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VORTEX-INDUCED VIBRATION FOR HEAT TRANSFER ENHANCEMENT
Junxiang Shi, Steven R. Schafer, Chung-Lung (C.L.) Chen* Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO, United States
ABSTRACT A passive, self-agitating method which takes advantage of
vortex-induced vibration (VIV) is presented to disrupt the
thermal boundary layer and thereby enhance the convective
heat transfer performance of a channel. A flexible cylinder is
placed at centerline of a channel. The vortex shedding due to
the presence of the cylinder generates a periodic lift force and
the consequent vibration of the cylinder. The fluid-structure-
interaction (FSI) due to the vibration strengthens the disruption
of the thermal boundary layer by reinforcing vortex interaction
with the walls, and improves the mixing process. This novel
concept is demonstrated by a three-dimensional modeling study
in different channels. The fluid dynamics and thermal
performance are discussed in terms of the vortex dynamics,
disruption of the thermal boundary layer, local and average
Nusselt numbers (Nu), and pressure loss. At different
conditions (Reynolds numbers, channel geometries, material
properties), the channel with the VIV is seen to significantly
increase the convective heat transfer coefficient. When the
Reynolds number is 168, the channel with the VIV improves
the average Nu by 234.8% and 51.4% in comparison with a
clean channel and a channel with a stationary cylinder,
respectively. The cylinder with the natural frequency close to
the vortex shedding frequency is proved to have the maximum
heat transfer enhancement. When the natural frequency is
different from the vortex shedding frequency, the lower natural
frequency shows a higher heat transfer rate and lower pressure
loss than the larger one.
NOMENCLATURE H (mm) Height of the channel
L (mm) Length of the channel
W (mm) Width of the channel
D (mm) Diameter of cylinder
d (mm) Distance from cylinder axis to inlet
𝐮𝐟 (m/s) Fluid flow velocity
𝐮𝐠 (m/s) Mesh velocity
𝑢𝑖𝑛 (K) Inlet velocity
ρ (kg/m3) Density
𝐶𝑝 (kJ/kg·K) Specific heat capacity
K (W/m·K) Thermal conductivity
𝑇𝑖𝑛 (K) Inlet temperature
𝑇𝑤 (K) Wall temperature
𝑓 Frequency (Hz)
St Strouhal number, = 𝑓𝐷/𝑢
𝑅𝑒𝐷 Reynolds number, = 𝑟𝑢𝑚𝐷/𝑚
INTRODUCTION Single-phase based convective heat transfer has been
widely used in various engineering applications. In recent
decades much effort has been devoted to enhancing the thermal
and hydraulic performance of single-phase channel flow [1] by
considering different mechanisms. It has been considered that
the cooling capacity of the laminar channel flow is hampered
by the dominant diffusive transport within the thermal
boundary layer. Hence, most work has been devoted to
enhancing the heat transfer performance by emphasizing
boundary layer disruption. Encouraging results have shown that
surface interruption by means such as vortex generators [2-5]
Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014
November 14-20, 2014, Montreal, Quebec, Canada
IMECE2014-37594
1 Copyright © 2014 by ASME
can significantly enhance the heat transfer coefficients.
However, surface interruption also increases the pressure drop
and thus requires higher pumping power.
Due to the development of microelectro-mechanical
systems (MEMS), heat transfer enhancement with active flow
control technologies have been implemented in the last decade.
Synthetic jet actuators were introduced to induce momentum
flux and improve the heat transfer coefficient of the heat sink
[6-8]. Ma and Chen et al.[9] and Yu and Simon et al.[10] used
vibrating fins actuated with piezoelectric material to break the
thermal boundary layer, and thereby enhance the convective
heat transfer coefficient. Yu and Simon et al concluded that the
turbulence induced by the tip of the plate substantially
increased the heat transfer and that the translational motion of
the plate performed better than the flapping motion. Air-jet-
impingement cooling [11, 12], which uses a high-pressure
pump to generate a jet of compressed air directed at a heat sink
surface, is also an effective method for reducing the thermal
resistance of the thermal boundary layer.
