Humidification and Dehumidification (HDH)

Description

This project simulates a Humidification Dehumidification (HDH) desalination system using ANSYS Fluent, a core clean water engineering method for producing fresh water from saline sources. The system operates in two coupled stages: a humidifier (evaporator), where hot salt water is sprayed and evaporates into dry air to produce salt-free humid air, and a dehumidifier (condenser), where that humid air contacts cold tube surfaces and condenses into fresh water. This two-step cycle mirrors real-world HDH desalination units, making it directly relevant to sustainable, low-energy freshwater production.

Methodology

The system is modeled in two separate simulations reflecting its two stages. The 3D geometry is built in DesignModeler and meshed in ANSYS Meshing — a hybrid structured/unstructured mesh of 206,928 cells for the humidification chamber, and an unstructured mesh of 553,086 cells for the dehumidification chamber. Cooling tubes in the dehumidifier are simplified using a constant-temperature thermal wall boundary rather than explicit pipe modeling.

In the humidification simulation, salt water is introduced as a discrete phase using the Discrete Phase Model, with droplets (1e-6 m diameter) surface-injected over 10 seconds and evaporating into the dry air stream. The Species Transport model tracks the resulting water vapor concentration without chemical reaction, and the internal membrane packing is represented as a porous medium defined by porosity, viscous resistance, and inertial resistance.

In the dehumidification simulation, the VOF multiphase model captures the distinct liquid water and water vapor phases, with evaporation-condensation mass transfer defined via Lee's equations based on saturation temperature and phase-change frequency.

Conclusion

The humidification stage results, evaluated at the final simulation second, show water vapor mass fraction and discrete particle concentration contours, along with animations tracking droplet spray behavior and vapor mass fraction buildup over time — confirming steady production of humid, salt-free air. The dehumidification stage results show velocity, temperature, mass transfer rate, and phase volume fraction contours, confirming condensation occurring in regions where vapor temperature drops below saturation near the cold tube surfaces, with the highest freshwater yield concentrated around those cooled surfaces. Together, these results validate the HDH system's core function: converting saline water into clean, potable water through a fully coupled evaporation-condensation cycle.