Transient heat transfer problem
Transient heat transfer problem
(OP)
Hello
I'm performing transient heat transfer analysis and since i've never done it before I have encountered some problems. The task is to compare the results made in FEA software to an experiment. I have a low carbon steel plate ( 0.2x0.1x0.01 m dimensions ) which is excited by a laser on a top surface ( 30 Watts in 0.1s , laser spot radius = 0.0045m ) and then thermal imaging camera aquires image.

Then Im doing analysis in MSC Marc software.
Here's data :
Model units : m
MATERIAL PROPERTIES :
Thermal conductivity = 54 W/m*C
Specific heat = 465 J/kg*C
Density = 7863 kg/m^3
Intial temperature = 23.7586 C
Heat flux = 30W / Area , where Area = pi * 0.0045^2 which gives 471570 W/m^2
Surface heat flux = 471570 * exp(-((time-0.95)^2)/0.01) - assuming gauss distribution function

Natural convection coefficient = 10 W/m^2*C
Ambient temperature = 23 C
Film applied on whole top and bottom surface

Analysis type transient with 200 increments, which gives constant time step 0.008s , total time 1.6s
Finally , im comparing the temperatures 5 mm away from the heat source and here's what i get :

I've expected the result to be different, but not that much....
I'd be very grateful if someone could verify input data / units and provide me guidance what could be wrong here.
Regards
Mike
I'm performing transient heat transfer analysis and since i've never done it before I have encountered some problems. The task is to compare the results made in FEA software to an experiment. I have a low carbon steel plate ( 0.2x0.1x0.01 m dimensions ) which is excited by a laser on a top surface ( 30 Watts in 0.1s , laser spot radius = 0.0045m ) and then thermal imaging camera aquires image.

Then Im doing analysis in MSC Marc software.
Here's data :
Model units : m
MATERIAL PROPERTIES :
Thermal conductivity = 54 W/m*C
Specific heat = 465 J/kg*C
Density = 7863 kg/m^3
Intial temperature = 23.7586 C
Heat flux = 30W / Area , where Area = pi * 0.0045^2 which gives 471570 W/m^2
Surface heat flux = 471570 * exp(-((time-0.95)^2)/0.01) - assuming gauss distribution function

Natural convection coefficient = 10 W/m^2*C
Ambient temperature = 23 C
Film applied on whole top and bottom surface

Analysis type transient with 200 increments, which gives constant time step 0.008s , total time 1.6s
Finally , im comparing the temperatures 5 mm away from the heat source and here's what i get :

I've expected the result to be different, but not that much....
I'd be very grateful if someone could verify input data / units and provide me guidance what could be wrong here.
Regards
Mike





RE: Transient heat transfer problem
RE: Transient heat transfer problem
In general though, you should use symmetry to reduce the model size. With a smaller model you could then put more elements into the centre and surrounding region, and through the thickness, to reduce the mesh size. Also use a very small initial time step, and allow automatic time steps. It may be that the constant value you've chosen isn't small enough to capture the change in temperature for this severe transient.
RE: Transient heat transfer problem
5 mm away in which direction?
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RE: Transient heat transfer problem
It has an eliptical shape, because camera is mounted near laser at some angle so image is a bit deformed. I've done some math and converted the distance from pixels to mm , the point that is marked is 5mm away from the center. Now , i've increased to power from 30W to 178W which gives flux = 2806947 W/m^2 ,reduced gauss function width by half to adjust the amplitude and here's what i get :
For now it's a bit sloppy ( I will adjust it later ), but now, since it's pure conduction , how can I adjust drop in temperature to fit experimental result?
Regards
Mike
RE: Transient heat transfer problem
In general though, the experimental results show the typical saw tooth variation in temperature, where temperatures increase rapidly and then tend to some steady state value before falling away quickly as the heat source is removed. Your FE results don't show that same variation at all even though you've increased the heat source some how to achieve the same peak value. I'd advise getting the geometry right and I'd still advise refining the mesh to see how that effects the results. In general the heat source causes a local surface change in temperature, and with only 5 elements through the thickness you're not going to pick that up accurately. That may affect results along the length of the plate.
RE: Transient heat transfer problem
How did you account for emissivity and reflectivity, which are not necessarily equatable, since your laser is only a single wavelength, but emissivity is broadband.
One obvious problem is that you somehow decided to make the time step and the camera frame time the same, which is not correct. The camera only records at 60 Hz, but that does not mean that the physical process does not proceed in between the frame exposures. Your time step is forcing all the action to be locked into the 60 Hz time step, but that's unrealistic.
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RE: Transient heat transfer problem
RE: Transient heat transfer problem
I also forget to mention that the steel sample is painted in black .
RE: Transient heat transfer problem