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


    Thermal expansion coefficient of air as applied to convective heat transfer


    The thermal exchange coefficient used in calculating the convective heat transfer is dependent on the Rayleigh number.

    For horizental tubes the Rayleigh number is calculated with the following formula


    Ra = gB(Ts-Ta)D^3/va


    In this formula B denotes the thermal expansion coefficient of air.

    The Lugano testers in the report state about this coefficient :


    β[K–1] is the volumetric thermal expansion coefficient, which, for an ideal gas (applied here to air for simplicity) is= 1/T


    Indeed, for ideal gases the thermal expansion coefficient is 1/T.

    While for many calculations air can be treated as an ideal gas, this is not true for it's volumetric thermal expansion coefficient.

    In those cases that the thermal expansion coefficient does not follow the ideal gas law 1/T the data can be found in tables. Such a table also exist for the thermal expansion coefficient of air.

    The differences for air between the tabulated values and the ideal law can be found in the folowing figure.





    As can be seen the difference between the ideal thermal expansion coefficient and the real value can be quite large. The errors are for different temperatures shown in the following table


    ---Temp-----------Error

    ------K----------------%

    ----250------------- -0.8

    ----300--------------12.8

    ----350--------------15.2

    ----400--------------12.4

    ----450--------------11.0

    ----500--------------10.6


    These errors result in an incorrect calculated convective heat transfer coefficient.

    However since in the final calculation of the convective heat transfer coefficient the Rayleigh numer is raised to the power .25 for the temperature ranges used in Lugano, the error will be lower and reduced to a value between 2.3 % and 4.2 %.

    The question is if the Lugano testers applied 1/T for the thermal expension coefficient for simplicity only as they state in their example calculation in the Lugano report, only for the calculation of the dummy run, or for all calculations.

    If they used 1/T throughout then their calculated convective heat transfer was somewhat under estimated.

    Lugano rods stacking dimensions


    In order to calculate the convective heat transfer correction factor for the Lugano rods, we need to know what the distances between the tubes are.

    I had thought that the distances between the tubes would still be rather large, in the range of 5 a 6 mm.

    However if we look at the following picture where we have drawn the outlines of the tubes, we can already see that distances between the tubes are rather small.





    To calculate the distances in an accurate way, a picture of the set up of the rods was loaded in a CAD program.

    Using perspective information derived from the support frame to correct the view of the rods to a front view I was able to reconstruct the distances between the tubes.

    Since only the placement of 2 tubes could be determined, it was assumed that the third one was symetrically placed with respect to the vertical axis.

    The calculations showed that the rods where equilateral stacked with 1 mm spacing between the rods.


    In principle we now could have calculated with the equivalent diameter method the correction factor (see earlier post in this thread).

    However since the equivalent diameter method is intended for calculating the correction in bank of tubes, where the distances between the tubes are still rather large, I wonder if that method gives accurate results for such small distances.

    The intention is now to simulate a single tube and also three stacked tubes in a CFD (Computional Fluid Dynamics) program in order to calculate an accurate convective heat transfer correction factor.

    As stated earlier I still have to gain enough experience with the CFD program to do that, so it can take a while before there will be any results (Or maybe no results at all).

    I am modelling some predictions for a new test rat device. It looks like a smooth alumina cylinder of 10 cm long x 1.5 cm diameter will be a useful size, although I may increase the diameter to 2 cm for a better IR target. No ribs, end caps, rods, etc.


    Preliminary calculations show a potential Lugano-Style COP of 4.9 at a real tube temperature of 730 C, although I am still working out the effect of the re-iterations.

    Convection should be limited to about 38 W while radiant power should be about 145 W (real), while the Lugano Protocol should appear to give about 70 W convection and 820 W radiant power at a supposed 1400 C.

    This is a bit sketchy since the heater coil that can easily fit this size is capable (on paper) of 823 W, but a lower power means fighting either a resistance wire length-diameter curve or having very little voltage control (tiny adjustment, huge changes in output).


    I am still checking where the best "COP" boosts are to be gained, by adjusting the geometry of the device and absolute power delivered to it and temperature out of it. I would like to get a "COP" of 12, but have to stay below 1600 C (IR).

    I am wondering what the purpose of your test is.

    We know already that wrong emissivities used on the Optris will yield positive COP's for heater coil type devices.


    Nevertheless if it is any aid for you then If you can supply a dimensional drawing of your device with also the dimensions and position of the heater coil I could make a CAD model of it and use that for a FEM thermal simulation.

    That would give us also an idea how well the FEM modelling software fits the experiments.

    The next days till after the weekend I am off to Berlin, but could maybe after that find some time for it.

    LDM ,

    I am just having some fun. Optimizing the Lugano effect is one good way to understand it.

    The plan, in flux, is a smooth alumina tube 10 cm x 1.5 cm, with a 16 ohm 24 ga Kanthal coil wound at 9mm ID with 1 mm spacing between coils, for a 1cm OD diameter coil, under 2.5 mm of alumina.

    It looks as though I will increase the coil and/or alumina diameter, maybe shortening the overall length in the process.

    I am using a simple online convection-radiation calculator to estimate heat flux, and steam-engine.org for the coil design.

    Between the two, it should be feasible to estimate approximate tube performance at various temperatures.

