Learning Forum

[abdellatif.denine]

Hello everyone,

I'm having trouble displaying radiation in APDL. I know that we can display surface loads defined with the /PSF command and body forces similarly. However, I can't seem to display the radiation that I've defined with the following commands:

ALLSEL
*get,n_max,node,,num,max
n,n_max+2,-7,16.1

type,13
mat,13
real,13
cmsel,s,n_air_chaud
nsel,a,,,n_max+2
esurf,n_max+2
allsel

mp,emis,13,epsi
r,13,factform,stefan

d,n_max+2,temp,Tair_chaud
KBC,0

Does anyone have any idea how to display radiation in APDL?

Thanks in advance for your help!

[dlooman]

It appears you're modeling radiation with a surface effect element like surf152.  With such an element radiation is defined as a boundary condition, it's implicit based on the keyopt(9) setting.  It's more accurate to model radiation with the Radiosity method and the surface load label rdsf.  With this method you would be able to display the radiation boundary condition.

[abdellatif.denine]

Hi Dlooman,

Thank you for your reply and suggestion regarding the use of the Radiosity method and the surface load label rdsf for modeling radiation. I understand that this method might offer more accurate results, but I would appreciate it if you could elaborate on how exactly it is more accurate compared to using the SURF151 element.

Indeed, I am modeling the radiation using the surface element 151 with keyoption (9) setting. I currently use SURF151 because it allows me to easily calculate the heat flux exchanged between each surface and the extranode. Calculating this with the rdsf method is as straightforward. Could you provide more insights or examples on how the Radiosity method handles this aspect?

Thank you for your assistance.

[dlooman]

With surf151 you can't model surface to surface radiation and you must specify the form factor if it is less than 1.  Surf151 would be OK for modeling radiation to the night sky from a surface that is fully exposed to the sky.

[abdellatif.denine]

I should have specified that I am modeling the radiation of a surface with the air at an ambient temperature. In my model, the air is represented by the extranode added using the keyoption (5). I don't have any problems with the modeling process. However, I am unable to display the radiation boundary condition (BC) applied to the surface, as I can with convection.

P.S. I am using this method because I can easily get the heat flux (W/m²) exchanged between the surface and the extranode representing the ambient air using the following commands:

allsel                              ! Selects all entities
nsel,s,node,,n_max+1                ! Selects the extranode
*get,total_heat_flow_rate,node,n_max+1,RF,heat
heat_flux = total_heat_flow_rate/Length_of_the_surface

Getting the heat flux when using the rdsf method is very complicated since the only ways to get it are either fsum or prrsol,heat, which provide the sum of all reactions (in our case, the total heat flow rate [W/linear meter]) and need to be divided or multiplied by the number of elements on the surface.

[dlooman]

Couldn't you just plot the surf151 elements?  If you want to display them on top of the other elements you could use /pnum,type,1 to distinguish between them.  The surface normal is implied by the underlying elements so a vector really isn't needed.