Humidification Dehumidification (HDH) CFD Simulation, ANSYS Fluent Tutorial

Description

This CFD Project is about humidification dehumidification (HDH) system via ANSYS Fluent software. HDH system is a model of water desalination methods. The mechanism of this method is based on humidification and dehumidification processes. One side of the system is the evaporator or humidifier, and the other is the condenser or dehumidifier.

The cold water enters the spiral tubes inside the dehumidifier (condenser). Then it takes the heat of the steam inside the shell. Heat reception continues until the steam condenses, and as a result, the water inside the tube is heated. This hot water enters the humidifier (evaporator), and the hot water is sprayed on the filter plates of the evaporator.

These droplets are mixed with the dry air out of the condenser and form wet steam or humidified air. The resulting steam is pure, and its impurity settles in the evaporator. This pure steam is reheated by a heater and enters the condenser.

Now, the inside of the condenser shell is in contact with the cold feed water pipes (as mentioned before). It undergoes humidification or distillation of pure water from the steam. The mechanism of this system consists of two steps (humidification and dehumidification). So, two simulations have been done for this problem. The first simulation investigated humidification.

Dry air flows from the bottom of the chamber to the top. At the same time, droplets of hot salt water are sprayed through several holes inside the chamber. The droplets in the membrane part of the chamber collide with the airflow and, after evaporation, produce humid air.

This steam is pure and free of salt. The second simulation investigated dehumidification. The flow of moist air created from the previous step enters the chamber. Inside the chamber, spiral tubes are considered to carry the cooling water flow. The contact of hot steam with the spiral tube’s cold surface causes condensation and freshwater production.

For simplicity, the pipe modeling is omitted, and the pipe wall is defined with a thermal boundary condition of constant temperature (cooling temperature). The geometry of the present project is modeled in three dimensions with Design Modeler software.

Then the model meshed with ANSYS Meshing software. The mesh of the first model is hybrid (a combination of structured and unstructured). The number of production cells is equal to 206,928. The mesh of the second model is unstructured, and 553,086 cells are created.

Methodology: Humidification Dehumidification (HDH)

In the first simulation, the salt water is sprayed from the chamber’s top. So it is necessary to define the discrete phase. Two approaches can be used to solve fluid equations and investigate fluid behavior. In the Eulerian approach, the fluid is assumed to be a continuous medium, and elements inside the fluid should be considered for solving equations.

In the Lagrangian approach, the fluid is considered as discrete particles, and the behavior of the fluid should be investigated particle by particle. In the humidification chamber, dry air enters the chamber as a continuous fluid and water spray droplets as a discrete phase.

So, in this simulation, the Discrete Phase Model has been used. The discrete phase is Unsteady and interacts with the continuous phase. After activating the discrete phase model, it is necessary to define the injection. The type of particle spraying is surface.

The particles are sprayed in the form of droplets, which turn into wet vapor as a result of evaporation. The diameter of the particles is equal to 1e-6 m, and it is released in 10 seconds. Since dry air enters the chamber and moist steam is produced inside the chamber, a Species Model should be used.

For this purpose, the Species Transport model has been used. This model deals with the solution of defined gas species transport equations. In addition, there is no need to define the chemical reaction between species.

Also, membranes are placed in the middle part of the humidity chamber to help the phase change process. A Porous Medium has been used to model these membranes. The porosity (equivalent to the ratio of void volume to total volume), viscous resistance (inverse of permeability), and inertial resistance must be determined in porous options.

In the second simulation, there is no need to define a discrete phase because no splashing occurs. Only the phase change from steam to water occurs in the dehumidifier chamber. The pure steam produced from the previous step turns into fresh water. Therefore, a Multiphase Model is used until two phases, including water liquid and water vapor, can be used simultaneously.

The multiphase model of VOF (Volume of Fluid) has been used in this problem. This model can completely separate two phases from each other and provide a distinct boundary between the two phases. Also, a Mass Transfer between water and steam must be defined for a phase change.

This phase change is defined in terms of Evaporation-Condensation. The evaporation-condensation mechanism is based on Lee’s Equations. For these equations, the saturation temperature and frequency of evaporation and condensation must be determined.

Conclusion

After the end of the first simulation, the contours related to the mass fraction of air and water vapor and the concentration of discrete particles have been obtained. Since this simulation is unsteady, the results are only related to the last second of the simulation.

Also, two animations have been prepared to understand this problem’s results better. One investigates the contour of changes in the mass fraction of produced h₂O on a two-dimensional plane. The plane is passing through the chamber. The other investigates how water droplets spray over time using Particle Tracking.

The results show that, over time, the droplets are regularly sprayed downwards from the upper openings of the chamber to be ready to meet the dry air. Also, the results prove that with time, h₂O is gradually produced and moves towards the upper exit of the chamber.

This shows that the simulation is done correctly because the goal of the problem was to produce humid air. In addition, the plot of the mass fraction of h2o produced over time has been obtained. This plot proves that as time goes by, more steam is produced.

After the end of the second simulation, two-dimensional contours of velocity, temperature, mass transfer rate (from steam to water), and volume fraction of water and steam have been obtained. This simulation has been performed steadily and independently of time.

The results show the phase change in the regions around the cold spiral tube. The temperature contour shows that the vapor temperature in the vicinity of the cold pipe has decreased and reached below the saturation temperature. So, condensation (turning vapor into water) must take place.

The contour of the mass transfer rate also shows well that the phase change occurs in regions with a temperature lower than the saturation temperature. The negative sign indicates the phase change from vapor to liquid (it becomes positive when the phase change from water to vapor takes place).

Also, the contour of changes in the volume fraction of water and vapor confirms the occurrence of condensation. It is visible in the results that the largest amount of produced water is obtained in the vicinity of cold pipes.