[Stoves] venturi system -ratios of air and gas?

[ajheggie at gmail.com]

[Default] On Fri, 8 Jan 2016 00:31:19 +0100,"Boll, Martin Dr."
<boll.bn at t-online.de> wrote:

> A happy New Year to all stovers!
>
>In a venturi system like of a propane gas-burner the driving gas has pressure about 30 to 50 millibar.
>- What ratio have the volumes of gas and air, to make our admired blue flame, as it does in gas-flames?

As you thought it is a bit more complicated, the stoichiometric
(chemically exact amounts to react for complete combustion) mass ratio
of methane (same principle for propane but numbers are simpler) and
oxygen is about 4:1 but as you have to allow for the other
constituents of air, principally nitorgen, this means you actually
have to supply about 17 kg of air for every 1 kg of gas.

Volume ratio is  different but can be calculated by knowing that each
mole of gas occupies the same volume. From the mass calculation with
molar values rounded to intergers.


CH4+2O2+7N2=CO2+2H20+7N2

So 1 volume of gas reacts with 9 volumes of air to give 10 volumes of
combustion products at standard temperaure and pressure, of course as
you are aware after the flame the temperature has increased to 2000C
so the volumes are much larger.

In practice a little more air is supplied to keep the flame clean.

>-What ratio(range) must have air and woodgas , to get the same blue flame?

The blueness is a feature of the gas and air being premixed.

The stochiopmetric mass ratio for wood is about 5 parts air to 1 part
dry wood, because wood is already partially oxidised. This relates to
about 4.4 M3 of air to 1 kg of wood. In practice much more air is
used, this higher need for excess air is one reason a wood flame is
never going to get as hot as a gas flame.

Who volunteers to check my workings?

Andrew