Browsing by Author "Mohamed, Omair A. A."

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Investigations into the evolution of heated liquid films
(2024-08) Mohamed, Omair A. A.
The evolution of the free surface of a heated liquid film is directly tied to the performance and efficiency of various industrial systems. Therefore, we investigate the spatiotemporal evolution of heated liquid films across a range of different settings by formulating distinct of hydro-thermal models taking into account the effects of inertia, thermocapillarity, evaporation, gas shear, and thermal radiation, where we direct our modeling efforts in each problem on the most dominant physical phenomena. In liquid flows characterized by relatively low Reynolds numbers belonging to the drag-gravity flow regime, we model the hydrodynamics of the film using the long-wave expansion (LWE) methodology and perform linear stability analyses focused on the thermocapillary and evaporative instabilities, as they have a primary influence on the film’s evolution in this flow regime. Consequently, the evaporation process is governed by the competition between thermodynamic disequilibrium and diffusion effects dependent on the interface’s curvature. We modify the kinetic-diffusion evaporation model of Sultan et al. [Sultan et al., J. Fluid Mech. 543, 183, (2005)] and combine it with long-wave theory to derive a governing equation encapsulating the coupled dynamics. We then utilize linear stability theory to derive the system’s dispersion relationship, in which the Marangoni effect has two components. The first results from surface tension gradients driven by the uneven heat flux, while the second arises from surface tension gradients caused by imbalances in vapor diffusion. These two components interact with evaporative mass loss and vapor recoil in a rich and complex manner. Moreover, we identify an evaporation regime where a volatile film is devoid of evaporation instabilities. Furthermore, we investigate the effect of film thinning on its stability at the two opposing limits of the evaporation regime, where we find its impact in the diffusion-limited regime to be dependent on the intensity of evaporative phenomena. Finally, we conduct a spatiotemporal analysis which indicates that vapor diffusion effects are correlated with a shift towards absolute instability. In the second problem, we study the spatiotemporal evolution of an evaporating liquid film sheared by a gas and consider both the inertial and thermal instability modes, where the shearing gas is modeled by imposing a constant shear stress along the liquid’s interface. Interestingly, it’s inclusion in the problem allows the utilization of a one-sided evaporation model, which is precisely the transfer-rate-limited case of the first system we investigated. Once more long-wave theory is used to derive the an evolution for the liquid film which incorporates the role of the shearing gas. Afterwards, linear stability theory is used to investigate the temporal and spatiotemporal characteristics of the flow, where it is found that the evaporation of the film promotes absolute instabilities and can cause convective/absolute transitions. We also find that counter-flowing shearing gas can suppress the inertial instability affirming similar conclusions found by previous studies for a strongly confined isothermal film. Furthermore, the evolution interface equation was solved numerically to explore the film’s nonlinear stability. Moreover, we employ self-similarity analysis to probe the shear stress’s effect on the film’s rupture mechanics. In the third problem we research, the liquid flow’s Reynolds number is relatively high, and hence we utilize the weighted-residual integral boundary layer (WIBL) technique [C. Ruyer-Quil and P. Manneville,” Eur. Phys. J. B, 15, 357, (2000)], and direct our attention at directly simulating the temperature field across the film using reduced models. The WIBL hydrodynamic equations are derived expressions obtained via the boundary layer approximation, while the thermal profile is modeled by employing an asymptotic expansion which produces a hierarchy of models in which enhanced sophistication is offset by higher complexity and computational cost. These models are solved numerically revealing how the temperature field across the film is governed by a balance between the conduction across both the liquid film and the solid surface, and their respecitve radiative emissions, wherein these two transfer phenomena are linked through two corresponding dimensionless numbers associated with both the liquid film and the solid surface.
Open Access
Spatio-temporal evolution of evaporating liquid films sheared by a gas
(2019-11) Mohamed, Omair A. A.
