14 Replies Latest reply on Feb 10, 2014 1:07 PM by Jared Conway

    Radiation Absorbant Wall

    Mark Keown

      Caution using Radiation Surface with 'Absorbent Wall'.  Make a block and put some power to it.  Have a plate next to it with low conductivity and  apply radiation with Absorbent Wall.

      The plate can get hotter than the block.  I don't know but I call this a bug.  If you find this material I want some.

        • Re: Radiation Absorbant Wall
          Jared Conway

          You'll need to post some more details, could be lots of things. Might be worth posting your model. What are you trying to do?

            • Re: Radiation Absorbant Wall
              Mark Keown

              File attached:

              Power to block

              Wall with selected type

              Problems- It Just can not be

              1) Absorbent wall hotter than the source

              2) Black wall colder than ambient air or ambient environment

               

              My version of Flow 2014 can only be set to Discrete Transfer - Discrete Ordinace not an option.

                • Re: Radiation Absorbant Wall
                  Jared Conway

                  i don't have sp2.0 installed yet so i'm not able to take a look at your flow model. i'll try getting it installed later today

                   

                  why have you chosen to use an absorbant wall? generally the recommended practice is to start with blackbody walls and then adjust the surfaces based on their material and coating. absorbant walls are relatively specific situations and personally can't think of any specific cases where somoene has used it so i'm curious what your application is that you've chosen to use it.

                   

                  also in another post you mentioned that you found reference that discrete ordinance is suggested for electronics cooling problems. i dug through a couple of the docs and couldn't find that reference, where did you find it.

                    • Re: Radiation Absorbant Wall
                      Mark Keown

                      The ref to the Electronics Module is in:

                      Flow Sim 2014 Online User's Guide

                      - Radiation #2

                       

                      The attached model is created so you can set different radiation settings and see how the solution reacts.  A heat source is in the center and three wall may be configured with radiation surfaces.  This is to test if the results are the same using one method as another.

                       

                      One would think that an Absorbent wall would have a absorbitivity of 1 and emissivity of 0.  But the results are out of this world - the absorbent wall ends up hotter than the source - and hotter than everything in the domain!

                       

                      The other situation is just as puzzling.  How can a Blackbody wall (e = 1, abs = 1) end up cooler than everything in the domain?

                      Regards

                        • Re: Radiation Absorbant Wall
                          Chris Michalski

                          It is not a bug, it is a specialized boundary condition that is only valid under very limited circumstances.

                           

                          It is hotter than the source because the source emits energy and the wall absorbs it.

                           

                          In reality that wall would emit also, so without internal power it can't exceed the source.  But you chose a condition that eliminates that emission.

                            • Re: Radiation Absorbant Wall
                              Mark Keown

                              Chris,  That may be but they should call it a black hole not a wall.  But the simulation gives the wall a temperature - so how dose the energy get from the colder object to the hotter wall? and just what would that special circumstance be?

                               

                              OK make a block.

                              Simulation:

                              External, Radiation, Environment 20°C, Ambient air 20°C, Wall Absorbent.

                              NO power, ON solar

                              Solve.

                               

                              What is the temperature of the block 20°C? - or much hotter than 20°C?

                               

                              I can see the situation where you have a window into outer space and all radiation that hits the window just 'disappears'.  BUT that is not what is going on.

                                • Re: Radiation Absorbant Wall
                                  Jared Conway

                                  mark, what are your walls made out of?

                                   

                                  or are you just asking theoretically how the absorbant wall would be used?

                                    • Re: Radiation Absorbant Wall
                                      Mark Keown

                                      Walls can be made of any material.

                                      Using any of the simulation commands is dangerous if you have not proven to your self how they work.  So yes this is an exercise to see how the command works.

                                      If I do not have a setup issue and you get results like I have my original message is correct that the Absorbent should be used with extreme caution.  I would like to hear the situation where Absorbent Wall is applicable - if this can not be explained then Absorbent Wall IS a bug.

                                       

                                      Issues that remain

                                      1) Absorbent wall hotter than the source.

