Another dumb question – some experimental results

Discussion in 'Stove Forum' started by Twoberth, Feb 28, 2020.

  1. Twoberth

    Twoberth United Kingdom Subscriber

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    Following on from the discussion here, I measured operating pressure, fuel consumption and boil times for a No.1 stove and a 210, and got some interesting results!

    Experimental method

    I tried to compare apples with apples, so I chose two stoves with fairly unused burners, cleaned the pipes out on the burners (they were very clean anyway) and fitted replacement jets. The jets were both 0.32mm, as I squashed them in a vice until they were a push tight fit on a 0.32mm pricker. Incidentally the burner pipes have an OD of 6mm and 8mm, so significantly different.

    DSC09481.JPG DSC09473.JPG DSC09474.JPG DSC09476.JPG DSC09484.JPG DSC09485.JPG

    Then I measured the tank capacities by filling completely (tilting the tanks), and then filled each to ¾ capacity.

    The airscrew was removed from the No.1 and a flexible tube fitted with a Schrader valve at one end was connected over the airscrew pipe and over the air vent hole. The hose was connected securely over the air screw vent hole with a crimped hose clamp, which is not shown in this photo.

    DSC09477.JPG

    A spare filler lid was used on the 210 with the air screw removed and a brass pipe brazed over the air hole. The Schrader valve and tube were connected as for the No.1.

    DSC09487.JPG DSC09480.JPG

    Before the tests both stoves and all the joints were leak tested under pressure by dunk tests.

    I did the tests on each stove one after the other, but the procedure was exactly the same. I fired up the stove as normal, then pumped to get a full throttle flame (about 20 pumps) and left the stove to stabilise for 5 mins.

    DSC09499.JPG DSC09505.JPG

    I then checked the pressure with the push on pressure gauge (0-60psi range) and then did the tea test, then checked the pressure again. I left each stove running for 15 mins total, then switched off, allowed them to cool and then drained the stove and check the fuel volume.

    The ‘tea test’ was done by putting 400ml of cold tap water into a cold MSR kettle, and measuring the time to achieve a rolling boil with the lid on. The lid was briefly lifted periodically to check when a rolling boil was achieved. The burner top to kettle bottom distance was the same in both cases.

    DSC09489.JPG DSC09492.JPG

    As you can see from the above flame shots, both burners were operating normally with no evidence of excess fuel/insufficient air mixing. Neither was there any soot on the kettle bottom after the tea tests.

    DSC09508.JPG

    Results

    210

    Tank capacity was measured as 530cc, so the tank was filled with 400 cc for the tests.

    No.1

    Tank capacity was measured as 2.125 pints (1200cc), so the tank was filled with 900 cc for the tests.

    DSC09512.JPG

    Because I was so surprised by the low operating pressures...

    DSC09500.JPG DSC09501.JPG DSC09503.JPG DSC09506.JPG DSC09507.JPG

    I decided to run the tests again without the pressure measuring connections which I thought may be leaking. I replaced the original filler caps and air screws so the stoves were as originally. I got the same results for boil tests and fuel consumption, so no leaks!

    Then I checked the pressure gauge for accuracy on my wife’s car tyres, which I had just pumped to 35psi at the gas station. Reading with my gauge 35psi!

    DSC09510.JPG

    Errors

    My estimates are….

    Error in boiling time measurements +/- 5 secs

    Error in fuel measurements is +/- 5cc

    Error in pressure is +/- 1psi

    Discussion
    Well, I am surprised at the low pressures and the fact that they are equal for both stoves.

    The fuel consumption data I measured are 160cc/hr for the 210 and 240cc/hr for the No. 1. This is similar to those reported by @kerophile here (193 and 279 respectively).

    The boil times for the No.1 are 30% faster than the 210 which is in line with reported power output differences.

    So why does the No.1 kick out more mass flow than the 210?

    PS When selecting the burners, I took the nipples out before cleaning the pipes, and after cleaning I blew through the burners with my mouth. The difference in resistance was very noticeable - even though the burners were relatively unused and clean. I still suspect that this is part of the answer.
     
  2. Tony Press

    Tony Press Australia Subscriber

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    This is both an eccentric and excellent piece of work, @Twoberth. :thumbup:

    The boiling time differences do not surprise me, but your measurements are indeed very interesting.

    Thank you.

    Tony
     
  3. flangset

    flangset Norway Subscriber

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    I find this quite interesting. Thank you for doing and presenting this! Difficult to draw any conclusion though, but accumulating information for further re-thinking. Experiments like this encourage new knowledge!

    I'm not sure if the boil time is significant since they are two different stove designs that might have different interactions between flames and kettle. But that tow stoves with same nozzle diameters that are run at the same tank pressure have different fuel consumptions is a bit difficult to understand. The numbers are there, it must be some assumption I make that must be wrong...

    Now, back to my pizza and beer!
    ;-)
     
    Last edited: Feb 28, 2020
  4. Twoberth

    Twoberth United Kingdom Subscriber

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    Thanks.
    If anyone wants to try to duplicate the pressure experiments I am happy to send them the modified 210 filler cap and the tube with the Schrader valve fitted in exchange for posting the results here.

