Journal of KONES Internal Combustion Engines 2003, vol. 10, 3-4
VARIOUS EXPERIMENTAL METHODS TO STUDY HEAT TRANSFER
FROM THE HEATED RIB-ROUGHED WALL TO A STEADY OR
PULSATING FLOW
Wojciech Jarosinski
Motor Transport Institute
03-301 Warszawa ul. Jagiellońska 80
email: [email protected], fax: (022)8110906 tel. (022)8141237
Abstract
The heat exchanger performance is mostly limited by the poor gas side heat transfer. This motivates attempts
to increase the effectiveness of gas side heat transfer processes in numerous types of heat exchangers in
automotive industry mostly used for cooling. The efficiency of compact heat exchangers can be improved by
means of boundary layer modification. The objective of this work is to study the mechanism of convective heat
transfer from the heated wall of a duct to a flow of air. The surface of the heated wall is flat or rib-roughened.
Rib-roughened surfaces are used to induce turbulence and to enhance the heat transfer. Interferometer,
schlieren and PIV methods assisted by high-speed video camera are used to study the mechanism of heat
transfer from the heated wall to the flow. These methods are accompanied by measurements of the flow velocity,
instant temperature, turbulence and frequency analysis of turbulence parameters. The range of Reynolds
numbers studied was from 1000 to 12000. Finally the influence of a steady, oscillatory and pulsating flow on the
heat transfer is investigated and compared. The low frequency oscillations from 0.083 to 0.25 Hz are applied in
experiments to be comparable with frequencies used in literature. Frequencies of the pulsating flow are used in
the range from 0.2 to 5 Hz. The results are presented in a form of a time-averaged Nusselt number as a function
of a Reynolds number calculated on the basis of the oscillatory and pulsating flow parameters. It is found that
turbulence promoters on the heated wall enhance heat transfer, while low frequency pulsation of the mean flow
deteriorates it. Both effects are explained by the processes in the boundary layer sticking to the heated wall of
the investigated channel.
Introduction
Many techniques have been developed recently to enhance convective heat transfer.
Investigations of these techniques are carried out both for practical and cognitive reasons.
Surfaces, periodically interrupted along the streamwise direction by ribs or grooves have been
widely used as the most effective means of heat transfer enhancement. Turbulence promoters
are applied to induce turbulence into laminar part of the flow. Such solution contribute to
improvement of the performance of heat exchangers by reducing their sizes. Still the open
question remains the real mechanism of convective heat transfer from the heated ribroughened wall of a channel to a flow of air. Formation, development and decay of vortical
structures, during flow of water along a rib-roughened surface were investigated
experimentally relatively long time ago [1]. It was found that dynamics of vortical structures
depends on the dimensionless rib spacing (the pitch to height ratio) and on the geometry of rib
edge. Later experimental and numerical investigations in stationary flow with rib-roughened
and grooved surfaces showed that application of any ribs or special inserts do not enhance
heat transfer as long as the Reynolds number is low [2-4].
In a recent experimental work Djenidi et al. [5] have explored the structure of a turbulent
boundary layer over a grooved wall made up of two-dimensional square cavities located
transversally to the flow direction. The Reynolds number ranged from 900 to 2300. The
visualization revealed outflow of fluid from the cavities as well as inflows into the cavities. It
was observed that outflows of fluid from the cavities into the overlying flow take place
randomly and are associated with the passage of near-wall quasi-streamwise vortices, similar
to those found in a smooth wall turbulent boundary layer. Relative to a smooth wall layer,
there is a visible increase in magnitudes of the Reynolds stresses and a smaller streamwise
variation of the local skin friction coefficient.
It was shown in a series of investigations [6-9] that hydrodynamic instability of laminar
flow can increase significantly the heat transport rates. The low Reynolds number flow
instability can be introduced by the designer, who can define special geometry of heat
exchanger passages, or by external excitation of natural resonant frequency of the channel
with appropriate modulation. The second method is applicable both for laminar and turbulent
flows. The self- sustained oscillations destabilize laminar thermal boundary layers and mix
the near-wall fluid with the fluid in the core region.
