9 Replies Latest reply on Feb 21, 2017 7:58 AM by Alexey Yuritsyn

    Wall conditions

    Alexey Yuritsyn

      Hello, everybody. I have a problem with, I suppose, easy question. just have limited time and need to get correct answer.

      stainless tube L = 2000 mm. Inlet-> Methane 1100 C. 1,5 m3/h. outside air 20 C.

      So I need to find out whether a heat exchanger is requited or not.

      Everything is clear but I never clearly understood WALL CONDITIONS...what should I choose and why?  Heat transfer coefficient or wall temperature? I suppose heat tran coef. but what number? I've done a heat exchanger exmeriment before but they just put 5 W/m2*K without explanation why, where it should be taken from.

       

      And when other parameters must me applied? like surface heat generation rate and wall temperature?

       

      Thank you for explanation in advance and sorry for asking elementary questions.

       

      Model.jpg

       

      Wall conditions.jpg

        • Re: Wall conditions
          Holger Parchmann

          Hello Alexey,

          if you define a wall temperature you get a boundary condition that doesnt change. It is all the time the same temperature.

           

           

          If you want to take into consideration that the temperature of the outer wall can change then you have to define another wall condition. Wall temperature and heat transfer coefficient could be fine if you know the proper value. The coefficient depends on the used materials and on the state of the flow if it is turbulent or not. If you dont know the value it could be better to run an external analysis, then the software calculates the heat transfer itself.

          BR Holger

          • Re: Wall conditions
            Amit Katz

            Heat transfer coefficient is tricky. Usually when we are in school learning heat transfer basics this number is given to us in a homework assignment, but in reality there is no physical property that it relates to. Heat transfer coefficient is something you define from the data after the experiment is run, you can't numerically derive it from physical properties of materials alone.

             

            In your case what I would do is model a section of pipe and a large bounding container around it, which will have surfaces defined as environmental pressure. Make sure you leave plenty of room for air around the pipe. I would not recommend modeling the entire length of the pipe, because your cell count will be astronomical. You will have to use some judgement here and balance your domain size with your time limits and computational resources.

            This model will allow you to simulate heat exchange between the pipe and ambient air. Hopefully it should give you some idea of the heat transfer rates and the amount of heat loss from your working fluid per unit length.