A DSMC Model on Droplet Clusters to Explore the Limiting Factors of Modified Surfaces Promoting Enhanced Dropwise Condensation

2015 ◽  
Author(s):  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Limiting Factors ◽  
Future Research ◽  
Computational Domain ◽  
Dropwise Condensation ◽  
Cold Wall ◽  
Ambient Conditions ◽  
Droplet Distribution ◽  
Droplet Sizes

Condensation is a physical process that occurs when a vapor is cooled and/or compressed to its saturation limit. Condensation becomes important in a variety of engineering applications such as in heat exchangers used for distillation purposes. In such instances, higher condensation efficiencies are desirable. Research to improve condensation has focused on dropwise condensation as it has been shown that it can be significantly more efficient than filmwise condensation. Recent investigations of dropwise condensation on nanostructured surfaces suggest that enhanced dropwise condensation can be attained as the average droplet sizes are reduced for clusters growing through dropwise condensation. This, in turn, significantly enhances the heat transfer coefficients of dropwise condensation. This paper summarizes a computational model developed to explore the mechanisms leading to this enhanced dropwise condensation. A Direct Simulation Monte Carlo (DSMC) approach is used here to investigate the mechanisms and limitations of enhanced dropwise condensation for these surfaces aiming to reduce the average droplet sizes of condensation. For computational purposes, several idealizations are assumed by the model, which include: (1) The condensation droplet clusters are assumed to have uniform size, corresponding to an average droplet size observed in actual dropwise condensation scenarios; (2) Due to the assumed uniform droplet distribution, symmetry can be observed from the droplet cluster, so a small but symmetrical cross section of the droplet distribution is used for the computational domain; and (3) Supersaturated steam condensing on a cold wall is assumed for most of the simulations. The mechanisms at play that are deliberately explored are: (1) The effects of surface wettability by using a model that considers droplet conduction variations with varying contact angle; (2) The changes of interfacial resistance with droplet curvature by introducing a surface tension model based on the Tolman length; and (3) The dynamic interactions between neighboring droplets by choosing our computational domain to be a symmetrical cross section that encompasses surrounding droplets in an appropriate fashion. The ambient conditions that were investigated were: (1) Varying atmospheric pressure; (2) Varying amounts of wall subcooling for the droplets; (3) Varying accommodation for water molecules condensing on the droplet; and (4) The introduction of air into the assumed supersaturated steam condensing on the cold wall. To investigate the overall and combined effects of the aforementioned mechanisms on enhanced dropwise condensation through reduced droplet sizes, the simulations were run for droplets with radii between 1 micrometer down to 5 nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer with decreasing droplet sizes breaks down as droplet sizes become smaller due to more prominence of the mechanisms hindering condensation for the reduced droplet sizes. As the model breaks down, a peak heat transfer is reached, and heat transfer is further reduced as the average droplet sizes continue to decrease. The predictions of this particular DSMC model are compared to previous work investigating similar effects. The implications of our observations and potential impact to current and future research in the area is discussed in detail.

DSMC Modeling of Dropwise Condensation With Variable Wettability

2012 ◽  
Author(s):  
Hector Mendoza ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Computational Time ◽  
Future Research ◽  
Direct Monte Carlo Simulation ◽  
Effective Diameter ◽  
Dropwise Condensation ◽  
Atmospheric Conditions ◽  
Interfacial Resistance ◽  
Transfer Coefficients ◽  
Droplet Sizes