Experiments show beneficial influence of turbulence promoters on heat transfer
enhancement for turbulent flow through a channel [4, 10-16]. Systematic investigations of the
influence of ribs shape on effectiveness of heat transfer in a channel were carried out in [4]. It
was found that the most effective rib spacing in turbulent flow is a rib-pitch-to- height ratio of
p/e = 10 when one considers both heat transfer and friction factor. The turbulence promoters
show their best effect in the region with Reynolds numbers Re = 1500-6000. It was found that
the local heat transfer always has maxima located just before the top of the rib, minima just
behind the ribs and intermediate values in the spacing between the ribs. The local heat transfer
minima behind the ribs were explained by stationary air flow bound in recirculation zones
behind the ribs. The flow bound in such vortices have only a little exchange with the main
flow, and therefore cause poor heat transfer in these areas.
In a recent study Olson and Sunden [13-15], following earlier works of Han et al. [10],
have investigated the establishment of large secondary flows in rectangular ducts with ribroughened walls. Secondary flows are created by rib configuration with angle of attack
different than 90°. These flows are expected to be additional factor in enhancing heat transfer.
Numerical investigations of heat transfer and friction in a rectangular duct were carried out
by Fodemski [17-19] and Sunden [14]. Calculated predictions were found to be qualitatively
in good agreement with published results. Reasonable agreement with temperature
distribution measured by holographic interferometry was obtained [17-18].
Opinions diverge on the dynamics of vortices formed in the cavity between the ribs and on
the effects of rib shape and angle of attack on the heat transfer coefficient in rib-roughened
ducts. The model of flow in a in rib-roughened wall vicinity presented by Williams and Watts
[1] is different from that of Tauscher and Mayinger [4]. Williams and Watts consider the
periodical shedding of vortices from the cavity to the main flow as an important factor of heat
transfer. Tauscher and Mayinger, and many other authors, consider standing recirculation
zones behind the ribs, with about 6% of the flow bound in such vortices, as having only a
little exchange with the main flow, and therefore causing poor heat transfer in these areas.
Han et al. [10] found, in their study, that the rib shape had only a modest effect on the heat
transfer coefficient. In contrast, other investigators, among them Tauscher and Mayinger [4],
show considerable influence of the rib shape on the heat transfer. Contrasting opinions on the
effect of rib angle of attack on the heat transfer coefficient are discussed in the paper of Olson
and Sunden [13].
More reliable experimental data are required to verify the mechanism of heat transfer from
the heated rib-roughened wall to a flow of air. Empirical knowledge in a form of different
parametric relations between channel geometry, flow and heat transfer can be apparatus
dependent. It is necessary to measure the instant flow and temperature parameters. In the
present study visualization and thermoanemometric methods are used to investigate the heat
transfer process in its dynamics. Additionally heat transfer from the heated wall to the
pulsating flow is investigated and the results are compared with those for the steady flow. The
comparison is made for the same mean mass velocity.
Experimental details
The wind tunnel was built to investigate the mechanism of convective heat transfer from a
smooth or rib-roughened heated wall of a rectangular duct to a pulsating or steady flow (Fig.
1). The basic part of the wind tunnel is a rectangular flow duct with a heated plate located at
the bottom wall of that duct. The duct is connected to a suction fan. The average volume flow
rate is measured behind the fan. The duct used in experiments is 60 mm wide, 20 mm high
and 400 mm long.
