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Pal et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945
Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86-89
Research Paper
EFFECT OF RELATIVE LONG-WAY LENGTH ON NUSSELT
NUMBER AND FRICTION FACTOR OF CONCAVE AND
CONVEX STREAMLINED PROTRUDED ARTIFICIALLY
ROUGHENED DUCT OF SOLAR AIR HEATERS Singh Sukhbir Pal
a, Mahajan Tarun
b, Kumar Pardeep
c
Address for Correspondence a,b,c
Sri Sai College of Engineering and Technology, Badhani, Punjab -145001
ABSTRACT: This paper presents result of an experimental investigation of heat transfer and friction loss for a range of system and
operating parameters, to study Nusselt number and friction factor of concave streamlined protruded artificially roughened
duct of solar air heaters. Enhancement of heat transfer and pressure drop of roughened absorber plate has been observed.
Experimental data may be used for designing such collectors for practical applications having investigated type of roughness
geometry.
KEY WORDS: Solar air heater, artificial roughness, Heat transfer coefficient, Nusselt number and Friction factor.
INTRODUCTION
Energy is a basic need for human being for generation
and economic development of nation. Energy
resources may be classified as conventional and non-
conventional energy resources. Global energy crisis,
environmental threats and to meet increasing energy
demand, need of alternative and clean energy sources
have to be increased. Solar energy is available in
abundance on earth in the form of radiation may be
converted into thermal energy and used for heating
and other applications. Solar air heater is a cheapest
way for converting solar energy into thermal energy
absorbs incoming solar radiation, converting it into
thermal energy at absorbing surface, and transferring
the energy to a fluid flowing through the collector.
These have found several applications including space
heating, crop drying, etc. The thermal efficiency of flat
plate solar air heater has been found to be low because
of low convective heat transfer coefficient between
absorber plate and flowing air which increases
absorber plate temperature leading to higher heat
losses to the environment resulting in low thermal
performance. Thermal Performance of solar air heaters
may be increased by increasing convective heat
transfer coefficient, by using fins or creating the
turbulence on the heat transferring surfaces [1].
Artificial roughness is basically a passive heat transfer
enhancement technique by which thermo hydraulic
performance of a solar air heater can be improved. The
artificial roughness has been used extensively for the
enhancement of forced convective heat transfer, which
further requires flow at the heat transferring surface to
be turbulent. However, energy for creating such
turbulence has to come from the fan or blower and
excessive power is required to flow air through the
duct, which may further be creating turbulence only in
the region very close to the heat transferring surface.
Several investigators reported heat transfer coefficient
and friction loss for roughened duct of solar air heaters
and developed the correlations for Nusselt number
and friction factor of various types of roughness
geometries. Bhushan and Singh [2] reported various
artificial roughness geometries produced by several
methods such as by Casting, welding and wire fixation
in the form of transverse continuous ribs, transverse
broken ribs, inclined and V-shaped or staggered ribs;
rib formation by machining process in the form of
chamfered ribs, wedge shaped ribs, combination of
different integral rib roughness elements and by using
expanded metal mesh ribs etc. Prasad and Mullick [3]
utilized artificial roughness in the duct used in solar
heater in the form of small diameter wires to increase
heat transfer coefficient. Prasad and Saini [4]
investigated fully developed turbulent flow in the duct
with a small diameter protrusion wire on the absorber
plate. Muluwork et al. [5] compared thermal
performance of staggered discrete V-apex up and
down with corresponding transverse staggered discrete
ribs. Momin et al. [6] experimentally investigated
effect of geometrical parameters of V-shaped ribs on
heat transfer and fluid flow characteristics in
rectangular duct used in solar air heaters. Jaurker et al.
[7] experimentally investigated heat transfer and
friction characteristics of rib-grooved artificial
roughness. Saini and Saini [8] used expanded metal
mesh to create artificial roughness on absorber plate
and investigated effect of system and operating
parameters on heat transfer and friction loss. Saini and
Verma [9] investigated heat transfer and friction
characteristics of dimple shaped artificial roughness
geometry for Reynolds number 2000–12,000. Bhushan
and Singh [10] analyzed the effect of artificial
roughness on heat transfer and friction in solar air
heater duct having protrusions as roughness geometry.
Varun et al. [11] presented a review on artificial
roughness investigations reported in literature.