Recent heat transfer studies on nanostructured surfaces have shown that enhanced dropwise condensation heat transfer can be attained through clusters with reduced droplet sizes. A Direct Monte Carlo Simulation (DSMC) approach is used here to investigate the limitations of dropwise condensation as droplet sizes are reduced. The model is idealized by assuming uniform droplet distribution with an effective diameter. To minimize computational time, the model further uses symmetry to focus the analysis on a quarter segment of a single droplet condensing on a cold wall. The effects of surface wettability are explored by accounting for variations in droplet conduction with contact angle, while changes of interfacial resistance with droplet curvature are also incorporated into the model. To investigate the effect of reduced droplet sizes, the simulations were run for droplets with radii between 1μm down to 5nm. The simulations were run for standard atmospheric conditions at different levels of subcooling and accommodation. In all instances, the same behavior is observed as non-continuum effects become significant at reduced diameters, causing heat transfer coefficients to reach a maximum. The predictions of the DSMC model are compared to previous work attempting to capture similar effects. The significance of the observations for current and future research in dropwise condensation is discussed in detail.

scholarly journals Review of Micro–Nanoscale Surface Coatings Application for Sustaining Dropwise Condensation

Coatings ◽  
2019 ◽  
Vol 9 (2) ◽  
pp. 117 ◽  
Author(s):  
Shoukat Alim Khan ◽  
Furqan Tahir ◽  
Ahmer Ali Bozdar Baloch ◽  
Muammer Koc
Keyword(s):  
Heat Transfer ◽  
Noble Metals ◽  
Power Plants ◽  
Contact Angle Hysteresis ◽  
Surface Hydrophobicity ◽  
Future Research ◽  
Dropwise Condensation ◽  
Nanostructured Surfaces ◽  
Research Guidelines ◽  
Coating Methods

Condensation occurs in most of the heat transfer processes, ranging from cooling of electronics to heat rejection in power plants. Therefore, any improvement in condensation processes will be reflected in the minimization of global energy consumption, reduction in environmental burdens, and development of sustainable systems. The overall heat transfer coefficient of dropwise condensation (DWC) is higher by several times compared to filmwise condensation (FWC), which is the normal mode in industrial condensers. Thus, it is of utmost importance to obtain sustained DWC for better performance. Stability of DWC depends on surface hydrophobicity, surface free energy, condensate liquid surface tension, contact angle hysteresis, and droplet removal. The required properties for DWC may be achieved by micro–nanoscale surface modification. In this survey, micro–nanoscale coatings such as noble metals, ion implantation, rare earth oxides, lubricant-infused surfaces, polymers, nanostructured surfaces, carbon nanotubes, graphene, and porous coatings have been reviewed and discussed. The surface coating methods, applications, and enhancement potential have been compared with respect to the heat transfer ability, durability, and efficiency. Furthermore, limitations and prevailing challenges for condensation enhancement applications have been consolidated to provide future research guidelines.

Effects of Intersection Angles on Flow and Heat Transfer in Corrugated-Undulated Channels With Sinusoidal Waves

2006 ◽  
Vol 128 (8) ◽  
pp. 819-828 ◽  
Author(s):  
Jixiang Yin ◽  
Guojun Li ◽  
Zhenping Feng
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Three Dimensional ◽  
Computational Domain ◽  
Straight Channel ◽  
Flow Area ◽  
Contact Points ◽  
Flow And Heat Transfer ◽  
Different Characteristics ◽  
Steady Laminar

This paper reported three-dimensional numerical simulations of the steady laminar flow and heat transfer in corrugated-undulated channels with sinusoidal waves, aiming to investigate the effects of intersection angles (θ) between corrugated and undulated plate and Reynolds number (Re) on the flow and heat transfer. The simulations are conducted by using multi-channel computational domain for three different geometries. The code is validated against experimental results and then data for Nusselt number (Nu) and friction factor (f) are presented in a Re range of 100-1500, and intersection angle range of 30-150deg. The simulation confirms the changes of Nuu (averaged over undulated plate) and Nuc (averaged over corrugated plate) with θ representing different characteristics. As θ increases, Nu (Nuu or Nuc) is about 2–16 times higher for the corrugated-undulated configurations CP-UH1 and CP-UP1 and the concomitant f is about 4–100 higher, when compared to a straight channel having square cross section. The minimum of local Nu ( Nuu or Nuc ) is situated at the four contact points where the top plate touches the bottom one, and the high Nu is located upstream of the crest of the conjugate duct. Performance evaluation for the CP-UH1 channel shows that the goodness factors (G) are larger than 1 with the straight channel having a square cross section as a reference, and the 30deg geometry channel has optimal flow area goodness.