The phenomenon of vortex induced vibration (VIV)
has been widely investigated [13-19] due to its importance in
engineering. The results have demonstrated that the vortex
shedding can resonate with one of the fundamental natural
frequencies or harmonic components of the flexible structure.
However, most of the previous studies focused on the structures
themselves, and the effects of this passive flow-structure
interaction (FSI) on the heat transfer in the channels have not
been studied in theory and experiments. Heat transfer
enhancement by intensifying vortex interaction with the walls
has been numerically demonstrated by Fu and Tong [20] and
Celik and Raisee et al. [21]. In those works, a cylinder in a two-
dimensional channel is actively and transversely vibrated at low
Reynolds numbers (ReD<=500). The encouraging results prove
that the generated vortex dynamics can effectively disturb the
thermal boundary layer and increase the heat transfer
coefficient of the channel wall. In our recent work [22], we
presented the heat transfer enhancement of a channel flow
which exploited VIV. In this design, a cylinder with a flexible
plate was placed in a clean channel; the vortex shedding due to
the cylinder gave rise to the oscillation of the plate downstream.
The two-dimensional numerical results showed that an
enhancement of the average Nusselt number in comparison
with the clean channel and the channel with a stationary
cylinder can reach increases of up to 91.95% and 22.10% when
ReD=204.8. In our other work[23], we studied a three-
dimensional channel with a flexible cylinder, and the vortex
induced vibration of the cylinder was demonstrated to distinctly
enhance the convective heat transfer rate at different Reynolds
number.
In this work, we extend our investigation of vortex
induced vibration for heat transfer enhancement. A three
dimensional flow-structure-thermal interaction enabled model
is employed to study the influences of Reynolds number,
structural properties, and channel geometries on the heat
transfer enhancement by VIV.
MODEL DESCRIPTION As shown in the schematic in Figure 1, an insulated
cylinder with a diameter of D is placed at the centerline of the
channel with a height of H, a width of W, and a length of L.
The distance between the cylinder’s axis and the entrance is d.
The four sidewalls of the channel are heated at a constant
temperature. Table 1 lists the values of the dimensional
variables. Water is selected as the working fluid in the channel.
The thermal properties of water are assumed to be constant at
25ºC. The flow in the channel is assumed to be laminar.
L
d
D H
uin, Tin
x
y
W
z
y
Constant temperature boundary Tw
Top wall
Bottom wall
y
xz Side wall
Side wall
Figure 1 Schematic of the channel with a cylinder
Table 1 Dimensions (mm) of the channel with a cylinder
Length of channel - L 50
Width of channel – W 30
Diameter of cylinder - D 1.5
Height of channel – H 6
Distance from cylinder axis to channel entrance – d 5
The inlet velocities, inu , in the two cases are given as
0.075m/s and 0.1m/s, corresponding to ReD inu D ~ 126
and 168, respectively. The inlet temperature profile (equation
(1)) is given to study the influence on the thermal boundary
layer. Here, the mean temperature, mT , is 313.15K; the constant
temperature of the channel walls, wT , is 343.15K.
2( ) 1 ( / 0.5 )in w w mT T y HT T (1)
When vortices shed from the cylinder an asymmetric
pressure distribution develops between the upper and lower
surfaces of the cylinder, and a periodic so-called lift on the
cylinder is generated. This force will induce the vibration of the
cylinder in a cross-flow direction. In the studies of VIV, the
2 Copyright © 2014 by ASME
Strouhal number ( St ) ( St fD u ) is a dimensionless
parameter describing the relationship among the shedding
frequency, characteristic length, and velocity at free stream
conditions. The Strouhal number depends on the Reynolds
number, and its function has been well summarized in [24].
Numerical and experimental studies [14, 25, 26] have indicated
that the walls will suppress the vortex shedding when the
cylinder is placed in a confined channel. The block ratio of the
cylinder diameter over the height of the channel has a
significant influence on the aforementioned function if the ratio
is larger than 0.35. In this work the block ratio is 0.25, hence
the actual frequency of the vortex shedding is determined in the
simulation.
