Jet sizes and multi-fuel burners

Discussion in 'Stove Forum' started by Twoberth, Jun 18, 2019.

  1. Twoberth

    Twoberth United Kingdom Subscriber

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    In order to satisfy my curiosity and help my understanding of multi-fuel burners, jets and the use of air restrictors, I decided to calculate the theoretical jet sizes for various stove fuels. Below are the results.

    All calculations are for a hypothetical multi-fuel burner design for which the optimum kerosene jet size is 0.28mm diameter.

    The basic assumptions for the calculations were…

    1.The simplest chemical formula for each fuel has been used, (e.g. C12H26 for kerosene)

    2. The fuel burns completely, with only carbon dioxide and water produced.

    3. At a fixed tank pressure the velocity exiting the jet remains the same when the jet diameter is changed.

    4. If the velocity remains the same, the amount of entrained air (oxygen) remains the same regardless of the jet size.

    5. If the velocity remains the same, the volume of fuel gas exiting the jet is proportional to the square of the jet diameter.

    Assumptions 3 is not strictly true (see later comments), but it greatly simplifies the calculations. These are the results..

    Relative theoretical Jet sizes (kerosene = 0.28mm)

    Kerosene 0.28mm

    Petrol 0.34mm

    Butane 0.47mm

    Butyl alcohol 0.49mm

    Propane 0.54mm

    Propyl alcohol 0.56mm

    Ethyl alcohol 0.69mm

    Methyl alcohol 0.98mm

    So theoretically for a multi-fuel stove for which the optimum kerosene jet is 0.28mm diameter, the ethyl alcohol jet should be 0.69mm in diameter. Clearly, if an air restrictor tube is used, the alcohol jet diameter can be smaller and the alcohol fuel consumption rate will be lowered, but the resulting flame will be less powerful.

    In the real world it’s not this simple and other factors are involved and interact. For example, as you increase the jet diameter the fuel exit velocity doesn’t remain the same (assumption 3), it falls - so not only do you get more fuel you also entrain less air. Because of this the best diameter for alcohol is less than the theoretical 0.69mm, with the best results being reported with a jet of 0.50 – 0.60mm.

    In the good old days when the original burners were designed and built, I can imagine similar simple calculations would have been done initially, but then experimental trial and error would have homed in on the actual jet sizes that give the best flames for any fuel and burner configuration. Similarly, the hole size in air restrictors would probably have been finalised by experiment.

    The calculations are simple but boring, so rather than list them all the following is an example of the calculations, and compares ethyl alcohol with kerosene.

    Assumed chemical formulae

    Kerosene, C12H26

    Petrol, C8H18

    Butane, C4H10

    Butyl alcohol, C4H9OH

    Propane, C3H8

    Propyl alcohol, C3H7OH

    Ethyl alcohol, C2H5OH

    Methyl alcohol, CH3OH


    For complete (stoichiometric) combustion

    Ethyl alcohol

    C2H5OH +3(O2) = 2(CO2) + 3(H2O)

    Kerosene

    C12H26 +18.5(O2) = 12(CO2) + 13(H2O)


    So ethyl alcohol needs only about 1/6 of the oxygen used by kerosene to burn completely. Or put the other way, for a burner where the amount of entrained oxygen (from the air) is the same, the volume of alcohol needs to be about 6 times the volume of kerosene to use up all the oxygen .

    To be precise, the ratio is 6.17, and so the alcohol jet cross-section needs to be 6.17 times the area of the kerosene jet, or 2.48 times the diameter.


    2.48 x 0.28mm = 0.69mm


    Although I will remain a trial and error guy, drilling and squashing to change jet sizes in order to get a nice blue flame, the theoretical results help to explain (to me) some of the practical experiences from multi-fuel use.

    1. General
    The lighter the fuel (i.e. the smaller the hydrocarbon) then the larger the jet should be, as more fuel is needed to use up the oxygen in the entrained air relative to heavier fuels. (If hydrogen was used in this hypothetical burner, the jet would need to be 1.7mm diameter!)

    2. Petrol
    If the petrol contains a significant proportion of kerosene–type heavy ends, then the kerosene jet should be OK without a restrictor. If the petrol is very ‘light’ then it follows that an air restrictor (or a larger jet) may be needed.

    3. Propane and Butane
    The theory predicts a similar jet size for both propane and butane, which explains why either gas (and mixtures of the two) can be used without a jet change.

    4. Diesel
    Since diesel (in simplified form) is a similar size hydrocarbon to kerosene, it should burn satisfactorily using a kerosene jet.

    5. Alcohol
    All alcohols are not the same. The four commonly available alcohols require very different jet sizes. The most common alcohol used in stoves is ethanol (ethyl alcohol). This is confusingly known as ‘meths’ in the UK, which is short for methylated spirits (as a small amount of methyl alcohol is added to the ethanol to make it undrinkable!). It is also known as ‘denatured alcohol’.

    if you are routinely using ‘meths’ for fuel and change to ‘rubbing alcohol’ or ‘surgical spirits’ you may (depending on the brand) be changing from ethanol to the heavier isopropanol (propyl alcohol). Isopropanol is not an inherently dirty fuel, it just burns dirty with the normal ‘meths’ jet and really needs a smaller size.
     
  2. presscall

    presscall United Kingdom SotM Winner Subscriber

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    @Twoberth
    Excellent work! Pretty much ties in with my own experience with an Optimus Nova I adapted to burn butane by drilling a spare jet out to 0.4mm. I found 0.45mm (near to your calculated optimum) ran a bit too rich.

    John
     
  3. Twoberth

    Twoberth United Kingdom Subscriber

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    @presscall
    Thanks John. All my calculated sizes will be too big due to my simple treatment of a complex process.

    While doing these calculations on entrained air burners I felt a renewed admiration for the elegance of the simple Bunsen burner with it’s variable air restrictor.
     
  4. presscall

    presscall United Kingdom SotM Winner Subscriber

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    Yes indeed, which principle a number of early gassies utilised with a similar holed sleeve that could be rotated on the riser tube.

    A Borde stove accomplishes a similar neat trick with a tapered needle/control spindle that screws in and out of relatively large bore jet outlet, creating a variable aperture to control the flame. The fuel/air mixture automatically self-regulates ... but messily, with a wonderfully irregular flame pattern.
     
  5. back view

    back view China Subscriber

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    Gasoline, kerosene, diesel, need to expand the pore size of 0.05mm, because they are not completely pure. Especially in countries outside Europe and America.