[Ali_qasim]

Hi everyone,
I am modeling a membrane energy exchanger for dehumidification of the air in a hot, humid climate. In this energy exchanger system, the air flows on one side of the membrane, and on the other, the liquid desiccant solution flows. Both fluids are separated by the semi-permeable membrane, which allows only moisture transfer but not liquid. I am modeling this system through the ANSYS FLUENT and UDF is used to model moisture transfer through the membrane.
As the moisture transfers through the membrane, it changes phase at the air-liquid solution interface. In ANSYS Fluent, the effect of this temperature change is not considered while solving the energy equation.
How can I include this effect of phase change of vapor (absorption on the solution-air interface) phenomenon on the performance of the system in terms of temperature change of solution and air?
Can I include this effect using Eulerian Wall Film?

Thanks

[bloodflow]

Originally Posted by Ali_qasim

Hi everyone,
I am modeling a membrane energy exchanger for dehumidification of the air in a hot, humid climate. In this energy exchanger system, the air flows on one side of the membrane, and on the other, the liquid desiccant solution flows. Both fluids are separated by the semi-permeable membrane, which allows only moisture transfer but not liquid. I am modeling this system through the ANSYS FLUENT and UDF is used to model moisture transfer through the membrane.
As the moisture transfers through the membrane, it changes phase at the air-liquid solution interface. In ANSYS Fluent, the effect of this temperature change is not considered while solving the energy equation.
How can I include this effect of phase change of vapor (absorption on the solution-air interface) phenomenon on the performance of the system in terms of temperature change of solution and air?
Can I include this effect using Eulerian Wall Film?

Thanks

Are you sure this energy change is not accounted for within the phase change process? If it is an in-built phase change model in Fluent I'm pretty sure its there.

The energy change would be m_dot*L_V where m_dot is the phase change mass flow rate in kg/m3/s, and L_v is the latent heat of vaporization (or condensation if the mass transfer is gas -> liquid).

You can add this with a DEFINE_SOURCE UDF.

[Ali_qasim]

Thank you for your response.
I am not using the in-built model of Phase Change ANSYS Fluent.
I am using the UDF for the vapor transfer from one side to another. Once the vapors transfer, they change their phase when they interact with the liquid. I need to tackle this effect of phase change on the temperature of the liquid in which it is absorbing. Your guidance would be a great help for me. Regards

[bloodflow]

Originally Posted by Ali_qasim

Thank you for your response.
I am not using the in-built model of Phase Change ANSYS Fluent.
I am using the UDF for the vapor transfer from one side to another. Once the vapors transfer, they change their phase when they interact with the liquid. I need to tackle this effect of phase change on the temperature of the liquid in which it is absorbing. Your guidance would be a great help for me. Regards

I assume your evaporation/condensation UDF returns the mass flow rate from one phase to the other, and a value of 0 elsewhere.

The temperature change when this phase change occurs from energy being released (condensation) or energy being absorbed (evaporation). The magnitude of this energy is the rate of phase change (kg/s) multiplied by the latent heat of evaporation/condensation (J/kg) for your species


You need to use a DEFINE_SOURCE UDF for this energy being released or absorbed during evaporation/condensation. It loops over all cells, and in cells where no phase change occurs, the value of the source is 0J. In cells where evaporation does occur, a suitable energy source is defined as m_dot*L_V. This will change the local temperature in cells with phase change.

[bloodflow]

PHP Code:

DEFINE_SOURCE(phase_change_energy,c,t,dS,eqn)
{
    /* Define source UDF's loop over all cells in the domain */
    /* A seperate phase change UDF has calculated the rate of phase change in each cell as m_dot, with units kg/s */
    /* If m_dot > 0 -> Phase change is liquid -> vapour -> Energy is absorbed -> Cell temp decreases */
    /* If m_dot < 0 -> Phase change is vapour to liquid -> Energy is released -> Cell temp increases*/
    /* If m_dot = 0 -> No phase change occured -> No energy is released -> No temperature change */ 

    real energy;
    real L_v = 2260*1000; /* Latent heat of vapourisation of water in J/kg */

    energy = m_dot*L_V; /* Energy source during phase change in J/s */

    return energy; /* Return value of energy source to cell */
}