NUMERICAL METHOD As the cylinder vibrates the flow field changes and
affects the deformation of the cylinder in return. Therefore, a
two-way FSI enabled solver is indispensable when studying the
heat transfer enhancement by the vibration of the cylinder. This
two-way FSI solution requires two solvers for the
computational fluid dynamics simulation (CFD) and
computational structural dynamics simulation (CSD) separately.
In the present work, two commercially available solvers,
ANSYS CFX 14.0 and ANSYS Transient Structural Module,
are employed. In view of the frequency, the time step is tuned
to 0.001(s) to capture the periodic vibration with sufficient
resolution. An unstructured quadrilateral grid is used for the
three dimensional computational domain. Turek and Hron [27]
provided numerous benchmarks for validating the FSI solver.
This solver has also been validated in our previous work [22].
In addition, three grids are employed to identify the grid
independence. The element number of the structural domain is
~20,000. The element numbers of the fluid domain are
~150,000, ~300,000 and ~600,000, respectively. The difference
in the average outlet temperature and pressure loss between the
~300,000 and ~600,000 node domains is less than 1%. Thus, in
the following discussion, the gird with ~300,000 elements is
used, considering the computational time and accuracy.
RESULTS AND DISCUSSION In this section we investigate the performance of a
clean channel (channel I), a channel with a stationary cylinder
(channel II), and a channel with a flexible cylinder (channel III).
Two Reynolds numbers, ReD (126 and 168), are investigated
with two inlet velocities. We will study the influence of the
material characteristics of the cylinder and the channel
geometries. The local and average Nusselt numbers are defined
in equation (2) to evaluate the heat transfer rate, where T is
the temperature gradient of the fluid at the channel wall
boundary. Since the vortex dynamics mainly interact with the
vertical walls (top and bottom shown in Figure 1), the average
Nu is obtained by integrating the local value over the channel
length, width, and vortex-shedding period .
0
( , )
1 1[( )
2
( ) ]
w m
W L topwall
W L bottomwall
HNu x t T
T T
Nu Nu dxdzWL
Nu dxdz
(2)
Performance of VIV at different Reynolds numbers
Figure 2 Lifts on the stationary cylinder when Re 168D
4
2 4
2
1 1(A πD I, πD A g)
4,
64
n
n
s
K EIgf
W
(3)
Figure 2 shows the lift coefficients on the stationary
cylinder at Re 168D . The lift coefficients, LC , are calculated
by 20.5 refF u D , where F is the force (Lift or Drag) and refu
is the reference velocity, which is taken as 0.1 m/s. The
frequency is estimated as 17.9 Hz, corresponding to a Strouhal
number of 0.2685. In channel III, the flexible cylinder is
composed of linear material and is fixed at both sides, hence
the Young’s modulus of the material can be used to estimate
the natural frequency by equation (3) in [28]. Here, A is the
cross-section area of the cylinder, I is the area moment of
inertia, is the load per unit length including weight, nK is
the constant of mode taken to be 22.4, and g is the
gravitational acceleration. Density s and Poisson’s Ratio are
taken as 2343.8 kg/m3 and 0.3, respectively. The Young’s
modulus of the cylinder at Re 168D is approximated as
4.1×105 Pa, corresponding to a natural frequency of 19.6 Hz.
The natural frequency of the cylinder is 109.5% of the vortex
shedding frequency. When the VIV occurs in channel III, the
amplitudes and frequency of the lift coefficients change
relevantly due to the fluid-structure-interaction. It is seen that
3 Copyright © 2014 by ASME
the lift coefficient upon the vibrating cylinder is much larger
than that upon the stationary one. As the cylinder vibrates the
flow velocity at the moving direction of the cylinder increases
due to the smaller cross section, as a result, the static pressure
and lift coefficient on the cylinder surface increase. The
frequency, calculated from Figure 3(a), is about 18.5 Hz, which
is 103.4% higher than the original one. This can be attributed to
the larger natural frequency of the cylinder. Figure 3(b) shows
the displacement of the center point of the cylinder (x=5mm,
y=0, z=0). It is worth mentioning here that the cylinder has not
only a periodic vibration along the y direction but also periodic
movement along the flow (x) direction.