    Using the Optris software and several MFMP .rav files, I have converted the emissivity-temperatures from a (good enough for now) 0.95 emissivity and various temperatures to Lugano equivalents (using Bob H's Lugano Plot 1 transcription from the MFMP report), and will only re-iterate once I settle on a best performance design, since it takes a lot of time to feedback temperature-emissivity changes.


    The overall plan is to build as simple, durable, and widest temperature control range as possible alumina heater with the highest possible Total-Spectral emissivity conflation COP while staying under 1350 C real T and 1600 C conflated T (the maximum reliable thermocouple and IR pyrometer temperatures. (Note that the Optris IR camera cannot display higher than 1524.7 C, so I may try and stay below that, but leave room to go a bit higher)


    https://www.thermal-wizard.com/tmwiz/default.htm


    http://www.steam-engine.org/co…30&awg=24&id=9&ll=10&ws=1

    Sooo...

    What happens is this... At around 860 C real temperature, regardless of the physical dimensions and input W, the conflation protocol hits a wall at a "COP" of about 5.5 .

    At this point I get a conflated IR temperature of 1600 C, the limit of my pyrometer, and well beyond the Optris camera capability.


    So what is required to go beyond a conflated COP of 5.5 are new spectral radiance measurement equipment, a material with a lower Total emissivity, and new materials in general to withstand the heat (or appear to withstand the heat of beyond 1600 C).

    It needs to be hotter, to get the best T^4 - Power bang for the buck.

    This looks, to me, something like the path to a QX or SK device.

    Conflate a plasma color temperature with an actual temperature, and voila!, COPs in the hundreds or thousands, and thousands of degrees C become possible...

    Quote from Shane D.

    Looks like Para has achieved 5 Sigma...congratulations! Have you started building the robotized factory to mass produce your masterpiece yet? :)


    The robots of course cannot be programmed until the final design is finalized, and certification has been received, however I can say that I have started towards something that could be construed to be a factory in some sense of the word factory. Now enough flapping of gums. Lavorare!

    Changing the planned heater to 5 cm long, 3.5 cm diameter with a 2.24 cm diameter coil, with a coil loop separation of 1.8 mm so the coil won't spit the outer alumina off the inside-the-coil alumina (hopefully). Plan also has 10 cm of coil leads (legs) doubled over, twisted, and bent into then out of the coil interior at each end to act as supports and electrical connection, so that no pedestals are required.

    So annoying.

    Wind the 24 Ga Kanthal over 22.4 mm, and the coil springs open to something like 32 mm OD, and the coil spring spacing jumps to 2.5 mm from 1.6 mm. Annealing the wire doesn't help.


    Wind the 24 Ga Kanthal over 14.4 mm, and the coil springs open to 15 mm OD, and the coil spacing stays at 1.6 mm if I wind quickly. If I wind slowly, the coil spacing increases, but the coil diameter stays the same...


    So to get the desired ~2.5 mm OD coil, I now need to make a third mandrel, guessing at where and how much this critical bend stiffness effect occurs, or I must change overall designs yet again. The 15 mm coil is reasonably stiff, so I will probably stick with that one.


    Oh, and I drilled another 1/16 hole 1 cm further up the shaft of that 7/8 bolt with the slag or whatever in the middle, and it went through no problem.

    • Official Post

    I have (for my sins) wound several hundred 0.9MM Kanthal coils by now. I do them under power using a lathe running in low gear. It took me a while to develop the technique. You definitely need an undersize mandrel, as you have discovered. I use a 15mm one for an 18mm coil and a 22mm one for a 30mm coil. The secret is to keep tension on the wire while you wind it, the more the merrier.I use a hand-held piece of broom-handle with a thin saw-cut in it, threading the wire through the cut. Squeezing the cut ends together controls the tension somewhat, and I can direct the wire along the mandrel easily. I have never made a video of this method though, as unless you are used to machine tools you are 'asking for trouble'.

    Alan Smith ,

    The coils for the Slab stayed so tight to the all-thread they were wound on, I had to twist the coil a bit to get it to expand a little, and at the same time thread the rod out of the coil.

    It seems to be a wire diameter vs how tight the turn radius is that affects the coil expansion rate.

    Most of the other coils I made were wound over top of alumina rods, and I just kept each end of the coil clamped in place and then fired them up to take the springiness out of them. At orange hot, the coils could be pushed around with a thin alumina rod a bit to even them out if needed. Of course this means the coils are energized and really hot, so being both careful and quick was necessary.

    The broom handle idea sounds useful. I was just gripping the wire with leather gloves on for tension, but I could imagine that garroting a finger off could happen if things went wrong.

    • Official Post

    Here's one on 'unpowered winding' I made a few years ago.




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    Here is the apparent 8-14 micron emissivity of Durapot 810 from one of two tests, which were essentially the same (this one has more steps).

    The thermocouple was embedded in the surface layer with the tip at the wire thickness depth, so it was just visible.

    In my opinion, the IR pyrometer (rated -50 C to 1600 C) is not very stable around 20 C, and works best (repeatable) above 45 C.


    Edit: 277 .9 C corrected to 257.5 C in the spreadsheet. (277.9 C was the internal temperature for that step)

    LDM ,

    Could you please produce a table, similar to the one I posted above, with your calculated emissivities for alumina at various temperatures?

    I would like to compare the temperature variance of the alumina (paint?) to the Durapot at similar temperatures.

    I won't bother with calculating power using the results, I just wan't see what the temperature difference might be.

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