Evaporation and Condensation inside the Desalination System, CFD Simulation ANSYS Fluent Training

Description

Evaporation and Condensation inside the Desalination System Description

The present simulation is about the evaporation and condensation inside the desalination system via ANSYS Fluent. This desalination system is a two-stage type, and the evaporation and condensation take place in the upper and lower parts.

The desalination mechanism in these two parts of the system is that impure water evaporates and turns into steam when it receives heat from the system’s floor. The hot steam then contacts the cold surface above it, and as a result, the condensation and the production of pure water occur.

These cold surfaces have a slope so that condensed water slides and is directed to the system outlet. Impure water initially has a temperature of 300 K and is at a certain height from each of the stages of the system.

The floor surface of the system has a temperature of 353 K, which causes heat transfer and evaporation. In this simulation, the VOF (Volume of Fluid) multiphase model defines several different fluids within the system.

Water, vapor, and air are the three main phases of the desired multiphase flow. Also, the mechanism of mass transfer between water and steam must be defined so that depending on the temperature and pressure of the internal space of the system, the evaporation (from water to vapor) or the condensation (from vapor to water) occurs. A UDF (user-defined function) defines the mass transfer process.

Geometry & Mesh

The present geometry is designed in a 3D model via Design Modeler. The computational zone of the model is related to the interior of two stages of the desalination system. On each stage, a sloping condenser plate, a sloping tray, and an outlet are designed. Also, a part of each stage is designed separately to define the initial water level.

The mesh of the present model has been done via ANSYS Meshing. Mesh is done unstructured, and the number of production cells equals 1936581.

Assumptions used in this simulation:

• Pressure-based solver is used.
• The present simulation is unsteady.
• Gravitational acceleration is equal to 9.81 m.s^-2.

 

Models
Multiphase Model		VOF (volume of fluid)
	number of eulerian phases	3 (air, water, vapor)
	interface modeling	sharp
	mass transfer	between water and vapor
Viscous		k-epsilon
	k-epsilon model	realizable
	near wall treatment	standard wall treatment
Energy		On
Boundary conditions
Bottom Wall		Wall
	wall motion	stationary wall
	temperature	353.15
Inner Wall (tray 1&2 and condenser 1)		Wall
	wall motion	stationary wall
	thermal condition	coupled
Outer Wall (stage 1&2 and condenser 2)		Wall
	wall motion	stationary wall
	heat flux	0 W.m-2
Outlet		Pressure Outlet
	gauge pressure	0 pascal
Methods
Pressure and Velocity Coupling		Coupled
	pressure	PRESTO
	momentum	first order upwind
	volume fraction	compressive
	turbulent kinetic energy	first order upwind
	turbulent dissipation rate	first order upwind
	energy	second order upwind
Initialization
Initialization methods		standard
	gauge pressure	0 pascal
	velocity	0 m.s-1
	air volume fraction	1 (patch: water = 1)
	temperature	300 K

Evaporation and Condensation inside the Desalination System Results

After calculation, 3D and 2D contours related to temperature, pressure, velocity, and volume fraction of air, water, and vapor are obtained. The contours show that water is up to one level from each stage and evaporates as heat is received from the system’s floor and steam is gradually produced.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.