Figure 3 Displacement of the center point of the cylinder when
Re 168D (a) Y-displacement with time (b) x and y
displacement in one period
Figure 4 Vorticity contours (Left) and Temperature fields at xy
plane (z=0) (a) channel I (b) channel II (c-d) channel III, when
Re 168D
Since the maximum deformation of the cylinder
occurs at its center, where z=0, the temperature field and vortex
dynamics at the xy cross plane (z=0) and xz cross plane (y=0)
are used to investigate the fluid dynamics and heat transfer. In
order to investigate the vortex dynamics and corresponding
influence on the heat transfer, Figure 4 illustrates z-directional
vorticity contours and temperature fields in three channels at
plane z=0. In channel II the vortices shed downstream of the
cylinder and arrange close to the centerline of the channel. The
vortex shedding is damped along the downstream, which is
consistent with [21]. Previous work has asserted that the
channel with a cylinder performs better in terms of heat transfer
than the clean channel, as the generated vortex shedding
disturbs the development of the thermal boundary layer
downstream. In channel III, the vibrating cylinder at a higher
frequency reinforces the interaction between the vortex
structures and wall shear layers. As shown in Figure 4c and 4d,
the vibration of the cylinder results in an up-and-down motion
of the vortex cores and moves these cores closer to the wall
shear layers. It is noticed that the number of the vortex cores in
channel III increases downstream with the increase in strength.
It is considered that the vibrating cylinder breaks the vortex
structures into multiple vortex structures, which is beneficial to
the hydrodynamic mixing process and heat transfer. In channels
II and III the vortex dynamics facilitate mixing the hot fluid
from the walls to the center of the channel and vice versa. After
studying the temperature fields seen in Figures 4 there is no
doubt that channel III performs better than channel II. In
channel II the cold fluid is broken into smaller segments by the
vortex/boundary-layer interaction. These segments arrange like
a train similar to how the vortex cores do. In channel III, the
stronger interaction assists the colder segments in penetrating
the thermal boundary layers and moving downstream.
Obviously, these motions increase the interfacial area between
the hot and cold fluid, and eventually enhances the heat transfer
rate. The results re-confirmed the conclusion drawn in our
previous two-dimensional investigations [22].
Figure 5 illustrates the enhancement of the local Nu on
the top walls of channel II and III when Re 168D . Due to the
periodic motion of the cylinder, the local Nu on the whole
bottom wall is close to that of the top wall. In channels II and
III the local Nu at three downstream positions is improved by
the vortex shedding; including the middle of the channel and
two portions near the horizontal sidewalls. The enhancement of
the local Nu at several regions is more significant in channel III.
To explain this phenomenon, vorticity at four cross-sections
along the flow direction (x=15mm, 25mm, 35mm and 45mm)
are investigated in channel III, as shown in Figure 6. We can
clearly observe the movement in vorticity at 15mm as the
vibration of the cylinder. This cross-flow movement increases
the mixing of the fluid within this region and thereby the heat
transfer rate. The vorticity downstream also has similar
dynamics.
4 Copyright © 2014 by ASME
Figure 5 Local Nusselt number on the top walls of (a) channel
II and (b) III when Re 168D
Figure 6 Vorticity contours at four sections (x=15mm, 25mm,
35mm, and 45mm) in channel III when Re 168D
Figure 7 compares the local Nusselt numbers at the
same time step; the walls are located at the xy plane z=0. For
channel I, Nu decreases monotonically due to the increase in
the thermal boundary layer thickness and approaches the fully
developed value. For channels II and III, the disruptions of the
thermal boundary layer lead to a non-monotonic variation of
Nu. The vibrating Nu along the downstream can be attributed
to the disturbed thermal boundary layer shown in Figure 4. Nu
increases near the 5mm distance from the entrance as the
cylinder located here decreases the thickness of the thermal
boundary layer. When the flow passes the cylinder the
thickness of the thermal boundary layer increases, and
consequently Nu decreases. Downstream of the cylinder the
interaction between the vortices and the thermal boundary layer
begins to enhance Nu. For channel II, this enhancement
increases gradually until reaching the outlet. Therefore, two
mechanisms contribute to the heat transfer enhancement; one is
squeezing the thermal boundary by the structure, and the other
one is disrupting the thermal boundary layer through vortex
interaction. Therefore, the local Nu of channel III is expected to
be higher than that of channel II. It is seen that Nu increases
sharply just behind the cylinder and then decreases
monotonically. Even so, the downstream Nu is always bigger
than that of channel II. The strengthened vortex dynamics and
the interaction with the wall shear layer can explain the
variation of Nu. It is apparent that the maximum enhancement
occurs close to the cylinder.
