Learning Forum

[anthony.spinozzi]

Hey everybody! I am working on a heat flow CFD model for a solar crop dryer that will be deployed and tested in Guatemala. The basic operation of the dryer is a large tube of transparent plastic is inflated by air blown from a fan. This air that then fills the volume within the dryer, which is heated from solar radiation and passed over corn kernels to dry them. We are less so interested in fully modeling the entire system (with the corn included), and more so interested in learning about the energy transferred to the fluid volume (the inside of the dryer). To represent the dryer volume for the simulation, I have created a very thin-walled 'shell' that represents the overall shape of the plastic enclosure/bubble we are going to be creating. The hole at the front is for a fan, which will blow air towards the rectangular side, which is going to be held in shape by a frame and an active vent placed there to release pressure and heat.

Dryer outer structure

In Fluent, I have created a valid volume mesh for the inner fluid region, and created boundary conditions for the fan inlet and the pressure outlet (I should be able to adjust the pressure to emulate the active vent changing the outlet pressure on the real dryer). Below is the mesh as it is visible in Fluent:

Fluent mesh with flow in/out BCs

 

In fluent, I have the Viscous SST k-omega enabled and Radiation enabled. Please let me know if I need to change anything in these panels, or if my radiation model needs to be set to a specific one (setting model to "Off" didn't seem to fail, but it could very well be what i need to change):

Viscous settings

Radiation settings

In Radiation, I have enabled Solar loading with Solar Ray Tracing enabled, using Solar position and time dta to simulate the dryer in Guatemala around mid-day. here are my Solar Calculator settings, and the console printout:

Solar calculator settings

Solar Calculator Values

Do I need to adjust the sunshine factor or the XYZ/North-East positioning, or can I leave those alone?

for Boundary Conditions, I also need some guidance. I have successfully set the inlet and outlet BCs and the interior fluid volume conditions are all set. But the exterior wall is where I am struggling. I assume we want a Radiation wall, to participate in Solar Ray Tracing? Should I use the 'Radiation' option on Thermal wall BC? Should I use Heat Flux? Here is what I have set now. I would also like to know how to find material settings for specific plastics or what values I should set here for Absorpivity and Transmissivity, or if I can leave them be.

Radiation Wall BC

For reporting, I want to know the radiation heat transfer rate, or more specifically how much heat energy is transferred to the air via solar radiation. I know for a fact I am setting this up wrong, as the values I get are all over the place or sometimes just stuck at zero. Here is the panel for my report definition:

Report definitions settings - please help

Or if you know a better way to get readable/understandable results please let me know. Any and all guidance is appreciated and thanks for reading through.

 

 

[SVV]

Hi,

 

You can enable radiation model if you want to solve radiation heat transfer in the fluid volume or between surfaces of the dryer that might exist due to the temperature difference caused by the solar radiation heat load on the system. If the radiation heat transfer is negligible, you can ignore it. A solar load model is used to calculate radiation effects from the sun’s rays that enter a computational domain.

You need to modify the solar load calculators for your model. Solver will use this to compute the direction vector and other inputs which it uses to calculate the solar load. More information on this can be found in 15.3. Modeling Radiation.

I assume you are using shell conduction. If you want to specify a solid material layer over the dryer without actually creating the geometry for solid, you can use shell conduction layer. Under shell conduction settings, you can find option to specify material type. A semi-transparent material requires specification of absorptivity and transmissivity. This value is your input for the material you are using. The thermal boundary condition is the condition which the wall is exposed to other than the solar heating. For example, if the wall is heated, you can apply heat flux boundary condition and provide value.

 

You can use solar heat flux variable available under Wall fluxes to report solar heat flux entering through a wall.