Vapor Recoil Research Articles

Mechanism of keyhole formation and stability in stationary laser welding

The formation and stability of stationary laser weld keyholes are investigated using a numerical simulation. The effect of multiple reflections in the keyhole is estimated using the ray tracing method, and the free surface profile, flow velocity and temperature distribution are calculated numerically. In the simulation, the keyhole is formed by the displacement of the melt induced by evaporation recoil pressure, while surface tension and hydrostatic pressure oppose cavity formation. A transition mode having the geometry of the conduction mode with keyhole formation occurs between the conduction and keyhole modes. At laser powers of 500 W and greater, the protrusion occurs on the keyhole wall, which results in keyhole collapse and void formation at the bottom. Initiation of the protrusion is caused mainly by collision of upward and downward flows due to the pressure components, and Marangoni flow has minor effects on the flow patterns and keyhole stability.

The effect of heat conduction in the vapor on the dynamics of downflowing condensate

A vapor fills the gap between two vertical plates, one hot and one cold. The temperatures are adjusted so that condensate forms on the cold wall. It is the dynamics of the system that is examined. The paper extends the one-sided model of evaporation–condensation to account the heat conduction in the vapor phase, which turns out to be important in many condensation problems. For the considered flow, both vapor recoil and Marangoni effect are stabilizing; as a result, the condensate becomes unstable at nonzero Reynolds numbers in contrast to the usual film flow down a vertical wall. A nonlinear evolution equation is derived and analyzed for the interaction of viscous shear and evaporation–condensation. It turns out that the one-sided model of heat and mass transfer gives a very good description of the initial stage of thin-film growth; in later stages, however, the heat conduction through the vapor becomes important when the film is sufficiently thick.

Gas “Wets” a Solid Wall in Orbit

When coexisting gas and liquid phases of a pure fluid are heated through their critical point, large-scale density fluctuations make the fluid extremely compressible and expandable and slow the diffusive transport. These properties lead to perfect wetting by the liquid phase (zero contact angle) near the critical temperature Tc. However, when the system's temperature T is increased to Tc, so that it is slightly out of equilibrium, the apparent contact angle is very large (up to 110°), and the gas appears to “wet” the solid surface. These experiments were performed and repeated on several missions on the Mir space station using the Alice-II instrument, to suppress buoyancy-driven flows and gravitational constraints on the liquid–gas interface. These unexpected results are robust, i.e., they are observed under either continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6 and CO2). Possible causes of this phenomenon include both a surface-tension gradient, due to a temperature gradient along the interface, and the vapor recoil force, due to evaporation. It appears that the vapor recoil force has a more dominant divergence and explains qualitatively the large apparent contact angle far below Tc.

Gas spreading on a heated wall wetted by liquid.

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.

A calculation model for the evaporation recoil pressure in lasermaterial processing

A physical model is proposed to calculate the recoil pressure actingon the surface of an evaporating material irradiated by a high-intensity laserbeam, such as encountered in laser material processing. Laser attenuation inthe laser-induced plasma plume, reflection at the material surface and theenergy consumption for material heating and melting are considered in thismodel. This evaporation model represents a revised form of that previouslyemployed by Chen Xi (1996, 1997, 1998) to calculate the evaporation-addedpressure acting on an evaporating particle exposed to a thermal plasma, buthere the special features associated with laser material processing areconsidered. The calculated recoil pressures compare favourably withavailable experimental data.

Experimental investigation of nanosecond pulsed Nd:YAG laser re‐melted pre‐placed powder beds

Describes the effects of the major process variables (Q‐switch pulse frequency, laser power, scan speed, scan spacing and scan length) on the production of single layer coupons. Results are compiled as a list of qualitative effects on the samples, such as degree of melting, shock compression effects, thermal stress cracking, etc. The results show that at certain pulse frequencies, evaporation recoil forces overcome the surface tension forces acting on the melt, improving cohesion compared to continuous wave (CW) lasing regime. The advantages lie in greater scan spacing and scan speeds enabling faster processing times for metallic objects built in this manner. The results also show the effect of power, scan speed, scan spacing and scan length on the morphology of the samples.

Growth of a dry spot under a vapor bubble at high heat flux and high pressure

We report a 2D modeling of the thermal diffusion-controlled growth of a vapor bubble attached to a heating surface during saturated boiling. The heat conduction problem is solved in a liquid that surrounds a bubble with a free boundary and in a semi-infinite solid heater by the boundary element method. At high system pressure the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force, a force coming from the liquid evaporating into the bubble. It is shown that at some typical time the dry spot under the bubble begins to grow rapidly under the action of the vapor recoil. Such a bubble can eventually spread into a vapor film that can separate the liquid from the heater thus triggering the boiling crisis (critical heat flux).

