2 Replies Latest reply on Jul 25, 2017 2:09 PM by Andrew Ross

    Phase change with steam

    Andrew Ross

      I am attempting to run a heat exchanger with steam through a helical coil. The steam starts as unsaturated with a quality of zero and I would like to know how much latent heat is absorbed. The heat is being gained from combustion and so I have the inputs to the coil to conduct from a hot environment of 600K and have the flame surface as a radiative surface of 180,000 W. I realize that combustion is a whole other story but I am happy with the boundary conditions of the heat source for now. It looks like this with the red transparent body approximating the flame volume:

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      I understand that SW Flow does not natively handle phase change but nonetheless I am following the instructions from S-019412 to define the steam. I have set up steam as an ideal gas material with a specific heat profile to account for the latent energy absorbed during phase change:

       

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      I am only interested in the phase change region which is why it is a flat line. The phase change "region" I have defined is over 1 degree K between 373.15K and 374.15 K. I only extended it beyond in both directions in case the simulation calculates the temperature past my phase change bounds (perhaps I should extend this region).

       

      My questions are:

      1) Should I not also be defining tables for thermal conductivity and dynamic viscosity since they too change significantly under the vapor dome? I have done this as follows:

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      2) I understand that there will be inaccuracies with this method but I am not clear on how to quantify them. Any explanation as to where the inaccuracies lie if the above table properties have been assigned well?

       

      I realize that other software handle phase change but I am interested in Flow only and how best to push the envelopes of its limitations in this capacity.

       

      Thanks in advance.

        • Re: Phase change with steam
          Bill McEachern

          Hi Andrew,

          First off, I m not much of a steam guy so forgive me if I am out to lunch on a thing or two, I presented a paper at SWW 200? (2007 maybe) with Bill Deziek(sp?) of SolidWorks and did confirm that this approach will work - i.e. abrupt changes in dependent properties does not cause program instabilities. I was illustrating an approximation of windshield defrosting - sublimation. Accuracy and error estimation were not really the goals - it was about program application and approaches.

          My comments are as follows:

          1. As you note, I would think you need to have your regime cover more than the 1K range for latent heat with the values at the extremities are for whatever phase properties apply and have then transition over a degree or two. the likelihood that temperatures go out of this range if even only temporarily, you need to have material properties available. The 1 K for the range seems unnecessarily tight unless there is some reason for having it that way that I don't understand. It is an approximation as are all calcs.
          2. If the stream is unsaturated (dry) and you are adding heat why would there be any phase change? I am not that current with steam so maybe this is not interpreted correctly by me or perhaps misstated by you. Depending on what's up in sorting this out I may have other suggestions on how you approach the whole problem
          3. Why not use the steam material available in SWX Flow? In my experience, which is not much with steam, it can be a bit unstable but I haven't used it much so take that with a grain of salt.
          4. As far as error quantification goes I would suggest you find in the literature some test case that you can compare your simulation approach to some experimental set up and see how it compares.
            • Re: Phase change with steam
              Andrew Ross

              Bill,

              Thank you for the response - I am very interested to read that paper you presented (if it's available). My gut reaction to this approach was that it would be unstable because of the sudden change in specific heat capacity but I am relieved to hear that is not necessarily the case.

               

              A couple comments back to you:

              1. I agree with your point about defining the regime to cover the extremities outside the phase change region. This makes sense to me in case the simulation finds itself in that temperature range even temporarily as you stated.

              I'm not convinced that the phase change region at 1K is too tight; however, since I expect to stay within the phase change region and not get to saturated steam, I suppose there is no harm in widening the temperature gap.

              2. The steam is not saturated steam which would be dry. It is wet steam (unsaturated) and enters the coil as just inside the vapor dome from the liquid side - essentially as saturated water.

              3. I understand that SWX Flow's steam material uses the superheated steam tables for its material properties (https://forum.solidworks.com/thread/69693). Since I am not intending to reach saturated vapor, this is not applicable for me. Having said, I may be wrong and reach saturation in which case point #1 should take care of any extremities.

              4. Thank you for the suggestion. I expect someone has tried this experimentally although I imagine that the specific application in industry is not common. It is my client's proprietary concept so unfortunately I cannot go into detail.

               

              Some other related questions:

              5. Do you have any insight about defining the viscosity and thermal conductivity over the phase change region as well as the specific heat capacity? I guess I am just not clear on why the proposed suggestion only talks about defining specific heat capacity. If I am interested in how well the steam absorbs heat and gets drier, I expect I would need to account for its change in ability to absorb that heat as it dries.

              6. I have had one successful run with this setup. Success for me in this case is a run that doesn't crash (had the SOLVER ABNORMALLY TERMINATED failure a few times) and produces a smooth increase in "temperature" across the phase change region. Here's the graph of "temperature" vs coil length:

              capture.png

              To explain, I've presented the increase in temperature as "wet steam quality" rather than temperature because of course the temperature doesn't actually increase (i.e. under the vapor dome the temperature remains at saturation temp and the steam gets drier). In my calculation results, I see a temp increase but I interpret this as a quality increase. I hope that makes sense.

               

              My comment is as follows. This whole system essentially sits inside a refractory lined furnace. When I add that refractory, which causes more radiation to hit the coil surface since it reflects and emits, the above graph becomes discontinuous with erratic behavior that just doesn't make sense to me. I hesitate to just throw more mesh at it although I should rule that out i suppose. I thought perhaps I had done something glaringly wrong in defining the thermal conductivity and viscosity curves. Any thoughts on that one?

               

               

              Thanks for taking the time to respond.

              Cheers!