Belenkiy et al. [12] used staggered array of concave
dimples in annular passages on the interior cylindrical
surfaces. Heat transfer augmentation as high as 150%,
compared to smooth surfaces were reported with
appreciable pressure losses. Kesarev and Kozlov [13]
present distributions of local heat transfer coefficients
inside a single hemispherical cavity, and indicate that
the convective heat transfer from the cavity is higher,
especially on the downstream portion, than that from
the surface of a plane circle of the same diameter as
the cavity diameter. Literature review revealed that
protrusions as artificial roughness is a good
technique for enhancing heat transfer coefficient of
air flowing through the duct and increases in friction
losses. Further, heat transfer coefficient of air
passing underside of the absorbing plate could be
enhanced by protruded concave streamlined
dimples. An experimental investigation has been
carried out to study the Nusselt number and friction
factor of roughened duct with concave streamlined
dimples protruded on the underside of absorber plate
of a solar air heater. The resulted increase in friction
loss can be further minimize by creating turbulence
only in the region very close to the absorber surface
i.e. in laminar sub-layer only [9].
EXPERIMENTAL SET-UP AND PROCEDURE To generate experimental data, a test rig has been
designed and fabricated; schematic and photographic
Pal et al., International Journal of Advanced Engineering Technology
Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86
views of experimental set-up are shown in Fig. (
and Fig. (2) respectively. It consists of a
mm x 840 mm x 70 mm) is made up of wooden ply
board of 20 mm thickness with length of entry, test
and exit sections are 900 mm, 1000 mm and 500 mm
respectively. Temperature and pressure drop
measuring instruments are provided in the
with surmounted electric heater assembly and
converging plenum (500 mm) was provided on exit
side of the air duct. To get uniform heating flux,
insulated electric heater (2200 mm x 82
been fabricated by combining series and parallel loops
of heating wire on mica sheet. Electric supply of
heater is being varied with help of Variac
U-tube manometer on calibrated orifice-meter is used
to measure the mass flow rate of air.
controlling of air flow rate, two gate valves one at
entry and other at exit of centrifugal blower
installed. To measure temperature of air and absorber
plate (22 SWG, GI sheet, 2400 mm x 880 mm),
copper-constantan thermocouples were used at
different locations as shown in Figs. (
Micro-manometer with kerosene oil as fluid was used
to measure the pressure drop across test section of the
duct. Photographic and geometrical views of
experimentally investigated protruded plate have
shown in Fig. (5) and Fig. (6).
Fig. 1: (a) Schematic of experiment set-up (b) Sectional
view of air duct at entry section.
Fig.2: Photographic view of experimental set
Table 1 shows range/value of parameters used in
present experimental investigation.
streamlined dimples were punched on the absorber
plate. Before starting the experimentation,
instruments/equipments like orifice meter, temperature
indicator, and temperature selector switch, micro
manometer and U-tube manometer were properly
inspected. All joints of the test rig were properly filled
with glue, to avoid leakage in any joint. Five values of
flow rate were used for each set and following data
were collected at an interval of one hour in each set of
experimentation:
• Pressure drop across orifice meter
flow rate of air.
• Pressure drop across test section of duct.
• Temperature of absorber plate at various
locations in test section of the duct.
• Temperature of air at various locations in test
section of the duct.
International Journal of Advanced Engineering Technology E-ISSN 0976
86-89
shown in Fig. (1)
consists of air duct (2400
mm x 840 mm x 70 mm) is made up of wooden ply
board of 20 mm thickness with length of entry, test
and exit sections are 900 mm, 1000 mm and 500 mm
Temperature and pressure drop
are provided in the air duct,
surmounted electric heater assembly and
was provided on exit
To get uniform heating flux, well
20 mm) have
ing series and parallel loops
Electric supply of
of Variac (0-250V). A
meter is used
to measure the mass flow rate of air. For smooth
wo gate valves one at
entry and other at exit of centrifugal blower was
To measure temperature of air and absorber
plate (22 SWG, GI sheet, 2400 mm x 880 mm),
constantan thermocouples were used at
in Figs. (3) and (4).
manometer with kerosene oil as fluid was used
ressure drop across test section of the
. Photographic and geometrical views of
experimentally investigated protruded plate have been
up (b) Sectional
view of air duct at entry section.
Photographic view of experimental set-up.
shows range/value of parameters used in
present experimental investigation. Concave
on the absorber
Before starting the experimentation,
equipments like orifice meter, temperature
indicator, and temperature selector switch, micro
tube manometer were properly
the test rig were properly filled
Five values of
flow rate were used for each set and following data
were collected at an interval of one hour in each set of
Pressure drop across orifice meter to measure
Pressure drop across test section of duct.