Fig. 1. Wind tunnel with rib-roughened heated plate
The heated plate is located along the duct at a distance of 125 mm from the inlet edge. The
plate has a sandwich structure (Fig. 2). An aluminum plate of dimensions 150 mm × 60 mm ×
7 mm faces directly the air stream. A heat flux transducer, 5 mm thick and with the diameter
equal to 50 mm, adjoins the upper aluminum plate in its central part. This transducer is
surrounded by a rubber layer, also 5 mm thick, characterized by the same thermal
conductivity coefficient as the transducer. Another aluminum plate, of the same dimensions
and parameters as the upper one, is situated under the transducer. The lowest layer consists of
a firebrick profile, in which a heater coil of 600 W maximum power was located. The heater
power can be controlled in a wide range. The sandwich plate is surrounded at the bottom by a
foamed brick which is a good heat insulator. The upper aluminum plate faces the measuring
duct surface and provides a surface for heat transfer to the flowing air. Three thermocouples
are located on the upper plate to measure an average surface temperature. The plate surface
temperature tw, used in the calculations of the heat transfer coefficient, is determined by
means of the thermocouple mounted in the middle part of the plate, directly over the heat flux
transducer. The remaining two thermocouples are used to control whether the plate heating
conditions remain isothermal. An air gap of 0.5 mm, is maintained between the sandwich
plate elements and the duct wall to avoid occurrence of a thermal bridge. Despite the fact that
a good thermal insulator around the sandwich plate is used, some heat losses through the
insulation were observed.
Fig. 2. Sandwich structure of the heated plate.
Fig. 3 Volume meter system.
The average volume flow rate was determined in this duct behind the fan by inflating a
given volume of a vessel in a measured time (Fig. 3).
To obtain a steady flow it was necessary to block the rotating plate in horizontal position.
Pulsating flow can be generated with a given frequency from 0.2 to 5 Hz by setting the
rotating plate in motion (small electric motor). The tests were carried out for smooth and ribroughened rectangular duct and for steady and pulsating flow. Transverse ribs 2 mm × 2 mm
were used. The pitch to height ratio was 10.5. The rib height to hydraulic diameter ratio was
0.067. The aspect ratio was H/W=1/3. The Reynolds number varied from 1000 to 11300. In
all experiments, before the test began, thermal steady state had been established. The system
required from three to six hours to reach such state. Steady state was obtained once the
temperature of the wall surface tw and the temperature of the inlet air t0, as well as the heat
flux q, had reached a stationary level. The stationary values of the parameters were recorded
at the end of each stage. The temperatures measured in various places of the plate differed by
no more than 2°C. The temperature t0 was measured at the duct inlet.
Heat transfer measurements
The Reynolds number for a pulsating or steady flow was determined from the relation:
Re = 2dw/ν
(1)
where 2d is the duct height, w is the average velocity in the duct cross-section, and ν is the
kinematic viscosity.
In order to determine the heat transfer coefficient h, which is defined as:
h = q/(Tw - T0)
(2)
a heat flux q flowing through the sandwich plate was measured perpendicularly to the plate
surface, and the temperature gradient between the wall surface and the flowing air was
determined as ∆T = tw - t0.
The Nusselt number was determined from the relation:
Nu = hl/λ
(3)
where l is the linear dimension of the plate (measured along the streamline), and λ is the
thermal conductivity.
Fig. 4. The Nusselt number versus the Reynolds number (1- steady flow in a smooth channel, 2 - steady flow in a
rib-roughened channel, 3- pulsating flow with frequency f=2 Hz in a smooth channel, 4- pulsating flow
with frequency f=2 Hz in a channel with rib-roughened surface of the wall).
The convective heat transfer from a smooth or rib-roughened heated wall of a rectangular
channel to a pulsating or steady flow was calculated on the basis of data determined in
experiments. The results of calculations are shown in Fig. 4. It can be easily seen from this
figure that the ribs located on the heated plate enhanced heat transfer to the flowing cold air in
comparison with the smooth channel. On the other hand heat transfer to the pulsating flow
showed that there are no advantages resulting from the application of such flow. However,
heat transfer from the heated plate to the pulsating flow is much more effective in the case of
rib-roughened channel, than of smooth one. Therefore, it can be noted that the pulsating
motion does not contribute to heat transfer enhancement for the laminar and turbulent flow
velocity range under investigation. This is contrary to a series of publications suggesting an
application of such flow for heat transfer enhancement. This negative result finds its
theoretical support in the paper of Valujeva et al. [20]. It has been also signaled previously in
publications related to some experimental works [21, 22].