scholarly journals Near Field Condensation

2020 ◽  
Author(s):  
Xiao Yan ◽  
Feipeng Chen ◽  
Chongyan Zhao ◽  
Yimeng Qin ◽  
Xiong Wang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Population Density ◽  
Near Field ◽  
Condensation Heat Transfer ◽  
Droplet Growth ◽  
Dropwise Condensation ◽  
Droplet Transfer ◽  
Droplet Sizes ◽  
Departure Size ◽  
Contact Line Pinning

Abstract Dropwise condensation represents the upper limit of condensation heat transfer. Promoting dropwise condensation relies on surface chemical functionalization, and is fundamentally limited by the maximum droplet departure size. A century of research has focused on active and passive methods to enable the removal of ever smaller droplets. However, fundamental contact line pinning limitations prevent gravitational and shear-based removal of droplets smaller than 250 µm. Here, we break this limitation through near field condensation. By de-coupling nucleation, droplet growth, and shedding via droplet transfer between parallel surfaces, we enable the control of droplet population density and removal of droplets as small as 20 µm without the need for chemical modification or surface structuring. We identify droplet bridging to develop a regime map, showing that rational wettability contrast propels spontaneous droplet transfer from condensing surfaces ranging from hydrophilic to hydrophobic. To demonstrate efficacy, we perform condensation experiments on surfaces ranging from hydrophilic to superhydrophobic. The results show that near field condensation with optimal gap spacing can limit the maximum droplet sizes and significantly increase the population density of sub-20 µm droplets. Theoretical analysis and direct numerical simulation confirm the breaking of classical condensation heat transfer paradigms through enhanced heat transfer. Our study not only pushes beyond century-old phase change limitations, it demonstrates a promising method to enhance the efficiency of applications where high, tunable, gravity-independent, and durable condensation heat transfer is required.

scholarly journals Numerical simulation of superheated jets using an Eulerian method.

2017 ◽  
Author(s):  
Konstantinos Lyras ◽  
Siaka Dembele ◽  
C. Madhav Rao Vendra ◽  
Jennifer Wen
Keyword(s):  
Heat Transfer ◽  
Turbulent Flows ◽  
Bubble Formation ◽  
Ambient Conditions ◽  
Local Pressure ◽  
Two Phase ◽  
Rapid Phase ◽  
Droplet Sizes ◽  
Flash Boiling ◽  
Fully Eulerian Approach

Flash boiling is the rapid phase change of a pressurised fluid that emerges in ambient conditions below its vapourpressure. Flashing can occur either inside or outside the nozzle depending on the local pressure and geometry and the bubble formation leads to interfacial interactions that eventually influence the emerging spray. Lagrangian methods which exist in literature to simulate the flash atomisation and inter-phase heat transfer employ many sim- plifying assumptions. Typically, sub-models used for the break-up, collisions and evaporation introduce an extensive empiricism that might result in unrealistic predictions for cases like flashing. In this study, a fully Eulerian approach is selected employing the Σ − Y model proposed by Vallet and Borghi. The model tracks liquid structures of any shape and computes the spray characteristics comprising a modified version for the transport equation of the sur- face density. The main goal of this study is to investigate the performance of this model in flash boiling liquids using the Homogeneous Relaxation Model (HRM) developed by Downar-Zapolski, a model capable of capturing the heat transfer under sudden depressurisation conditions accounting for the non-equilibrium vapour generation. The model in this present study considers that the instantaneous quality would relax to the equilibrium value over a given timescale which is calculated using the flow field values. A segregated approach linking the HRM and Σ − Y is implemented in a compressible formulation in an attempt to quantify the effects of flash boiling in the spray dynamics. The developed model is naturally implemented in RANS in a dedicated solver HRMSonicELSAFoam. Results from simulations of two-phase jets of different subcooled fluids through sharp-edged orifices show that the proposed approach can accurately simulate the primary atomisation and give reliable predictions for the droplet sizes and distribution. Strong effects of the flashing and turbulent mixing on the jet are demonstrated. The model istested for turbulent flows within small nozzles and was developed within the open source code OpenFOAM.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4667

