Exp Fluids (2011) 51:611–620
DOI 10.1007/s00348-011-1082-6
RESEARCH ARTICLE
Development and characterization of a variable turbulence
generation system
A. Marshall • P. Venkateswaran • D. Noble
J. Seitzman • T. Lieuwen
•
Received: 8 November 2010 / Revised: 15 March 2011 / Accepted: 18 March 2011 / Published online: 1 April 2011
Ó Springer-Verlag 2011
Abstract Experimental turbulent combustion studies
require systems that can simulate the turbulence intensities
[u0 /U0 * 20–30% (Koutmos and McGuirk in Exp Fluids
7(5):344–354, 1989)] and operating conditions of real
systems. Furthermore, it is important to have systems
where turbulence intensity can be varied independently of
mean flow velocity, as quantities such as turbulent flame
speed and turbulent flame brush thickness exhibit complex
and not yet fully understood dependencies upon both U0
and u0 . Finally, high pressure operation in a highly preheated environment requires systems that can be sealed,
withstand high gas temperatures, and have remotely variable turbulence intensity that does not require system shut
down and disassembly. This paper describes the development and characterization of a variable turbulence generation system for turbulent combustion studies. The system
is capable of a wide range of turbulence intensities
(10–30%) and turbulent Reynolds numbers (140–2,200)
over a range of flow velocities. An important aspect of this
system is the ability to vary the turbulence intensity
remotely, without changing the mean flow velocity. This
system is similar to the turbulence generators described
by Videto and Santavicca (Combust Sci Technol
76(1):159–164, 1991) and Coppola and Gomez (Exp Therm
A. Marshall (&)
School of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332, USA
e-mail: andrew.marshall@gatech.edu
URL: http://soliton.ae.gatech.edu/labs/comblab5/
P. Venkateswaran D. Noble J. Seitzman T. Lieuwen
School of Aerospace Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332, USA
Fluid Sci 33(7):1037–1048, 2009), where variable blockage
ratio slots are located upstream of a contoured nozzle.
Vortical structures from the slots impinge on the walls of
the contoured nozzle to produce fine-scale turbulence. The
flow field was characterized for two nozzle diameters using
three-component Laser Doppler velocimetry (LDV) and
hotwire anemometry for mean flow velocities from 4 to
50 m/s. This paper describes the key design features of the
system, as well as the variation of mean and RMS velocity,
integral length scales, and spectra with nozzle diameter,
flow velocity, and turbulence generator blockage ratio.
List of
A
E
ReD
Rel
SL
U(r)
U0
Ur(r)
Uh(r)
l
m_
u0
u0tot
ui
v0
w0
Ds
m
q
q(s)
s
symbols
Nozzle exit area
Power spectral density
Geometric Reynolds number
Turbulent Reynolds number
Laminar flame speed
Mean axial flow velocity
Spatially averaged flow velocity
Mean radial flow velocity
Mean azimuthal component of flow velocity
Characteristic longitudinal length scale
Mass flowrate
RMS of axial velocity fluctuations
Total turbulence intensity
Instantaneous axial velocity perturbation
RMS of radial velocity fluctuations
RMS of azimuthal component of velocity
fluctuations
Time lag width for autocorrelation calculation
Kinematic viscosity
Density
Autocorrelation coefficient
Lag time
123
612
sint
smax
Exp Fluids (2011) 51:611–620
Integral time scale
Maximum lag time for autocorrelation calculation
1 Introduction
This paper describes a turbulence generator system developed for turbulent combustion studies. Understanding the
behavior of turbulent flames at conditions that realistically
simulate the operating conditions of practical devices such
as boilers and gas turbine burners continues to be an
important need. Typical systems operate at flow velocities,
U0, ranging from a few m/s to values of several hundred
m/s, relative turbulence intensities, u0 /U0, up to 30%, preheat temperatures of several hundred Kelvin, and pressures
from one to tens of atmospheres (Koutmos and McGuirk
1989). However, there is relatively little fundamental data
of turbulent flame properties at these conditions, due to the
severity of the environment. For example, consider the premixed, turbulent flame literature, where measurement of
quantities such as turbulent flame speed, flame brush
thickness, or various measures of flame front topology
(e.g., fractal dimension, surface density) is required. Much
of the turbulent flame speed data in the literature is
obtained at normalized turbulence intensity, u0 /SL, values
less than fifty, with the majority of it less than ten or even
five. Very little high pressure, pre-heated turbulent flame
speed data are available. Moreover, much of the data
showing sensitivities of turbulent burning velocity to turbulence intensity have been obtained while varying mean
flow velocity and turbulence intensity simultaneously,
despite the fact that it is known that the flame exhibits
separate dependencies upon these quantities (Shepherd
et al. 1998). There exists a large literature on the use of
turbulence generating systems to actively manipulate the
length scales and turbulence intensities in combustion and
aerodynamics. These involve the use of perforated plates,
spinning/oscillating rods, and auxiliary fluid jets. The rest
of this section briefly reviews several prior approaches
and discusses their suitability for turbulent combustion
applications.
