Numerical Simulation of Effect of Different Initial Morphologies on Melt Hydrodynamics in Laser Polishing of Ti6Al4V

As a surface finishing technique for rapid remelting and re-solidification, laser polishing can effectively eliminate the asperities so as to approach the feature size. Nevertheless, the polished surface quality is significantly sensitive to the processing parameters, especially with respect to melt hydrodynamics. In this paper, a transient two-dimensional model was developed to demonstrate the molten flow behavior for different surface morphologies of the Ti6Al4V alloy. It is illustrated that the complex evolution of the melt hydrodynamics involving heat conduction, thermal convection, thermal radiation, melting and solidification during laser polishing. Results show that the uniformity of the distribution of surface peaks and valleys can improve the molten flow stability and obtain better smoothing effect. The high cooling rate of the molten pool resulting in a shortening of the molten lifetime, which prevents the peaks from being removed by capillary and thermocapillary forces. It is revealed that the mechanism of secondary roughness formation on polished surface. Moreover, the double spiral nest Marangoni convection extrudes the molten to the outsides. It results in the formation of expansion and depression, corresponding to nearby the starting position and at the edges of the polished surface. It is further found that the difference between the simulation and experimental depression depths is only about 2 μm. Correspondingly, the errors are approximately 8.3%, 14.3% and 13.3%, corresponding to Models 1, 2 and 3, respectively. The aforementioned results illustrated that the predicted surface profiles agree reasonably well with the experimentally measured surface height data.

Studying the stratification of hydrodynamic fields for laminar flows of vertically swirling fluid

The article proposes an approach to estimating the number of stratification points in hydrodynamic fields. The article provides a method allowing one to estimate from above the number of zero points of hydrodynamic fields (points and stratification zones). The application of the proposed methodology is illustrated by several examples of the analysis of the exact solution to the problem of describing steady laminar flows of a viscous incompressible fluid in an infinite horizontal layer. In example 1, convection is induced by setting the shear stress field at one of the layer boundaries. The features of the background temperature profile, which is a seventh-degree polynomial, are discussed. It is shown that this component of the temperature field is a nonmonotonic function and that the obtained exact solution for the temperature field can describe the stratification of the considered fluid layer into one, two or three zones relative to the reference value. Example 2 illustrates evaluating the number of the zero points of the velocity field components in a vertically swirling fluid, in which convective flows are initiated by thermocapillary forces at the upper boundary of the layer. The exact solution studied in this example is a sixth-degree polynomial, which can have at most two zeros inside the region under consideration. This means that this exact solution is able to describe the stratification of the fluid layer into three zones, in each of which the test speed takes values of the same sign.

The Influence of Surfactants, Dynamic and Thermal Factors on Liquid Convection after a Droplet Fall on Another Drop

The regularities of the processes and characteristics of convection in a sessile drop on a hot wall after the second drop fall are investigated experimentally. The movement of a particle on a drop surface under the action of capillary force and liquid convection is considered. The particle motion is realized by a complex curvilinear trajectory. The fall of droplet with and without surfactant additives is considered. Estimates of the influence of the thermal factor (thermocapillary forces) and the dynamic factor (inertia forces) on convection are given. The scientific novelty of the work is the investigation of the simultaneous influence of several factors that is carried out for the first time. It is shown that in the presence of a temperature jump for the time of about 0.01–0.1 s thermocapillary convection leads to a 7–8 times increase in the mass transfer rate in drop. The relative influence of inertial forces is found to be no more than 5%. The fall of drops with surfactant additives (water + surfactant) reduces the velocity jump inside the sessile drop 2–4 times, compared with the water drop without surfactant. Thermocapillary convection leads to the formation of a stable vortex in the drop. The dynamic factor and surfactant additive lead to the vortex breakdown into many small vortices, which results in the suppression of convection. The obtained results are of great scientific and practical importance for heat transfer enhancement and for the control of heating and evaporation rates.

The effect of the liquid layer thickness on the evaporation intensity

An experimental study of the influence of thermo-capillary forces and shear stresses with the side of the gas flow to the evaporation flow rate has been made. The experiments were carried out at various thicknesses of the liquid layer and constant gas velocity. The influence of the thickness of the liquid layer on the evaporation flow rate (the intensity of evaporation) has been analyzed. It is shown that the thermocapillary forces have a direct effect on the evaporation flow rate of the liquid layer.

Nonlinear Stability Of Free Thin Films: Thermocapillary And Van-Der-Waals Effects

In the present work the dynamics of a non-isothermal thin viscous film, with fully mobile interfaces, is studied in the case when the inertial, viscous, capillary, van der Waals and thermocapillary forces are important. The film is laterally bounded by a frame, whose temperature is higher than the environmental one. The stability of the static film shapes is examined numerically by a linear and non-linear analysis. The results show that the film rupture is mostly governed by the dynamics, but it could be delayed or enhanced by the thermocapillary convection and the heat transfer with the surrounding environment.

Spreading and bistability of droplets on differentially heated substrates

An axisymmetric drop spreads on a radially heated, partially wetting solid substrate in a rotating geometry. The lubrication approximation is applied to the field equations for this thin viscous drop to yield an evolution equation that captures the dependence of viscosity, surface tension, gravity, centrifugal forces and thermocapillarity. We study the quasi-static spreading regime, whereby droplet motion is controlled by a constitutive law that relates the contact angle to the contact-line speed. Non-uniform heating of the substrate can generate both vertical and radial temperature gradients along the drop interface, which produce distinct thermocapillary forces and equivalently flows that affect the spreading process. For the non-rotating system, competition between surface chemistry (wetting) and thermocapillary flows induced by the thermal gradients gives rise to bistability in certain regions of parameter space in which the droplets converge to an equilibrium shape. The centrifugal forces that develop in a rotating geometry enlarge the bistability regions. Parameter regimes in which the droplet spreads indefinitely are identified and spreading laws are computed to compare with experimental results from the literature.