CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Heat transfer coefficient from CFX

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   September 14, 2007, 01:51
Default Heat transfer coefficient from CFX
  #1
Sulfi
Guest
 
Posts: n/a
Hi all,

I am trying to find out the htc distribution in a 3d aerfoil used for a axial turbine. The usual trend of htc is maximum at the leading edge and then reduces down as we go away from LE to the TE. But i am getting a reverse trend. I am prescribing aerfoil wall temp as 1000F and the inflow velocity is 1126F. Any suggestions why CFX is giving such an inverse trend(The qflux is also max at the trailing edge)?. Do i need to use the tbulk for proper prediction?

Thanks, Sulfi
  Reply With Quote

Old   September 14, 2007, 10:19
Default Re: Heat transfer coefficient from CFX
  #2
Glenn
Guest
 
Posts: n/a
The HTC profole characteristic from LE to TE of an airfoil will depend on the shape of the airfoil. It is not uncommon for the HTC magnitude to rise as one traverses aft from the stagnation point (line). For example, on the suction side of an airfoi, the flow may be rapidly accelerating immediately aft of the leading edge and could easily result in an HTC which rises over this region above the stagnation HTC value. Look at your flow field around the airfoil shape and see if this may be the case. Are you sure the HTC should be monotonically reducing as you progress aft of the LE? Why?
  Reply With Quote

Old   September 14, 2007, 19:16
Default Re: Heat transfer coefficient from CFX
  #3
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Also, as Sulfi mentioned the wall heat transfer coefficient is based on a local estimate of the fluid bulk temperature which often varies by quite a bit over the domain. Activating the expert parameter mentioned will fix the bulk temp to a defined temperature and this is how most wall heat transfer coefficients are reported in the literature.

Glenn Horrocks
  Reply With Quote

Old   September 15, 2007, 08:02
Default Re: Heat transfer coefficient from CFX
  #4
Sulfi
Guest
 
Posts: n/a
Glenn, Thanks for the reply! From journals, As Glenn pointed i have come across couple of cases where the htc at suction side is more than the the LE htc value. I would like to validate my case for building confidence. The difficulty here is that i am using steam properties. Could you suggest some open literature where i can get some data for 1st stage nozzle validation for steam turbine.

Thanks Sulfi
  Reply With Quote

Old   September 18, 2007, 19:18
Default Re: Heat transfer coefficient from CFX
  #5
Wooster
Guest
 
Posts: n/a
Regarding the Bulk temperature setting, what would be a good approximation? I'm looking for a HTC in one of my models as well and I've been wondering how the bulk temp effects the thermal problem. Thanks!

-W
  Reply With Quote

Old   September 19, 2007, 01:04
Default Re: Heat transfer coefficient from CFX
  #6
sulfi
Guest
 
Posts: n/a
You need to set the Tbulk as inlet temperature.

Sulfi
  Reply With Quote

Old   September 19, 2007, 18:17
Default Re: Heat transfer coefficient from CFX
  #7
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Wooster - the Tbulk temperature does not affect the simulation, only the way the HTC is reported in the results file. In the simulation heat transfer at the wall is calculated directly as a heat flux with no direct use of HTC's. The HTC is calculated from the wall heat flux.

Sulfi - You can set the Tbulk to be anything you like. Inlet temperature would be a good guess for most applications.

Glenn Horrocks
  Reply With Quote

Old   September 20, 2007, 13:52
Default Re: Heat transfer coefficient from CFX *NM*
  #8
valypoor
Guest
 
Posts: n/a
  Reply With Quote

Old   September 24, 2007, 12:51
Default Re: Heat transfer coefficient from CFX
  #9
wooster
Guest
 
Posts: n/a
Thanks guys.

I'm looking for an easy way to just select the heated surface and figure out a heat transfer coefficient for a numerical model. Looks like I'll have to setup an equation in Pre for later use in Post. I'll play around with the bulk temp and see how that works for me.

-W
  Reply With Quote

Old   September 25, 2007, 06:31
Default Re: Heat transfer coefficient from CFX
  #10
sulfi
Guest
 
Posts: n/a
Hi,

Anybody aware of validation database(or journal where we can find the standard airfoil information) for htc around Gas turbine airfoil.

Thanks in advance, sulfi
  Reply With Quote

Old   September 29, 2007, 21:41
Default Re: Heat transfer coefficient from CFX
  #11
Pete
Guest
 
Posts: n/a
The initial guess for tbulk does effect the reporting of the HTC in POST. Could this make a difference if you are using CFX convection for your thermal analysis?
  Reply With Quote

Old   March 23, 2011, 15:10
Default
  #12
New Member
 
Join Date: Jan 2010
Posts: 13
Rep Power: 7
Karkoura is on a distinguished road
Hi,

I am having the same problem. Could you please let me know where you can modify the Tbulk in CFX?

