Purely Buoyant Diffusion Flames: Some Experimental Results. Final Report.
Purely Buoyant Diffusion Flames: Some Experimental
Results. Final Report.
McCaffrey, B. J.
NBSIR 79-1910; 49 p. October 1979.
Combustion Institute/Eastern States Section. Chemical
and Physical Processes in Combustion. 1978 Fall
Technical Meeting. November 29-30, 1978-December 1,
1978, Miami Beach, FL, 8/1-4 pp, 1979.
Available from: National Technical Information Service
(NTIS), Technology Administration, U.S. Department of
Commerce, Springfield, VA 22161.
1-800-553-6847 or 703-605-6000;
Order number: PB80-112113
buoyancy; diffusion flames; plumes; scaling
Measurements of temperature and velocity using
thermocouples and an impact probe were made in the near
field of a purely buoyant diffusion flame produced by a
porous refractory burner. Based on time-averaged center
line value of V and delta T together with photographic
records the flame can be conveniently divided into three
distinct regimes: (1) a continous flame region,
starting from the surface of the burner with V equal to
zero at the surface and rising with the height above the
burner. Z, to the 1/2 power. Delta T is constant over
this regime. Higher up is (2), an intermittent regime,
with pulsating flame (~3 Hz) exhibiting approximately
constant V and delta T falling with z to the first
power. Still higher is (3) the plume region which is,
most of the time, free of flames with V~z-1/3 and delta
T~z(-5/3) as predicted by conventional plume theory.
Throughout the three regimes and indistinguishable among
these is the consistency of the buoyancy relation,
[equation omitted] which has a value of approximately
0.9, a factor 2.5 times previous estimates in the flame
region and confirming the recent correlation
measurements of Cox(1). Different heat release rates,
[equation omitted], can be scaled to a "universal" fire
if the length is normalized as z/[equation omitted](2/5)
and the velocity scale as v[equation omitted](1/5). The
flames regime is thus independent of [equation omitted].
In the radial direction for time-averaged quantities
only the plume region appears resonably Gaussian. The
data in the flame and intermittent regimes do not fall
as rapidly as that dictated by a Gaussian distribution.
In all three regimes the velocity profile is wider than
the temperature profile. Large scale, low frequency p
fluctuations are about 35% of the time-averaged signal
on the center line throughout the three regions.
Radially the fluctuating to time-averaged signal ratio
rises from the center line value and approaches 100% in
the wings. Elementary spectral analysis indicates that
most of this energy is concentrated in a narrow band
centered around 3 Hz. Implications of these results for
flame entainment calculations and heat release rates
will be discussed.