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

Fri Jan 8 19:04:07 CST 2016

All very interesting.

A couple of comments:

>For a particular fuel under a particular operating condition, the size of
the injector (a.k.a. "tiny hole") determines the level of air entrainment,

I was not referring to this (though it is true for a pre-mixer). I was
referring to the small hole that limits the propane flow into the (not
premixing) propane burner head such as one finds with the 'screw on' top.

Like this:

There is a jet inside that limits the flow, and there is no pre-mixed air,
just a lot of tiny flames. When extinguished it doesn't *pop*.

I have a BBQ that runs on propane and it has multiple flamelets on long
tubes. I don't think there is a pre-mixer in there. I looked:. When
converting that same unit to natural gas there are pre-mixers on each line.
That is a basic difference.

My gas stove has a lot of pre-mixing but I doubt it is 'full'. It is
definitely using around the burner as well.  But this one (American) looks
like it has a pre-mixer at the bottom:

This one definitely does - you can see the adjuster:

>It is not true that "propane should generally not be premixed but natural
gas should be". It will depend on the situation and application.

I can't quibble. I am just reporting what I see and some reasons why. If a
natural gas flame is not pre-mixed it needs more time and space to burn
well.  For a given number of Joules it needs more volume.  This can be
demonstrated (be careful!) by closing the premixing hole in a gas stove
burner and watching what happens. To meet the USA requirement of 0.8% CO/CO2
ratio the flame has to be completed just before getting to the pot. If the
pre-mixing air is inadequate, it can't do it. The flames are too long.

>Any flame operates equally well in any combustion mode. 

That I have not experienced. It would be interesting to see.  The combustion
in my furnace (which is a pre-mixed natural gas flame x 4) is terrible if I
close the air pre-mix holes. In fact there is a detector in place to shut it
off if that happens.  Is there a common device burning natural gas that does
not use a premixed flame? 

Maybe this one is not pre-mixed:

http://www.cuisinartbbqs.com/en/ceramic_heat_technology.php

Propane Model # G35801 / 85-3112
Natural Gas Model # G35802 / 85-3113

Photo of it running (is the flame colour Photo-shopped?)

http://www.cuisinartbbqs.com/en/ceramic_smallspaces_photos.html#images/g3580
1-2_pic07.jpg

A 300 kW natural gas burner without pre-mixing is modelled here
<http://www.academia.edu/3648228/Simulation_of_Non-Premixed_Natural_Gas_Flam
e>  and please note that the gas comes through 24 ports and has carefully
placed air directors to get the fuel mixed with it. Huh. There is a pretty
big advantage to mixing first on a small scale, it seems.

For the numbers, if I am not too far off, a kg of natural gas will contain
about 309 moles of C,H and N, consume 124 moles of O2 and produce 14.65 m3
of combustion products with no excess air at all (CO2MAX of 10.6%).

A kg of Propane (99%) needs 113 moles of O2, creates ?13.14 m3 of products
and has a CO2MAX of 12.6%. Not so different, is it?

Propane is easier to light 470° v.s. 580° and it doesn't need as much air
mixed in it to ignite: 2.1% v.s. 5%. Maybe the reason for natural gas to be
(frequently) pre-mixed is just to make it easier to light with a ceramic
sparker.

Regards

Crispin

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