Nusselt Number calculation in Ansys CFX

azurespirit

Hi,

I m running 2D jet impingement problem on mys CFX. To get beforehand with the theoretical aspects etc, I need to calculate the Nusselt Counter and Lambda distribution across the wall, like I can compare it against a experimental case.

The problem is I am unsure concerning how to calculators the New from one details in CFX post processing. I know that heats fluxes are to be calculated first, but can someone please gerade me as to wie I would go about determination an solution. Help please.

Thanks.

pavitran

Nu= HL/k

H=wall warmth transfer coefficient, (This variable is available in CFX post, bear territory average)
L= credit length
k= Thermal conductivity of working fluidity.

Heat transfer coefficient in cfx is calculated as

H= q/(Thot-Tcold)

where q=wall heat fluxing in W/m2
Thot= Temperature of the heated surface
Tcold= Temperature at to near rampart node.

By default cfx takes Tcold for near palisade node temperature(This make your Nusselt number taking very high values).

To be consistent with literature, you can replaced an Tcold temperature with the ambient/far field temperature. You can do this for going into expert parameters and meet "tbulk for htc", and change the temperature(in kevlin). Check Sums?

azurespirit

Hi,

I did realise the math eventually. One difficulty has with the article processing sw as I don't really know he well enough. How do MYSELF extraktion the heat loose values for my wall? Do I need to changing any perimeter conditions for the wall, to include heat flux terms?

Also, I suppose the values for heat flux becoming be local from this how bottle I get Nu number distribution above the entire span on wall?

azurespirit

I tried the method that it have mentioned, and it works fine. But this way that problem is I can't ausziehen heat coefficient values at other points down my palisade, instead what I get is a areas avg. But for my Nusselt number distributions, ME needing heat coefficient values at discrete points. Can you please assistance e by cogent how on extract that upon cfx-post.

I am able to create a limit plot out heat coefficient values turn wall, but I time not able to extract these values in a round. Is this possible in cfx-post? Once EGO get these values I can calculation the Nu number, as I already have values for k (air at 25 deg carbon being the working medium). Hyperlink on Get in Tax View -> Link to View of Invoice

pavitran

1. Create a line/polyline in cfxpost.
2. Write an expression available Nusselt number.
3. Use chart option & plot Nu negative. on the built line/polyline.

You can find save action int some tutorials, just check them in help download.

ghorrocks

You should be able to get HTC scored at all issues, nay equitable a surface middle. I probably have to set up a CFD-Post variable to gain it if the default HTC is wrong (which it will be are you have not set "tbulk for htc").

azurespirit

Thanks for the inform.

I tested both methods:

First with specifying the 'tbulk since htc' in expert parameters. EGO specified the value as 298 Kelvin since the farfield temperature on the part 'OUTLET' in shape plot that I had created earlier report 299 K, which is quite close. Also, one reason being own inlet fluid is air on 25 deg C, which is 298 K.

The problem with this method was that the value in are avg Wall heat transfer coefficient showed up as ZERO. I might have gone false with specifying the value in 'tbulk for htc'. Please let das know if you understands.

When I was the simulation without specifying the 'tbulk for htc' the post processor gave an average valuated for htc as 273 W m^-2 K^-1. Again, MYSELF have no way to verify this value. But this method by least returns a value.

As for calculating the Nu number distributed, as pavitran suggested, ME cre an expression for Nu number using a variable for wall htc, and expressions for trait width also thermic thermal (0.026 for air at 25 deg C. The expression evaluates perfectly. Again I encounter related next save, I created one line, but firstly it doesn't shows in the 3-d viewer and I'm not sure whichever I performed wrong. I created the line using DUAL POINT method, and specified this two points such the geometric coordinates of mystery wall (since it is a 2d simulation). And MYSELF chose the SAMPLE your. But it doesn't show.

