2 Replies Latest reply on Nov 6, 2010 8:19 AM by Andrew DeMedeiros

    Lumped Capacitance Analysis

    Andrew DeMedeiros

      I am trying to run a transient analysis of a lumped capacitance problem, it is a school assignment. Unfortunately I am having a very difficult time getting my simulation to work out correctly. The piece in question is a steel bar, 6cm in diameter and 20cm long. Calculating out the respective biot number proves that it is in fact a lumped capacitance system. The problem provides us with various material properties (Density, Cp,k) and with the convection coefficient of the fluid. Intial temperature and fluid temperature are given also.

       

      It is required that I find the temperature at time=50s, doing this out by hand I come to about 270C or so. The problem arises when I try to simulate it, I set the study to transient, with a time of 50s and a step of 1s. I set the initial temperature of the piece, and I set the respective convection parameters, after meshing and running I get ridiculous results. It tells me that the temperature of the piece is 93K, so -180C. This is impossible as my fluid temperature is 20C.

       

      Can anyone help me out with this?

       

      Problem parameters are:

       

      Diameter=6cm

      Length=20cm

       

      K= 43 W/mK

      C = 473 J/kgK

      Density = 7800 kg/m3

      h = 110 W/m2K

      To = 300 °C

      Tfluid = 20 °C

       

       

      Thanks!!!

        • Re: Lumped Capacitance Analysis
          Regis WACHEUX

          Andrew,

          The issue comes from the intial temperature setting. "Internal temperature" is not defined for all the nodes in the Thermal Loads boundary conditions, but only for the exposed faces nodes. In that study, the internal nodes temperatures go from a "0K" (unknown) to a heated temperature of ~90K...

           

          a) in the transient study, select the thermal load initial temperature for the Solid Body, instead of the 3 exposed faces (only).

          or

          b) set a first steady-state thermal analysis with a convection (whatever value), and a temperature of 300°C.

          That model thermal result at steady state will have the 300°C initial temperature for all nodes.

          Then set a 2nd study, as thermal transient, with the initial temperatures from the steady-state (1st) thermal study.

           

          I have run the studies in these ways, and both solutions work fine, in line with the analytical results.

           

          b.r.

          Regis.