J. A. Wharton Æ J. L. Ellzey Æ D. G. Bogard
An experimental study of turbulence intensities and non-uniformities
in the exit ﬂow from a porous combustor
Received: 5 December 2003 / Accepted: 15 August 2004 / Published online: 13 April 2005
Ó Springer-Verlag 2005
Abstract In this study, experiments were performed on a
two-section porous burner operated on propane and air.
The burner consisted of an upstream section of reticu-
lated yttria stabilized zirconia with 23.6 pores per cen-
timeter (ppc) followed by a downstream section of
3.9 ppc, composed of the same material. The velocity
and turbulence intensity of the exit ﬂow for reacting and
non-reacting conditions were measured. The velocity
proﬁles for both reacting and non-reacting ﬂow were
very non-uniform. The turbulence intensity for the
reacting ﬂow increased with distance due to turbulence
created by the non-uniformities. Blow-oﬀ occurred ﬁrst
on one side of the burner, but otherwise the ﬂame in this
burner was stable.
Porous burners and reactors are currently under inves-
tigation for various applications such as radiant heaters
and gas-turbine combustors. Many studies have focused
on operating range (Smucker and Ellzey 2004; Mathis
and Ellzey 2003; Viskanta and Gore 2000; Khanna et al.
1994; Hsu et al. 1993) and radiant output (Mital et al.
1998; Howell et al. 1996) but little emphasis has been
placed on understanding the nature of the exit ﬂows of
these burners. The characteristics of the exit ﬂow are
particularly important for gas-turbine combustors in
which the velocity and turbulence intensity play a large
role on heat transfer to the turbine airfoils downstream.
The ﬁrst study of exit ﬂow from a reticulated porous
material was conducted by Hall and Hiatt (1994) who
examined a non-reacting ﬂow exiting a single porous
media section with 4 to 12 pores per centimeter (ppc).
They measured velocity and turbulence intensity up to
0.8 cm downstream of the porous media. The mean
velocity distribution across the top of the porous section
showed a jet-like structure out of the pores, with local
means as high as two times the area mean. The negative
mean velocities between the pores indicated that recir-
culation was occurring in these regions. In addition, the
presence of non-uniformities that were much larger than
the average pore size implied some length scale other
than the pore diameter was aﬀecting the ﬂow. The tur-
bulence intensities ranged from 5 to 60% for various
ﬂow rates and pore sizes, and local turbulence intensity
peaked where the velocity gradients were the steepest.
They also found that as the pore size increased the tur-
bulence intensity increased as well.
While exit ﬂows from porous burners in which reac-
tion takes place within the matrix have never been
studied, the eﬀects of non-uniformities have been no-
ticed in other ways. Hsu et al. (1993) noted that the
ﬂame tilted signiﬁcantly after repeated use of a ceramic
burner. They theorized that this could be due to the
formation of ﬁssures in the ceramic matrix due to the
high thermal gradients generated by combustion. They
also noted that this did not occur in all cases, meaning
that the individual structure of the porous media con-
tributed to this eﬀect.
Viskanta and Gore (2000) observed deterioration of
the ceramic after continuous operation of the burner.
They noted that the structural damage resulted in non-
uniform temperature of the ﬂame, which could be
visually observed. Blocked pores which could result in
ﬂow non-uniformities have been observed by Younis
and Viskanta (1993 ). They examined two diﬀerent types
of media, an alumina specimen and a cordierite ceramic
foam. The alumina structure had fewer blocked pores
than the cordierite. This indicated that material and
manufacturing techniques are important in creating a
uniform porous media.
Earlier work on exit ﬂows from porous media
considered only non-reacting ﬂows. In this paper, the
J. A. Wharton (&) Æ J. L. Ellzey Æ D. G. Bogard
Department of Mechanical Engineering,
University of Texas, Austin, TX, USA
Experiments in Fluids (2005) 38: 701–707