Numerical Investigation of Natural Convection Heat Transfer from Square Cylinder in a Vented Enclosure

2011 ◽  
Vol 110-116 ◽  
pp. 4451-4464 ◽  
Author(s):  
Ghalib Y. Kahwaji ◽  
Abbas S. Hussien ◽  
Omar M. Ali
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Cross Section ◽  
Convection Heat Transfer ◽  
Grid Generation ◽  
Flow Patterns ◽  
Horizontal Cylinder ◽  
Natural Convection Heat Transfer ◽  
Computational Domain ◽  
Convection Heat

In the present work, the natural convection heat transfer from horizontal cylinder with square cross section situated in a square enclosure, vented symmetrically from the top and the bottom was investigated numerically. The work investigate the effect of the Ra, enclosure width and opening size of the enclosure on the streamlines, isotherms and heat transfer results. The numerical work included the solution of the governing equations in the vorticity-stream function formulation which were transformed into body fitted coordinate system. The transformations are based initially on algebraic grid generation and elliptic grid generation to map the physical domain between the heated horizontal cylinder and the vented enclosure into a computational domain. A hybrid scheme finite volume based finite difference method was used. The study included the following ranges of the studied variables:- 0 < Ra ≤ 6.5× 105 1.5 ≤ W/H ≤ 4 0.375 < O/H ≤ 4 The numerical results were compared with experimental results, which showed good agreement. The effect of cylinder cross section, Ra, enclosure width, and opening size on the Nu, mass flowrate, flow patterns and isotherms were investigated. The results show that the cylinder cross section has a large influence on the results especially the Nu. The Nu is proportional with Ra and inversely proportional with enclosure width and opening size. The flow patterns and isotherms display the flow and temperature behaviors with changing studied variables. The results show that the starting of natural convection heat transfer depended on the cylinder cross-section, enclosure width and opening size in addition with Ra. In addition, the results display that the hydrodynamic and thermal boundary layer thickness decreases with increasing Ra. Nomenclature

Experimental Investigation of Aerial Spray Characteristics

2010 ◽  
Author(s):  
Ali Bagherpour ◽  
Gordon Holloway ◽  
Ian M. McLeod
Keyword(s):  
Experimental Study ◽  
Experimental Investigation ◽  
Cross Section ◽  
Laser Diffraction ◽  
Spray Application ◽  
Droplet Distribution ◽  
Droplet Sizes ◽  
Droplet Trajectory ◽  
Biological Dose ◽  
Doppler Interferometer

An important characteristic of sprays is their statistical distribution of droplet sizes. Knowledge of the droplet distribution is particularly important for pesticide applications because droplet size affects droplet trajectory, probability of contact with foliage, and the biological dose to target pests. This work describes an experimental study of an aerial spray application in a wind tunnel environment at realistic flight speeds (67 m/s) using a full-scale rotary atomizer turning at 8600 rpm. Comparative measurements of water droplet velocity and diameter were made a 3 component Artium Phase Doppler Interferometer (PDI) and a Sympatec Helos Vario Laser Diffraction (LD) instrument. Distance from the atomizer to the measurement cross section was varied to observe the effects of the atomizer wake on the results.

