[Pavel Mihailovski]

Hello everyone,

I have a somewhat serious problem and I'm in desperate need of your help. Namely, my situation is as follows.

I am modeling industrial drying of a sludge in a pilot-like laboratory chamber. I have the drying air recirculating for about an hour or so, drying the sludge, gaining moisture from the wet material.

My question is as follows:

Does anyone of you guys know how can you add a time-dependent relative humidity (or water vapor mass fraction) in the ever-upcoming recirculating air, which enters the chamber through a velocity inlet boundary condition?
The fresh air (relative humidity at the beginning of the process is equal to the surroundings rel. humidity) enters the chamber and gains moisture from the sludge during the simulation (transient).

I ask this because what I get from Fluent is two options under the species tab which defines the vapor species mass fraction:
1. Constant
2. Input parameter (which I do now know at this point, whether or not it
suits the problem)

Of course, the problem would have been easily solved if I didn't have a physical discontinuity in the model, namely, the circulation side is cut off, divided in an upper (velocity inlet) boundary, and a lower (pressure outlet) boundary for the recirculation of the air. If the discontinuity did not exist, then the flow of air wouldn't have been disrupted, meaning that the gained moisture (dm/dt was experimentally obtained), would have been easily transported to the upper side of the recirculation, since the fluid flow wouldn't have been disrupted.

The geometry was cut in such a way because I had no idea how to model the heat pump which heats the drying air during its circulation.

So, long story short:

The relative humidity or the mass fraction of water in the inlet fresh air is changing with respect to time. I have this function written down, but since there is no user-defined option available in the species dialog box (besides the constant option, and the input parameters option), I ask you whether you know how can this "input parameter" work in this case?

[Pavel Mihailovski]

Hello everyone,

I have a somewhat serious problem and I'm in desperate need of your help. Namely, my situation is as follows.

I am modeling industrial drying of a sludge in a pilot-like laboratory chamber. I have the drying air recirculating for about an hour or so, drying the sludge, gaining moisture from the wet material.

My question is as follows:

Does anyone of you guys know how can you add a time-dependent relative humidity (or water vapor mass fraction) in the ever-upcoming recirculating air, which enters the chamber through a velocity inlet boundary condition?
The fresh air (relative humidity at the beginning of the process is equal to the surroundings rel. humidity) enters the chamber and gains moisture from the sludge during the simulation (transient).

I ask this because what I get from Fluent is two options under the species tab which defines the vapor species mass fraction:
1. Constant
2. Input parameter (which I do now know at this point, whether or not it
suits the problem)

Of course, the problem would have been easily solved if I didn't have a physical discontinuity in the model, namely, the circulation side is cut off, divided in an upper (velocity inlet) boundary, and a lower (pressure outlet) boundary for the recirculation of the air. If the discontinuity did not exist, then the flow of air wouldn't have been disrupted, meaning that the gained moisture (dm/dt was experimentally obtained), would have been easily transported to the upper side of the recirculation, since the fluid flow wouldn't have been disrupted.

The geometry was cut in such a way because I had no idea how to model the heat pump which heats the drying air during its circulation.

So, long story short:

The relative humidity or the mass fraction of water in the inlet fresh air is changing with respect to time. I have this function written down, but since there is no user-defined option available in the species dialog box (besides the constant option, and the input parameters option), I ask you whether you know how can this "input parameter" work in this case?

[Ahmed Saeed Mansour]

Hello dear Pavel, I am gonna study your problem in details soon, then I can tell you...
Thanks

[mahesh1819]

Hi we need to simulate relative humidity in the space. Please see the description below:
1. We have a space having hot air fan duct (refer fig. MassFlowInlet-Marked) which we have defined as mass flow inlet (refer fig. MassFlowInlet-Momentum, MassFlowInlet-Species, MassFlowInlet-Thermal) and there is no outlet.
2. Side walls are defined as per SideWall-Boundary Condition
3. We are stuck how to define inner air which is at 30 deg and higher relative humidity (Refer fig.CellZone Input)
4. And not getting any results in H20 mass fraction. (Fig.H2O-MassFractionResults)
Please review and help where we are wrong.

[vinerm]

For a closed system with only inlet, you have to ensure that the fluid is treated as compressible. Otherwise, it won't allow any incoming mass because there is not space for it. Initialize whole domain at 313 K and a mass fraction for the vapor. Apply a particular value of mass fraction at the inlet. This is to be done under the Species tab at inlet. And if extra vapor is coming only from the ducts, then you do not need a mass source. Mass source implies there is something within the space that is generating mass.

[mahesh1819]

Thank you Vinerm, I shall check and tell you

[mahesh1819]

Dear Vinerm,
I have tried the analysis and below are the outcomes:
1. Do we need to specially define the fluid as compressible? Can we proceed solving with pressure based solution?
2. I have initialized the whole domain inside the room with no mass source and defined the mass fraction of H2O in the source term as shown in "fig. mass fraction" Please tell it is correct or not.
3. Actually the room volume is very big (13000 m3) and we adding only little mass which is 1kg/s of hot air and it is approximately taking 2-3 hrs for getting the required RH. So the question is do we need to solve this in steady state or transient?
Please help. Thanks in advance.

