Numerical Study on Evolution of Sessile Water Droplets During Evaporation

2012 ◽  
Author(s):  
Dinghua Hu ◽  
Huiying Wu ◽  
Xinyu Wu
Keyword(s):  
Contact Angle ◽  
Numerical Study ◽  
Concentration Field ◽  
Marangoni Effect ◽  
Simple Expression ◽  
Vapor Concentration ◽  
Contact Radius ◽  
Iteration Algorithm ◽  
Dimensionless Time ◽  
Water Droplets

The evaporation of sessile water droplets has been investigated numerically in this paper. A numerical model based on the quasi-stationary assumptions is established for describing the evaporation dynamics. Droplets are assumed to be pinned and axisymmetric on a substrate during the evaporation, and the influence of Marangoni effect on the evaporation has been taken into account. In a self-consistent way, the vapor concentration field in the gas phase, the velocity field in the liquid phase, and the temperature field in both liquid and gas phases have been calculated using the finite element method (FEM). A novel iteration algorithm is proposed to study the time evolution of contact angle. From the numerical results, we derived a universal correlation of the contact angle with the dimensionless time. The correlation, which is validated against experimental data from open literature, is independent on the contact radius, substrate temperature and ambient humidity. Base on this correlation, a simple expression has been further developed to estimate the evaporation time, and the predictions are in good agreement with the experimental results from open literature. The work in this paper provides an extensive understanding of the evaporation of sessile droplets.

Numerical Study of Water Droplet Evaporation on a Superhydrophobic Surface

2013 ◽  
Author(s):  
Zhenhai Pan ◽  
Susmita Dash ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Contact Angle ◽  
Water Droplet ◽  
Evaporation Rate ◽  
Numerical Study ◽  
Liquid Droplet ◽  
Current Model ◽  
Solid Substrate ◽  
Analytical Models ◽  
Vapor Concentration ◽  
Constant Contact Angle

A comprehensive numerical model is developed to predict evaporation of a water droplet from an unheated superhydrophobic substrate. Analytical models that only consider vapor diffusion in the gas domain, and assume the system to be isothermal, over-predict the evaporation rates by ∼25% compared to experiments conducted on such surfaces. The current model solves for conjugate heat and mass transfer in the solid substrate, liquid droplet, and surrounding gas. Evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. A droplet evaporating in a constant-contact-angle mode with an initial volume of 3 μl and contact angle of 160 deg is considered at an ambient temperature of 21°C and 29% relative humidity, to match conditions of related experiments. The interface cooling effect suppresses the evaporation rate significantly; however, natural convection in the gas and liquid domains has a negligible impact on the evaporation rate. The local evaporation flux along the droplet interface predicted by the model is compared to that predicted by an analytical diffusion-based model. The numerically calculated total evaporation rate agrees with experimental results to within 2%. The large deviations between past analytical models and the experimental data on superhydrophobic surfaces are reconciled.

scholarly journals Influence of Substrate Material on Flow in Freezing Water Droplets—An Experimental Study

Water ◽  
2021 ◽  
Vol 13 (12) ◽  
pp. 1628
Author(s):  
Erik Fagerström ◽  
Anna-Lena Ljung ◽  
Linn Karlsson ◽  
Henrik Lycksam
Keyword(s):  
Turbine Blades ◽  
Maximum Velocity ◽  
Substrate Material ◽  
The Arctic ◽  
Natural Phenomenon ◽  
Contact Radius ◽  
Freezing Process ◽  
Water Droplets ◽  
Symmetry Line ◽  
Influence Of Substrate

Freezing water droplets are a natural phenomenon that occurs regularly in the Arctic climate. It affects areas such as aircrafts, wind turbine blades and roads, where it can be a safety issue. To further scrutinize the freezing process, the main objective of this paper is to experimentally examine the influence of substrate material on the internal flow of a water droplet. The secondary goal is to reduce uncertainties in the freezing process by decreasing the randomness of the droplet size and form by introducing a groove in the substrate material. Copper, aluminium and steel was chosen due to their differences in thermal conductivities. Measurements were performed with Particle Image Velociometry (PIV) to be able to analyse the velocity field inside the droplet during the freezing process. During the investigation for the secondary goal, it could be seen that by introducing a groove in the substrate material, the contact radius could be controlled with a standard deviation of 0.85%. For the main objective, the velocity profile was investigated during different stages of the freezing process. Five points along the symmetry line of the droplet were compared and copper, which also has the highest thermal conductivity, showed the highest internal velocity. The difference between aluminium and steel was in their turn more difficult to distinguish, since the maximum velocity switched between the two materials along the symmetry line.

Numerical Study of Bicomponent Droplet Vaporization in a High Pressure Environment

1996 ◽  
Author(s):  
J. Stengele ◽  
H.-J. Bauer ◽  
S. Wittig
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Gas Phase ◽  
Numerical Study ◽  
Droplet Evaporation ◽  
Single Component ◽  
Vapor Concentration ◽  
Diffusion Limit ◽  
Evaporation Process ◽  
Droplet Vaporization

