Author: Austin Palya Publisher: ISBN: Category : Electronic dissertations Languages : en Pages : 162
Book Description
The goal of the current work is to perform numerical modeling and identify important phenomena associated with vapor plume and jet flows induced by irradiation of metal targets with short pulse or continuous wave (CW) lasers, discover and explain the mechanisms responsible for vaporized material motion in applications of lasers for material processing and analysis such as deep laser drilling, laser-induced breakdown spectroscopy (LIBS), and selective laser melting (SLM) of metallic powders. The simulations of laser-induced vapor expansion into a background gas are performed with a combined computational model, including the thermal model of irradiated targeted and a kinetic model of multi-component gas flows. The latter is implemented for simulations in the form of the Direct Simulation Monte Carlo method. Based on this model, two major problems are considered. In the first problem, vapor plume expansion under conditions of spatial confinement when the plume, which is induced by irradiation of a copper target by a short-pulse laser, propagates inside a cavity or trench, is considered. The simulations identify two major effects, the focusing effect, appearing due to transient motion of shock waves inside the cavity, and the confinement effect, induced due to overall deceleration of the plume with increasing background gas pressure, as two major mechanisms affecting removal of vaporized material out of the cavity and formation of high-density and high-temperature regions in the plume core. Due to the trade-off between these effects, an optimum background gas pressure exists, when the efficiency of the vapor removal from the cavity is maximized. At later stages of plume expansion, the simulations also reveal a suction effect, when the vapor flow at the cavity throat can be temporarily directed into the cavity, inducing a decrease of the overall efficiency of vapor removal. The balance between the focusing and confinement effects is studied in a range of the background gas pressure, for various background gas species, and various geometrical parameters of the cavity and laser beam. It is also shown that application of double laser pulses with short inter-pulse separation can be beneficial for both laser drilling and LIBS. In the second problem, a vapor jet induced by irradiation of a stainless steel target by a CW laser is simulated under conditions specific for SLM of metallic powders. The generated vapor jet is then used to predict motion of powder particles that can be efficiently entrapped into the ambient gas flow induced by the vapor jet. It is shown that powder particles in a broad range of their diameters can be efficiently entrained into the gas flow and, thus, removed from the irradiated surface. These results are in agreement with experimental observations of the surface denudation effect in SLM.
Author: Joseph Hun-wei Lee Publisher: Springer Science & Business Media ISBN: 1461504074 Category : Science Languages : en Pages : 391
Book Description
Jets and plumes are shear flows produced by momentum and buoyancy forces. Examples include smokestack emissions, fires and volcano eruptions, deep sea vents, thermals, sewage discharges, thermal effluents from power stations, and ocean dumping of sludge. Knowledge of turbulent mixing by jets and plumes is important for environmental control, impact and risk assessment. Turbulent Jets and Plumes introduces the fundamental concepts and develops a Lagrangian approach to model these shear flows. This theme persists throughout the text, starting from simple cases and building towards the practically important case of a turbulent buoyant jet in a density-stratified crossflow. Basic ideas are illustrated by ample use of flow visualization using the laser-induced fluorescence technique. The text includes many illustrative worked examples, comparisons of model predictions with laboratory and field data, and classroom tested problems. An interactive PC-based virtual-reality modelling software (VISJET) is also provided. Engineering and science students, researchers and practitioners may use the book both as an introduction to the subject and as a reference in hydraulics and environmental fluid mechanics.
Author: Kedar Ashok Pathak Publisher: ISBN: Category : Aerodynamics, Hypersonic Languages : en Pages : 141
Book Description
The flow field induced by the ablation plume in the presence of background gas is simulated numerically. The study of plume flow that occurs in laser ablation is important for it can yield information on ablation process itself and the properties of end product for which the ablation is carried out. Unsteady compressible axisymmetric Navier-Stokes equations govern the plume flow. The major challenge involved, even in this simplified model of plume dynamics, is twofold: (i) the time scale of simulation spans six orders of magnitude, from nanosecond to millisecond, and (ii) the high nonlinearity of governing equations because of high pressure, temperature and injection velocity of plume. A computational model is developed that can account for the entire range of time scale and high nonlinearity. This model is a combination of numerical methods and includes multi-time step and multi-size grid technique. The uniqueness of model lies in choosing the combination of numerical methods and handling multi-size grid interface in a conservative way. The combination of numerical methods is decided after comparing the results of few numerical methods for a single plume. The plume dynamics for single plume is explained with the help of proposed post-processing model based on vorticity dynamics. The model not only helps in understanding the expansion dynamics of plume but also provides quantitative comparison amongst numerical methods. The validity of nano-to-micro second range viscous and inviscid models of plume dynamics is discussed by means of evaluation of source terms in the vorticity transport equation. The role of turbulence is evaluated by millisecond-scale modeling of plume expanding in surrounding furnace gas with imposed turbulent gust. The results for multiple plumes typical for real life ablation are presented and discussed. Shielding of laser beam by previously ejected plume in multiple laser hits is important because it changes energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. To account for this shielding effect, shielding models are developed and implemented. The quantity of ablated mass due to the shielding effect is evaluated. Ionization of carbon plume and its impact on plume dynamics and shielding is studied. An iterative procedure is developed to determine the local equilibrium temperature affected by ionization. It is shown that though shielding due to the presence of ionized particles in carbon plume is small, the effect of ionization on plume dynamics can be considerable. Shielding effect is calculated for laser pulses with different time interval between pulses. The effect of high temperature and low density of plume are controversary and cause shielding behavior to be non-monotonic with pulse number. It is shown that the non-monotonic dependence of the delivered laser energy with the pulse number and the difference in shielding characteristics between planar and axisymmetric formulations increase with the time duration between two consecutive pulses. The developed numerical methodology is employed to study the heat transfer modulation between the Thermal Protection Shield (TPS) and the gas flow occurring because of ejection of under-expanded pyrolysis gases through the cracks in the TPS in hypersonic flight. The simulations are performed for an axisymmetric bluff body flying at Mach 7. The influence of the geometry of the TPS on heat transfer pattern is studied for two representative shapes. The results are presented for three different flight altitudes (low-ground level, moderate-20km and high-30km). At the low altitude the plume pressure is lower than the pressure behind the detached front shockwave and the plume propagates slowly along the wall surface. At high and moderate altitudes, the plume path and consequently, convective heat transfer between the TPS and the plume depends on the plume interaction with the bow shock wave. The effect of viscosity for the plume injection conditions and free stream Mach number considered is found to be negligible at simulated altitudes. However, the effect of initial pressure of pyrolysis gas on the plume dynamics is significant. The presence of the blast wave associated with under-expanded plume alters the heat transfer and increases mixing. Finally, the enhanced heat transfer caused by the emergence of multiple pyrolysis plumes is investigated.
