Experimental and Numerical Investigations of Sand-Water Slurry Jets in Water.

摘要

Sand-laden jets can be found in many engineering applications, such as, marine bed capping, mining operations, hydro-transport, dredging material disposal, and discharge of industrial and domestic wastewater. Understanding the dynamic interactions of the sand particles and its ambient are important for proper design and optimizing the engineering systems. Mass, momentum and energy of the system can be influenced by interactions of the suspended particles within the jet. As a result, the hydrodynamics of the flow of particle and fluid velocities, turbulence, and shear stresses are affected by particle interaction mechanisms which increase the complexity of the system and mixing phenomenon. The conducted experimental studies and numerical modeling in this thesis are new in terms of the fundamental understanding, flow characteristics and numerical techniques in simulation of sediment laden jets in water. The obtained results of this study can be used in many other environmental problems, such as marine construction, building artificial islands, deep ocean mining and discharge of sewage sludge into water bodies.;Part of this thesis focused on the numerical investigation of sand and slurry jets. Effects of the controlling parameters of the jets such as, particle size, nozzle diameter, initial velocity and particle concentration were investigated and it was found that in contrast with single-phase water jet, the water-phase spreading of slurry jets is function of nozzle size and particle concentration. Numerical experiments revealed that the water-phase centreline velocity of slurry jets increased with increasing particle concentration. Empirical formulations were introduced to show these strong correlations between densimetric Froude number, particle concentration and the velocity decay of slurry jets.;Particle-laden jets are commonly observed when particles are released instantaneously into water bodies but less attention has been devoted to study the starting of particle-laden jets and jet front with relatively high particle concentrations. It was found that the jet front terminal velocities, u_f∞, of small particles were as large as 5 times of the individual particle settling velocity, u_{f ∞}. Experimental investigation of sand jet front revealed that the slope of correlation between the normalized frontal velocity and axial distance was found to be ⅕ whereas this slope for a single-phase water jet was known to be ⅓.;An interesting flow feature that has long been associated with starting jets is the formation of the vortex structure. The formation of the vortex is largely due to the roll up of the jet shear layer as it is introduced into the ambient. Vortex structure of sand jet front was studied experimentally by employing the Galilean decomposition and the swirling strength techniques. In study of the turbulent modulation, it was found that smaller particles attenuate the turbulence much faster that larger particles and logarithmic formulations were developed for prediction of turbulent modulation on solid-gas and solid-liquid turbulent jets.;Effect of particle size on turbulence modulation of sand and slurry jets were investigated with numerical simulation and it was found that the turbulent kinetic energy of the water phase decreases with increasing particle size. Grouping effect of particles on variation of the drag coefficient of the slurry jets was studied by the employing the fundamental conservation of mass and momentum equations. It was found that particle concentration can reduce the drag coefficient of particle cloud since particles tend to travel behind the wake generated by frontal particles.;Part of this thesis focused on the physical characteristics and underlying dynamics of particles for designing purposes such as designing the nozzle size of the jet, choosing the most suitable jet velocity, and the initial sand concentration of the slurry jet for a given particle size range. In order to optimize slurry disposal and capping thickness, laboratory experiments were conducted to understand the structure and dynamics of slurry jets and the development of sand deposition with time. Variations of the aspect ratios with time were studied and power law correlations were introduced for prediction of the mound dimensions. Proposed models can be employed for estimating the mound dimensions on slopes up to 10 degrees with the average error of 4.8%.