Flow visualization
The following methods for visualization of heat flow and transfer were used in the study:
smoke visualization, PIV method, and also interferometer and schlieren methods. A Fizeau
interferometer was used with a field of vision 100 mm in diameter. Schlieren method was
employed with a single mirror 0.3 m in diameter and 2.5m radius of curvature. The density
gradients images were recorded by a standard video camera Panasonic type S-VHS-C model
NV-S70E and by a high-speed video camera Redlake of Motion Scope Company. The
interval between successive frames of Redlake camera was 0.002 s.
a)
b)
Fig. 5. Two selected schlieren images of pulsating flow through rectangular channel with rib-roughened heated
plate: a) minimum flow velocity; b) maximum flow velocity. Mean Reynolds number Re=5000. Regular
CCD video camera
It was found that only a few of the considered methods could be used effectively. A very
traditional smoke visualization method can be applied only for very slow laminar flows,
which are of marginal interest. However, it can be used successfully to detect qualitatively the
character of the secondary flows, which are also present in turbulent motion. On the other
hand the weak point of very modern and sophisticated PIV method is a limited number of
images, which makes it difficult to record step by step the unstable processes, to reveal their
dynamics.
a)
b)
Fig.6. Two selected schlieren images of pulsating flow through rectangular channel with rib-roughened heated
plate: a) minimum flow velocity, b) maximum flow velocity. Mean Reynolds number Re=5000. Highspeed video camera Redlake
The schlieren method is very helpful in studying very complex nonhomogenuities, such as
irregular vertical flows with gradients of density. It can be used together with the
interferometer method to study in detail the heat transfer mechanism. Just the same as in the
case of interferometer method a standard CCD video camera can be used with satisfactory
good records for very slow flows, while in the case of fast turbulent flows a picture from this
camera represents some statistical image of the phenomenon. In Fig. 5 two frames are shown
from video tape with record of pulsating flow, illustrating the influence of slow and fast air
flow on character of the images.
Only high-speed video camera can properly record vertical structures in turbulent motion
generated by ribs and heated by the wall.
Fig. 6 shows two frames from record of high-speed video camera Redlake which
correspond to conditions similar to those shown in Fig. 5.
Using the schlieren method one can observe turbulent motion of vertical structures with
different Reynolds numbers and the influence of Reynolds number on the scale of vertical
structures and on the rate of mixing process. The higher is the Reynolds number, the smaller
are volumes exchanging heat and faster is the mixing process. The mechanism of heat transfer
from the heated wall to the flow of air can be observed in its dynamics by watching motion of
flow recorded by a high speed video camera, but reproduced with the rate typical of a
standard video cassette recorder. Growth of the Reynolds number is followed by increased
pulsating frequency of small vertical structures moving along the duct with crosswise
fluctuations and increasing intensity of mixing process.
The interferometer method applied to heat transfer phenomena is a proper tool because
generally in such conditions variations of the refraction index determined by this method
depend only on temperature. The interferometer method is usually used to qualitative and
quantitative evaluation of density and temperature fields. Applied to investigation of heat
transfer from the heated wall of a rectangular duct to a flowing air, it makes possible to
observe variation of temperature field as a function of channel geometry and flow velocity
(Figs. 7 and 8). A standard CCD video camera proved to be sufficient to record interferometer
images of flow up to Reynolds number Re < 2000 (Figs. 7 and 8a), but in the range of higher
Reynolds numbers corresponding to transient or turbulent flows its framing frequency was too
low (Fig. 7b and 8b).
a)
b)
Fig. 7. Fizeau interferometer images of flow through rectangular channel with rib-roughened heated plate: a)
laminar flow with Re=1000; b) turbulent flow with Re=5000. Fringes situated perpendicular to flow.
Regular CCD video camera
a)
b)
Fig. 8. Fizeau interferometer images of flow through rectangular channel with rib-roughened heated plate: a)
laminar flow with Re=1000; b) turbulent flow with Re=5000. Fringes situated parallel to flow. Regular
CCD video camera
Interferograms might be used to calculate the distribution of temperature in the channel.
During calculation it is necessary to measure displacement of fringes in a field under
investigation in comparison with their position in a standard field with well known density.
