5 Replies Latest reply on Jul 1, 2013 11:13 PM by Jared Conway

    Emissivity’s dependence on thickness?

    Jerry Pham

      Hello,

       

      The tutorial “150W Halogen Floodlight” example uses Emissivity Coefficient:

      emissivity.jpg

      but there is a question on the exact definition for this: it ϵ thickness-dependent?

      According to http://en.wikipedia.org/wiki/Emissivity:

      Although it is common to discuss the "emissivity of a material" (such as the emissivity of highly polished silver), the emissivity of a material does in general depend on its thickness. The emissivities quoted for materials are for samples of infinite thickness (which, in practice, means samples which are optically thick) — thinner samples of material will have reduced emissivity.

      Most emissivities found in handbooks and on websites of many infrared imaging and temperature sensor companies are the type discussed here, total emissivity. However, the distinction needs to be made that the wavelength-dependent or spectral emissivity is the more significant parameter to be used when one is seeking an emissivity correction for a temperature measurement device.

       

      I wonder in doing the calculations, does Solidworks make any adjustment according to the actual thickness of a surface?

       

       

      Jerry

        • Re: Emissivity’s dependence on thickness?
          Bill McEachern

          I am thinking that optical depth or thickness they are eferring to is in relation the wave lengths of the light not any sort of swx dimensions. In the base package it is black body radition anyway. If you have the HVAC module which handles spectral sensitivities and absorption in solids it would still likely be considered "thick". It will be whatever number you give it.

          • Re: Emissivity’s dependence on thickness?
            Chris Michalski

            Jerry, from my limited knowledge in this, most anything modeled in SW is going to be considered optically thick.  In order for the emissivity to vary with thickness you are considering materials which are so thin that photons do not encounter atoms on their passage through the material.

            The emissivity SW applies is based on the entire projected area relating to an atom or molecule in the solid, not materials so thin that an appreciable percentage allows direct radiation through the material avoiding diffraction and reflection.

            What physicaly thickness that eqautes to would take some research based on individual materials.

            • Re: Emissivity’s dependence on thickness?
              Jared Conway

              "I wonder in doing the calculations, does Solidworks make any adjustment according to the actual thickness of a surface?"

               

              have you found such a correction factor in your research?

               

              my suggestion would be to find an example that shows the effect of thickness and compare it in flow simulation but my guess is there is no correction. how would flow simulation know the thickness of a part relative to the surface is?

               

              like bill mentioned, this is black body radiation which is what almost all standard calculations are based on and the only adjustments is when you get to the HVAC module.

               

              another question you might ask yourself, does your application potentially "suffer" from this limitation? if so and flow simulation didn't exist, how would you incoporate an adjustment?

                • Re: Emissivity’s dependence on thickness?
                  Jerry Pham

                  halogen.jpg

                  Jared,

                   

                  For the 150W halogen lamp example in the tutorial, the quartz glass is actually modeled as a "semi-transparent" absorptive surface. Is this an example that the surface material is not assumed to be optically thick (otherwise it will be opaque and no light could come out).

                   

                  How does Solidworks determine the amout of radiative energy been absorbed by the software? Does it use Lambert's law and do the calculation incorporating the surface thickness?

                   

                   

                  Jerry

                    • Re: Emissivity’s dependence on thickness?
                      Jared Conway

                      with radiation, components are either opaque to radiation or transparent to radiation. the option you're looking at is from the HVAC module which allows the component to absorb some vs none/all of the radiation. the methods are all described in the technical reference. please review the types of radiation that are used by flow simulation. I think you're getting radiation and light mixed up.