Turbulent jet flows are very common in engineering applications. One example is that of fuel injection in internal combustion engines, which is closely related to the combustion process. Because of the widespread use, the resulting emissions of such engines have a significant impact on human health and the environment. For a long time, research has sought to improve the mixing in developing turbulent jets to reduce the level of pollutants. Findings have indicated that injection unsteadiness can be used to improve the spray quality. Furthermore, it has been demonstrated that important spray characteristics can be linked to physical phenomena occurring in the region close to the nozzle. In this work, the breakup of an intermittently injected jet is investigated using numerical simulations. Cases of both single-phase and two-phase conditions are studied, characterizing the pulse breakup for different injection timing and varying fluid properties. For single-phase pulsation, mixing efficiency is shown to be connected to the generation of different secondary flow structures and their interaction. The breaking of symmetry along the pulse, responsible for the increased the mixing, is explained through a consideration of vorticity transport. This sequence shows local mixing is faster in the trailing region of pulses that are long enough to form secondary vorticies in the corresponding region. The study is extended to include effects of acceleration and deceleration during injection. The mixing rate depends on the accumulation of jet fluid within the generated flow structures. A rapid injection increase or decrease is found to promote the jet mixing and spreading by triggering jet fluid shedding and destabilization of such flow structures closer to the nozzle. Slow velocity changes promote separation of the injected fluid which instead suppresses near-nozzle mixing. Simulations of intermittent injection of liquid into quiescent gas have also been performed. Primary breakup of liquid pulses is assessed by considering the increase of the liquid-to-gas interface area and volumetric decrease over time. The disintegration process for these cases are less sensitive to the surrounding gas flow because of the higher jet inertia. Increased injection frequency and lower injection to non-injection ratio, is observed to stimulate primary breakup. This is due partly to a stretching action near the nozzle, and partly to a stronger relative influence of collision between liquid pulses. / <p>QC 20160504</p>

In the last few years a new method for surface preparation has evolved, namely thermo-abrasive blasting. This technique utilises a high enthalpy thermal jet to propel abrasive particles. The thermo-abrasive blasting gun, also called a thermal gun, is based on the principles of High Velocity Air Fuel (HVAF) processes. Nozzles used for thermo-abrasive blasting are subjected to thermal loading, wear and mechanical stresses. Therefore, the nozzle geometry and materials are critical for reliable performance of a thermo-abrasive system. In this investigation, the thermal stresses developed in the nozzle materials for thermo-abrasive blasting were analysed. The analytical and the computational models of the thermo-abrasive gun and the nozzle were developed. The computational fluid dynamics, thermal and structural finite element analyses have been employed in this study. The nozzle materials investigated were tungsten carbide, hot pressed silicon carbide, nitride-bonded cast silicon carbide and SIALON. The simulation and experimental results show that the highest thermal stresses occur during the first two minutes from the start of the thermal gun. However, thermal stresses are also high after the system is shut off. The nozzle geometry was optimised, which provided high cleaning rates with evidence of improved thermal loading, based on the experimental results. A new design of the thermal gun and the ignition method associated with a HVAF system were developed in this study. It is also concluded that the computation fluid dynamic and the finite element technique can be used to optimise the design of thermo-abrasive blasting nozzles. / Thesis (Ph.D. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2004.

In the last few years a new method for surface preparation has evolved, namely thermo-abrasive blasting. This technique utilises a high enthalpy thermal jet to propel abrasive particles. The thermo-abrasive blasting gun, also called a thermal gun, is based on the principles of High Velocity Air Fuel (HVAF) processes. Nozzles used for thermo-abrasive blasting are subjected to thermal loading, wear and mechanical stresses. Therefore, the nozzle geometry and materials are critical for reliable performance of a thermo-abrasive system. In this investigation, the thermal stresses developed in the nozzle materials for thermo-abrasive blasting were analysed. The analytical and the computational models of the thermo-abrasive gun and the nozzle were developed. The computational fluid dynamics, thermal and structural finite element analyses have been employed in this study. The nozzle materials investigated were tungsten carbide, hot pressed silicon carbide, nitride-bonded cast silicon carbide and SIALON. The simulation and experimental results show that the highest thermal stresses occur during the first two minutes from the start of the thermal gun. However, thermal stresses are also high after the system is shut off. The nozzle geometry was optimised, which provided high cleaning rates with evidence of improved thermal loading, based on the experimental results. A new design of the thermal gun and the ignition method associated with a HVAF system were developed in this study. It is also concluded that the computation fluid dynamic and the finite element technique can be used to optimise the design of thermo-abrasive blasting nozzles. / Thesis (Ph.D. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2004.