The interferogram used for calculations of the temperature is shown in Fig. 9 and the
calculated distribution of the temperature in the rectangular channel in Fig. 10.
A relatively new tool which offers unparalleled possibilities for studying flow patterns is
the PIV method. Using this method, an instant flow velocity field in a selected channel cross
section can be determined. A serious disadvantage of the PIV method is that changes of flow
structure cannot be followed in short time intervals.
Fig.9. Interferogram used for calculation of the of the Fig.
rectangular channel related to the channel.
Fig. 11. Digitized high resolution PIV image of flow.
10.
Calculated temperature in a section
temperature in a section of the rectangular
middle part of the interval between 1st and
2nd rib on the interferogram (x is
coordinate normal to the heated plate).
Fig. 12. Velocity vectors calculated basing on the
picture from Fig. 11
(2D)  01 Dec 1998 01.vec
18
Y mm
16
14
12
10
8
6
10
15
X mm
20
25
Speed
1.40
1.33
1.26
1.19
1.12
1.05
0.98
0.91
0.84
0.77
0.70
0.63
0.56
0.49
0.42
0.35
0.28
0.21
0.14
0.07
0.00
20
18
16
14
12
10
8
6
10
15
X mm
20
25
Speed
1.40
1.33
1.26
1.19
1.12
1.05
0.98
0.91
0.84
0.77
0.70
0.63
0.56
0.49
0.42
0.35
0.28
0.21
0.14
0.07
0.00
20
18
16
Y mm
Speed
1.40
1.33
1.26
1.19
1.12
1.05
0.98
0.91
0.84
0.77
0.70
0.63
0.56
0.49
0.42
0.35
0.28
0.21
0.14
0.07
0.00
20
(2D)  01 Dec 1998 21 .vec
Y mm
(2D)  01 Dec 1998 51 .vec
14
12
10
8
6
10
15
20
25
X mm
Fig. 13. Streamlines in cross-section of the duct, X- distance across the duct, Y- distance along the duct. Rib
obstacles at the edges of the drawings on their left side. Displacement of vertical structures between rib
obstacles. Mean flow velocity corresponds to Re=2000.
This makes it difficult to record the unstable processes step by step, in order to reveal their
dynamics. The examples of PIV application for studying of the flow structure in the vicinity
of the heated rib-roughened wall are shown in the succeeding figures.
Records of PIV image of flow similar to that shown in Fig. 11 were used to calculate the
flow structure and the velocity field between the rib obstacles (Figs. 12, 13). Measurements
revealed fluctuations of vortical structures generated by the rib obstacles.
Conclusions
Studies on the visualization of air flow near the wall with rib-roughened obstacles
confirmed that the dynamics of vortex structure development near the wall is qualitatively
similar to that determined in water tunnel (Williams et al., 1970). As in that case, during the
flow a vortex is formed behind the rib obstacle. The vortex increases gradually and flows
down towards the next rib obstacle. Then, it flows over the rib and breaks up. The influence
of Reynolds number on the scale of vertical structures and on the rate of mixing process can
be observed. The higher the Reynolds number, the smaller are the volumes of exchanged heat
and the faster is the mixing process. The mechanism of heat transfer from the heated wall to
the flow of air can be observed in its dynamics by watching the flow motion recorded by a
high speed video camera, but reproduced with the rate typical of a standard video cassette
recorder. Growth of the Reynolds number is followed by increased pulsating frequency of
small vertical structures moving along the duct with crosswise fluctuations and increasing
intensity of mixing process. The motion of vertical structures from the heated wall to the
stream core is the main factor of heat transfer from the wall to the stream. Finally, it can be
noted that interferometer and schlieren methods completed with a high-speed video camera
are a very effective tool to analyze heat transfer from the heated wall to the flow. Also PIV
method is very effective in studying the flow structure. It was found that the ribs on the heated
wall, as turbulence promoters, considerably enhance heat transfer by introducing turbulence
into the boundary layer. It was also found that low frequency pulsation of the mean flow
deteriorates heat transfer because they periodically slow down the flow and the mixing
process near the wall.
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