10 Replies Latest reply on Feb 6, 2014 3:24 PM by Eric Ryan

    Large Heat sink

    Mark Keown

      I have a large heat sink 750 mm wide x 350 mm tall x 80 mm fins.  What methoid do I use to enter the known resistance (sink-to-air) and not have to mesh all the fins.  Can I make a simple block and enter a resistance value from sink to air?

        • Re: Large Heat sink
          Jared Conway

          tkae a look at porous medium or heat sink simulation

          • Re: Large Heat sink
            Eric Ryan

            Hello Mark,

             

            I am working on a similar problem with an arrangement of folded fin heat sinks.  I was wondering if you were able to solve your problem using porous medium, or the heat sink simulation feature.  The heat sink feature did no seem to fit what I am trying to do.   I have been looking at the porous medium with heat conductivity enabled.  I am trying to figure out how to define the specific heat, thermal conductivity, and the matrix and heat exchange definition.  Any thing you could share on your results would be helpful.

             

            Thanks,

             

            Eric

              • Re: Large Heat sink
                Mark Keown

                Eric,

                 

                Yes - using porous medium to represent a sink is a mission and not useful or practical.

                 

                My solution:

                For the model use use Initial Mesh of #2 or #3.  This is to keep the number of cells from going in the millions.

                Use local mesh around the sink and components where heat is applied as follows:

                 

                For the local mesh around sinks un-tick Automatic Settings  and set:

                  - Solid Fluid / Small Solid #2

                  - Refining Calls / tick Refine all cell / #3

                  - Narrow Channel / Characteristic across 3, NC Refinement #3, Enable Max Height and enter your sink height

                 

                For the local mesh around other components:

                  - Automatic Settings / tick narrow channel / set refinement high like #6 to #9

                 

                When solving just solve for the mesh and check the number of cells.  If the number of cells is too high - lower the refinement settings.

                 

                If you find improvement please let me know.

                  • Re: Large Heat sink
                    Eric Ryan

                    Mark,

                     

                    After looking at the porous medium idea yesterday I also came to the same conclusion.  Thanks for the input on mesh settings.  That is the other area I have be delving in to and definitely seems to have the most promise.   I will let you know if I find anything to add.

                     

                    Thanks,

                     

                    Eric

                      • Re: Large Heat sink
                        Jared Conway

                        Why is porous not possible?

                         

                        Setup a heat sink.

                         

                        Setup some simulations to determine the heat sink inputs.

                         

                        Plug info into porous.

                         

                        Use the info everytime you use that heat sink.

                         

                        Check blog.hawkridgesys.com where we went part of the way with the flow part.

                          • Re: Large Heat sink
                            Mark Keown

                            If you are using a fan porous may be used - but it has a limitation. 

                            - You have to know how much power is going to the fins (porous).  If no other thermal path is present this is fine - but of multiple thermal paths are considered how do you know how much power is going each way?

                            - Porous can not be used with natural convection.

                            - Radiation (emissive / solar) can not be entered - with large heat-sinks emissivity should always be considered.

                            - You have to know the flow resistance of the sink - which you will not know that is why you bought CFD to figure that out.

                            - No consideration is given for by-pass air flow.

                            - ** And the killer - you have to know the thermal resistance of the sink - typically that is what you are trying to confirm or determine.

                            If you have all that information a $2 calculator will give you the answer.

                            Or am I off base?

                             

                            Message was edited by: Mark Keown

                             

                            Message was edited by: Mark Keown

                              • Re: Large Heat sink
                                Bill McEachern

                                You do not need to be using a fan.

                                You do not need to know how much power is going where.

                                It does work with natural convection.

                                I believe you are right on radiation - they are fluid cells so no surfaces to apply the emmissivity to.

                                Flow resistance of the sink you calculate using flow sim - sub scale model required.

                                Thermal resistance you calculate using flow sim - sub scale model.

                                The general method for use in system level work is attached.

                                 

                                PM with heat transfer method.pptx.jpg

                              • Re: Large Heat sink
                                Eric Ryan

                                Jared,

                                 

                                Can post a direct link to the blog post you are referring to, or even just the title?  I searched through the blog site but could not fine one relating to porous medium or heat sinks. 

                                 

                                I have been using the porous medium to simulate heat sinks for purely air flow studies.  I have run flow models of a single heat sink at various airflow rates to determine the values to use for the equivalent porous medium.  This has worked farily well.  Now I am trying to figure out how to use the heat conductivity settings, but have been coming to some of the same conclusions that Mark mentioned. 

                                 

                                ER

                                  • Re: Large Heat sink
                                    Jared Conway

                                    http://www.hawkridgesys.com/blog/flow-simulation-2013-parametric-studies/

                                     

                                    There is no magic button to make the conversion and it isn't as easy as the flow porosity. Also it will not be exact because of some of the things that mark mentions but it will be pretty close. Start with something reasonable that has a known solution and try it.

                                     

                                    I think one thing mark is saying above is that you can use a heat sink simulation if you have a fan. For a heat sink only you have to develop the conversion.

                                      • Re: Large Heat sink
                                        Eric Ryan

                                        Jared,

                                         

                                        Thanks for the link.  I glanced at this one, but it did not click that using the new Parametric Studies capabilities could be used in determining the thermal parameters for a porous medium.  I will have to take a look at that functionality.  In the past I have manually run parametric studies by just cloning the study multiple times then batch running them.  This new feature sounds like it would be in improvement on that.

                                         

                                        I also agree that trying this on a smaller scale model, with a known solution is the way to start.

                                         

                                        ER