Figure 7 Local Nusselt number at xy plane (z=0) when
Re 168D
Table 2 lists the average Nusselt numbers of the three
channels. When Re 168D , channel II increases Nu from
1.52 to 3.355 (+120.7%), and channel III increases to 5.078
(+234.8%). For channel I, the increasing Reynolds number
(inlet velocity) cannot improve the average Nu distinctly
because the heat diffusion within the thermal boundary layer
plays a dominant role in heat transfer. It is noticed that Nu decreases slightly as ReD increases in channel I, which can be
attributed to the given inlet temperature profile. It is reasonable
that channel III performs better than channel II at the three
Reynolds numbers in view of the previous discussion.
Table 2 Average Nusselt number in three channels (Percentage
in the bracket is heat transfer enhancement compared to
channel I)
ReD 168
I/ Nu 1.52
II/ Nu 3.355 (+120.7%)
III/ Nu 5.078 (+234.8%)
It is known that the disruption of the velocity
boundary layer will lead to a greater pressure loss throughout
5 Copyright © 2014 by ASME
the channel. Moreover, the drag due to the structure also
contributes to the pressure loss of channels II and III. Figure 8
shows the drag coefficients upon the cylinder in channels II and
III. It can be seen that the VIV increases the drag coefficients in
a manner similar to that of the lift coefficients. Therefore, it is
not surprising that channel III has a larger pressure loss than
channel II. To evaluate the pressure loss coefficients of
different channels, the average value over a period is taken into
account and listed in Table 3. The pressure loss coefficients are
computed by 20.5 refP u . Comparing the thermal and
hydraulic performances between channels II and III, it is seen
that channel III increases the pressure loss by 16.79%, while
simultaneously improving the average Nu by 51.4%.
Table 3 Pressure drop coefficients in three channels
(Percentage in the bracket is pressure loss coefficient compared
to channel I)
ReD 168
I / cP 1.58
II / cP 2.74 (73.4%)
III / cP 3.2 (102.5%)
Figure 8 Drag coefficients in channel II and III when Re 168D
Effects of material properties
Celik and Raisee et al. [21] assert that the thermal
performance has the maximum improvement when the
vibrating frequency is 75% of the vortex shedding frequency.
Meanwhile, the amplitude of the vibration could have
significant effects. In this work, the cylinder is self-agitated due
to the FSI. Therefore, the vibrating frequency and amplitude
not only depend on the vortex shedding process but also the
properties of the cylinder, including Young’s Modulus and
density. To study the effects of the characteristics of the
cylinder, the Reynolds number is set as 126. When Re 126D ,
the original vortex shedding frequency is 13.4 Hz. If the density
and Young’s modulus of the cylinder is 2343.8 kg/m3 and
2.30625×105 Pa, respectively, the corresponding natural
frequency of the cylinder is 14.7 Hz, which is close to 13.4Hz.
Then, the density is tuned to be 1042 kg/m3 and 4166.8 kg/m3,
as a result, the natural frequency is 22 Hz and 11.02 Hz. The
vertical displacements of the cylinder are compared in Figure 9.
The cylinder with a density of 2343.8 kg/m3 has the largest
displacement because its natural frequency is closest to the
original vortex shedding frequency. Due to the difference
between the original vortex shedding and natural frequency
increases, the cylinder has a smaller deformation which can be
attributed to the weakened resonance. The frequency in Figure
9 is 14.3 Hz, 13.8 Hz, and 11.9 Hz for the cylinder, which is
88.8%, 103.0% and 106.7% of the original vortex shedding
frequency. Apparently, the cylinder with the smallest natural
frequency has a greater impact on the oscillating frequency of
the cylinder. Here, the cylinder with the greatest natural
frequency has the smallest deformation. As such, it is
reasonable to consider that this cylinder will have the lowest
heat transfer enhancement.