Many turbulent flame studies use grid or perforated plategenerated turbulence. While the characteristics of gridgenerated turbulence are documented extensively (Liao
et al. 2003), the range of achievable turbulent fluctuations is
on the order of 2–10% (Natarajan et al. 2007; Baines and
Peterson 1951). Moreover, either the grid or the flow
velocity must be changed in order to alter the turbulence
intensity. Hardware changes are inconvenient for combustion studies, particularly high pressure ones involving
pressure vessels, as it requires shutting down and disassembling the experiment for each test point. Mean velocity
123
changes are problematic as they confuse the underlying
sensitivity of the turbulent flame property to potentially
separate dependencies on u0 and U0 (Driscoll 2008).
High turbulence intensity levels have been achieved by
active-grid turbulence generators in wind tunnels, which
use oscillating rods with diamond-shaped agitator wings to
add turbulent kinetic energy to the flow (Makita 1991;
Mydlarski and Warhaft 1996, 1998). The rods are oscillated independently using a random frequency generator to
change the direction of oscillation. Using this configuration, Makita (1991) reported relative turbulence intensities
up to 37% at 10 grid spacing lengths behind the turbulence
generator. However, the turbulence was highly anisotropic
at this location, and he recommended taking measurements
at least 50 grid spacing lengths downstream, where the
turbulence was more isotropic. At this location, the relative
turbulence intensity was approximately 16%. While this
system significantly increases the turbulence levels compared to traditional grid-generated turbulence, there are
drawbacks. The cost of implementing such a system is high
compared to some of the other turbulence generation systems, e.g., the design described by Makita (1991) uses 15
stepper motors and motor drivers.
Variable turbulence levels in high pressure environments can be readily achieved using fan-stirred bombs
(Abdel-Gayed et al. 1984). This is a transient experiment
where the flame generally is initiated at the flow centerline
and expands spherically. Fans are used to stir the mixture at
the center of the bomb and generate isotropic turbulence
(Burke et al. 2007). While very useful for some types of
studies, the fact that the experiment is unsteady and that
there is no average flow along the flame limits the range of
applicability of data obtained from these systems.
Another method of generating turbulence, developed
independently by Thole et al. (1994) and Shavit and Chigier
(1996), is based on injecting high speed jets normal to the
mean flow. The advantage of this system is that the turbulence intensity can be varied independently of the mean
flow by adjusting the ratio of the transverse jet velocity to
the main flow velocity. In the study by Thole et al. (1994),
the configuration was implemented in a wind tunnel to
study surface heat transfer rates in gas turbines. They were
able to achieve relative turbulence intensities of 20% while
maintaining uniform mean and turbulence quantities across
the span of the test plate. In the study by Shavit and Chigier
(1996), the principle was applied to generate turbulence in a
twin-fluid atomizer, where the effect of turbulence characteristics of the outer air jet on the atomization of the inner
liquid jet was investigated. In their study, they were able to
achieve relative turbulence intensities ranging from 10 to
24% over mean flow velocities ranging from 27 to 53 m/s.