    You will just need some crimp type tube clips and a push on tyre pressure gauge to repeat the experiment on both the 210 and the No.1 stoves.

    Just let me know.
     
  5. snwcmpr

    snwcmpr Subscriber

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    Are the fuel tubes going up from the tank the same diameter?
    Could hydraulic or pneumatic pressuers be involved in the differences?

    I am just trying to picture ALL of the differences to pinpoint the cause of the difference.

    Ken (Not an engineer)
     
  6. dwarfnebula

    dwarfnebula United States Subscriber

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    One way to try and reduce the variables would be to repeat the test with exactly the same jet swapped between the two burners.

    I would suppose that the vaporization of the fuel results in a dynamic pressure change in the burner, and the smaller expansion volume of the smaller tube slows vaporization or something? That’s all a WAG.
     
  7. Twoberth

    Twoberth United Kingdom Subscriber

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    Good question Ken. At first glance they look markedly different, but they are not. They have the same ID. A 8mm rod just fits down to the base of both tanks.

    DSC09519.JPG DSC09520.JPG DSC09521.JPG

    Most big tank tubes thin down inside the tanks. In fact for the tanks below (an Optimus 00) and a Monitor 2 pinter), the bigger tank has the smaller internal tube.

    NOTE; NO STOVES WERE HARMED IN THE TAKING OF THIS PHOTO.:lol:

    DSC09522.JPG

    Going back to the stoves in question, the burner tubes have different diameters. As mentioned in my experimental section, the ODs of the tubes are 6mm for the 210 and 8mm for the No.1.

    As best I can measure the IDs are 4mm and 5mm at the base of the burners,

    DSC09523.JPG DSC09528.JPG

    but as @JP2 illustrated here and below, the ID within the burner varies from base to jet.


    [​IMG]
     
  8. Twoberth

    Twoberth United Kingdom Subscriber

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    Even though I was convinced there were no leaks, I wasn't happy with the robustness of the flexible tube assembly, so I brazed Schrader valves onto the two filler lids.

    DSC09541.JPG

    Not surprisingly I got the same results.

    Since seeing is believing, here is a short video of the pressure test on the No.1 stove.



    Big roaring Primus, and roaring at only 10psig!

    If I can get there, I will bring these caps along to Newark.
     
  9. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi @Twoberth . Interesting measurement results.

    The internal pressure within the tanks is essentially the same in both 1 pint and 2pint capacity stoves, when operating.

    The jet aperture diameter is identical in the two stoves.

    The power output from the burner of the 2 pint stove is proportionally greater than from the burner of the 1 pint stove.

    The fuel mass transfer of the larger stove must therefore be greater than in the smaller stove burner.

    I would suggest that there is a back pressure from the burner flame, which determines the actual maximum fuel mass transfer which can be achieved by a given burner design.

    The back pressure would seem to be higher in the 1 pint stove burner than in the 2pint burner.


    Best Regards,
    Kerophile.
     
  10. Twoberth

    Twoberth United Kingdom Subscriber

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    @kerophile
    Well yes, it must be something like that. Back pressure and/or flow resistance in the tubes, or......

    In which case Grigoriy’s original post here is incomplete, and there is a third factor controlling mass flux and so five factors that affect power. So I would suggest modifying Grigoriy's original list to...

    stove power depends on 5 things
    1) jet diameter
    2) pressure
    3) burner dimensions and geometry
    4) premixing capacity
    5) vapor produce capacity

    I am pretty sure now that these two stoves, left to their own devices, operate at roughly the same pressure, and for the same reason I am fairly convinced that if I put a No.1 burner on a 210 stove tank I will get the higher power output. So it must be the burner dimensions and geometry. QED.

    However I am uneasy with this, as I don't understand the mechanisms inside the black box of 'stove dimensions and geometry'.

    My original question here was basically 'How can two stoves operating at the same pressure and with the same nozzle diameter have different power outputs'. If the answer is 'because of the burner dimensions and geometry' then we are no further forward.

    Much more we don't understand e.g.

    What happens if I have a burner with the overall size of a 210 burner, but with No.1 diameter tubes, and vice versa?
     
  11. Twoberth

    Twoberth United Kingdom Subscriber

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    George, I was going to finish with your sign off 'aren't stoves fascinating' but when I read it at the end of my post it sounded a bit patronising and disrespectful, which would never be the impression I would want to give you.

    However, you are spot on as usual. Stoves ARE fascinating.
     
  12. itchy

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    @Twoberth

    Wow. Beautiful job and very convincing.

    Totally debunks my best guess. And, I don't see how the pressure in the number 1 burner can be be different than what you have measured.

    One way to increase fuel flow though the same size jet at a given pressure is to reduce the vapor temp and thus increase the density of the fuel vapor. Is it possible that the larger burner while putting out more fuel (and heat) is also operating at lower temperature than the small burner? That the large burner might actually be cooler is counter intuitive to me. But if it was able to more efficiently transfer absorbed heat to the liquid kerosene it might be possible that the vapor also absorbs less heat, and is thus more dense as it exits the jet than it would be in a smaller (hotter) burner.

