Emissivity

[HVSmith]

Hello - I'm working through some transient thermal analysis and all seemed to be going well. I thought I had the radiosity solver cracked. The results I am getting are very reasonable.

However...

I set up my material properties as such:

MP,DENS,1,2000 !DENSITY [kg/m3]
MP,EMIS,1,0.8 !EMISSIVITY [-]
MPTGEN,0,15,300,200
MP,C,1,12.7,2.872,-0.00145,3.12E-7,-2.38E-11 !HEAT CAPACITY [J/kg.K]
MP,KXX,1,241.54,-0.241,1.088E-4,-2.144E-8,1.531E-12 !THERMAL CONDUCTIVITY [W/m.K]

Varying the emissivity has no effect on the solution. I've varied it 0 < EMIS < 1 but it doesn't make any difference.

Since I'm generating a number of temperatures and using C and KXX as polynomials to be evaluated at these temperatures do I need to do the same for EMIS? Or am I defining it wrongly somehow?

I'm convinced that changing the emissivity should affect the solution:

Power radiated = area * stefanBoltzmann * emissivity * Temp^4

Thanks for any advice!
H

 

[flash3780]

For the radiosity solver, I think that you want to put in your emissivity using the SF command. Something like this:

/prep7
! Enclosure 1
sf,rad_1a,rdsf,0.950,1
sf,rad_1b,rdsf,0.900,1

! Radiation options
stef,5.67e-8
toffst,273.15
hemiopt,10
tunif,500

/solu
rsymm,,0,y  ! For y-symmetry
rsymm,,0,z  ! For z-symmetry
rsurf       ! Create SURF252 elements on radiating surfaces

I found this link helpful:

http://www.ansys.com/staticassets/A...article/AA-V1-I1-View-Factoring-Radiation.pdf

 

[HVSmith]

OK - great - thanks.

I was using the following command later on in the script:

SFA,ALL,1,RDSF,0.8,1

so in fact it was just my stupid mistake of not realising I should change the emissivity value here, the MP,EMIS,1,0.8 command seems only to have an effect if I specify a range of emissivities and then tell SFA RDSF to use these.

Again, thank you for pointing out my massive oversight.

That document is a very good overview.