Study of the mechanism of steel cutting with the weakly focused radiation from a repetitively pulsed CO2 laser

A study was made of the process of steel plate cutting with a weakly focused radiation from a repetitively pulsed CO2 laser with the output of no more than 500 W without the use of a gas jet. The regimes were determined in which the removal of the melt under the action of the vapour recoil momentum proved to be more efficient than the melt removal under continuous irradiation. The physical mechanism of the process was proposed and its numerical model was elaborated. A comparison with experiments showed that this model provides a qualitatively adequate description of the experimental data and that it can be employed for the evaluation of remote cutting with high-power laser radiation.

ChemInform Abstract: The Phase Transition of Gases and Liquids

Abstract The behavior of gases and liquids during a phase transition is severely affected by gravity effects which unavoidably drive the denser phase downwards and the lighter phase upwards. When such gravity effects are suppressed a number of new and surprising behavior are observed. The studies are performed mostly in the vicinity of a critical point where the behavior can be expressed in scaled units of a natural length scale (the correlation length of the order parameter fluctuations) and a natural timescale (the characteristic relaxation time of the order parameter fluctuations). We will review the main results obtained with binary liquids and simple fluids. Firstly, a phase change is initiated by quenching down the fluid from above the coexistence curve, where the fluid is homogeneous, to inside the coexistence curve, where its phase separates into two phases. The main result is the recognition that the hydrodynamics of coalescence eventually induces the pattern morphology and the phase transition kinetics. The volume fraction, is the key parameter which decides whether drops fuse because of random, Brownian collisions [pattern of drops, growth law in (time) 1/3 ], or coalesce in a continuous process where the flow due to a coalescence event induces another coalescence (growth proportional to time, interconnected drop pattern). The presence of a wall modifies by its geometry and wetting properties the phase development. Coalescence is constrained and leads to new growth laws in the immediate vicinity of the wall. We emphasize that these phase ordering processes are quite general; they can be applied with success to quite different ordering situations, such as the sorting of embryonic tissues, an important process in morphogenesis. Secondly, when a gas–liquid, two-phase fluid, is heated from below the critical point to above, the process appears to be driven by the wetting and thermal properties of the boundary layer near the heating wall. Under terrestrial gravity, the dynamics is driven by a Rayleigh–Taylor instability. Under weightlessness, the liquid/vapor contact angle on the wall is modified in such a way that the gas seemingly “wets” the wall. We propose the vapor recoil force to be at the origin of this non-equilibrium “wetting” transition.

Laser Short Pulse Heating and Elastic-Plastic Wave Generation

Laser induced shock processing improves the mechanical and tribological properties of metallic surfaces. During the high power intensity laser beam interaction with the solid substrate ablation of the surface occurs, which in turn results in a recoil pressure developing in the interaction zone. This triggers the generation of elastic-plastic waves that propagate in the solid substrate and as a consequence a plastic region is developed in the vicinity of the substrate surface. In the present study, short pulse laser heating of steel is considered, and the Fourier and the electron kinetic theory models pertinent to laser heating are introduced. The governing equations of heating are solved numerically with the appropriate boundary conditions considered. The recession and vapor front velocities, and vapor pressure and recoil pressures are determined for different laser power intensities. The elastic-plastic wave propagation in the solid substrate is modelled and findings are compared with the results of the previous studies. It is found that the rapid rise of the surface temperature occurs at high laser power intensities and the recoil pressure generated at the interface substantiates, which in turn results in elastic-plastic wave propagation in the solid substrate. This develops a plastic region in the order of 0.9 µm in the surface vicinity of the solid substrate and the time required for this deformation is in the order of 25 ns. In addition, the depth of the plastic region developed in the present study agrees well with the previous results.

