Cfd analysis of evacuated solar still

Computational fluid dynamics analysis of an evacuated solar still . . .
The basic idea behind an evacuated solar cell is meeting the current energy needs with the use of renewable sources of energy like solar energy. The underlying principle behind this is using solar collectors with evacuated tubes which has the basic characteristic: low heat loss by convection resulting from the vacuum isolation.
These evacuated solar cells can be used for a variety of purposes including meeting hot water requirements, and building upon a new multistage evacuated solar distillation system to desalinate drinking water and purifying them for drinking purposes. The later is still under research and if proven successful will be the next biggest contribution of solar energy to mankind.
The analysis of natural circulation rate within a chamber and evacuated chambers with single & two open ends were performed with the objective to allow better heat removal from the tube and avoid effects such as internal re-circulation and stagnation of cold water.
Numerical approach
The heat transfer by radiation is modeled using the heat flux incident on the lateral side of the tube. The fluid thermal analysis is basically done by solving the Navier-Stokes equation and the energy equations for incompressible fluid whose density may vary with temperature.
S=(ρ-ρ_ref )g
(ρ-ρ_ref )=ρ_ref β(T-T_ref )
Here, S is the source term, g is gravitational acceleration, ρ is the corrected density, ρ_ref is the reference density, β is the thermal expansivity, T is the studied temperature, and T_ref is the reference temperature.
In the conventional model with single end tube which has two water flows moving in opposite directions inside the tube dis-favors the extraction of thermal energy. To start with the cfd analysis three different tilt angles were evaluated: 30, 45 and 60 degrees. The initial temperature of the water is 300 °K and uniform heat flux applied on the tube surface was 500 W/m2. The geometries are meshed with tetrahedron elements. After performing a detailed CFD analysis it is seen that a stagnation zone forms in the collector tank of the solar still where cold water does not get heated up. This is generally due to the small tilt angles i.e 30 degrees.
At 45 degrees the stagnation region is present but with much reduced intensity and at 60 degrees the region becomes negligible.
Modified collector geometry
An additional lateral connection resulted in increased turbulence inside the collector with a reduction in the stagnation zone. In this arrangement the highest achievable temperature is still lower than the conventional model as water is being passed on only once and the amount of water that has to be heated up will also increase.
An inferior connection creates a turbulence of less intensity, this connection achieves a slightly higher temperature and the re-circulation and stagnation zones do not exist in the system.
Effect of varying circumferential heat distribution
Evacuated tubes can be used with a concentrating reflector to increase radiation on each tube. Simulation results reveal that flow rate increases as heat distribution shifts from bottom to top half of the tube. It is also shown that between extreme bottom heating and extreme top heating the natural circulation rate through the tubes increases by almost two fold. When heating on the bottom half is more dominant, cold fluid from the tank penetrates further into the tube.
While in the higher temperature ranges water viscosity is lower and density gradient is larger for the same temperature difference, the flow rate through the tube increases as the temperature increases.

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