Water Distiller (Domestic) CFD Simulation, ANSYS Fluent Training

Description

This simulation is about a home water distiller via ANSYS Fluent software. One of the ways to water desalination is the process of water distillation. In this problem, a small-scale desalination device for home use has been investigated. This system’s main mechanism is heat transfer and phase change.

First, the water is heated by the model’s floor heater until it starts to evaporate. This produced steam is pure and free of salt or bacteria and pollutants. Now, this pure steam goes into spiral tubes. A fan cools these water pipes. The steam in the vicinity of these pipes undergoes a phase change due to cooling and turns into fresh or pure water.

In this simulation, a simple and small-scale water distiller is designed. This system consists of three parts. Its lower part is the evaporator, and the water in it is vaporized. Its upper part is the condenser and is responsible for cooling the steam pipe.

A spiral tube is also considered a channel for transferring vapor from the evaporator to the condenser. The process of condensation and freshwater production occurs due to the decrease in temperature inside this spiral tube.

simplifying the modeling, the definition of the heater in the evaporator or the fan in the condenser is omitted.

Instead, a fixed value for temperature is used in these sections. The evaporator temperature equals the constant value of 373 K (equivalent to saturation temperature), and the condenser temperature equals the constant value of 363 K. Also, with the patch tool, water is defined up to a certain level inside the evaporator tank.

The simulation process is time-dependent and unsteady. So, with the start of time, the water evaporates in the evaporator, and the resulting vapor inside the pipe leads to condensation. This problem aims to investigate the rate of the phase change (evaporation and condensation) and the amount of freshwater produced from this system.

The geometry of the present project is modeled in three dimensions with Design Modeler software. Then the model meshed with ANSYS Meshing software. The model’s grid is unstructured, and 478,805 cells have been created.

Methodology: Water Distiller CFD Simulation

In this simulation, three different phases are used. Water and vapor are constantly changing phases and transforming into each other. In addition to them, air also acts as a coolant inside the condenser. So a multiphase flow must be used instead of a single fluid. The VOF (volume of fluid) model has been used to define the multiphase model.

This multiphase model can completely separate different phases from each other and display a distinct boundary between phases. For the boundary between the water phase and the vapor phase in the evaporator tank and the condenser tube to be clear, the VOF model is the best option.

Also, the separation boundary between the two phases does not have a layer transition. So the Sharp option has been used to interface between the phases. Air is defined as the primary phase of the model, and water liquid and water vapor are defined as the secondary phases of the model.

When the multiphase model is used, a parameter called volume fraction is provided for the secondary phases of the model. This parameter solves their transport equations. A phase change occurs between the water and steam phases. So a Mass Transfer between these two phases is defined.

This mass transfer is defined based on the evaporation-condensation mechanism. This mechanism deals with the phase change process between liquid and vapor. In the evaporation-condensation mechanism, Lee’s equations calculate the mass transfer rate.

These equations depend on the saturation temperature and the frequency coefficient of evaporation and condensation. When the saturation temperature is set at 373.15 K, it means that if the temperature of the fluid rises above it, evaporation (phase change from water to steam) occurs. Also, if the fluid’s temperature falls below it, condensation (phase change from steam to water) occurs.

Conclusion

After the end of the simulation solution, the contours related to temperature, phase change rate between water and steam, and the volume fraction of each of the water and steam phases have been obtained. These contours are presented on a two-dimensional plane passing through the middle of the model.

Since the simulation is unsteady, the presented contours are only for the last second of the simulation (10th second). Also, to better understand the system’s behavior, several animations have been presented to investigate the mass transfer rate between phases (rate of evaporation and condensation), fluid temperature, and volume fraction of produced water over time.

A plot of the amount of freshwater produced inside the condenser tube over time is also presented. One plot shows the volume-average volume fraction of produced water in the system, and the other shows the mass flow rate of freshwater leaving the system.

The contours of temperature and mass transfer rate inside the tube are completely consistent with each other. Wherever the temperature falls below the saturation temperature, condensation has occurred. A negative sign for the phase change rate indicates the occurrence of condensation (from vapor to liquid).

Inside the evaporator, water begins to evaporate from its level and creates steam on top of the evaporator tank. When this vapor enters the condenser tube, it is faced with a decrease in temperature, and condensation is achieved. Hence, some water inside the pipe is produced as fresh water.

The plots correctly state that more condensation has occurred over time, and as a result, the rate of freshwater production increases. So, in general, it can be concluded that this water desalination system works properly, and the evaporation and condensation inside it happen clearly.