The phase transition of gases and liquids

The behavior of gases and liquids during a phase transition is severely affected by gravity effects which unavoidably drive the denser phase downwards and the lighter phase upwards. When such gravity effects are suppressed a number of new and surprising behavior are observed. The studies are performed mostly in the vicinity of a critical point where the behavior can be expressed in scaled units of a natural length scale (the correlation length of the order parameter fluctuations) and a natural timescale (the characteristic relaxation time of the order parameter fluctuations). We will review the main results obtained with binary liquids and simple fluids. Firstly, a phase change is initiated by quenching down the fluid from above the coexistence curve, where the fluid is homogeneous, to inside the coexistence curve, where its phase separates into two phases. The main result is the recognition that the hydrodynamics of coalescence eventually induces the pattern morphology and the phase transition kinetics. The volume fraction, is the key parameter which decides whether drops fuse because of random, Brownian collisions [pattern of drops, growth law in (time) 1/3], or coalesce in a continuous process where the flow due to a coalescence event induces another coalescence (growth proportional to time, interconnected drop pattern). The presence of a wall modifies by its geometry and wetting properties the phase development. Coalescence is constrained and leads to new growth laws in the immediate vicinity of the wall. We emphasize that these phase ordering processes are quite general; they can be applied with success to quite different ordering situations, such as the sorting of embryonic tissues, an important process in morphogenesis. Secondly, when a gas–liquid, two-phase fluid, is heated from below the critical point to above, the process appears to be driven by the wetting and thermal properties of the boundary layer near the heating wall. Under terrestrial gravity, the dynamics is driven by a Rayleigh–Taylor instability. Under weightlessness, the liquid/vapor contact angle on the wall is modified in such a way that the gas seemingly “wets” the wall. We propose the vapor recoil force to be at the origin of this non-equilibrium “wetting” transition.

Role of beam absorption in plasma during laser welding

The relationship between beam focus position and penetration depthin CW laser welding was studied numerically and experimentally for differentwelding conditions. Calculations were performed using a transient hydrodynamicmodel that incorporates the effect of evaporation recoil pressure and theassociated melt expulsion. The simulation results are compared withmeasurements made on a series of test welds obtained using a 1650 W CO2laser. The simulations predict, and the experiments confirm, that maximumpenetration occurs with a specific location of the beam focus, with respect tothe original sample surface, and that this relationship depends on theprocessing conditions. In particular, beam absorption in the plasma has asignificant effect on the relationship between penetration and focus position.When the process parameters result in strong beam absorption in the keyholeplasma, the maximum penetration will occur when the laser focus is at or abovethe sample surface. In a case of weak absorption however, the penetrationdepth reaches its maximum value when the beam focus is located below thesample surface. In all cases, the numerical results are in good agreement withthe experimental measurements.

Dynamics of explosive boiling of drops at the superheat limit

A physical model for explosive boiling of drops is presented. Loss of stability of the liquid-vapor interface results in occurrence of evaporation fronts. Their propagation in a metastable liquid is determined by the vapor recoil momentum. Detachment of drops from the interface is due to thermocapillary forces. The validity of the model is supported by comparison of calculations with experimental data.

Temporal evolution of the temperature field in the beam interaction zone during laser material processing

This paper presents a numerical simulation of the temperature field resulting from the impingement of a laser beam on a metal surface. The calculations were based on a hydrodynamic physical model of laser-material interaction, which includes the effect of evaporation recoil pressure on melt flow and the related convective heat transfer. The computations follow an experimentally verified physical model of melt hydrodynamics and the model of laser-induced evaporation developed by Anisimov. The simulations indicate that convective heat transfer which is induced by recoil pressure is significant for absorbed laser intensities from 0.5 MW cm-2 to 10 MW cm-2. This range corresponds to laser welding, cutting and drilling due to melt ejection (hydrodynamic drilling). The simulations also show that the motion of the melt from the centre of the beam interaction zone towards the periphery results in a secondary maxima in the temperature distribution, which in turn can lead to instability of the melt flow and temperature field fluctuations. The model also predicts that the cooling rate in and around the fusion zone is strongly influenced by the recoil-pressure-induced melt flow. At the centre of the beam interaction zone, the cooling rate is much higher (~10 times), and at the periphery, much slower than predicted by a pure heat conduction model.

Excimer laser ablation removal of thin chromium films from glass substrates

The excimer laser has been widely used for the removal of thin metal films . In this work, excimer laser ablation was applied to remove chromium films from glass substrates. To understand the removal mechanism and to seek the optimal processing conditions for pattern generation, a one-dimensional heat flow model was formulated which incorporated melting and vaporization. Morphological investigations were carried out experimentally on samples having different chromium film thicknesses that were irradiated with a KrF excimer laser. The thresholds for film damage, complete removal and glass damage were determined experimentally and compared with the calculated results. The measured single-shot ablation rate is about twice as high as the numerical predictions based on a heat conduction theory. The removal mechanism of the chromium film is found to be divided into two steps of pure vaporization and melt expulsion by vapor recoil pressure. Optimal processing conditions were determined to enhance the accuracy and quality of film removal for pattern generation. Finally, the slit disk of a rotary encoder was manufactured using the optimal processing conditions established.