                                      1.1) Absorbent wall getting hot when there is NO energy input.

                                      2) Black wall colder than ambient air or ambient environment when there is NO energy input.

                                      Thanks for helping -

                                        • Re: Radiation Absorbant Wall
                                          Jared Conway

                                          hi mark, i don't know of any material that is fully absorbant. i suspect some materials have some absorption capabilities but that the absorbant wall is in the software as an example to let users know that the option exists. similar to how there is a blackbody wall but that most surfaces aren't true black bodies. same goes for white bodies.

                                           

                                          as for the temperature increasing, per the technical reference, it is absorbing radiation only. i can't see your example because id on't have sp2.0 but i suspect you have heat going into your cube which is a black body radiation to the walls/environment and also heating the air and environmental radiation turned on which is heating/cooling the walls and cube depending on the temperature differences. so i think from an energy balance perspective, the temperature of absorbing wall comes from a balance of the radiation energy from the walls of all the components pointed towards it + the environmental radiation and convection. which i think depending on conditions could be higher because it is always absorbing radiation. for the other walls, they also have radiationg leaving them as blackbodies. i'm not sure if you've played around with it but with the surface paramters you can output heat power and radiation heat fluxes to develop a heat balance to understand the system. have you tried simplifying your situation to just a single wall with environmental radiation and an absorptive wall? what would you expect in that case and what do you get out of the software? what do the surface parameters read? if there is a bug, you may want to submit to your reseller so that it can get logged and fixed.

                                           

                                          as for the comments you found in the help about DO method, as noted previously, I think for most cases the DT method is still similarly accurate for most cases. especially if you make the correct considerations for mesh convergence...etc. maybe you could work with your reseller to get an evaluation of HVAC to check the differences. but as i noted, it really isn't required unless you have partially transparent objects. (transparent to radiation, which can be confusing because it is also called absorption by solids) it does also say at the end of the statement to move to DT method if the discretization method of DO isn't significant to add accuracy. but i agree if DO is helpful in EC, it might be nice to have it in the EC module as well as HVAC.

                                            • Re: Radiation Absorbant Wall
                                              Jared Conway

                                              so i had to do some testing with our nvidia GRID over an ATT mifi and tried out what i suggested above.

                                               

                                              1 wall

                                              no gravity

                                              no heat in

                                              just environmental radiation

                                              goals on temperature, heat power and radiation

                                               

                                              11.828 watts go in (radiation)

                                              11.828 come out (convection)

                                                • Re: Radiation Absorbant Wall
                                                  Mark Keown

                                                  What is the temperature of the 'block' and environment?

                                                  If the temperature of the block is above ambient temperature(environment and air) then how did the energy get there?

                                                  Heat / radiation must go down hill. 

                                                   

                                                  IF:

                                                  White Wall:        emissivity = 1, abssorptive = 0

                                                  Blackbody Wall: emissivity = 1, abssorptive = 1

                                                  one would think that:

                                                  Absorbent Wall: emissivity = 0, abssorptive = 1

                                                   

                                                  But this is not what you have.

                                                   

                                                  If the case can be made that this 'Absorbent Wall' command is part of an engineering solution then fine - but I do not think you can.  Second the documentation should explain what the 'Absorbant Wall' is - that is add the the wall will absorbe regardless of it's temperatue.  This must be so to get the energy balance that you tested.

                                                    • Re: Radiation Absorbant Wall
                                                      Jared Conway

                                                      mark, i think you've got it and would agree that some additional documentation would be helpful. you can submit that through the enhancement request portal.

                                                       

                                                      as for the specific application, it might be helpful to take a step back and draw up a diagram that explains the situation without CFD (hand calcs) and then compare against CFD instead of trying to "Explain" what the CFD is doing.

                                        • Re: Radiation Absorbant Wall
                                          Jared Conway

                                          when you say online user guide, do you mean the help file? how are you getting to it?

                                           

                                          i'm with chris, i think you're associating absorbant wall with a whitebody?