Numerical Prediction of Dropwise Condensation Performances Over Hybrid Surfaces, Under the Action of Gravity and Vapor Shear

2020 ◽  
Author(s):  
Nicola Suzzi ◽  
Giulio Croce ◽  
Paola D’Agaro
Keyword(s):  
Heat Transfer ◽  
Flow Rate ◽  
Thermal Energy ◽  
Heat Transfer Performance ◽  
Droplet Growth ◽  
Nucleation Density ◽  
Computational Domain ◽  
Transfer Performance ◽  
Dropwise Condensation ◽  
Structured Surfaces

Abstract A Lagrangian model following the history of every droplet belonging to an evolving droplets population, originally developed to simulate pattern evolution in the framework of in-flight icing phenomenon, is used in order to simulate dropwise condensation over different shaped micro-structured surfaces. Both the mechanical and the thermal energy balances are solved for every droplet, allowing to predict droplet velocity and condensing flow rate. Coalescence phenomenon is also implemented. The model in the present form is an evolution of the code presented at ICNMM 2019, introducing the effect of vapor shear, a physical model of the evolution of the dynamic contact angle during droplet growth and a prediction of condensing flow rate through the solution of thermal energy balance, thus taking into account the influence of the droplet size. Shared memory parallelization is also carried out decomposing the computational domain into different subdomains, allowing the efficient simulation of a larger number of droplets. Here, the model is validated and used to predict the heat transfer performance of hybrid condensation surfaces, both plane and curved, under the action of both gravity and vapor shear. Starting from literature proposals, several patterns, each characterized by a complex composition of patches with different wettabilities, are numerically investigated and the configuration ensuring the best heat transfer performance and liquid drainage is identified. The sensitivity of the solution with respect to the uncertainty on the estimate of some parameters, such as nucleation density, is also discussed.

scholarly journals Dropwise condensation on solid hydrophilic surfaces

2020 ◽  
Vol 6 (2) ◽  
pp. eaax0746 ◽  
Author(s):  
Hyeongyun Cha ◽  
Hamed Vahabi ◽  
Alex Wu ◽  
Shreyas Chavan ◽  
Moon-Kyung Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Contact Angle Hysteresis ◽  
Bond Number ◽  
Condensation Heat Transfer ◽  
Past Century ◽  
Dropwise Condensation ◽  
The Past ◽  
Order Of Magnitude ◽  
Droplet Sizes

Droplet nucleation and condensation are ubiquitous phenomena in nature and industry. Over the past century, research has shown dropwise condensation heat transfer on nonwetting surfaces to be an order of magnitude higher than filmwise condensation heat transfer on wetting substrates. However, the necessity for nonwetting to achieve dropwise condensation is unclear. This article reports stable dropwise condensation on a smooth, solid, hydrophilic surface (θa = 38°) having low contact angle hysteresis (<3°). We show that the distribution of nano- to micro- to macroscale droplet sizes (about 100 nm to 1 mm) for coalescing droplets agrees well with the classical distribution on hydrophobic surfaces and elucidate that the wettability-governed dropwise-to-filmwise transition is mediated by the departing droplet Bond number. Our findings demonstrate that achieving stable dropwise condensation is not governed by surface intrinsic wettability, as assumed for the past eight decades, but rather, it is dictated by contact angle hysteresis.

scholarly journals Численное моделирование процесса получения мультикремния методом направленной кристаллизации

2020 ◽  
Vol 90 (7) ◽  
pp. 1080
Author(s):  
С.А. Смирнов ◽  
В.В. Калаев
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Crystal Growth ◽  
Dislocation Density ◽  
Directional Solidification ◽  
Cross Section ◽  
Thermal Stresses ◽  
Vertical Cross Section ◽  
Computational Domain

Numerical simulation of silicon multi-crystal growth by directional solidification with a square crucible is considered. We validate the use of 2D geometry of the vertical cross section as a computational domain. The model describes melt hydrodynamics, global heat transfer, thermal stresses and the evolution of the dislocation density in the crystal. The sensitivity of the stresses and dislocation density in the Si crystal to the parameters of the Alexander-Haazen model is analyzed.

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