Rossi Lugano/early demo's revisited. (technical)

Simulating convection with CFD - update 2

In my previous post I wrote :

Next step now is to do some additional simulations to gain more insight on the effect of using higher levels of meshes (smaller mesh sizes) instead of using a boundery layer. This since if the smallest mesh size is less then the thicknes of the first (smallest) boundery layer, the simulation results are expected to be to be as good as with a boundery layer. (But the calculation overhead much larger due to the greater amount of meshes)

Thus I started simulating the bare tube again using meshing levels 1, 2, 3 and 4 and without a boundery layer. A higher messing level then 4 was not possible due to exceeding the maximum number of meshes allowed for my version of the program.

I did not do a rerun for level 1, since that was already simulated and the result presented in my previous post. For all 4 meshing levels a cross sectional view was made. These are presented in the following figure.

Each case was again simulated with a tube temperature of 445 degree C and an environmental temperature of 21 C.

From the reported heat flux for each simulated case the heat tranfer coefficient was calculated.

The results can be found in the following table :

Level-----------heat transfer coefficient-------------deviation (%)

---1------------------------10.84------------------------------- -18.4

---2------------------------12.84--------------------------------- -3.4

---3------------------------13.03--------------- ----------------- -1.9

---4------------------------13.24--------------------------------- -0.3

The reported deviation is the deviation from the value calculated with the method used in the Lugano report, the value being 13.28

For the smallest grid simulated, the deviation from the value derived from the calculation method used in the Lugano report is only -0.3%

Level 4 simulation with one layer per mesh size

Meshing a ribbed area requires many more meshes then for a bare tube as used in the simulations above.

This is because the area between the ribs must be meshed with a small mesh size for proper simulation of the convective heat between the ribs.

In order to reduce the number of meshes in order to stay within the limits of my program I deceided for the bare tube to do an experiment with one layer of meshes per mesh size instead of the 4 layers as used in the examples shown in the figure above. This reduces the number of total meshes.

The result of that simulation was that the calculation of the convective heat transfer coefficient had for this case a value of 13.28, the same as the value calculated with the method in the Lugano report.

My conclusion is that a level 4 simulation without a boundery layer will give results very close to the values obtained with the method used in the Lugano report.

Lugano ECAT convection correction

Due to the close spacing of the ribs of the Lugano ECAT, the convective heat transfer of the ribs is less effective. Thus there needs to be applied a correction to the calculated convective heat transfer of the ribs.

Without a correction the convective heat transfer is calculated based on the calculated convective heat transfer coefficient for the base tube with a diameter of 20 mm.

The convected power is then calculated by multiplying the convective heat transfer coefficient of the tube by the area of the fins and the difference temperature of the surface and the ambient temperature. The formula :

Q = h x Af x (Ts - Ta)

---------Q--------The power

---------Af-------The area of the fins

---------Ts-------Temperature of the surface

---------Ta-------Ambient temperature

However due to the fins being close to each other and their respective convective heat flows interacting, the convective heat transfer will be less effective and will result in a lower convected power.

Evaluating the above formula it means that the convective heat transfer coefficient has a lower value then one would expect when calculating it based on a bare tube. (which is the basis for the calculation in the Lugano report). The question now is how to determine the correct value of the convective heat transfer coefficient for a finned area as used on the Lugano ECAT.

The method followed was to calculate with the CFD software the convected power for two different section lengths of a finned area, the second section having halve the length of the first section.

The sections where simulated with a temperature of 445 degree C and an environmental temperature of 21 degree C.

Subtracting from the power of the longer section the power of the halve section yields the power of a halve section.

Since for both simulated sections the convected power of the sides at the ends is equal, this power of the sides is then automaticcaly removed by the subtraction and only the power of the finned area is calculated in this way.

Deviding the calculated convected power of the halve section by the fin area of the halve section and by the difference between section temperature and ambient temperature gives the convective heat transfer coefficient.

This convective heat transfer coefficient obtained in this way was found to have a value of 9.99

Since the value for a bare tube of 20mm diameter, tube temperature of 445 degree C and an ambient temperature of 21 degree C is 13.28 the convective heat correction factor becomes :

-------------------------Correction factor = 9.99/13.28 = .752

The conclusion is that due to having the fins next to each other without additional spacing in between, the convective heat transfer of the finned area is lower then one would expect based on the calculation in the Lugano report.

Quote from Paradigmnoia

LDM,

Do you mean that with everything else unchanged, a tube would run hotter with the fins (with a 2 cm valley bottom diameter), than a plain bare tube with a 2 cm diameter, notwithstanding the apparent extra surface area caused by the fins using the fin design used for the Lugano device?

