Nonpremixed flame control with microjets
R. Ganguly, Ishwar K. Puri
Abstract The hydrodynamic control of buoyant nonpre-
mixed ﬂames is investigated by injecting high-momentum
ﬂuid through a central microjet. The resulting ﬂame
characteristics are mapped for jets of different strengths.
The ﬂame height decreases linearly with an increase in the
microjet Froude number as the ﬂow changes from a
buoyancy-dominated to a momentum-controlled regime.
The ﬂame luminosity is reduced by injecting stronger
microjets. The jets alter the ﬂame structure by establishing
strong entrainment of the ambient air from the quiescent
surroundings. The introduction of an inert species as the
microjet ﬂuid has a similar qualitative effect as air.
Microjet assistance is as effective as partial premixing for
reducing the ﬂame height and luminosity.
A fuel jet burning in an oxidizing environment is a well-
studied nonpremixed ﬂame conﬁguration (Burke and
Schumann 1928; Mitchell and Saroﬁm 1980; Li et al. 1995).
Although most practical nonpremixed combustion sys-
tems function with an oxidizer coﬂow, this arrangement
has practical relevance for understanding ﬁre dynamics
and combustion in dump combustors. Several investiga-
tions have been performed to characterize the conditions
that enhance the combustion efﬁciency (Leahey et al.
2001), reduce soot formation (Saito et al. 1999; Saito et al.
1998; Saito et al. 1997; Ohisa et al. 1999), and control the
ﬂame shape and spread (Saito et al. 1999; Becker et al.
1981) in these applications.
Kimura (1965) provided early observations that buoy-
ant laminar jet ﬂames are characterized by gravity-induced
ﬂickering. A more recent investigation by Hamins et al.
(1992) addressed the effect of burner diameter, jet Rey-
nolds number, and fuel properties on the ﬂickering
behavior. For slot burners, Roper (1977) and coworkers
(Roper et al. 1977) identiﬁed three regimes of ﬂame
behavior: buoyancy controlled, transition, and momentum
controlled depending on whether the Froude number was
very small, of the order of unity, or very large. Heat and
mass transport in ﬂames established with extremely low
velocity fuel jets on small burners can be dominated by
molecular diffusion rather than by convection or buoy-
ancy. Ban et al. (1994) observed that it is possible to almost
eliminate the effect of gravity and produce a nearly
spherical nonpremixed ﬂame when the dimensionless
Peclet number Pe (=ud/D, where D denotes the pertinent
mass diffusivity) has a value smaller than ﬁve. Baker et al.
(2002) performed a more extensive parametric investiga-
tion of micro-slot nonpremixed ﬂames and determined
that for low values of a diffusion-to-buoyancy parameter,
the ﬂame shape is buoyancy controlled. Momentum was
shown to play a more dominant role in ﬂames associated
with low values of the diffusion-to-momentum ratio.
Soot formation is useful in some furnaces to enhance
radiative heat transfer. There are several methodologies
available to control ﬂame luminosity for initially nonpre-
mixed ﬂames, one of which is partial premixing (Alder
et al. 2000; Goldstein et al. 2002). Mitrovic and Lee (1998)
observed that the initial addition of air to the fuel stream
at high equivalence ratios (in the range from 20 to 40)
increases soot formation. Kaplan and Kailasanath (2001)
correlated the ﬂame structure of inverse nonpremixed
ﬂames with the relative placement of the fuel and air
streams. They showed that an inverse ﬂame produces far
less soot than a ﬂame established in the usual conﬁgura-
tion with an inner fuel core and an outside air annulus for
the same fuel–air ratio. (However, soot oxidation in in-
verse ﬂames declines considerably prior to the cessation of
soot surface growth, leading to soot emissions from these
We have explored the feasibility of hydrodynamic control
of nonpremixed ﬂames by injecting ﬂuid through a high-
momentum jet of submillimeter radius. We will refer to
Experiments in Fluids 36 (2004) 635–641
Received: 21 August 2003 / Accepted: 27 October 2003
Published online: 26 February 2004
Ó Springer-Verlag 2004
R. Ganguly, I. K. Puri (&)
Department of Mechanical and Industrial Engineering,
m/c 251, University of Illinois at Chicago, 842 W. Taylor St.,
Chicago, IL 60607–7022, USA
Department of Power Engineering,
Jadavpur University, 700032 Calcutta, India
This research was supported partly by the National Science
Foundation Combustion and Plasma Systems Program through
Grant No. CTS–9707000 for which Dr. Farley Fisher is the Pro-
gram Director, and partly by the NASA Microgravity Research
Division through Grant No. NCC3–688 for which Dr. Uday Hegde
serves as the technical monitor.