Heat transfer between I beam and environment

[barthez89]

Hi, I've got a question about heat transfer. I'm doing thermal-structural analysis in Patran and I'm not sure what boundary conditions I should use in thermal analysis.
I want to analyze displacements in I beam in different ambient temperature. Should I define convection or radiation in this situation (or maybe both)? And if both types of heat transfer occurs is it too big simplification to assume that radiation would not be analyzed. I would prefer not to use radiation becouse I've got a problem with defining proper view factors.

 

[prex]

Assuming a steady state calculation and if there is no heat source, you don't normally need the heat transfer coefficients between steel and air, just assume the beam is in equilibrium with the ambient.
Otherwise I usually take 10 W/sqm/degC over the whole exposed surface (in still air, the double in windy environment): this value includes the radiation contribution (about 50% of the total).

prex
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[corus]

For a relatively simple analysis then prex's approach is correct, and the value of htc is about right for a transient thermal analysis. You could be more accurate and have different values of natural convection depending on whether the surfaces of the I beam are vertical or upward or downward facing. Again, radiation can occur between surfaces of the I beam as well as to the ambient, and you could determine view factors based upon surfaces that were at right angles to each other. I'd only go to this level if the beam was particularly hot, say if it had just been rolled, and radiation would have a significant effect. If it's just an I beam in the open air, used in construction for example, then just set it to the ambient as prex suggests.

 

[boyze]

Can you tell us about the heat sources and your requirement?

One can have radiation with little convection and conduction and of course the other permutations and combinations.

With the heat source are you interested in steady and/or transient conditions? For the beam are you interested in thermal induced deflections and/or stresses? If the temperatures are high enough then you can get into the material property dependency and even perhaps the load dependency issues.

If this is an I beam in a open building with varying ambients then as suggested you can probably just assume the beam is at the ambient. On the other hand if the beam is supporting a piece of high precision equipment in a high temperature radiation chamber then a more sophisticated approach might be warranted.







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[barthez89]

There is no heat source. Just I beam in the open air. And for now I'm interested in steady state analysis. So what BC I should exactly create in Patran? Should I choose convection with option: To Ambient and just define ambient temperature? Sorry if I don't get some obvious thing that you wrote but English is not my mother tongue so I could miss something.

 

[chicopee]

Well, if you beam is along the equatorial belt, radiation would be a problem in addition to convection; if it is in the shade, radiation would not be an issue; if it is potentially exposed to the element of fire, then radiation may be the prevalent factor. For now the rule provided by REX seems reasonable for lack of specificity.

 

[corus]

If it's in open air and at ambient then there's no need to run a separate thermal analysis. Just set the temperature everywhere to ambient as part of your loads.

 

[barthez89]

I changed my assumptions a little. Now instead of analyzing just I beam I'm analyzing gantry crane and I've got a question. Is it possible that for 50 degC displacement are smaller than for -30 degC? I've done just structural analysis with temperature as one of loads like corus said. It was steady state analysis on 1D beam element. Gantry crane stands on 4 swivel casters. In 2 of them I took away all DOFs (these are with brakes). Other 2 can move in X and Z axis and rotate in Y axis. There is 30 kN force applied in the middle of I beam. I've attached screens with cross sections of the crane and with deflections. Gantry crane is about 3 meters high, 3.6 meters wide and has 2 meters in depth.

 

[barthez89]

Units are meters, Newtons, Pascals. And I defined material with Elastic Modulus = 2.1E+011, Poisson Ratio = 0.3 and Thermal Expanssion Coefficient = 1.2E-005

 

[corus]

To understand what's going on in a structure it's often better to separate the thermal loads from the mechanical loads and then combine them later. However, in this case you can work it out if you consider the rotational stiffness of the joints in combination with the general thermal expansion to understand the displacement under the dominant thermal loads. For the 50deg C case the horizontal member expands and pushes the vertical members outwards so that the horizontal member would deflect upwards, due to the rotational stiffness of the joint. Conversely the -30 deg C case would cause the horizontal member to deflect downwards as it shrinks in length. When you add in the mechanical load, which would deflect the horizontal member downward at the centre for both cases, the nett effect is to produce lower displacements for the 50 deg C case.

 

[goutam_freelance]

I feel the thermal strains should be measured from the strain free state, which should correspond to the temperature at which the erection is made, unless yielding occurs subsequently due to thermal stress.