Fluids

Fluids

air closed in steel box, during heating box and air

    • andre8
      Subscriber

      Anyone can help me solve the problem in ansys - air in a steel box, in a heating box up to 300 C


      I need to know what will be the effect of heating a steel box to 300C. Inside the box is closed air (I mean, how to calculate the stresses in a steel box from the preservation of heated air).


       


      I do not have to much experience in ansys.

    • peteroznewman
      Subscriber

      I assume you mean the steel box is sealed so that when it is heated, no air can escape and the air pressure inside the box increases due to the thermal expansion of the air.  Are you familiar with the Ideal Gas Law?   PV = nRT


      The problem is not very well stated because a box has stress concentrations due to the edges, so the stress also depends on the radius at the square edge. A better problem would have been to ask about a steel sphere as an ideal sphere has no stress concentration. The thickness of the steel will also effect the stress.


      The problem also lacks another important piece of data, the temperature of the air and box when the box was sealed.

    • andre8
      Subscriber

      thanks for answer, 


      I would like to analyze situation when air is closed in box for example 300x300x100mm (thicknes = 10mm), this box is made by welding on full penetration six pieces of plate, next it will stay for a month in temperature 20C so we can assume that temperatur steel box and air will be the same, during welding we will have a smal hole (radius = 5mm) in one plate, closed by welding when temperature of steel box and air reaches the value 20C. I'm aware of welding stresses, but I would like to omit them and consider only influance pressure from expansion air after heating, considerated in the same time expansion of steel box (non-linear material).


      I would like to analized sucha a situation in ansys, I builded model of box in this program in analysis system "Static structural" but I think that in this system I can't analyzed situation with air closed in box

    • peteroznewman
      Subscriber

      You didn't say if you knew about the Ideal Gas Law.


      The volume enclosed in the box is 9 liters (assuming the thickness is on the outside).


      To make this simple, assume the box doesn't change shape when the temperature increases. When we do the analysis, we can measure a small volume change due to the sides of the box bowing out very slightly due to the pressure, but with a 10 mm thick wall, that is going to be practically zero. At 1 mm it may have been measurable. However, we can easily estimate the volume increase in the box due to thermal expansion.


      The coefficient of expansion of steel is 1.2e-5/C and the temperature change is 280 C so the box grows to 9.091 L because 30 cm grows by 1.2e-5*280*30 = 0.1008 cm and 1/3 of that for the 10 cm dimension.


      Temperature in the Ideal Gas Law must be in Kelvin, so the two temperatures are 293 K and 573 K.


      Atmospheric pressure is 101325 Pa, which is what we assume was the pressure when the box was sealed.


      The Ideal Gas Law is used to calculate the pressure of the air when the temperature increases from 293 K to 573 K while the volume increases from 9 L to 9.091 L.



      ANSYS Static Structural is interested in the pressure difference between the inside and the outside of the box, so subtract off the atmospheric pressure.


      If you apply a pressure to the inside of your box of 94845 Pa, that will generate the stress caused by raising the temperature of a sealed steel box containing air (or any gas) of the dimensions given from 20 to 300 C.


      Regards,


      Peter

    • andre8
      Subscriber

      Thanks a lot for your help


      Regards,


      Andre

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