Figure 9 Y-directional displacements of the cylinder with
different properties at Re 126D
Table 4 Average Nu and Pressure loss coefficients of the
channels with three cylinders at Re 126D
Nu p
I 1.561 1.068
II 3.038
(+94.6%)
1.751
(+63.9%)
III 3( )s kg m
2343.8
4.5671
(+192.6%)
2.03
(+90.0%)
III 3( )s kg m
1042 3.358 (+115.1%) 1.87 (+75.1%)
III 3( )s kg m
4166.8 4.108 (+163.2%) 1.86 (+74.2%)
Table 4 lists the thermal and hydraulic performance by
the average Nu and pressure loss coefficient. The cylinder with
the greatest natural frequency loses more heat transfer
6 Copyright © 2014 by ASME
enhancement than the others, but the corresponding pressure
loss is not the smallest one. On the contrary, the cylinder with
the smallest natural frequency has the smallest pressure loss
and an intermediate heat transfer enhancement. Therefore, it is
concluded that for the same Young’s modulus, the natural
frequency of the cylinder should be close to the original vortex
shedding frequency to reach the maximum heat transfer
enhancement, otherwise, it is better for the frequency to be as
small as possible.
Effects of channel geometries
It is noticed that the Young’s modulus of the solid
cylinder is much smaller than various metals and alloys. To
overcome this shortcoming, the cylinder may be designed as a
hollow tube. Meanwhile, several sorts of polymers can satisfy
different properties, for instance, the Young’s modulus of
polybutadiene is about 1.6x106 Pa, and that of polyurethane is
25x106 Pa. When the width of the channel increases, the length
of the cylinder extends, therefore the Young’s modulus of the
cylinder should increase at the same flow conditions. In this
part, the width is taken as 63mm. The Young’s modulus of the
cylinder is chosen as 4x106 Pa, the density is considered as
1200 kg/m3 and 900 kg/m3, and the consequent natural
frequency is 19.4 Hz and 22.5 Hz. The performance of both
cylinders is listed in Table 5. It can be found that the cylinder
with larger density and corresponding lower natural frequency
has higher thermal performance enhancement and lower
pressure loss than the other one.
Table 5 Average Nu and Pressure loss coefficients of the
channels with two cylinders at Re 168D
Channel I II III3( )s kg m
=1200
III 3( )s kg m
=900
Nu 1.54 3.31
(+114.9%)
4.56
(+196.1%)
4.44
(+188.3%)
p 4.23 7.58
(+79.1%)
8.76
(+107.1%)
8.89
(+110.2%)
CONCLUSION A cylinder which is self-agitated due to vortex induced
vibration is introduced to enhance the convective heat transfer
coefficient of the single-phase channel flow without any
additional power. A three-dimensional model with an FSI
enabled solver is used to demonstrate the performance of this
approach at different Reynolds numbers and in different
channels. Three channel configurations are studied: a clean
channel, a channel with a stationary cylinder, and a channel
with a flexible cylinder. The results demonstrate that the
channels with VIV perform the best at all of the flow
conditions as the periodic vibration of the cylinder strengthens
the vortex interaction with the walls, and thereby enhances the
mixing process. The properties of the cylinder, and different
Reynolds numbers and channel geometries are investigated. If
the Young’s modulus is the same, the VIV reaches the
maximum performance once the natural frequency of the
cylinder is close to the original vortex shedding frequency.
When the natural frequency is different from the vortex
shedding frequency, the lower natural frequency is better than
the larger one when considering hydraulic and thermal
performance. Therefore, the channel with the VIV is considered
to be a promising way to further enhance the heat transfer rate
of the single-phase channel flow with acceptable pressure
penalty.
ACKNOWLEDGEMENTS This research is sponsored by startup funding from University
of Missouri and Dr. Myers, Joseph at ARO under contract No.
W911NF-12-1-0147.
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