Lab 7 - Diffusion Flames
(Recommended material: Chapter 7 – Non-premixed combustion)
 Video demonstrations of this lab can be found on YouTube at
http://www.youtube.com/user/FndmtlsofCombustion
Generally, in most of the combustion phenomena that we observe, the fuel and
oxidizer are separate and do not mix until the instant of combustion. Examples include candle flames, cigarette lighters, fireplaces, wildland fires, and droplet combustion in diesel engines. This type of flame is referred to as a diffusion flame (or
non-premixed flame). In contrast to premixed flames, in diffusion flames the fuel
and oxidizer must diffuse into one another before combustion occurs. Because the
process of diffusion (mixing) is slow compared to the chemical reaction rate, the
rate of non-premixed combustion is usually controlled by the diffusion rate. The
height of the flame is determined by the time required for complete mixing to occur which, in turn, is related to the fuel flow rate. Mixing by diffusion depends
strongly on whether the flow is laminar or turbulent.
In this lab, the height of the flame formed by a methane jet issuing into air will
be measured for a range of flow velocities and burner diameters. The flow velocity
will range from the lowest flow for which a flame can be maintained to the largest
flow before blow-off. For each test point, measure the flame height and volume
flow rate of the methane (note the tube inside diameters). The variation in temperature along the height of a jet flame will be measured by placing a thermocouple
inside the reaction zone (outer edge) over the height of the flame.
1
1
Figures from Lawrence Berkeley Laboratory (left) and Sandia National La-
boratories (right).
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Exercises
1.
2.
3.
Qualitatively plot the jet-flame height as a function of the fuel flow-rate for
each diameter.
Plot the measured temperature as a function of height from the jet outlet.
Compare to theory and discuss your findings.