poisson distribution, consistent with the high particle Stokes numbers (86 ≤ ≤ 514), and are not affected by the presence of radiative heating, implying that any flow phenomena induced by thermal gradients in the flow negligibly influence the particles in the near field of the jet for the conditions investigated here.Elliott W. Lewis, Timothy C.W. Lau, Zhiwei Sun, Zeyad T. Alwahabi, Graham J. Natha