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Surface Evaporation of a Water Droplet CFD Simulation

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In this project, the process of evaporation of a water droplet into the air space is simulated.

 

This ANSYS Fluent project includes CFD simulation files and training movies.

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Description

Project Description

In this project, the process of evaporation of a water droplet into the air space is simulated. Therefore, the Multiphase model is used to simulate the initial phase of air and the secondary phases of water and water vapor. A droplet surface evaporation in the air should be simulated. The purpose of the simulation is to investigate the droplet behavior during the falling and the amount of vapor produced in the air due to the water surface evaporation. No external factor as a boundary condition affects the drop, and the downward movement is based solely on the force of gravity. The time taken to process the downward movement of water within the air space is assumed to be 12 seconds.

Assumption

There are several assumptions used for the present simulation:

The solver is Pressure-Based and the simulation is Transient (unsteady) because the problem is to investigate the phase change of the droplet over time. Also, the effect of the Earth’s gravity on the model is considered because gravity is the sole cause of the downward falling of the droplet.

Geometry & Mesh

We design the 2-D geometry of the present model using Design Modeler software. We define the specific airflow domain as a plane of 3 cm * 30 cm. The meshing of the present model is performed by ANSYS Meshing software. The mesh is Structured and face meshing is used and the element number is 94800.

Surface Evaporation of a Water Droplet CFD Simulation

We present summaries of the problem definition and problem-solving steps in the table:

Models (Surface Evaporation of a Water Droplet)
LaminarViscous model
MixtureMultiphase model
air, water, vaporeulerian phases
Onslip velocity
OnImplicit body force
OnEnergy
Boundary conditions (Surface Evaporation of a Water Droplet)
WallWalls type
0 W.m-2mixture
Solution Methods (Surface Evaporation of a Water Droplet)
Simple Pressure-velocity coupling
PRESTOpressureSpatial discretization
first-order upwindmomentum
first-order upwindvolume fraction
first-order upwindenergy
Initialization (Surface Evaporation of a Water Droplet)
Standard – patchInitialization method
0 m.s-1velocity (x,y)all the zone
0water volume fraction
0water volume fraction
305 Ktemperature
1water volume fractiondroplet
300 Ktemperature

How to define a Water droplet using PATCH:

To define the existence of a droplet and move it along the y-axis in the air, we must define the shape of the drop in appropriate coordinates in the air using the Adapt Region and select the Circle mode and then Mark it. After initializing the model with a zero volume fraction for water (ie, there is only air in the model), using the Patch option, set the volume fraction for the water phase to the defined circular area, equivalent to one. We consider that in the whole model space, there is air and only in the defined circular area there is water. The initial droplet formation is in the middle of the width of the air domain (15 mm in the x-direction) and 3 mm from the upper edge of the air domain in the y-direction and the radius for the droplet is also equal to 0.002 m.

Using Implicit Body Force in Mixture Multi-Phase Model

Since in the present model, we define the effect of gravity as the main cause of the downward motion of the droplet, it is necessary to apply the effect of volume forces on the model because in this case, the effect of pressure gradient terms and volume forces on the momentum equation is significant compared to the viscosity and transport terms.

UDF for Surface Evaporation of a Water Droplet

As water droplets evaporate in space, the phenomenon of mass transfer between the two phases of water and vapor should be used. As the evaporation of the present model is not boiling, the surface evaporation type should be used. The major difference between the boiling and surface evaporation is that the boiling evaporation occurs when the water temperature reaches saturation temperature and the boiling phenomenon occurs, while the surface evaporation can occur at a lower temperature than the saturation temperature; and it happens on the surface of the liquid. We use the Evaporation-Condensation type mass transfer to define boiling evaporation, whereas, in the present model, we use the UDF code to define surface evaporation.

We write the UDF model based on the specific relationship of surface water evaporation within a space containing airflow. In the following equation, Jw denotes the rate of evaporation from the water surface (kg.s-1.m-2), Pw denotes the partial pressure of saturated steam at water temperature (Pa), Pa denotes the partial pressure of water vapor (Pa), and Va represents the average airflow rate (ms-1).

Droplet

In the present model, the pressure difference is 2339.2 Pa and the airflow velocity is 3 m.s-1.

 

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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