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Step Solar Still CFD Simulation

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The desalination thermal method needs a heat source to evaporates saline water and then vapor distillation.

 

This product includes CFD simulation files and a training movie using ANSYS Fluent software.

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Description

Step Solar Still Problem Description

The present study investigates the performance of a step solar desalination unit. The present model consists of a small chamber with a sloping glass surface on both sides and steps within it, where saline water flows on the surface of these steps. Solar radiation heat transfer passes through the glass to the surface of the water in the enclosure to evaporate the water surface on the step walls. The resulting vapor meets the cold glass surface and undergoes a distillation process. Pure water from hot vapor distillation, moves down the slope of the glass plate and discharges as pure water.

We use the Species Transport method to solve this problem. This assumes that there is a mixture of water and water vapor inside the chamber interior. In fact, the simulation focuses solely on the amount of distilled water in the system, and the purpose of the present study is to investigate the amount of distilled water on the cold surface of the sloping glass plate. Radiation is also used to create surface evaporation on the running water on the stair walls. The radiation heat transfer modeled in this problem is solar energy.

Assumptions for Desalination CFD Simulation

The simulation is STEADY and the solver is Pressure-Based. The effect of the earth’s gravity on the flow is considered equal to 9.81 m.s-2. In this problem, the glass plate slope causes the downward movement of the water from the vapor distillation because of the gravity.

Geometry & Mesh of the Step Solar Still

The present 3-D model was designed by Design Modeler software. Since the geometry is symmetric, only one-quarter of the geometry is modeled. This quadrilateral geometry consists of two sloping glass and steps carrying water flow on it.

An unstructured mesh was carried out by ANSYS Meshing software. The element number is 809037.

CFD Simulation

Here is a summary of the steps to define and solve the problem in the table:

Models (desalination)
Laminar Viscous model
Species transport Species
water (liquid) – water (vapor) species
Solar Ray Tracing Radiation model
504 W.m-2 direct solar irradiation
on Energy
Boundary conditions (desalination)
wall Walls type
311.75 K free stream temperature glass walls
25 W.m-2.K-1 heat transfer coefficient
1 mass fractions for h2O (l)
0 Absorptivity
1 transmissivity
311.75 K free stream temperature heat wall
25 W.m-2.K-1 heat transfer coefficient
1 mass fractions for h2O (l)
1 Absorptivity
0 transmissivity
311.75 K free stream temperature wall
25 W.m-2.K-1 heat transfer coefficient
1 mass fractions for h2O (l)
Solution Methods (desalination)
Simple   Pressure-velocity coupling
PRESTO pressure Spatial discretization
first-order upwind momentum
first-order upwind energy
first-order upwind h2O (l)
Initialization (desalination)
Standard Initialization method
311.75 K temperature
0 m.s-1 velocity (x, y)
0 h2O (l)

Species Transport for desalination

This method assumes that there is a mixture of air and vapor inside the chamber interior and does not simulate distilled water flow. In fact, at the beginning of the simulation, the space inside the chamber is filled with air only; the bottom plane of the chamber is considered as the surface of the water, and since the water evaporates on the surface, it is assumed that the surface consists of vapor. It is also assumed that the sloping plate where the steam collides with, is composed of vapor only.

The equation governing the flow of gaseous species present in the simulation is as follows:

speies

In the above equation, Yi represents the gas species present in the equation, which includes air and vapor in the present model. The first term on the left is equivalent to the time-dependent term (if unsteady) and the second term is the equivalent of the transport term. The first term is the equivalent of the diffusion term (including mass and thermal diffusion), the second term is the equivalent of the chemical reactions term (if any) and the third term is for heat sources (if any).

The two mass diffusivity and thermal diffusion coefficient parameters indicate mass diffusion and thermal diffusion, respectively. such that the mass diffusion coefficient is equal to the constant value of 1.00314⨯10-5 m2.s-1 and the thermal diffusion coefficient is equivalent to the conductivity coefficient to specific heat capacity ratio, which is equal to 0.0002 kg.m-1.s-1.

Solar Load Model

In general, heat transfer occurs in three categories, including Conduction heat transfer, Convection heat transfer, and Radiation heat transfer. Since the thermal source of the phase change factor on the surface of water contained in the model is the solar radiation energy, the Radiation model is used to simulate the present problem. The radiation energy model is generally divided into two categories of radiation between different surfaces (including P1, S2S, DTRM, DO and MC methods) and Solar-Ray Tracing, which currently, the solar ray-tracing model is used.

In the Solar Calculator section, the Latitude and Longitude, Time Zones, Hours and Days of radiation in the geographic Region are defined. In the Sun Direction Factor section, the direction of the solar radiation to the desalination is determined. Direct Solar Irradiation of 504 W.m-2 is also considered. In addition, by enabling the Use Direction Computed From The Solar Calculator option, the solar radiation direction to the model is defined based on the input data to the Solar Calculator.

Boundary Condition for desalination

All of the walls in the present model, including the stepped walls carrying water flow and sloping glass plates, are affected by the convection heat transfer by their surroundings ambient, such that the surrounding air temperature is 311.75 K and the heat transfer coefficient is defined as 25 Wm-2.K-1. Also, the glass plate under the transparent environment has a transmission coefficient of 1 and zero absorption coefficient of the received solar radiation, while the walls of the inside steps have an absorption coefficient of 1 and a transmission coefficient of zero. Also, the mass fraction of water on sloping glass panels and step walls is equal to 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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