Mass Transfer: Dehumidifier, VOF Multi-Phase Model

Description

This simulation models a dehumidifier using ANSYS Fluent. A dehumidifier is an air-conditioning device that reduces and maintains the humidity level of the air. It is used to improve people's health and thermal comfort, eliminate musty odors, and prevent mildew growth by removing water from humid air.

This project investigates a humidification–dehumidification system based on a phase-change process. These systems consist of two sections: the evaporator and the condenser. First, the airflow inside the copper pipes is heated and compressed by the compressor and then directed to the condenser section. In this section, the temperature of the air inside the pipes is reduced by the blowing fan, and condensation occurs. The resulting liquid then moves to the evaporator section, where it absorbs heat, evaporates, and returns to the gas phase. This heat is drawn from the humid air blown over the pipes, and because the humid air gives up its heat, dry air is obtained.

In this study, the humid air is treated as water vapor. Therefore, a multiphase model consisting of water and vapor must be defined. The mass transfer between vapor and water is then defined as an evaporation–condensation type, so that steam turns into liquid water when the temperature drops below the saturation temperature. The amount of water produced by the phase change between vapor and water indicates the degree of dehumidification. Since the two phases of water and vapor are entirely separate from each other, the Volume of Fluid (VOF) model is used. Accordingly, a chamber is designed with spiral tubes, so that the pipes carry a flow of cold water while the chamber contains water vapor. The vapor enters the chamber at a saturation temperature of 373.15 K and a velocity of 0.05 m/s and contacts the surface of a pipe carrying water at a temperature of 358.15 K and a velocity of 0.01 m/s.

Geometry & Mesh

The present geometry is designed as a 3D model using Design Modeler. The computational zone is the interior of a dehumidifier, which consists of a chamber with spiral tubes. Steam flows inside the chamber, and cold water flows inside the spiral pipes. The mesh of the present model is generated using ANSYS Meshing. The mesh is unstructured, and the number of cells produced is equal to 1,522,772.

Set-up & Solution

Several assumptions are applied in this simulation. A pressure-based solver is used, and the simulation is steady. The effect of gravity is considered, with the gravitational acceleration defined as 9.81 m/s².

For the models, the realizable k-epsilon model is selected to account for turbulence, together with the standard wall function for near-wall treatment. The multiphase flow is captured using the VOF model with two Eulerian phases (water and vapor), sharp interface modeling, and an evaporation–condensation mass transfer mechanism. The energy equation is also enabled.

The boundary conditions are defined as follows. The wet-air inlet is set as a velocity inlet with a velocity magnitude of 0.05 m·s⁻¹, a temperature of 373.15 K, a water volume fraction of 0, and a vapor volume fraction of 1. The cool-water inlet is likewise a velocity inlet, with a velocity magnitude of 0.01 m·s⁻¹, a temperature of 358.15 K, a water volume fraction of 1, and a vapor volume fraction of 0. The dry-air outlet is defined as a pressure outlet with a gauge pressure of 0 Pascal, and the cool-water outlet is also a pressure outlet with a gauge pressure of 0 Pascal. The inner wall is a stationary wall with a coupled thermal condition, while the outer wall is a stationary wall with a heat flux of 0 W·m⁻².

Regarding the solution methods, the pressure–velocity coupling is handled with the Coupled scheme. The PRESTO! scheme is used for pressure, and the modified HRIC scheme is used for the volume fraction. First-order upwind discretization is applied to the momentum, turbulent kinetic energy, turbulent dissipation rate, and energy equations.

Finally, the solution is initialized using the standard method. The gauge pressure is set to 0 Pascal and the velocity to 0 m·s⁻¹ throughout the domain. The chamber is patched with a vapor volume fraction of 1 and a temperature of 373.15 K, while the tube is patched with a vapor volume fraction of 0 and a temperature of 358.15 K.