Acetone as a tracer for mixture fraction time-series measurements
in turbulent non-reacting jets
A. Lakshmanarao, M. W. Renfro, G. B. King, N. M. Laurendeau
Abstract Mixture fraction time series were measured us-
ing laser-induced ¯uorescence of seeded acetone in tur-
bulent non-reacting jets. Power spectral densities obtained
using the seeding technique, especially in the shear layer,
appear to be consistent with turbulence theory. The
technique can be used for many gaseous mixtures of
The mixture fraction is an important scalar for the
description of turbulent diffusion ¯ames. In this commu-
nication, we demonstrate a technique for making time-
series measurements of mixture fraction that is viable for a
large class of non-reacting ¯ows. Our eventual goal is to
compare time-series measurements of mixture fraction in
turbulent non-reactive jets with scalar time-series mea-
surements in diffusion ¯ames.
Acetone vapor was introduced into a gaseous jet, and
the acetone concentration was monitored via laser-in-
duced ¯uorescence. Acetone generates a high ¯uorescence
signal on account of its high seeding density and its broad
detection spectrum (Lozano et al. 1992). Time series were
measured in three turbulent jets and were used to evaluate
power spectral densities (PSDs) associated with ¯uctua-
tions in the mixture fraction.
Measurements were obtained in jets nominally com-
posed of 81.6% air and 18.4% helium (v/v), in which
suf®cient acetone was introduced by bubbling some of the
air through an acetone bath maintained at 35 °C. The
maximum acetone mole fraction was 10% at the jet exit.
The mixture ¯owed into still air through a 3.4-mm di-
ameter tube. The near-®eld region (x/D £ 20) of three jets
was investigated, with Reynolds numbers of 5,000, 9,000
and 13,000, respectively. The required excitation and
detection systems are shown in Fig. 1.
Acetone excitation at 286 nm occurred via a frequency-
tripled Ti:sapphire laser (Renfro et al. 2000) with a repe-
tition rate of 80 MHz and a pulse width of 2 ps (FWHM).
The average laser power was 10 mW (0.13 nJ/pulse). Using
a suitable arrangement of lenses, the acetone ¯uorescence
signal was focussed into a 0.25-m monochromator, which
was adjusted for detection at a wavelength of 410 20 nm.
An adjustable slit placed at the entrance to the mono-
chromator de®ned a probe volume with a diameter and
length of approximately 70 and 500 lm, respectively. The
¯uorescence was detected with a Hamamatsu HS5321
photomultiplier tube (PMT) and a photon-counting sys-
tem (Packet al. 1998). This system had suf®cient speed
and memory for detailed time-series measurements.
The ¯uorescence signal is directly proportional to the
acetone concentration in the probe volume. However, the
mixture molecular weight must be considered to convert
the measured acetone number density to a mass fraction.
Fortunately, even in the present case where the fuel density
differs from that of ambient air, a nearly linear relation-
ship exists between the ¯uorescence signal and mixture
fraction (within 5%). Hence, the mixture fraction can be
directly computed using the ¯uorescence signal obtained
at a given measurement location.
Radial pro®les of mean mixture fraction were obtained
by normalizing the ¯uorescence signal obtained at differ-
ent locations by that obtained just upstream of the jet exit.
These mean pro®les were sampled at 10 Hz and averaged
over 25 s. The pro®les displayed behavior similar to that
expected for scalars in turbulent non-reactive jets. In
particular, for all locations outside the jet potential core
(x/D ³ 5), the mixture fraction pro®les were found to
collapse when normalized by their appropriate centerline
values and expressed in terms of the usual similarity
variable, r/x (Dowling and Dimotakis 1990). This collapse
was found for all jets studied, regardless of axial location
(x/D ³ 5) or jet Reynolds number.
Time-series measurements of mixture fraction were
performed at selected locations, chosen to encompass a
range of mixture fraction values and gradients. The time-
series were sampled at 4 kHz, and 50 time series of 4,000
points each were collected at each measurement location.
PSDs of mixture fraction ¯uctuations were calculated for
each measured time series and averaged to obtain a smooth
curve. To facilitate comparisons, each PSD was normalized
by its rms value. The shot-noise component of the ¯uo-
rescence signal, which has a relatively ¯at spectrum (Gas-
key et al. 1990), was subtracted from the signal PSD, as with
our previous OH measurements (Renfro et al. 2000).
Experiments in Fluids 30 (2001) 595±596 Ó Springer-Verlag 2001
Received: 19 October 1999/Accepted: 17 October 2000
A. Lakshmanarao, M. W. Renfro (&)
G. B. King, N. M. Laurendeau
School of Mechanical Engineering
Purdue University, West Lafayette
IN 47907-1288, USA
Tel.: +765-494-9117; Fax: +765-494-0539
This workwas sponsored by the Air Force Of®ce of Scienti®c
Research with Dr. Julian Tishkoff as technical monitor. We are
grateful to Dr. Douglas Thomsen (General Electric) for sharing
his experiences with acetone seeding.