Micrometer-Sized Water Droplet Impingement Dynamics and Evaporation on a Flat Dry Surface

Abstract

A comprehensive numerical and experimental investigation on micrometer-sized water droplet impact dynamics and evaporation on an unheated, flat, dry surface is conducted from the standpoint of spray-cooling technology. The axisymmetric time-dependent governing equations of continuity, momentum, energy, and species are solved. Surface tension, wall adhesion effect, gravitational body force, contact line dynamics, and evaporation are accounted for in the governing equations. The explicit volume of fluid (VOF) model with dynamic meshing and variable-time stepping in serial and parallel processors is used to capture the time-dependent liquid-gas interface motion throughout the computational domain. The numerical model includes temperature- and species-dependent thermodynamic and transport properties. The contact line dynamics and the evaporation rate are predicted using Blake's and Schrage's molecular kinetic models, respectively. An extensive grid independence study was conducted. Droplet impingement and evaporation data are acquired with a standard dispensing/imaging system and high-speed photography. The numerical results are compared with measurements reported in the literature for millimeter-size droplets and with current microdroplet experiments in terms of instantaneous droplet shape and temporal spread (R/D(0) or R/R(E)), flatness ratio (H/D(0)), and height (H/H(E)) profiles, as well as temporal volume (inverted A) profile. The Weber numbers (We) for impinging droplets vary from 1.4 to 35.2 at nearly constant Ohnesorge number (Oh) of approximately 0.025-0.029. Both numerical and experimental results show that there is air bubble entrapment due to impingement. Numerical results indicate that Blake's formulation provides better results than the static (SCA) and dynamic contact angle (DCA) approach in terms of temporal evolution of R/D(0) and H/D(0) (especially at the initial stages of spreading) and equilibrium flatness ratio (H(E)/D(0)). Blake's contact line dynamics is dependent on the wetting parameter (K(W)). Both numerical and experimental results suggest that at 4.5 < We < 11.0 the short-time dynamics of microdroplet impingement corresponds to a transition regime between two different spreading regimes (i.e., for We < or = 4.5, impingement is followed by spreading, then contact line pinning and then inertial oscillations, and for We > or = 11.0, impingement is followed by spreading, then recoiling, then contact line pinning and then inertial oscillations). Droplet evaporation can be satisfactorily modeled using the Schrage model, since it predicts both well-defined transient and quasi-steady evaporation stages. The model compares well with measurements in terms of flatness ratio (H/H(E)) before depinning occurs. Toroidal vortices are formed on the droplet surface in the gaseous phase due to buoyancy-induced Rayleigh-Taylor instability that enhances convection.

Simulation of the spreading of a gas-propelled micro-droplet upon impact on a dry surface using a lattice-Boltzmann approach

“…As pointed out by Ebrahim and Ortega [3], immediately after impact, the droplet experiences a collapsing regime, in which a spherical droplet begins to deform to a lamella [18] with a defined contact line. For moderate to high impact conditions, this regime has very short duration and therefore is usually neglected.…”

“…The simulation is validated with the experimental data of Briones et al [18]. The impact of micro-sized droplets in free-fall in ambient air and under a stagnation air flow are modeled in order to study the hydrodynamics of unassisted and gas assisted droplet impact, respectively, at these very low droplet diameters.…”

“…The We, Re, Oh and Ca numbers are defined as The experimental data of Briones et al [18] was used to validate the three-dimensional LBM simulation of this study. Operating conditions for each impact case are listed in Table II and were exactly matched in each simulation.…”

“…There are however few dynamic models to compute the instantaneous droplet diameter [34][35][36]. Several of these models are presented in Table III and are compared against the experimental data of Briones et al [18], Figure 5 presents the error in predicting the maximum spreading diameter, ξ max , using these models. The correlation of Scheller and Bousfield [37] and the model of Son et al [16] are accurate for all case studies (1 < We < 12) to within ±25%.…”

“…Therefore, the heat transfer is speculated to be enhanced due to the formation of a larger contact area and thinner liquid film on the target surface. However, the effect of the gas jet flow on the Table III with the experimental data of Briones et al [18] dynamics of the droplet impact is not fully understood.…”