Thanks
Karkoura is offline   Reply With Quote

Old   March 23, 2011, 17:36
Default areaInt(Wall Heat Transfer Coefficient) problem
  #13
New Member
 
Kojot
Join Date: Mar 2010
Posts: 4
Rep Power: 7
karolrecko is on a distinguished road
Hi everyone!

I have a problem. Why if I set function in CFX Post:

areaInt(Wall Heat Transfer Coefficient)@wall

I get a bad unit:

[kg m^2 s^-3 K^-1]=[W/K]? Where is a m^2?

How is Ansys calculated this?

best regards

KR
karolrecko is offline   Reply With Quote

Old   March 24, 2011, 10:36
Default
  #14
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 495
Rep Power: 11
singer1812 is on a distinguished road
??

[W m^-2 K^-1] = [J s^-1 m^-2 K^-1] = [N m s^-1 m^-2 K^-1] = [kg m s^-2 m s^-1 m^-2 K^-1] = [kg s^-3 K^-1]

Integrated over an area

[kg s^-3 K^-1]*[m^2] = [kg m^2 s^-3 K^-1]

which is what you got.

Sure you didnt want areaAve(h)@wall or areaInt(h)@wall/area of wall?
singer1812 is offline   Reply With Quote

Old   March 24, 2011, 16:02
Default
  #15
New Member
 
Kojot
Join Date: Mar 2010
Posts: 4
Rep Power: 7
karolrecko is on a distinguished road
Thanks for your reply!!

I calculated flow over a cylinder. Cylinder has a 294K and fluid has 293K.
I cut the cylinder in mean hight by plane and take polyline by boundary intersection- wall of cylinder and plane (green line on the picture)

http://www.fotosik.pl/pokaz_obrazek/...204239445.html

Difference temperature between cylinder and fluid is 1K, so

q=h(Twall-Tfluid)
Twall-Tfluid=1K
so
q=h

why if I take this chart:
http://www.fotosik.pl/pokaz_obrazek/...9266fa28c.html
it isn't the same like this:
http://www.fotosik.pl/pokaz_obrazek/...1ae6c5eb3.html
karolrecko is offline   Reply With Quote

Old   March 24, 2011, 16:14
Default
  #16
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 495
Rep Power: 11
singer1812 is on a distinguished road
Did you specify Tbulk as 293K or are you using first node temperature for your h?

If you just specified 293K as the fluid inlet, and asked for a wall h, without specifying the Tbulk, CFX will use the first node temp for its h calcs, which probably wont be 293K, it will be something higher.

That would drive your delta T down, in which you would expect your charts to show a higher value for h, which it does.
singer1812 is offline   Reply With Quote

Old   March 24, 2011, 16:21
Default
  #17
New Member
 
Kojot
Join Date: Mar 2010
Posts: 4
Rep Power: 7
karolrecko is on a distinguished road
I set the Initialisation domain temperature - 293K and fluid inlet 293K. that's wrong?

Where I might specify the Tbulk temperature?
karolrecko is offline   Reply With Quote

Old   March 24, 2011, 16:25
Default
  #18
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 495
Rep Power: 11
singer1812 is on a distinguished road
Your initializing just fine.

But if you want a fixed Tbulk for any h calcs you need to set that Tbulk. It doesnt have to be fluid temperature, you can set it to whatever you want.

Go into the expert parameter section under Discretisation Tab>>Miscellaneous>>tbulk for htc to set the Tbulk to a fixed value.
singer1812 is offline   Reply With Quote

Old   March 24, 2011, 16:47
Thumbs up
  #19
New Member
 
Kojot
Join Date: Mar 2010
Posts: 4
Rep Power: 7
karolrecko is on a distinguished road
Thanks a lot!! Problem is sloved.Singer1812, I owe you one.
karolrecko is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Heat transfer coefficient - what is waht Stan FLUENT 20 March 11, 2014 11:42
BOILING HEAT TRANSFER COEFFICIENT John CFX 3 April 4, 2012 17:50
calculate heat transfer coefficient in repeated geometry mullenc525 CFX 2 December 21, 2010 14:57
Distribution of surface heat transfer coefficient along the axial direcion of a tube andred FLUENT 0 November 16, 2010 22:13
negative heat transfer coefficient Sigrid Andreae CFX 1 February 26, 2005 08:13


All times are GMT -4. The time now is 21:54.