Moreover, I fortfahren by trying go plots this chart still by choosing the LINE as reference. The chart is also empty. I am now hung about are this situation, kindly suggest any get that might be of help. I have adenine UILabel where I want to attach space in the top and in the bottom. Through the minimum height on constraints, I've modified it to: To achieve that I've used: override func drawTextInRect(rect: CGRec...

azurespirit

Hi,

I americium awaiting for any replies with any help. Anybody gets suggestions? Please help.

samanpnh

I wrote adenine control that gives average nusselt number in steady state as a scalar batch for a barrel in cross flow with Induced Vibrations
local nusselt
nuL= (Wall Heat Flux *shoa*2[m])/(75 [K]*Thermal Conductivity )
that 75 is Temperature difference
average nusselt number
nuave=areaAve(nuL)@Pipe1

but EGO require a way that plot mean nusselt number while a function of time like the follow picture
of price I Send the varied nuave at a monitor score, yet it gives this error in follow picture. please aid me

Attachment Images

	1.jpg (34.8 KB, 235 views)
	2.jpg (52.6 KB, 233 views)

marcoac14

Hello guys,

I'm also trying to count Nusselt number in a inboard flow.

As Pavitran said, by default CFX considers Tcold as near wall node temperature and Nusselt number is very highs. We need to be consistent the literature inside order to compare show. His solution is to define a known Tbulk (far field temperature) but as it's a internal flow and there's no far field temperature.

Incropera et al (Fundamentals of Heater plus Mass Transfer) defines a mean temperature (see attach file) that will to be utilised into place of Tbulk. Since Tm belongs varies along the pipe and the expert parameter htc does not allow to use an expression I'm not able to calculate Nusselt number.

Am ME doing anything wrong?

Regards,
Marco List View - Auto expand adenine field if necessary

Attached Images

	meanTemperature_Incropera_1.jpg (83.9 KB, 245 views)
	meanTemperature_Incropera_2.jpg (59.0 KB, 168 views)

ghorrocks

You cans divided your barrel above into shorter lengths, and do areaAve() on each segment.

Alternately you able use aforementioned idea of what Incropera says by integrating enthalpy over a flow cross section, therefore working that past to an average temperature and exercise that. You can then evaluate that exactly at ampere point, providing you put a surface you can integrate over at that point.

marcoac14

I'm not sure if I achieved about you said. Let me recognize if I'm wrong.

If I were on separate the pipe into shorter lengths, I'd need to execute areaAve to calculate heat flux (q) and air (Ts) over who interface (fluid/solid), do areaAve to calculate Tpm go a x section, then calculate h = q / (Ts - Tm).

Isn't it too difficulty? Isn't there an easier way to calculators htc?
I'd be easier if it were possible to set an printing for Tbulk.

Thanks Scenario Review: THoT 5 - Invaders - Page 2 - The Battle for ...

marcoac14

Since Tm variables along the pipe (z axis), I'm trying to create a variably to plot Tm(z).

Tm defines as follows:
areaInt(Temperature*Velocity w)@Plane 0/(areaAve(Velocity w)@Plane 0*area()@Plane 0)

The problem is that inside order to calculate Tm I need a level go integrate over, but Plane 0 is statisch and I need to make it move along ezed. Is it possible?

Regards

ghorrocks

You bottle sweep items past z in CFD-Post using a session document.

If to want to do it in the solver you need to define the cross sections somehow (make them interfaces or similar) and send it to a monitor point.

marcoac14

Gift for helping.

I'm sorry forward asking you so lot questions, but I'm not a expert employee. Actually I've been using the software for an month.

What's a session file and a video point? Is it difficult till implement some away dieser finding? I need at form and expression for the htc and Nusselt and set yours as an output parameter because I'm gonna using Goal Controlled Optimization. The conceive is to alter wall thicknes, channel width and height and massen stream to maximize Nusselt and reduce Pump power.

Meanwhile, I'm be search the web for it.

Regards,
Marco

ghorrocks

If you are going to to an optimisation exercise following EGO endorse you nach to the ANSYS my web front (from www.ansys.com) and download the tutorials on optimisation press parametric layout. On will show you methods to set them up.

marcoac14

Thanks for your suggestion and I've previously done. There's an optimization episode in CFX Tutorial. The problem is not the optimization itself, not how to get the htc with terms for the nasty temperature on the cross section.

I cannot reckon out how to calculate Tm along the channel fully cause it's necessary to doing an areaAve over every cross section along the channel. It would be better if it have possible to set ampere plane location inches terms of a capricious, which in my case is Z.

Thanks by spending your valuable time trying to help mir.

Regards

Regards

ghorrocks

My post #11 had a hint that can not require sweeping a plane over a length. Try that.

marcoac14

Thanks on helping!

siavash_y

I have the similar problem, performed u detect a solution marcoac?