Author: Gordon N. Taub Publisher: ISBN: Category : Languages : en Pages : 164
Book Description
Flows where a narrow body of fluid, carried by initial momentum or buoyancy, entrains an ambient fluid are common in the environment. Volcanoes, smoke exiting a chimney or rising from a forest fire, tornadoes and deep water oil spills are just a few examples. If the main driving force is momentum we term such a flow a jet. On the other hand, if the main driving force is buoyancy we typically refer to the flow as a plume. Perhaps more common, initial momentum and buoyancy are both present in which case we can refer to the flow as either a buoyant jet or a forced plume. Forced plumes can be either single phase, where buoyancy is caused by either a difference in temperature or some other scalar phenomenon, or multiphase where buoyancy is due to the injection of particles, bubbles or drops of material other than the ambient fluid into the flow. In this study single and multiphase jets and plumes will be examined. A laminar study will be presented which examines the effect of the addition of a small amount of nonzero angular momentum to jets and plumes. The results of direct numerical simulations of a pure jet, a pure plume and a forced plume, in the turbulent, moderate Reynolds number regime are presented and compared. Each simulation was run long enough so that time averaged third order statistics converged, allowing each term of the turbulent kinetic energy equation and Reynolds stress transport equations to be calculated directly. The results of laboratory experiments of a pure jet and multiphase plume, conducted at Ecole Nationale Superieure De Mecanique et D'Aerotechnique (ENSMA) located in Poitiers, France will be presented. Preliminary results of multiphase plume simulations using the Equilibrium-Eulerian technique will be discussed.
Author: Omid A. Ranjbar Publisher: ISBN: Category : Electronic dissertations Languages : en Pages :
Book Description
Ablation of materials by nanosecond laser pulses involves expansion of a laser-induced vapor plume into a background gas. The absorption of the incident laser radiation by the plume can substantially decrease the amount of laser energy absorbed directly by the target, and, correspondingly, the amount of the ablated material. This plasma shielding effect limits the overall efficiency of industrial laser systems designed for material removal applications. The goal of the present work is to numerically study the expansion process of plumes induced by irradiation of a metal target by bursts or groups of nanosecond laser pulses and to reveal the implications of the interaction between plumes induced by individual pulses for the efficiency of material removal. The plume expansion induced by irradiation of a copper target in argon background gas is studied based on one- and two-dimensional hybrid computational models that include a hydrodynamic or kinetic model of plasma plumes. The hydrodynamic model is based on finite-difference solution of gas dynamics equations. The kinetic model is implemented in the form of the direct simulation Monte Carlo (DSMC) method. In this work, the generalization of the DSMC method for plasma flows is developed. The effects of laser fluence, spot size, inter-pulse separation, and background gas pressure are thoroughly studied. The numerical simulations of plume expansion induced by a burst of pulses indicate the formation of complicated flow structures with cascades of the primary and secondary shock waves and strong interaction between plumes induced by individual pulses. The simulations reveal the plume accumulation effect when the plumes induced by preceding pulses in a burst change conditions of propagation of plumes generated by subsequent pulses. The degree of plasma shielding increases with increasing number of laser pulses due to the plume accumulation effect. It results in reduction of the effectiveness of material removal by the subsequent pulses. The degrees of the plasma shielding and plume accumulation effects strongly depend on the inter-pulse separation and laser spot size. The trade-off between the plume accumulation and thermal accumulation effects maximizes the ablation depth per pulse at a certain value of the time delay between pulses.
Author: Austin Palya
Author: Joseph Hun-wei Lee
Author: 
Author: J. Laasonen
Author: Zheng Wang
Author: Kedar Ashok Pathak
Author: Pauline H. Gurewitz
Author: United States. National Bureau of Standards
Author: Gordon N. Taub
Author: Omid A. Ranjbar