This approach does not appear to have been implemented in
combustion applications.
Exp Fluids (2011) 51:611–620
Videto and Santavicca (1991) developed a system where
the flow passes through slots in a blockage plate, leading to
the generation of vortical structures. These vortical structures impinge on the walls of an axially contracting section,
breaking down into fine-scale turbulence before exiting the
burner (Bédat and Cheng 1995; Kortschik et al. 2004;
Videto and Santavicca 1991). They were able to achieve
isotropic turbulence profiles across the burner width, with
relative intensities up to u0 /U0 * 40%. The turbulence
level can be varied by either changing the plate gap width,
thereby changing the flow velocity through the gap, or by
varying the mean flow velocities. The configuration was
only tested for velocities up to 2 m/s, so there is some
question as to whether these levels will be achievable at
higher mean flow velocities. Bédat and Cheng (1995)
iterated upon this design by utilizing azimuthal slots to
enable use of the turbulence generator in axisymmetric
geometries. Coppola and Gomez (2009) employed a similar strategy of laser cutting slots with various shapes into
metal plates through which the flow passes.
The approach described in this paper was motivated by
these latter studies, with some generalizations to enable
remote operation in a high pressure, pre-heated flow. This
approach was pursued for these studies for several reasons.
First, as noted above, very high levels of spatially uniform
turbulence intensity can be generated at a fixed flow
velocity. Second, the device is amenable to remote variation of blockage ratio. The rotating plate design utilized
here was selected because of minimal problems with
sticking over the wide range of air pre-heat temperatures
used, and the single access point made high pressure
sealing straightforward. Key features of this system and its
characteristics are described in the rest of this paper.
2 Turbulence generator setup
This section describes the experimental facility with particular emphasis on the design and implementation of the
turbulence generator. It was important in the development
of this facility to design a robust system capable of operating over a range of mean flow velocities, inlet temperatures, and pressures. As such, careful consideration had to
be taken in regards to the selection of materials and the
method of sealing the system from the surrounding
environment.
Figure 1 shows a schematic of the experimental facility
in which the velocity characterization studies were conducted. The facility consists of a contoured nozzle burner
used for turbulent pre-mixed flame experiments. The
smoothly contoured nozzle was designed in order to suppress boundary layer formation along the walls and create a
uniform top-hat velocity profile at the exit. The air supply
613
Fig. 1 Schematic of the experimental facility. Dimensions in mm
is delivered from blow down tanks that store compressed
air from the main facility compressors. The air flow is
metered upstream of the burner assembly using a subcritical orifice plate. Once the flow is near the burner, a small
percentage of the flow is bypassed through the seeder and
then rejoined to the main air line to ensure that there is no
loss of mass flow rate. The seeded flow then enters the
plenum through four ports and passes through a layer of
ball bearings to minimize ‘‘jetting’’ effects from the smaller
reactant feed lines.
The flow then passes through the turbulence generator
plates, shown in Fig. 2. The turbulence generating plates
are secured 84 mm upstream of the burner exit, as shown in
Fig. 1. Both plates have an identical annular slot pattern
milled in them so the turbulence intensity can be varied by
rotating the top plate, resulting in a change in the blockage
ratio, depicted in Fig. 2. This design is motivated by the
systems developed by Videto and Santavicca (1991) and
Bédat and Cheng (1995). The main flow passes through
these slots, generating vortical structures that then impinge
on the inclined wall of the converging section of the nozzle, breaking down into finer scale turbulence.
At very high blockage ratios, the mixture passes through
the slots at an angle, leading to swirl in the flow, as shown
in Fig. 3a. This effect was reduced somewhat by the
addition of straighteners shown in Fig. 3b. We used the
criterion that the swirl velocity remains less than 20% of
the mean axial flow velocity, which limited the maximum
usable blockage ratio to 93% for all flame speed
experiments.
