Learning Forum

[Arun mathew]

Hello,

I would like to inquire about the appropriate simulation approach for the system I want to model. The system consists of a tank that has one inlet and is filled with porous material. The porous material is initially filled with dry air, and moist air and liquid water enter the tank through the inlet, which is open to atmospheric conditions. The bottom part of the tank is at a constant high temperature, which causes evaporation and condensation to occur in the tank.

1. Should I use a multiphase model or a species transport model, or both?
2. The flow through the porous media is generally laminar. Is this applicable in this case?
3. How do I set a constant relative humidity at the inlet, and how do I measure the partial pressure of water vapor?

Thank you in advance.

 

[Rob]

You probably want both species and multiphase if the gas is a mix of air and vapour. 

Setting a constant mass fraction on the boundary is simple. RH is a function of pressure, temperature and volume fraction so you'd need to use an expression or UDF: neither will be simple. 

Re laminar/turbulent, that's for you to decide. Given the channel size do you think it will be turbulent? What about the head space? 

[Arun mathew]

Thank you for your message and for providing me with this helpful information.

The gas is a mixture of air and water vapor. Additionally, liquid water also enters through the inlet. I am curious to know if I need to consider species flow even if there is no liquid flow through the inlet. I am a bit confused about it.

I assume that the flow in the tank is laminar. Regarding the term headspace, I am not entirely sure what it means in this context. If you are referring to the condition outside the inlet, then it is simply atmospheric conditions.

[Rob]

Headspace would be the gap above the porous zone. In Chemical Engineering it's typically the air space at the top of a mixing tank above the liquid. 

For the gas phase you have a species mixture of vapour and air. For the evaporation you can either code up everything, or use a multiphase model. I'm lazy so would choose the latter.... 

[Arun mathew]

Thank you for your guidance and assistance. The headspace measures less than 5 cm.

In terms of the gas phase, I have chosen a mixture template that includes air and water vapor. However, for the water phase, I have selected a mixture template with only water in its liquid form. I am uncertain if it is necessary to consider water as a mixture template in this case.

Could you please provide guidance on how to configure the conditions for the occurrence of evaporation and condensation?

[Rob]

You don't need a mixture for the liquid phase, just use water-liquid as a species. 

From there, use the phase interaction options (evap & condensation models) in the multiphase model panel. 

[Arun mathew]

Thank you for your reply.

I have some concerns about configuring the evaporation and condensation conditions in Fluent. Here is the specific scenario I intend to simulate:

Initially, the air (dry air+ water vapour) entering the tank has a temperature of 25°C and a relative humidity of 50%. Additionally, liquid water is also introduced into the tank at the same temperature.

To create a conducive environment for condensation, the temperature of the bottom wall should be set below the dew point temperature, which is assumed to be 10°C. With an air temperature of 25°C and 50% RH, the calculated dew point temperature is 14.5°C.

My objective is to observe condensation phenomena under these prescribed conditions. I kindly request your guidance on how to establish these parameters effectively within Fluent. Any suggestions or available options for this simulation would be highly appreciated.

Regarding the materials properties tab, I am uncertain whether I should modify the standard state enthalpy and reference temperature for both water in the liquid phase and water vapor. I would appreciate your advice on this matter as well.

[Rob]

Have a look at the evaporation-condensation model in the Phase panel. That should cover the phase change part. 

It's a often a good idea to have the same reference for both materials, the difference in enthalpy is then the latent heat.