[vinerm]

1. If there is only inlet and no outlet, you have to define the material as compressible, else, no fluid can enter until some of the existing fluid is removed. If there is an outlet as well, then compressible fluid is not required. For compressibility, you have to enable Ideal Gas for density of the mixture material.

2. Mass source is required only if you do not want to include any inlet of humidity. If humidity is coming along from the duct, then you do not need a mass source. Both may be required if humidity is coming from inlet duct as well as being generated somewhere in the room due to some reason.

3. Transient or steady-state depend upon the objective. If the objective is to study how long does it take to reach an equilibrium or a particular RH, then it has to be transient and the initial condition has to be accurate, i.e., if room already has an RH of 2%, then it will take longer to achieve RH of, say, 10% then if the initial value is 4%.

[mahesh1819]

First of all Thank you so much Vinerm for your continuous help.
I have tried by selecting the Ideal Gas for mixture template. Then it has asked to enter the specified density value as '0'. I did the same and run the analysis. But at 40th iteration came across the error as shown in the figure and simulation stopped. Request your help on this in resolution.
Thanks in advance,

[vinerm]

Did you initialize with a proper pressure value? Since operating density and operating pressure, both, are set to 0, you need to ensure that a proper pressure and temperature is used in the Initialization tab.

[mahesh1819]

I have given pressure and temperature as per fig 1&2. please suggest if there is any mistake. It is getting solved but the error showing as per fig. 3. I hope I am not giving proper pressure and temperature values.

[vinerm]

The first image is relevant but the data in the second one does not affect simulations; these are reference values used to determine coefficients as output. You have to look at the Initialization Panel, ensure that Standard is selected, and that pressure has a positive value.

[mahesh1819]

Thank you Vinerm.
I have done using Standard initialization and the ensured positive value for pressure as shown in fig. 4. But still the error is persisting and the showing absolute pressure is limited to 1 as shown in fig.5. Even I have tried with '0' for pressure value in the initialization tab and the same error is coming. Could not figure out why the error is coming. Please suggest what to do.
Thanks in advance

[vinerm]

1 Pa pressure almost pure vacuum. You have to use a much higher pressure. Atmosphere is at 101325 Pa. So, use that.

[mahesh1819]

As it was for gauge pressure I have initially tried with '0' (which is atmospheric pressure) and after that '1'. As you suggested tried with 101325 Pa.
In all the cases same error as that of fig. 5 is reflecting. No idea what to do.

[vinerm]

Have you set your operating pressure to 0? If yes, then that's correct because it should be set to 0 whenever ideal gas formulation is used for the density. Since operating pressure is set to 0, though Fluent shows Gauge Pressure in labels yet it behaves like absolute pressure. I'd suggest you test you case without running species equations and see if it works alright. Go to Solution Controls > Equations and then deselect equations for species. Initialize with 101325 Pa as pressure and 0 mass fraction for all species except the one that you have provided as inlet, e.g., if you have two species in your domain, air and vapor, and vapor is secondary specie, i.e., for which Fluent asks for mass fraction, then set it to 0. The case must be run as transient since there is no outlet.

[mahesh1819]

Dear Vinerm,
Yours is good idea actually. I have tried the way as you suggested by keeping the species equation OFF and putting the mass source terms to '0'. Even after that faced divergence in temperature as shown in Fig. 6

[vinerm]

Are you running the simulation as steady-state or transient? It has to be transient since there is no outlet, hence, no steady-state exists for the system.

[Morice]

Originally Posted by AAA
;144141

Hi

Use the Species model and define the properties of the "mixture species" in the materials window to have H2O and Air (H2O must be the top one). Choose "volume-wheighted" for density and "mass-weighted" for thermal conductivity and viscousity. Then specify the moisture mass fraction for inlets to the domain (this is the humidity ratio in the psycrometric chart). Specify a "constant" mass fraction value for the body of the sheep.

I am modelling humidity/water vapour concentration in a greenhouse. I was able to follow the methods stated above. In addition, I wanted to specify the specific heat of water vapour as a function of temperature i.e. C water vapour = f(T). I have been unable to do this. In addition, can we activate the Boussinesq model for air density when modelling humidity?
Thanks. Much appreciated

[vinerm]

There are three materials in a case with two components; two materials are the components themselves and one is their mixture. For the mixture, you might be using mass or volume weighted properties; use mass-weighted if density ratio is high. However, for individual components, you can still apply some of the material properties; not all though. So, you can go to Material Properties of Vapor and assign the properties as function of temperature. The material properties that you can assign for individual components depends on the options chosen for the mixture. If mixture property is based on some mixing law, then that particular property has to be given for individual component, otherwise, the property from the mixture is used. Boussinesq is not compatible with Species Transport.