123
614
Exp Fluids (2011) 51:611–620
Fig. 2 Schematic of the
turbulence generating plates:
a fully open and b partially
closed
Fig. 3 Flow characteristics
a without and b with flow
straighteners
After passing through the turbulence generator plates,
the flow impinges on the walls of the contoured nozzle, as
detailed in Fig. 4. This is an important design element, as
nozzles with too large diameter or blockage plates with too
small diameter of the open area allow the large-scale
structure generated at the blockage plate to exit the nozzle
without impinging upon the walls of the contoured nozzle.
Since we are aiming to achieve homogeneous turbulence
with no narrowband spectral features, this is undesirable.
As such, the inner diameter of the radial slots was set to
30 mm, 1.5 times larger than our largest nozzle diameter.
Measurements and characterization studies were conducted
under isothermal flow conditions with burner diameters of
12 and 20 mm to achieve different ranges of length scales
and assess their influence on the turbulent flame properties.
The turbulence generator is a unique aspect of this
experimental facility, and substantial effort was invested to
meet key goals that were derived from shortcomings of
turbulence generators used in other studies. The criteria set
forth in designing the turbulence generator were to (1) have
the ability to vary the turbulence intensity without changing out plates or changing mean flow velocity, (2) access a
wide range of turbulence intensities, (3) have uniform exit
mean and turbulent quantities, (4) be able to operate at high
air temperatures and pressures, (5) be remotely operable,
123
and (6) have very thin boundary layers to prevent flashback
of high flame speed fuels, such as high H2 mixtures. The
need for remote operability and continuously variable turbulence intensity was motivated by the need to access a
range of turbulence intensities in high pressure situations
without having to shut down and cool the experiment to
replace blockage plates. Furthermore, due to the influence
of the mean flow velocity on the turbulent flame speed, we
wanted the ability to change the turbulence intensity
independently of the mean flow velocity.
The turbulence generator system consists of a 3-mmthick bottom plate that is secured to the plenum and a
6-mm-thick top plate attached to a central shaft. This
central shaft passes through the flange as shown in Fig. 5.
A significant amount of work was put into the design of the
pass-through assembly in order to ensure that the system
would not leak at high pressures. The system was designed
so that increased chamber pressure would induce a force
imbalance on the pass-through components (hemispherical
nut and outer seal), thereby effectively enhancing their
ability to seal. This pass-through has been leak tested at
pressures up to 10 atm. Outside the flange, the central shaft
is coupled to a DC stepper motor through a 50:1 worm and
worm gear. This system has been tested to successfully
rotate the turbulence plates at inlet temperatures up to
Exp Fluids (2011) 51:611–620
615
Fig. 4 Detailed views of the
turbulence generator plates and
contoured nozzle (12 mm
nozzle diameter shown) a with
dimensions and b isometric cutaway. Dimensions in mm
3 Flow field characterization
Fig. 5 Detailed view of the pass-through assembly for the central
shaft of the turbulence generator
600 K and pressures up to 10 atm. In addition, the worm
and worm gear were chosen for the low amount of backlash
inherent in their design. The plate’s angular position is
measured with an optical encoder, attached to the other end
of the central shaft, to an accuracy of ±0.1°. The range of
blockage ratios possible with this setup is 69–97%, corresponding to angular slot openings from 30° to 2°. The 30°
angular slot opening corresponds to the fully open position,
where the two plates are aligned. It will be shown later that
the turbulence intensity increases monotonically with
increasing blockage ratio.
This section presents typical results illustrating the flow
field characteristics. Data were obtained with a TSI
3-component Laser Doppler velocimetry (LDV) system,
using 5 lm alumina (Al2O3) particles. Data were taken in
two perpendicular radial cuts 1 mm above the nozzle exit.
