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GavinBRumble10-Marble
10-Marble
March 5, 2013
Question

Units again, Edith

  • March 5, 2013
  • 4 replies
  • 1817 views
I feel a bit like Archie Bunker sometimes.rehashing tired old complaints.
Here I am looking at a Thermal problem.



The support site (TPI 8663) says this about units:



The following are the consistent set of the units to use when defining the
thermal
material properties in Pro/MECHANICA 18.0 pre-defined material libraries:

Convection Film Coefficient(h):

MNS W/m^2-degC
mmNS mW/mm^2-degC
FPS Btu/hr-ft^2-degF
IPS Btu/hr-in^2-degF



Yet, when I go to enter the film coefficient when using IPS, I see the
following displayed (WF4 thru Creo2):

that is lbf/(in sec F) in case the image is stripped.



Am I to go on faith that despite what is says, the calculations are actually
being performed using Btu/hr-in^2-degF?



Thanks, and can you get me a beer while you're at it?



Gavin B. Rumble, PE

Solid Engineering

336-224-2312




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4 replies

G
GavinBRumble10-MarbleAuthor
10-Marble
March 5, 2013

I feel a bit like Archie Bunker sometimes…rehashing tired old complaints. Here I am looking at a Thermal problem.


The support site (TPI 8663) says this about units:


?) pre-defined material libraries:

Convection Film Coefficient(h):

MNS W/m^2-degC
mmNS mW/mm^2-degC
FPS Btu/hr-ft^2-degF
Btu/hr-in^2-degF?


Thanks, and can you get me a beer while you’re at it?


Gavin B. Rumble, PE
Solid Engineering
336-224-2312

I
iceblink11-Visitor
1-Visitor
March 6, 2013
The calculations are performed in what it says in (Creo2): Home tab,
Setup, Units

The input is performed in whatever you select there.

The output shows what units it is in.


It's always good to do a simple calculation that you can check by hand,
to see if you understand what is going on.


And no more beer for you until you get it right.


Best regards,
Patrick Asselman


C
ChrisKaswer16-Pearl
16-Pearl
March 7, 2013
Gavin,

Yes, it is an equivalent compound unit. Here's how they are related (using Mathcad - I substituted Rankine for Fahrenheit):

[cid:image002.png@01CE1AA0.17E27650]

Cheers,

Chris

Christopher Kaswer
Principal Engineer
Covidien
Research and Development, Surgical Solutions
60 Middletown Avenue
North Haven, CT 06473
(203) 492-7167 (office)
(203) 492-8081 (fax)
www.covidien.com
G
GavinBRumble10-MarbleAuthor
10-Marble
March 7, 2013
Thank you, Sims, for your help. The answer is, if it doesn’t look like a
duck it is definitely not one. Pay no attention to the Help files which say
one thing that is clearly not going to help you
they should just say, “Do
the conversion, and eat your peas”.



The convection coefficient input screen does not give you an option for the
input units, and the units presented (lbf/in sec F
never seen before, but
at least they are presented) are not the “duck” that you found in the heat
transfer tables in your reference book, so you have to convert the
parameters to the units shown. This is actually therapeutic because by the
time you figure out that the $20k software you bought doesn’t do this for
you and your head has exploded, you need a nostalgic little exercise to calm
you down and give you the visual reward of a task well done. Btu’s convert
to lbf-ft and anyone with a name badge can convert the feet to inches and
the hours to seconds.



For the material parameters that come in with the PTC material definitions
you will have the same need to convert the units so you can verify that the
parameter matches the ones in your reference material. The good thing is,
these inputs can be entered in any one of a number of unit systems and the
Units Manager will correctly use them in the computation or convert them if
you change your mind (be sure to select the “convert” option that pops
up)
but you have to enter them correctly for the units you select.



I resist the urge to simplify things by converting my USC problems to SI
units because in the end I will be making engineering judgments based on my
years of experience and that experience just doesn’t warn me when the Joules
are too large or the MPa’s are too small.



[Summary of responses follows]



J.H.

No, don’t believe anything, not even me!!! Btu system is not consistent
with IPS system. The Btu system in Mechanica is leftover from the early
days before integrate mode and Rasna thought they were doing the right thing
using more accepted Btu engineering units rather than a true IPS system for
the material data base (at least they didn’t use the old military standard
at the time of mixing metric Watts and inches). When Rasna came out with
the integrated mode they actually had (and still do) two different materials
data bases, the old one for standalone mode using the BTU system and the new
one in Pro/E using true IPS. The materials are not interchangeable. I
stopped using IPS or Btu for thermal models long ago but in integrated mode
the units manager should now take care of it. I would recommend doing a
simple hand calc check case or better yet, since there is now a good units
manager, skip the IPS and Btu system and just use metric.



I’m going to have that beer now




D.L.

If it makes you feel any better, in ANSYS there are no units, and you are
responsible for entering values and interpreting results in a consistent
unit system. In that case, if you enter an H assuming energy units of BTU,
then you need to interpret your results in BTU.

As a primer, remember that Heat/Energy/Work units are expressed in
fundamental units of (force×length) or (power×time). As long as you enter
all "energy" values in "BTU", and interpret all energy results in that
sense, you might be ok. Units for the heat transfer coefficient "H" should
be in (power / (area×temp)), which is how it's displayed in the help file
(power = energy/time). In the input field, it looks like someone converted
to the (force×length/time) definition of power, and simplified the units,
which doesn't help at all in understanding what the heat transfer
coefficient means.

I think you're screwed in terms of understanding what was in their minds,
but you might be ok if you interpret all your results as a consistent set of
units. Run a test case.



P.A.

The calculations are performed in what it says in (Creo2): Home tab, Setup,
Units

The input is performed in whatever you select there.

The output shows what units it is in.

It's always good to do a simple calculation that you can check by hand, to
see if you understand what is going on.



And no more beer for you until you get it right.



Thanks again,



Gavin B. Rumble, PE

Solid Engineering

336-224-2312


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