Kinetic simulation of the non-equilibrium effects at the liquid-vapor interface

Phase change phenomena at microscale is important for novel cooling microsystems with intensive evaporation, so the development of reliable models and simulations are challenging. The vapor behaviors near its condensed phase are simulated using the non-linear S-model kinetic equation. The pressure and temperature jumps obtained numerically are in good agreement with the analytical expressions derived from the appropriate Onsager-Casimir reciprocity relations. The results of the evaporation flux are close to those given by the Hertz-Knudsen-Schrage formula, only when the values of the pressure and temperature at the upper boundary of the Knudsen layer are used. Comparison with recently measured temperature jumps are provided and disagreement with some experiments are discussed. © 2019 Elsevier Lt

Numerical investigation of the tangential stress effects on a fluid flow structure in a partially open cavity

Mathematical and numerical modeling of fluid flows in the domains with free boundaries under co-current gas flow is widely investigated nowadays. A stationary problem of fluid motion in a rectangular cavity with a non-deformed free boundary is studied in a two-dimensional statement. The tangential stresses created on the free boundary by an adjoint gas flow are considered to be a driving force for a fluid motion. The influence of the cavity geometry (cavity aspect ratio) and of the free boundary (length of the open part of the boundary) on the velocity field is investigated numerically. The simulations are carried out for different values of the gas Reynolds numbers. The characteristic values for the flow parameters as well as geometrical characteristics described in this paper are motivated by the main features of the CIMEX-1 experiments prepared for the International Space Station. The paper presents examples of the fluid flow structure in the open cavities and conclusions. © 2013 Pleiades Publishing, Ltd.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Instabilities in a horizontal liquid layer in cocurrent gas flow with an evaporating interface

The problem of a two-layer system consisting of a horizontal liquid layer in contact with its own vapor is considered. The liquid layer is bounded by a rigid wall from below, and phase change can occur at the interface. The flow of the vapor phase is driven by a constant pressure gradient in the streamwise direction. We have taken into account the effects of buoyancy, thermocapillarity, evaporation, and the dynamics of the vapor phase. A full linear stability analysis is performed using a Chebyshev spectral method. The influences of evaporation effect and the interfacial shear on the Rayleigh instability and the Marangoni instability have been studied. The results show that both the evaporation and the interfacial shear play important roles in the stability of the system. © 2012 American Physical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Nonmodal and nonlinear dynamics of a volatile liquid film flowing over a locally heated surface

The stability of a thin, volatile liquid film falling under the influence of gravity over a locally heated, vertical plate is analyzed in the noninertial regime using a model based on long-wave theory. The model is formulated to account for evaporation that is either governed by thermodynamic considerations at the interface in the one-sided limit or limited by the rate of mass transfer of the vapor from the interface. The temperature gradient near the upstream edge of the heater induces a gradient in surface tension that opposes the gravity-driven flow, and a pronounced thermocapillary ridge develops in the streamwise direction. Recent theoretical analyses predict that the ridge becomes unstable above a critical value of the Marangoni parameter, leading to the experimentally observed rivulet structure that is periodic in the direction transverse to the bulk flow. An oscillatory, thermocapillary instability in the streamwise direction above the heater is also predicted for films with sufficiently large heat loss at the free surface due to either evaporation or strong convection in the adjoining gas. This present work extends the recent linear stability analysis of such flows by Tiwari and Davis [Phys. Fluids 21, 022105 (2009) ] to a nonmodal analysis of the governing non-self-adjoint operator and computations of the nonlinear dynamics. The nonmodal analysis identifies the most destabilizing perturbations to the film and their maximum amplification. Computations of the nonlinear dynamics reveal that small perturbations can be sufficient to destabilize a linearly stable film for a narrow band of wave numbers predicted by the nonmodal, linearized analysis. This destabilization is linked to the presence of stable, discrete modes that appear as the Marangoni parameter approaches the critical value at which the film becomes linearly unstable. Furthermore, the thermocapillary instability leads to a new, time-periodic base state. This transition corresponds to a Hopf bifurcation with increasing Marangoni parameter. A linear stability analysis of this time-periodic state reveals further instability to transverse perturbations, with the wave number of the most unstable mode about 50% smaller than for the rivulet instability of the steady base state and exponential growth rate about three times larger. The resulting film behavior is reminiscent of inertial waves on locally heated films, although the wave amplitude is larger in the present case near the heater and decays downstream where the Marangoni stress vanishes. The film’s heat transfer coefficient is found to increase significantly upon the transition to the time-periodic flow