The distance between sequential data points was 1 mm.
The radial cuts were aligned so that one cut would pass
over a solid portion of the turbulence generator, and the
other cut would pass over open slots, as depicted in Fig. 6.
The purpose of this was to verify azimuthal symmetry in
the flow. Profile results presented later in the paper will be
shown only for a single cut, since there was no noticeable
difference between the two traverse directions.
Time-series velocity data were also acquired at the
burner centerline using a Dantec Streamline Constant
Temperature hotwire Anemometer (CTA) with a 5 lm
diameter, 1.25 mm length model 55p11 straight probe. The
system was calibrated over the range of velocities for flow
characterization using a Dantec 90H02 calibration unit to
an accuracy of 2%. The probe was held in place with a
straight probe holder at 1 mm above the nozzle exit centerline, oriented downward. Data were taken at 100 kHz
with sample sizes of 105 points.
Data were obtained at mean flow velocities from
_
U0 = m=qA
= 4–50 m/s, which correspond to geometric
Reynolds numbers of ReD = U0D/m = 5,100–64,000 and
turbulent Reynolds numbers of Rel = u0tot l/m = 140–2,200.
The following section starts with an overview of the mean
and turbulence profiles, followed by an investigation of
integral length scales and power spectra.
123
616
Fig. 6 Illustration of direction and orientation of cuts performed in
LDV velocity characterization relative to the turbulence generator top
plate
Exp Fluids (2011) 51:611–620
Fig. 8 Plots of a mean axial, radial, and azimuthal velocities and
b fluctuating axial, radial, and azimuthal and total fluctuating
velocities as a function of radial distance from the center of the
burner for U0 = 50 m/s at a blockage ratio of 81%
3.1 Mean and turbulence profiles
Figures 7, 8, and 9 plot representative profiles of the mean
and fluctuating axial, radial, and azimuthal velocities as a
function of the radial distance from the center of the 20mm-diameter burner at 50 m/s at three different blockage
ratios. Total turbulence intensity is calculated from
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u0tot ¼ ðu0 Þ2 þ ðv0 Þ2 þ ðw0 Þ2 .
These data show a monotonic increase in fluctuating
velocity with blockage ratio, as desired. The data also show
a well-defined top-hat mean axial velocity, along with low
radial velocity. The azimuthal velocity increases with
increasing blockage ratio, as discussed earlier. The radial
Fig. 9 Plots of a mean axial, radial, and azimuthal velocities and
b fluctuating axial, radial, and azimuthal and total fluctuating
velocities as a function of radial distance from the center of the
burner for U0 = 50 m/s at a blockage ratio of 93%
Fig. 7 Plots of a mean axial, radial, and azimuthal velocities and
b fluctuating axial, radial, and azimuthal and total fluctuating
velocities as a function of radial distance from the center of the
burner for U0 = 50 m/s at a blockage ratio of 69%
123
variation of the mean flow velocity is low, as noted in
Table 1. Also noted in Table 1 are maximum values of the
mean radial and azimuthal velocities relative to the mean
axial velocity. As shown in Figs. 7, 8, and 9, the radial
velocity is low, less than 0.05U0, and independent of
blockage ratio, while the mean azimuthal velocity increases monotonically with blockage ratio. These results are
typical for the other velocities and the 12 mm burner
diameter.
