4 Replies Latest reply on Apr 4, 2013 8:16 AM by Bill McEachern

    Using Flow to analyze enhanced heat exchange tubes

    David Paulson

      Although a preliminary discussion of this topic cam up in another discussion, it was somewhat off-post and I thought it useful to focus the discussion on the best means to model enhanced heat exchange tubes (such as Turbo-Chill manufactured by Wolverine  www.wlv.com) in Flow.  Analysis of heat exchange in bare tubes without any internal or external enhancements is very straightr forward.  The below example illustrates the effect of a bare tube with 2.5 GPM of water at 40 F. passing 1200 mm (47 in.) through a "tank" full of water at 100 F. :

       

      Test Pipe Assembly 2 (3).jpg

        Test Pipe Assembly 2 (1)_4.jpg

       

      With a bare 3/4" copper tube, as shown in the left image,  chilled water increased in temperature from 40 F. to an average temperature of 44.8 F.  The image on the right depicts a 3/4" copper tube that is enhanced with twelve internal spirals and "fins" spaced at approximately 24 per inch of tube length.  With these enhancements the water heats up from 40 F. to 46.5 F.  The simulation on the left takes about 10 minutes of processing time utilizizng four cores.  The simulation on the right takes a little over ten hours of processing time utilizing 4 cores (initial mesh of 6).  Typical commercial heat exhangers such as this utilize 200 to 500 or more such tubes and are arranged so that they will pass through the liquid 2-4 times or more, and with much less tempere difference than is shown above.  To simulate a four pass arrangement there would be four such tubes connected in a serpentine manner, and each tube would be at least twice as long.  I wouold estimate processsing time to be about 80 hours if enhanced tubes are used.  Not insignificant when it is desired ti investigate a number of tube sizes and fluid variables.

       

      Jared Conway suggested using a porous media to replace the fins on the tubes.  To do this I modeled the porous media with a thermal conductivity about 4 time that of copper for starters.  I did obtain a result that closely matched the result obtained witht he enhanced tubes:

       

      Test Pipe Assembly 2 (1)_3.jpg

      I was able to converge the leaving water temperature of 46.5 F.so it would appear that the porous is a very good approximation. 

      thanks, Jared.  The simulation time reduces to about four hours which is also helpful, but still scales to a large number. 

       

      Perhaps some of you might share the extent that you  have scaled your results in Flow and still get a result that can be trusted.

        • Re: Using Flow to analyze enhanced heat exchange tubes
          Jared Conway

          Nice. David, any idea how much the numbers are off with a coarser mesh? Just curious to know how close you can get with a coarser mesh and lower solve time.

           

          If the fluid didn't have to stay in the tube, there might be a way to replace the whole tube arrangement with a porous medium.

           

          But overall, nice application of the theory.

          • Re: Using Flow to analyze enhanced heat exchange tubes
            Bill McEachern

            Just an FYI note: You can get away with a much coarser fluid mesh and get accurate temperatures when mass or volume flow rates are used as prediction of the pressure drop is not critical to the mass flow rate determination which is a first order affect on temperature. I would take a look at the solid boundary mesh and ensure that the cut cell approximation has similar surface areas to the CAD model. I doubt you need as long a peice as you have used to validate and calibrate the prous media elements which would make make the process faster. I get that it would ot be as indicative if the goal is determining the speed up compared to an all up model. I presented these finding at a SWW conference a few years ago which showed that you could used very reduced fluid mesh densities for accurate temperature prediction when mass flow rates are specified as opposed to pressure pressure BC's.