An experimental investigation was conducted using particle image velocimetry to study the effect of nozzle geometry on turbulent free orifice jets. The nozzle geometries studied include the round, cross, flower, star, rectangular and elliptical nozzles (aspect ratio 2). The spread rate of the rectangular nozzle was 61% greater than the square nozzle while the elliptical nozzle was 45% greater than the round nozzle using the conventional half velocity width. The superior mixing capacity of the rectangular and elliptical nozzles is attributed to the axis-switching mechanism. Evaluation of the energy budget showed a higher level of production of turbulence and convection of the mean flow for the rectangular nozzle compared with the round nozzle. Two-point auto-correlation function revealed larger structures in the non-circular nozzles and in particular the rectangular nozzle. The Kolmogorov and Taylor microscales however, did not show any significant dependency on nozzle geometry. / October 2017

Cold spray is an additive manufacturing method in which powder particles are accelerated through a supersonic nozzle and impinged upon a nearby substrate, provided they reach their so-called critical velocity. True to its name, the cold spray process employs lower particle temperatures than other thermal spray processes while the particle velocities are comparably high. Because bonding occurs mostly in the solid state and at high speeds, cold spray deposits are distinguished for having low porosity and low residual stresses which nearly match those of the bulk material. One complication with the cold spray process is the tendency for nozzles to clog when spraying (in general) low-melting-point or dense metal powders. Clogging occurs when particles collide with the inner nozzle wall and bond to it rather than bouncing off and continuing downstream towards the substrate. The particles accumulate and eventually plug the nozzle passage. Clogging is inconvenient because it interrupts the spraying process, making it impossible to complete a task. Furthermore, when particle buildup occurs inside the nozzle, the working cross-sectional area decreases, which decreases the flow velocity and therefore the particle velocity, ultimately jeopardizing the particles’ ability to reach critical velocity at the substrate. In this work, computational fluid dynamics (CFD) is used to study various aspects of nozzle clogging. Nozzle cooling with supercritical CO2 as the refrigerant is investigated as a means to prevent clogging. The effects of nozzle cooling on both the driving gas and the particles are addressed. Simplified pressure oscillations at the nozzle inlet are imposed to determine whether such oscillations, if present, can cause clogging. Subsequently, more realistic and complicated flow oscillations are introduced to isolate a potential root cause of clogging. Finally, several novel nozzle internal geometries are evaluated for their effectiveness at preventing clogging. A recommendation is provided for a nozzle to be tested experimentally because it might completely prevent clogging.

This thesis aims to make advances in the accurate simulation of the ows in and around clustered module plug nozzles. The resulting simulations presented in this thesis are, as far as can be ascertained from available data, the most detailed to date in Europe. A comparison is made with results from other sources for clarication of this point. In the process of producing these solutions, two ow solvers have been developed. NSAXIMB is a general 2D multi-block ow solver,developed by the author, for the axisymmetric, Reynolds averaged Navier-Stokes equations. It was developed to allow simulation of axisymmetic plug nozzle congurations and the investigation of the effects of turbulence modelling on such ows. MERLIN is a general 3D, implicit, multi-block ow solver again for the RANS equations. MERLIN was developed by the Centre for Computational uid Dynamic at Craneld. Signicant input from this work has included a large portion of the structure of the mean ow solver and the extension of the advanced two equation turbulence modelling, incorporated in NSAX- IMB, to three dimensions. Of the turbulence models investigated the zonal models of Menter prove to be most effective in reproducing experimental results. These models out perform a more advanced non-linear eddy viscosity formulation, based on the work of Abid. In an effort to improve solution accuracy, grid adaptation software, based on node redistribution techniques has been developed for use in conjunction with the 3D ow solver. This work is demonstrated in conjunction with a basic test case before application to the clustered module plug nozzle conguration. Results for the complex block topology adopted in the 3D test case are shown to cause the adaptation process to fail. Further, it is shown that such a process may not be generalised for arbitrary topologies.

A study of thrust vectoring techniques of annular aerospike nozzles was conducted. Cold-flow blow-down testing along with solid modeling and rapid prototyping technology were used to investigate the effects of slot size, placement, geometry and orientation. The use of slot-type compound nozzles proved to be a feasible approach to thrust vectoring. Previous methods of thrust vectoring have proved to be difficult to implement in a cost effective manner or have had limited effectiveness or durability.