These figures also show that the turbulent fluctuations in
the axial direction, *3–5 m/s, are about half of the fluctuations in the transverse directions, *6–12 m/s. This is
due to the well understood phenomenon of turbulent flow
Exp Fluids (2011) 51:611–620
617
Table 1 Maximum values of normalized mean axial, radial, and
azimuthal velocity at three different blockage ratios
BR
(%)
|(U(r) - U0)/U0|max
(%)
|Ur(r)/U0|max
(%)
|Uh(r)/U0|max
(%)
69
3.1
4.6
6.9
81
1.4
5.8
13.3
93
1.8
4.3
21.3
dynamics through a contraction (Prandtl 1932; Taylor
1935). Essentially, the vortex tubes that are aligned with
the main axis of the burner are elongated as the flow
accelerates through the contraction, increasing their vorticity and increasing the transverse velocity fluctuations (v0 ,
w0 ). In addition, the vortex tubes aligned perpendicular to
the main axis are contracted, reducing their vorticity, and
decreasing the axial velocity fluctuations (u0 ). This is different from the results reported by Videto and Santavicca
(1991), who reported nearly isotropic turbulence. It is
common to use a contraction after a turbulence generator
(i.e., grid, perforated plate.) to improve the isotropy of the
turbulence (Comte-Bellot and Corrsin 1966), because the
resulting flow turbulence is strongest in the axial direction.
The contraction causes vortex stretching, which equilibrates the three components. However, this is generally a
weak contraction; an area contraction ratio of 1.27 and 2.6
was used in the studies of Comte-Bellot and Corrsin (1966)
and Videto and Santavicca (1991), respectively. Our area
contraction ratios are 40 and 14 for nozzle diameters of 12
and 20 mm, respectively. These high area contraction
ratios produce radially uniform velocity profiles as shown
above, as well as flashback-resistant burners, but also lead
to this anisotropy in turbulence intensity.
Figure 10 summarizes the performance of the turbulence generator, by plotting the total centerline turbulence
intensity, u0tot =U0 as a function of the blockage ratio. It
shows that the turbulence intensity monotonically increases
with blockage ratio. Note that the 12-mm burner has lower
turbulence intensities than the 20-mm burner at the same
blockage ratio, because the flow velocity through the
blockage plate gaps is lower for the smaller burner (note
that the nozzle exit velocity is the fixed parameter, not the
velocity through the plates).
One question that should be considered when performing combustion experiments (such as determining the turbulent flame speed) is the appropriate location to
characterize the turbulence intensity. This is an important
question since (1) the turbulence intensity varies radially
and axially, (2) there is strong shear-generated turbulence
at the jet edges, and (3) the location of the flame surface is
constantly changing. We do not propose an answer to this
difficult question in this paper, however, we do note that
Fig. 10 Dependence of burner centerline total turbulence intensity
(i.e., summed overall 3 fluctuating velocity components) upon
blockage ratio
the centerline turbulence intensity scales well with that at
other locations. To illustrate, Fig. 11 presents a comparison
between the shear layer (or, more precisely, u0tot at
r = 10 mm) and centerline turbulence intensities. Note the
one-to-one correspondence between the two, with u0tot
(r = 10 mm) = 0.8 u0tot (r = 0 mm) ? 6.9U0.
3.2 Flow length/time scales
This section describes measurements of the integral time
scale. Longitudinal integral time scales, sint, were calculated from the LDV time-series data at the burner centerline and also used to calculate a characteristic longitudinal
length scales by multiplying by the mean flow velocity,
Fig. 11 Comparison of turbulence intensity in the shear layer to
turbulence intensity along the nozzle centerline
123
618
Exp Fluids (2011) 51:611–620
l = sintU0. For u0 /U0 1, this characteristic length scale is
equal to the integral length scale, as per Taylor’s hypothesis. However, at the higher turbulence levels used in this
experiment, these two length scales are different (Tennekes
and Lumley 1972). The longitudinal integral time scale
was calculated from the normalized autocorrelation function as a function of lag time from the LDV time-series
data. Since the LDV data are sampled in unequal time
intervals, the autocorrelation function was determined from
Eq. 1 using the slotting technique of Mayo (1974) with the
local normalization improvement described by Tummers
and Passchier (1996),
PN1 PN
i¼1
j¼iþ1 ui uj ðkDsÞ
qðkDsÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
PN
PN1 2
2
ð1Þ
i¼1 ui ðkDsÞ
j¼iþ1 uj ðkDsÞ
for: k ¼ 0; 1; . . .; M 1
where (k - 1/2)Ds \ ti - tj \ (k ? 1/2)Ds, M - 1 =
smax/Ds, smax is the maximum lag time, and Ds is the
slot width. The integral time scale was defined using the
relationship,
Z1
sint ¼
qðsÞds
ð2Þ
Fig. 12 Autocorrelation coefficient plotted against normalized lag
time for U0 = 4 m/s, blockage ratio = 75%
0
Because of the high uncertainties associated with the
autocorrelation at large time lags (because of its low
value), an exponential expression of the form qðsÞ ¼
aebs þ ð1 aÞecs was fit to the autocorrelation function
and used to evaluate this integral, so that sint is given by
Eq. 3,
a 1a
sint ¼ þ
b
c
ð3Þ
A typical result is shown in Fig. 12. With our LDV system
and experimental configuration, it was difficult to obtain
data rates high enough to obtain the roll-off region at low
time lag constants. The roll-off is noticeable in Fig. 12;
however, it appears to start at an autocorrelation value of
approximately 0.9. This occurs in all cases and indicates
very low correlation time random noise in the
measurements.