The understanding of multicomponent droplet evaporation in a high pressure and high temperature gas is of great importance for the design of modern gas turbine combustors, since the different volatilities of the droplet components affect strongly the vapor concentration and, therefore, the ignition and combustion process in the gas phase. Plenty of experimental and numerical research is already done to understand the droplet evaporation process. Until now, most numerical studies were carried out for single component droplets, but there is still lack of knowledge concerning evaporation of multicomponent droplets under supercritical pressures. In the study presented, the Diffusion Limit Model is applied to predict bicomponent droplet vaporization. The calculations are carried out for a stagnant droplet consisting of heptane and dodecane evaporating in a stagnant high pressure and high temperature nitrogen environment. Different temperature and pressure levels are analyzed in order to characterize their influence on the vaporization behavior. The model employed is fully transient in the liquid and the gas phase. It accounts for real gas effects, ambient gas solubility in the liquid phase, high pressure phase equilibrium and variable properties in the droplet and surrounding gas. It is found that for high gas temperatures (T = 2000 K) the evaporation time of the bicomponent droplet decreases with higher pressures, whereas for moderate gas temperatures (T = 800 K) the lifetime of the droplet first increases and then decreases when elevating the pressure. This is comparable to numerical results conducted with single component droplets. Generally, the droplet temperature increases with higher pressures reaching finally the critical mixture temperature of the fuel components. The numerical study shows also that the same tendencies of vapor concentration at the droplet surface and vapor mass flow are observed for different pressures. Additionally, there is almost no influence of the ambient pressure on fuel composition inside the droplet during the evaporation process.

EXPERIMENTAL AND NUMERICAL STUDY OF THERMAL ANALYSIS OF HIGH-SPEED FRICTION BRAKING SYSTEM

2011 ◽  
Vol 10 (01) ◽  
pp. 135-142
Author(s):  
CHUNMEI ZHANG ◽  
YONGFENG LI
Keyword(s):  
Thermal Analysis ◽  
Friction Surface ◽  
High Speed ◽  
Material Selection ◽  
Numerical Study ◽  
Friction Pair ◽  
Maximum Temperature ◽  
Dimensionless Time ◽  
Braking System ◽  
Friction Braking System

Thermal analysis can be used as one of the basis for the friction pair material selection in high-speed friction braking system. In this study, the experimental results showed that surface temperature could be reduced by increasing the radius of the friction disk or thermal conductivity coefficient of disk material with stable braking; In the early stage of long braking, the temperature on the friction surface rises rapidly, but further braking does not lead to a significant rise in temperature; In the case of short braking, there is not enough time for the friction surface to reach the critical temperature, and the disk surface reaches the maximum temperature at the end of braking. During long braking, the dimensionless time capacity of the friction surface reaching the highest temperature is F0 ≈ 0.1F0s.

Numerical Study of a Small Droplet Movement in a Microchannel under Heat Source

2019 ◽  
Vol 894 ◽  
pp. 104-111
Author(s):  
Thanh Long Le ◽  
Jyh Chen Chen ◽  
Huy Bich Nguyen
Keyword(s):  
Contact Angle ◽  
Heat Source ◽  
Water Droplet ◽  
Stokes Equations ◽  
Numerical Study ◽  
Dynamic Contact Angle ◽  
Dynamic Contact ◽  
Initial Position ◽  
Immiscible Fluids ◽  
Two Phase

In this study, the numerical computation is used to investigate the transient movement of a water droplet in a microchannel. For tracking the evolution of the free interface between two immiscible fluids, we employed the finite element method with the two-phase level set technique to solve the Navier-Stokes equations coupled with the energy equation. Both the upper wall and the bottom wall of the microchannel are set to be an ambient temperature. 40mW heat source is placed at the distance of 1 mm from the initial position of a water droplet. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet and the thermocapillary on the receding side is smaller than that on the advancing side. The temperature gradient inside the droplet increases quickly at the initial times and then decreases versus time. Therefore, the actuation velocity of the water droplet first increases significantly, and then decreases continuously. The dynamic contact angle is strongly affected by the oil flow motion and the net thermocapillary momentum inside the droplet. The advancing contact angle is always larger than the receding contact angle during actuation process.

Evaporative Particle Deposition on Superhydrophobic Surfaces

2013 ◽  
Author(s):  
Mercy Dicuangco ◽  
Susmita Dash ◽  
Justin A. Weibel ◽  
Suresh V. Garimella
Keyword(s):  
Contact Angle ◽  
Particle Deposition ◽  
Contact Angle Hysteresis ◽  
Droplet Evaporation ◽  
Superhydrophobic Surfaces ◽  
Contact Radius ◽  
Droplet Shape ◽  
Substrate Heating ◽  
Solid Liquid ◽  
Constant Contact Angle

The ability to control the size, shape, and location of particulate deposits is important in patterning, nanowire growth, sorting biological samples, and many other industrial and scientific applications. It is therefore of interest to understand the fundamentals of particle deposition via droplet evaporation. In the present study, we experimentally probe the assembly of particles on superhydrophobic surfaces by the evaporation of sessile water droplets containing suspended latex particles. Superhydrophobic surfaces are known to result in a significant decrease in the solid-liquid contact area of a droplet placed on such a substrate, thereby increasing the droplet contact angle and reducing the contact angle hysteresis. We conduct experiments on superhydrophobic surfaces of different geometric parameters that are maintained at different surface temperatures. The transient droplet shape and wetting behavior during evaporation are analyzed as a function of substrate temperature as well as surface morphology. During the evaporation process, the droplet exhibits a constant contact radius mode, a constant contact angle mode, or a mixed mode in which the contact angle and contact radius change simultaneously. The evaporation time of a droplet can be significantly reduced with substrate heating as compared to room-temperature evaporation. To describe the spatial distribution of the particle residues left on the surfaces, qualitative and quantitative evaluations of the deposits are presented. The results show that droplet evaporation on superhydrophobic surfaces, driven by mass diffusion under isothermal conditions or by substrate heating, suppresses particle deposition at the contact line. This preempts the so-called coffee-ring and allows active control of the location of particle deposition.

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