The main purpose of this work is the investigation of the spatio-temporal characteristics of an evaporating liquid film under the in uence of inertia, hydrostatic pressure, thermocapillary effects, vapor recoil, and shear stress imparted by a gas. The effects of the shearing gas are included via the introduction of a constant shear agent quantity modeling the effect of a constant shear stress applied along the liquid interface. Subsequently, long wave theory is used to derive an interface evolution equation accounting for all the previous effects which then is used to analyze the linear stability characteristics of the film for different parameter combinations. First a temporal analysis is performed to determine the stable/unstable parameter sets, followed by spatio-temporal analysis to differentiate the absolute/convective stability domains. It is demonstrated that the shear agent acts as a modifier to the base ow's existing inertia and therefore doesn't change perturbation growth rates in a stationary base ow, however it does have a strong effect on the phase speed. Therefore it can cause convective/absolute transitions of thermal instabilities. As for its effect on inertial instabilities, namely the H-mode, positive values of the shear agent promote its growth, while negative ones suppress it, to the point of completely eliminating it for sufficiently negative values. As for the effects of evaporation it is found that the reduction in film height due to evaporation suppresses the advection of perturbations through the film and therefore promotes absolute instabilities. In order to investigate the non-linear evolution of the interface, the evolution equation is solved numerically. Initially, the interface evolution is simulated for infinitesimal perturbations over a periodic domain for the purposes of validation by comparison to the linear temporal stability results, and also to existing literature. Once the numerical procedure is validated, the non-linear evolution of the interface is studied. Finally, the shear gas's effect on film rupture location nd time are studied where it is found that the shear agent can strongly affect rupture location and time, but doesn't change the self-similar rupture mechanics as the minimum film height approaches zero.
Open Access
Spatiotemporal evolution of evaporating liquid films sheared by a gas
(American Physical Society, 2021-11-04) Mohamed, Omair A. A.; Dallaston, M. C.; Biancofiore, Luca
We study the spatiotemporal evolution of an evaporating liquid film sheared by a gas considering both inertial and thermal instabilities, the latter arising from a combination of evaporation and Marangoni effects. The shearing gas is modeled by imposing a constant shear stress applied along the liquid's interface. Following in the footsteps of Joo et al. [S. W. Joo et al., J. Fluid Mech. 230, 117 (1991)], long-wave theory is used to derive a Benney-like equation governing the temporal volution of the liquid interface under the effects of inertia, hydrostatic pressure, surface tension, thermocapillarity, evaporation, and gas shear. Linear stability theory is used to investigate the temporal and spatiotemporal characteristics of the flow, where it is found that the evaporation of the film promotes absolute instabilities and can cause convective-absolute transitions of the perturbations. It is also found that a strong enough counterflowing shearing gas can suppress the inertial instability, commonly known as the H mode, affirming similar conclusions found by previous studies for a strongly confined isothermal film. Additionally, our temporal stability analysis indicates that the thinning of the film reduces the phase speed of thermal perturbations, due to the increasing dominance of viscosity. However, our spatiotemporal analysis shows that the thinning of the film actually results in the growth of additional modes with higher group velocities resulting in faster contamination of the flow field. Moreover, the interface evolution equation is solved numerically to (i) simulate the film's interface evolution subject to finite perturbations and (ii) compare to the results of the linear stability analysis. We find qualitative agreement between the temporal dynamics of the linear and nonlinear instabilities. Our subsequent numerical nonlinear spatiotemporal stability analysis demonstrates that for weaker thermal instabilities, the wave-front dynamics are imposed by the nonlinearly saturated wave packet, while for stronger thermal instabilities, the wave-front dynamics are dictated by the linear dispersion relationship. We also study the effects of the dimensionless parameters on the rupture location and the time it takes for the fluid film to rupture. Finally, the shear stress's effect on the rupture mechanics of the film is studied using self-similarity analysis, where we identify the fate of the evolution equation's solutions.