Figure 13 summarizes the calculated l/D values at mean
flow velocities of 4, 30, and 50 m/s for the 20-mm burner
and 30 and 50 m/s for the 12-mm burner at various
blockage ratios. The data indicate that l/D do exhibit some
sensitivity to blockage ratio, but that they are not changing
proportionally to blockage gap width. The blockage gap
width changes by a factor of six from the fully open
position to the highest blockage ratio, while l changes at
most by a factor of two. The increase in length scale at the
highest blockage ratios is apparently a manifestation of the
growing presence of a large-scale organized structure and
123
Fig. 13 Comparison of characteristic longitudinal length scale,
l (normalized by burner diameter) as a function of blockage ratio
for the two burner diameters over a range of mean flow velocities
provides another reason to not obtain flame data at blockage ratios greater than about 93%.
3.3 Power spectra
Power spectra were obtained from velocity time-series data
using hotwire anemometry. The power spectra were
ensemble-averaged over 50 ensembles with 50% overlap.
Figure 14 presents the non-dimensionalized turbulent
power spectra at the burner centerline for several blockage
ratios at U0 = 10 m/s. The spectra show a smooth roll-off
with increasing frequency and no discrete peaks, as
Exp Fluids (2011) 51:611–620
619
generators, they have the disadvantages that the grid or
plate must be changed in order to access a different turbulence level at a given flow velocity. The turbulence
generator described in this paper was found to achieve a
range of relative turbulence intensities from 10 to 30% and
turbulent Reynolds numbers from 240 to 2,200 without
exchanging plates or varying the mean flow velocity. This
allowed us to independently study the effects of mean flow
velocity and turbulence on combustion characteristics such
as the turbulent flame speed.
The maximum useful turbulence intensities of the device
are primarily limited by the increasing azimuthal flow at
high blockage ratios. This is manifested by deterioration in
turbulent flow profiles, as shown in Fig. 9 and indications
of an organized structure in the flow, as shown in Fig. 13.
Fig. 14 Turbulent power spectra for U0 = 10 m/s at three different
blockage ratios
References
Fig. 15 Turbulent power spectra for BR = 77% at three different
mean flow velocities
desired. It also shows that the spectra have virtually the
same shape at each blockage ratio.
Figure 15 presents the turbulent power spectra at a fixed
blockage ratio of 77% at 4, 10, and 50 m/s for the 20-mm
burner to illustrate mean velocity effects. The figure shows
the collapse of the spectra at lower frequencies, but also
that increasing flow velocity broadens the high frequency
spectral range over which kinetic energy is distributed, as
also expected (Tennekes and Lumley 1972).