For a bare tube the convective power can be calculated as :

Q = h x At x (Ts - Ta)

---------h--------Convective heat transfer coefficient

---------Q--------The power

---------At-------The area of the tube

---------Ts-------Temperature of the surface

---------Ta-------Ambient temperature

For the finned tube the power is calculated as :

Q = C x h x Af x (Ts - Ta)

---------C--------Convective heat transfer correction factor for the finned tube

---------h--------Convective heat transfer coefficient

---------Q--------The power

---------Af-------The area of the fins

---------Ts-------Temperature of the surface

---------Ta-------Ambient temperature

C, the correction factor has a value of .752

h is for both cases 13.28

At, the area of a bare tube of 20 mm diameter and 200 mm length is 0.0125 m^2

Af, the area of a finned tube is 0.0263 m^2

For the surface temperature we take 445 degree C

For the ambient temperature we take 21 degree C

The calculated powers for both cases are then

Qt-------------70.75 Watt for the bare tube

Qf------------111.36 Watt for the finned tube

Clearly with fins the dissipated power by convection is larger and thus the temperature lower.

As an additional comment : C x h = 9.99, the convective heat transfer coefficient I derived for the finned area.

Hope this explains it for you.

Lugano dummy run recalculated - Final

The close agreement between applied electrical power and the confected and radiated thermal power makes it unlikely that the reported temperatures where inflated

Presented here is the, what I believe final, recalculation of the total convected and radiated power of the Lugano dummy run.

Compared to the first recalculation (post #226) the following changes have been implemented :

1. Stacked rod correction

In my first recalculation I included only on sets of rods while there are two sets.

This has been corrected.

2. Viscosity of air

In the calculation of the convective heat transfer coefficients I had wrongfully used the dynamic viscosity of air instead of the kinematic viscosity of air.

This has been corrected and all convective heat transfer coeffcients in the spreadsheet have been updated.

3. Convective heat transfer correction factor for the ribbed area

Due to having no spacing between the ribs, the convective heat transfer of the ribbed area is less efficient. Thus) a correction factor needs to be applied. This correction factor was, using CFD simulation (see post #389) , found to have a value of 0.752

The spreadsheet now includes this correction for the convective heat of the ribbed area.

The updated spreadsheet is included as an attachment to this post.

For information on additional changes compared to the original calculations of the Lugano report see the comments in the original post #226

The outcome of the spreadsheet calculation is that the difference between applied electrical power and the total convected and radiated thermal power is 1.62 %.

If the reported temperatures for the Lugano dummy run where inflated, then the difference would have been much larger. As such it is unlikely that the temperatures where inflated

Quote from Paradigmnoia

This simulation also shows that the MFMP thermal state report is deeply flawed in its conclusions about the dummy.

I find it a pitty that the MFMP did not cross check their measurements by calculating the convective and radiated thermal power and compare it with their electrical power setting.

If you do that you will see that the convective and thermal power is larger then their electrical power, which is indeed an indication that there is something wrong with their measurements.

Also I did in the past some thermal simulations with my thermal FEM program on my dogbone model and the temperatures the FEM program calculated where much lower then those reported by the MFMP.

For their 500 Watt run, about the power setting of the Lugano run, the simulated temperatures where close to the Lugano ones and thus much lower then those reportes by the MFMP.

What bugs me is what could have been gone wrong with their measurements.

Quote from Paradigmnoia

The reported Dummy temperatures were inflated, based on your work. Just not by much.

What is more interesting is the Dummy peak temperature. It is just below the point where emissivity-temperature errors really become apparent. Personally I doubt that is an accident.

What's important is that if they where inflated, what then the real input power must have been.

So the next thing for me to work on is making a spread sheet based on inflated temperatures and calculate what in that case the total convective and radiated thermal power would have been.

I think that the total power in that case is much lower. (But we will find out)

Quote from Paradigmnoia

In turn, this increases the MFMP Main Tube temperature relative to input power, skewing the simulation.

The simulation I did was on a model of the MFMP dogbone. (without rods)

The remark that the simulated temperatures where near the Lugano ones must be interpreted as that they where much closer to the Lugano ones then those of the MFMP, but indeed higher (skewed) then the Lugano ones.

However those simulations did at that time not take into the latest findings which I incorporated in my spreadsheet.

So If I can find time in the near future I can redo those simulation with my latest findings. But those simulations take a lot of my time.