Modeling and simulation of melt-layer erosion during plasma disruption

Metallic plasma-facing components (PFCs) e.g. beryllium and tungsten, will be subjected to severe melting during plasma instabilities such as disruptions, edge-localized modes and high power excursions. Because of the greater thickness of the resulting melt layers relative to that of the surface vaporization, the potential loss of the developing melt-layer can significantly shorten PFC lifetime, severely contaminate the plasma and potentially prevent successful operation of the tokamak reactor. Mechanisms responsible for melt-layer loss during plasma instabilities are being modeled and evaluated. Of particular importance are hydrodynamic instabilities developed in the liquid layer due to various forces such as those from magnetic fields, plasma impact momentum, vapor recoil and surface tension. Another mechanism found to contribute to melt-layer splashing loss is volume bubble boiling, which can result from overheating of the liquid layer. To benchmark these models, several new experiments were designed and performed in different laboratory devices for this work; the results are examined and compared. Theoretical predictions (A ∗THERMAL-S and SPLASH codes) are generally in good agreement with the experimental results. The effect of in-reactor disruption conditions, which do not exist in simulation experiments, on melt-layer erosion is discussed.

The spreading of volatile liquid droplets on heated surfaces

A two-dimensional volatile liquid droplet on a uniformly heated horizontal surface is considered. Lubrication theory is used to describe the effects of capillarity, thermocapillarity, vapor recoil, viscous spreading, contact-angle hysteresis, and mass loss on the behavior of the droplet. A new contact-line condition based on mass balance is formulated and used, which represents a leading-order superposition of spreading and evaporative effects. Evolution equations for steady and unsteady droplet profiles are found and solved for small and large capillary numbers. In the steady evaporation case, the steady contact angle, which represents a balance between viscous spreading effects and evaporative effects, is larger than the advancing contact angle. This new angle is also observed over much of the droplet lifetime during unsteady evaporation. Further, in the unsteady case, effects which tend to decrease (increase) the contact angle promote (delay) evaporation. In the ‘‘large’’ capillary number limit, matched asymptotics are used to describe the droplet profile; away from the contact line the shape is determined by initial conditions and bulk mass loss, while near the contact-line surface curvature and slip are important.

Two- and three-dimensional instabilities and rupture of thin liquid films falling on heated inclined plate

Long-wave instabilities in thin viscous films draining down heated inclined planes are studied. Linear stability analysis and nonlinear computations are performed via a long-wave evolution equation, which describes the effects of mass loss, wave propagation, mean flow, hydrostatic pressure, thermocapillarity, vapor recoil, and mean surface tension. Mean flow, thermocapillarity, and vapor recoil destabilize the flow, and give, respectively, surface-wave, thermocapillary, and evaporative instabilities. Nonlinear flow developments, examined by numerically integrating the evolution equation, show interesting interactions of the instabilities, including generation of longitudinal patterns.

Hydrodynamic processes in a melt bath under laser–arc interaction conditions

An investigation is reported of the physical features of the hydrodynamic processes taking place in a melt bath in the case of the laser–arc interaction with metals when the laser radiation intensity was in the range 0.5–2 MW/cm2. The velocity of the melt on the surface of the bath was determined experimentally. The dominant role of the vapor recoil pressure in the formation of liquid metal streams was noted. When the laser radiation intensity was ~ 2 MW/cm2 the melt particles participated in a vibrational process which was superimposed on a vortex motion of the melt and complicated the mixing mechanism. A model of the process was proposed and its main characteristics were determined.

The Use of Microwaves to Evaporate Liquids: The specific role of the dielectric loss factor on hydrodynamic instabilities

A linear hydrodynamic stability analysis of polar fluids evaporated under microwave irradiation is peiformed for liquids with various dependance of the dielectric loss with the temperature. The analysis predicts the onset of hydrodynamic buoyancy driven instabilities for liquids with an increasing dielectric loss with temperature by a self amplification effect of the local heating. This effect is confirmed experimentally for ethanol. For liquids with decreasing dielectric loss factor with temperature the analysis shows no occurrence of buoyancy instabilities as it is confirmed by experiments on water.For high values of the microwave power absorbed, the surface itself becomes unstable by mainly two mechanisms: the Marangoni and Hickman (vapor recoil) instabilities . The coupling between these two mechanisms is studied and compared with experimental data obtained for ethanol for various values of the microwave power absorbed.For water the surface instabilities are the prevailing destabilizing mechanisms. Re...