Sessile condensate droplets as quasi-static wall deformations in direct numerical simulations of channel flow with condensation (original) (raw)
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Introduction Condensation of vapor plays a crucial role in a wide range of large-scale energy systems. In particular, steam power plants and HVAC systems, which, respectively, account for 78% of global electric power generation 1, 2 and 10-20% of total energy consumption in developed countries 3 , rely on the process of vapor condensation. Besides steam power plants and HVAC systems, efficiency of several industrial applications such as water desalination 4-7 , water collection 8-10 and thermal management 11-13 depend on vapor condensation. Therefore, any improvement in the efficiency of vapor condensation process can lead to significant energy savings. Condensation can be categorized as either filmwise condensation (FWC) or dropwise condensation (DWC). Figure 1 shows the schematic and Fig. 2 shows the images illustrating FWC and DWC. In FWC, the condensate forms a liquid film on the surface. This liquid film provides additional thermal resistance to heat transfer between the surface and the vapor. On the other hand, in DWC, vapor forms distinct liquid drops
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2010
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International Journal of Thermal Sciences, 2008
A semi-analytic model for convective evaporation of sessile droplets from solid surface has been developed. The effect of turbulence on evaporation is incorporated into the model through the use of the friction velocity. The friction velocity is calculated from the wall shear stress that was obtained experimentally as a function of free stream velocity and turbulence intensity. In the model, the Reynolds number based on the friction velocity was used because it is more pertinent for sessile droplets than the Reynolds number based on the free stream velocity. Thus formulated model has been successfully validated with wind tunnel data, where a very good agreement between the model predictions and experimental measurements is observed. By utilizing the model, a unique evaporation master curve that correlates the normalized evaporation mass and the non-dimensional time corrected for the influence of the driving force was developed for all droplet sizes, free stream velocities, turbulence intensities, and the surrounding temperatures.
International Journal of Thermal Sciences, 2020
A theoretical framework is established to model the evaporation from continuously fed droplets, promising tools in the thermal management of high heat flux electronics. Using the framework, a comprehensive model is developed for a hemispherical water droplet resting on a heated flat substrate incorporating all of the relevant transport mechanisms: buoyant and thermocapillary convection inside the droplet and diffusive and convective transport of vapor in the gas domain. At the interface, mass, momentum, and thermal coupling of the phases are also made accounting for all pertinent physical aspects including several rarely considered interfacial phenomena such as Stefan flow of gas and the radiative heat transfer from interface to the surroundings. The model developed utilizes temperature dependent properties in both phases including the density and accounts for all relevant physics including Marangoni flow, which makes the model unprecedented. Moreover, utilizing this comprehensive model, a nonmonotonic interfacial temperature distribution with double temperature dips is discovered for a hemispherical droplet having internal convection due to buoyancy in the case of high substrate temperature. Proposed framework is also employed to construct several simplified models adopting common assumptions of droplet evaporation and the computational performance of these models, thereby the validity of commonly applied simplifying assumptions, are assessed. Benchmark simulations reveal that omission of gas flow, i.e. neglecting convective transport in gas phase, results in the underestimation of evaporation rates by 23-54%. When gas flow is considered but the effect of buoyancy is modeled using Boussinesq approximation instead of assigning temperature dependent density throughout the gas domain, evaporation rate can be underestimated by up to 16%. Deviation of simplified models tends to increase with increasing substrate temperature. Moreover, presence of Marangoni flow leads to larger errors in the evaporation rate prediction of simplified models.
Turbulence and heat exchange in condensing vapor-liquid flow
Physics of Fluids, 2008
Turbulence and heat exchange during condensation of a vapor stream countercurrently flowing to a subcooled liquid stream in a slightly inclined channel has been investigated by direct numerical simulation ͑DNS͒. Condensation rates and imposed pressure gradients have been varied, and capillary-gravity waves have been allowed to develop at the ͑deformable͒ vapor-liquid interface. These simulations extend our previous DNS of turbulence and scalar exchange in stratified gas-liquid flows without condensation. The previous studies indicated that for conditions in which the gas-liquid interface remained continuous, i.e., did not "break," scalar exchange rates on both the gas and liquid sides were largely determined by sweeps which brought high momentum fluid from the bulk flow to the interface. As sweep frequencies were found to scale with interfacial friction velocities, scalar exchange coefficients could be parametrized with a surface renewal theory. The issue addressed in the current work is how these findings are altered by condensation which acts somewhat like suction through a wall on the vapor side and injection through a wall on the liquid side. Both suction and injection have been found to affect shear stresses, turbulence characteristics, and scalar exchange rates, and hence similar effects might be expected during condensation. The present simulations indicate that the turbulence characteristics in both phases are affected, with turbulence intensities and Reynolds stresses being enhanced on the vapor side and attenuated on the liquid side. For a given imposed pressure gradient, the interfacial shear stress decreases as a result of the interfacial momentum exchange due to condensation. Interfacial waves are also found to be damped by condensation and the streamwise vortical structures on the liquid side are attenuated. The frequencies of sweeps and ejections, however, do scale with the interfacial friction velocity, reduced due to condensation, as does the liquid-side heat transfer coefficient. The simulations indicate that the scaling relationship between the interfacial friction velocity and the liquid-side heat transfer coefficient is similar to that in the absence of condensation, although the interfacial friction velocity itself is different, being dependent on condensation rates. As condensation rates depend in turn on the liquid-side heat transfer, their prediction becomes a coupled problem. A procedure for determining condensation rates as a function of imposed pressure gradient and liquid subcooling is derived from the simulations.