4 Concluding remarks
In this paper, we presented a turbulence generation system
developed for combustion studies. While grids, perforated
plates, or slots are commonly used for turbulence
Abdel-Gayed R, Al-Khishali K, Bradley D (1984) Turbulent burning
velocities and flame straining in explosions. Proc R Soc Lond
Ser A Math Phys Sci 391:393–414
Baines W, Peterson E (1951) An investigation of flow through
screens. Trans Am Soc Mech Eng 73(1):467–480
Bédat B, Cheng RK (1995) Experimental study of premixed flames in
intense isotropic turbulence. Combust Flame 100(3):485–494
Burke M, Qin X, Ju Y, Dryer F (2007) Measurements of hydrogen
syngas flame speeds at elevated pressures. In: 5th US combustion meeting, San Diego, p 14
Comte-Bellot G, Corrsin S (1966) The use of a contraction to improve
the isotropy of grid-generated turbulence. J Fluid Mech Digit
Arch 25(04):657–682. doi:10.1017/S0022112066000338
Coppola G, Gomez A (2009) Experimental investigation on a
turbulence generation system with high-blockage plates. Exp
Therm Fluid Sci 33(7):1037–1048
Driscoll JF (2008) Turbulent premixed combustion: flamelet structure
and its effect on turbulent burning velocities. Prog Energy
Combust Sci 34(1):91–134
Kortschik C, Plessing T, Peters N (2004) Laser optical investigation
of turbulent transport of temperature ahead of the preheat zone in
a premixed flame. Combust Flame 136(1–2):43–50
Koutmos P, McGuirk JJ (1989) Isothermal flow in a gas turbine
combustor—a benchmark experimental study. Exp Fluids
7(5):344–354
Liao SY, Jiang DM, Gao J, Huang ZH (2003) Measurements of
Markstein numbers and Laminar burning velocities for natural
gas–air mixtures. Energy Fuels 18(2):316–326. doi:10.1021/
ef034036z
Makita H (1991) Realization of a large-scale turbulence field in a
small wind tunnel. Fluid Dyn Res 8(1–4):53–64
Mayo WT Jr (1974) A discussion of limitations and extentions of
power spectrum estimation with burst-counter LDV systems. In:
International workshop on laser velocimetry, West Lafayette,
Indiana, Purdue University, pp 90–101
Mydlarski L, Warhaft Z (1996) On the onset of high-Reynoldsnumber grid-generated wind tunnel turbulence. J Fluid Mech
320(-1):331–368. doi:10.1017/S0022112096007562
Mydlarski L, Warhaft Z (1998) Passive scalar statistics in highPéclet-number grid turbulence. J Fluid Mech 358(-1):135–175.
doi:10.1017/S0022112097008161
123
620
Natarajan J, Lieuwen T, Seitzman J (2007) Laminar flame speeds of
H2/CO mixtures: effect of CO2 dilution, preheat temperature,
and pressure. Combust Flame 151(1–2):104–119
Prandtl L (1932) Herstellung einwandfreier Luftströme:(Windkanäle)
Shavit U, Chigier N (1996) Development and evaluation of a new
turbulence generator for atomization research. Exp Fluids
20(4):291–301
Shepherd IG, Bourguignon E, Michou Y, Gökalp I (1998) The
burning rate in turbulent Bunsen flames. Symp (Int) Combust
27(1):909–916
Taylor G (1935) Turbulence in a contracting stream. ZAMM-J Appl
Math Mech/Zeitschrift für Angewandte Mathematik und Mechanik 15(1–2):91–96
123
Exp Fluids (2011) 51:611–620
Tennekes H, Lumley J (1972) A first course in turbulence. The MIT
press, Cambridge
Thole K, Bogard D, Whan-Tong J (1994) Generating high freestream
turbulence levels. Exp Fluids 17(6):375–380
Tummers MJ, Passchier DM (1996) Spectral estimation using a
variable window and the slotting technique with local normalization. Meas Sci Technol 7:1541–1546
Videto B, Santavicca D (1991) A turbulent flow system for studying
turbulent combustion processes. Combust Sci Technol
76(1):159–164