    Clearly I'm grasping at straws.
     
  13. Lennart F Sweden

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    With the classic roarer the vapour jet stream hits the burner top wich creates a back pressure restricting the flow without affecting the pressure inside the vapouriser.
    A jet closer to the burner top makes more back pressure and it is more significant at faster flow present with higher tank pressure. This would explain why the flame doesn't improve very much when rising pressure above normal on most classic stoves.
    In most blowlamps with open flame outlet there is a back pressure too but only created by air and in some extent the area of the flame outlet.
     
  14. JP2

    JP2 Subscriber

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    Good work
    I was wondering if the skirt of a silent burner make a big difference for the output, how about the flame ring on a roster?

    (The skirt of the silent burner is the most misunderstood part of the kerosene-pressure stove. It's function is often misinterpreted as a kind of wind-shield for the flames. In fact it's something quite different. The skirt collects energy from the flames and conducts it to the lower parts of the burner where a remarkable amount of energy is necessary to vaporize the fuel)

    I wonder because the 2 flames ring used on the test are different.
     
  15. itchy

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  16. Twoberth

    Twoberth United Kingdom Subscriber

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    @Lennart F and @itchy
    Not so fast!

    I was warming to the back pressure theory, not least because two knowledgeable guys (@kerophile and @Lennart F ) were proponents of it.

    However one suggested difference in back pressure with no explanation, and the other suggested difference in back pressure with the explanation 'A jet closer to the burner top makes more back pressure'.

    Well guess what, when I measure the two burners the distance between the jets and the burner top is exactly the same!

    DSC09551.JPG

    In any case I can accept back pressure involved with a nozzle in a pipe, but less so with a nozzle exiting in the atmosphere. This is what I see in my head.

    DSC09555.JPG

    The top half of the photo shows a pipe where the back pressure impedes flow through the nozzle.

    The bottom half of the photo represents a stove burner venting into atmosphere. I suggest that the pressure where I have put 'Back Pressure?' must be less than atmospheric pressure, as this low pressure region is produced by the venturi effect of the vapour velocity exiting the jet, giving rise to an inward air flow through the space between the burner tubes. Very similar to what happens with a bunsen burner.

    Mute point anyway, as there is no difference in jet to burner heights.

    HOWEVER, there is a difference in burner tube internal diameter and vapourisation chamber size.
    Again, pictures from my head, showing a fat tube on the left and a thin tube on the right.

    DSC09554.JPG

    I think the restrictive effect (boundary layer effect) of the walls reduces the velocity and therefore the mass flow more in the smaller burner tubes.

    SUMMARY (and open to further debate)

    Difference in back pressure needs a difference in height between jet and burner top, which we haven't got.

    Difference in flow in the tubes needs a difference in tube inner diameter which we have got.
     
  17. Twoberth

    Twoberth United Kingdom Subscriber

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    @JP2
    The flame ring is another variable, and may have a effect. But I haven't got my head around it yet.
     
  18. snwcmpr

    snwcmpr Subscriber

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    All this talk of various pressures are quite interesting.
    But, I get lost in the 'virtual' image of those pressures.

    Have we identified all of the differences between the stoves?
    It seems to me that a simple approach would be to ID the differences. Because that is the source of the different output.
     
  19. Twoberth

    Twoberth United Kingdom Subscriber

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    It's fine to list the differences, but we then need to identify what effect these differences have (if any) on the mass flow, which inevitably involves a theoretical analysis.

    The physical and chemical processes involved in the operation of these little brass buggers is very complex. If you think these explanations are confusing, you should have a look in my head! :lol::lol:

    Differences

    Tank volume
    Air space above liquid in tank (when 3/4 full)
    Pump volume/ stroke
    Distance between tank top and burner jet
    Difference in riser tube diameters (above tank)
    Difference in burner tube diameters (ID and OD)
    Difference in air space between burner tubes
    Size of vaporisation chamber
    Radius of base of vaporisation chamber (if any)
    Size of gallery
    Size of flame ring
     
  20. kerophile

    kerophile United Kingdom SotM Winner Subscriber

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    Hi @Twoberth . You combine my suggestion and that of @Lennart F , and then dismiss both on the basis of the jet to vaporisation chamber distance being the same for the 2 burners.

    You accept that the fuel mass flow-rate is different between the 1 and 2 pint roarer burners and we are now looking for a mechanism to explain why this difference occurs.

    I suggested that back pressure from the combustion zone could be the explanation for the lower mass transfer of 1-pint stoves. You draw more diagrams and suggest that pressures may even be lower in some areas of the combustion zone due to Venturi effects.

    However we know that there is a huge increase of fluid volume in any combustion zone, and this results in higher pressures until the combustion products can disperse and cool.

    We are not considering equilibrium conditions in this stove situation and I would suggest that we have a higher pressures in the combustion zone of the burner which in turn influences mass flow from the jet.

    Best Regards,
    Kerophile.