This document describes the design of an axisymmetric aerospike nozzle to replace the conical converging-diverging nozzle of a commercially available hybrid rocket motor. The planar method of characteristics is used with isentropic flow assumptions to design the nozzle wall. Axisymmetric adjustments are made with quasi-one-dimensional flow approximations. Computational Fluid Dynamics (CFD) simulations verify these assumptions, and illustrate viscous effects within the flow. Nozzle truncations are also investigated. Development of a hybrid-rocket-specific data acquisition system is also detailed.

In the continuous casting of steels, clogging of the submerged entry nozzle has long been a persistent and costly issue. Previous modelling attempts have assumed that inclusions of different types exhibit the same degree of adhesion when colliding with the nozzle wall - an assumption not borne out by evidence in the literature. In this thesis, a dynamic clogging model is proposed which accounts for the effects of different degrees of inclusion-wall and inclusion-clog adhesion on clog formation and growth. The overall clogging model consists of several sub-models in order to account for the different physics. The melt flow and inclusion motion are modelled using an Eulerian-Lagrangian approach. The inclusion adhesion behavior is determined by the use of a stochastic model activated when an inclusion collides with a surface. A user defined sticking probability is used to determine if an inclusion sticks to a surface (Swall for wall collision or Sclog for clog collision) or instead bounces off. A macroscopic model is used to determine clog growth, where the volume of clog in a cell is tracked and used to determine when the clog grows into adjacent cells. Finally, a modified Kozeny-Carmen equation is used as a porosity model so that the presence of the clog affects and diverts the melt flow. The modified melt flow then alters subsequent inclusion deposition and clog growth. The model is used to investigate the effects of different degrees of inclusion adhesion on inclusion deposition and clog growth. Three scenarios are examined - 1) Inclusion deposition in a pilot scale nozzle, 2) Inclusion deposition in an industrial scale slide-gate controlled nozzle and 3) Clog formation and growth in a pilot scale nozzle. The deposition studies indicate that in a pilot scale nozzle, only a minority of inclusions ever collide with the nozzle (≈ 10%). In contrast, in the industrial scale nozzle there are far more inclusion collisions with the nozzle wall, ranging from 80% when the slide-gate is 20% open to 30% when the slide-gate is 100% open. Despite the differences in nozzle geometry and flow conditions, a similar effect on inclusion deposition is seen when Swall is varied. The effects of Swall can be divided into two regimes. When 0 ≥ Swall < 0.05 there is a sharp increase in the deposition ratio as Swall increases. When Swall > 0.05 there is a small and linear increase in the deposition ratio as Swall increases. This pattern is also seen in the study of clog formation and growth in a pilot scale nozzle. The effects of Swall or Sclog on clog volume can be divided into two regimes. As Swall or Sclog increases, there is a large increase in clog volume, until the sticking probability increases above 1E-2, then any further increase results in only a small increase in clog volume. In comparison to literature data the model successfully simulates the location of clog formation, the initial jump in clogging factor and the clogging factor growth rate in the later stages of clogging. However, the model underestimates the overall increase in clogging factor, resulting in a clogging factor at the end of the simulation which is half of that seen in the experiment. / Thesis / Doctor of Philosophy (PhD) / One of the ongoing challenges in the continuous casting industry is the occurrence of nozzle clogging. Over time, a buildup of material occurs within the submerged entry nozzle, called a clog. The clog leads to the partial or complete blockage of the nozzle, resulting in increased production costs. Since studying this phenomena experimentally is difficult due to the high temperature and opacity of the molten steel, modelling provides a useful alternative approach. However, previous modelling efforts regarding nozzle clogging have treated all inclusions as exhibiting the same adhesion behavior. This thesis aims to address this issue by presenting a dynamic nozzle clogging model which accounts for the effects of different degrees of inclusion adhesion. The model is used to study both inclusion deposition and clog formation. Results indicate that even a small amount of sticking probability results in a significant degree of inclusion deposition and clogging. The effect of sticking probability on clogging can be divided into two regimes, one where the clogging is very sensitive to the sticking probability and one where it is insensitive. Finally, the model was shown to run adequately even on coarser meshes (meshes with a smaller number of larger cells), indicating its utility in industrial applications, where it can be used to predict the location of clog formation and the clog growth rate.

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