Please use this identifier to cite or link to this item:
Type: Thesis
Title: Aerodynamics of Fibrous Particles
Author: Qi, Guo Qiang
Issue Date: 2016
School/Discipline: School of Mechanical Engineering
Abstract: This thesis presents the first measurements of the velocities and orientation of nylon fibrous particles with length to diameter ratios of between 35 and 60 where the density ratio between the two phases is of order 1,000. One set of data was obtained with the fibres settling in air at a fibre Reynolds number in the range of 10 – 100 based on the fibres’ lengths under conditions that avoided any influence of secondary flows and at a mean volume fraction of 10-5. The other data were obtained with the fibres transported in a turbulent co-flowing jet at the jet Reynolds number of 70,000 in the super-dilute regime. These data are of relevance to the combustion of biomass particles in furnaces and boilers. As such, the data will contribute to the replacement of fossil fuels with biomass, which is an attractive fossil-fuel alternative because it is renewable and the net greenhouse gas emissions are lower than for fossil fuels. The measurements described above were undertaken with a novel implementation of Particle Tracking Velocimetry (PTV) in which a fibre’s orientation, the vertical and horizontal components of velocity, were measured simultaneously based on each fibre’s two end-points. The laser used in the experiments was a Quantel Brilliant Twins doublecavity pulsed Nd: YAG 10 Hz laser. The thickness of the light sheet was about 5 mm, which is a value found experimentally to provide a good compromise between a sufficiently high fraction of fibres fully within the light sheet and a reasonable spatial resolution. The one key feature of this method is that the “part-in” fibres within the laser sheet were detected and rejected through an assessment of the signal intensity and signal Abstract VI intensity gradients. The other key feature is that the volume fraction of the fibres was measured by counting the number of particles in the viewing volume. Firstly, the drag coefficient of long aspect ratio fibrous particles has been investigated experimentally. A “sphericity” parameter has been widely introduced in previous work to define the drag coefficient of fibrous particles. However it is not suitable for long aspect ratio fibres. In the present work, the relationship between the drag coefficient and a fibre’s Reynolds number based on the diameter for a long fibre was derived and investigated. An equation was proposed to describe the relationship between the volume fraction and settling velocity. It was also found that the scatter of horizontal velocity increases significantly with the volume fraction. The equivalent diameter of a settling fibre in air is reported. Two previous models of the drag coefficients of fibrous particles were also assessed. Secondly, the influence of volume fractions of the fibrous particles on their settling velocities and orientations was investigated. It was found that the mean settling velocities of the fibrous particles increased significantly with the number density of the fibres for the low volume fractions. This is attributed to the fibres’ orientation transition from the horizontal to the vertical state because of increasing interactions between the fibres, together with the influence of the cloud-like motion on the fibres. The volume fraction also has a strong influence on the mean orientations of the fibrous particles such that the fibres’ orientation tends to be more vertical with an increase in the number density of the fibres. Abstract VII Thirdly, for a bulk settling motion of the fibrous particles, it has been found that the distributions of the fibres’ vertical and horizontal components of settling velocity are nearly Gaussian. The bulk mean settling velocity of the fibres is much higher than that of a single fibre. This is attributed to the bulk motion effect and orientation transition mentioned above. The orientations of the majority of the fibrous particles are nearly horizontal. A key new finding is that the horizontal velocity of the fibres, whilst settling in air, is preferentially aligned with the major axis of the fibres, because a horizontal fibre moving horizontally in this direction has the minimum drag force. Furthermore it has been found that the majority of fibres exhibit rotation and tumbling while settling in air, which contrasts with the previous measurements in water. This is attributed to the fibre’s pressure centre being behind the mass centre whilst settling. It has also been found that the fibres’ tumbling is inhibited by a decrease in the aspect ratio. Angular velocities of the fibrous particles and their distributions for four types of the fibres are reported. Fourthly, the influence of the aspect ratio of the fibrous particles on their settling velocities and orientations was investigated for aspect ratios of 35, 48 and 60. For fibres with a constant diameter but different length, it was found that the settling velocity normalized by that of an equivalent sphere (Vcx/Veq-sph) decreases with an increase in fibre length. For fibres with the same length but different diameter, both the mean settling velocity and the normalized settling velocity (Vcx /Veq-sph) decrease with an increase in diameter. For fibres with the same aspect ratio but different length and diameter, the normalized settling velocity (Vcx /Veq-sph) decreases with an increase in particle size. Abstract VIII Lastly, the measurements in a turbulent jet found both that the fibres’ most probable orientation tends to be approximately 50° to the axial direction and that there are few fibres that are aligned with the direction of the flow at the centre-line of the jet, which is consistent with the simulation of inertial fibres in a turbulent channel flow. However it contrasts with the previous work of small fibres in a turbulent pipe flow, where the fibres were reported as being predominately aligned with the direction of the flows at the centre-line of a pipe. This difference is attributed to the fibres’ inertia. The fibres’ inertia is significant for the present jet flow because the density ratio between the particle and fluid phases for the present case is three orders of magnitude larger than that of the turbulent pipe flow. At the centre of the co-flowing jet, the fibres’ axial velocity and orientation were found to change little with an increase in volume fractions in the super-dilute regime, which contrasts with the findings of the free-falling cases in which the settling velocity and orientation change significantly with the volume fraction. These differences are attributable to a reduction in the relative significance in the interactions between the wakes of the proximate particles. The fibres’ vertical angular velocity is the lowest at the centre-line of the jet and increases significantly in the radial direction, which is consistent with the previous simulations. At the centre-line of the jet, the fibres’ normalized radial velocity is much higher than that of the spheres with a similar Stokes number based on diameter. This is attributed to the dual effects of significant orientation of the fibres to the flow and to their tumbling, both of which induce a radial velocity that does not occur with spheres.
Advisor: Nathan, Graham J.
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2016
Keywords: aerodynamics
fibrous particles
free settling
turbulent jet
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
Appears in Collections:Research Theses

Files in This Item:
File Description SizeFormat 
Qi2016_PhD.pdf9.38 MBAdobe PDFView/Open

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.