Temperature of absorber plate at various
Temperature of air at various locations in test
• Voltage and current supplied to electric heater.Table 1: Range/Value of parameters.
Accuracy of experimental data was verified by
conducting experiments for a conventional smooth
duct. Experimental and predicted values of
number and friction factor were
rectangular smooth duct as reported by Momin et al.
[6].
Dittus-Boelter correlation for Nusselt number is
Nus=0.023Re0.8Pr
0.4
Blasius equation for friction factor is
25.0Re085.0 −=sf
Fig. (7) and Fig. (8) shows experimental and predicted
values of Nusselt number and friction factor for
smooth absorber plate. Error analysis based on the
procedure described by Holman [14] has been carried
out to find out uncertainties in measured/calculated
values of experimental data. Uncertainty in Reynolds
number, Nusselt number and friction factor values has
been estimated as 2.63%, 2.55%, and 4.04%
respectively. Good agreement in experimental and
predicted data ensures accuracy of collected data from
self designed and fabricated experimental set
Fig. 3: Different locations of thermo
measure absorber plate temperature.
Fig.4: Different locations of thermocouples used to
measure air temperature in the duct.
Fig. 5: Photographic view of protruded metallic plate
Fig. 6: Geometrical views of protruded metallic plate
ISSN 0976-3945
electric heater. Range/Value of parameters.
Accuracy of experimental data was verified by
conducting experiments for a conventional smooth
Experimental and predicted values of Nusselt
were compared for
as reported by Momin et al.
Boelter correlation for Nusselt number is
(1)
(2)
experimental and predicted
of Nusselt number and friction factor for
smooth absorber plate. Error analysis based on the
] has been carried
t uncertainties in measured/calculated
values of experimental data. Uncertainty in Reynolds
number, Nusselt number and friction factor values has
been estimated as 2.63%, 2.55%, and 4.04%
Good agreement in experimental and
res accuracy of collected data from
self designed and fabricated experimental set-up.
couples used to
temperature.
Different locations of thermocouples used to
measure air temperature in the duct.
Fig. 5: Photographic view of protruded metallic plate
Fig. 6: Geometrical views of protruded metallic plate
Pal et al., International Journal of Advanced Engineering Technology
Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86
Fig.7:Experimental and predicted Friction factor and
Reynolds number
Fig. 8: Experimental and predicted Nusselt number and
Reynolds number.
Data Reduction
Following equations were used for calculating
pressure drop across test section (∆Pt) and orifice plate
(∆Po), mass flow rate of air (m), velocity of
heat transfer rate (q), heat transfer coefficient (h),
Nusselt number (Nu) and friction factor (f):
( )t k tP g hρ∆ = ∆
( )o w wP g hρ∆ = ∆
0.5
4
2
1
od o
P Sinm C A
ρ θβ
∆=
− &
c
mV
Aρ=
&
( )p o iq mC T T= −&
Also,
( )ampmc TThAq −=
Therefore, from eqs. (7) and (8)
( )ampmc TTA
qh
−=
Where Tpm and Tam are mean temperature of valu
recorded for absorber plate and air at different
locations along test section of the duct. Reynolds
number, Nusselt number and friction factor values
were calculated by using the following relationships:
µρVD
=Re
k
hDNu =
2
2
4
tPDfLVρ∆
=
RESULTS AND DISCUSSION
Variation of heat transfer coefficient as a fu
mass flow rate of air for smooth and roughened
absorber plate is shown in Fig. 9. It has been observed
that heat transfer coefficient increases consistently
with positive slope for entire range of mass flow rate
of air. It is investigated that concave roughened plate
(s/e=14, l/e=16, e/D=.03928655) have highest heat
transfer coefficient as compared all other
convex roughened plates as well as smooth plate.
Variation of Nusselt number as a function of Reynolds
number for roughened and smooth absorber plate
shown in Fig. 10. Concave Roughened plate
s/e=14, e/D=0.03925688) has shown highest
International Journal of Advanced Engineering Technology E-ISSN 0976
86-89
Experimental and predicted Friction factor and
Experimental and predicted Nusselt number and
Following equations were used for calculating
and orifice plate
), velocity of air (V),
heat transfer rate (q), heat transfer coefficient (h),
Nusselt number (Nu) and friction factor (f):
(3)
(4)
(5)
(6)
(7)
(8)
(9)
are mean temperature of values
recorded for absorber plate and air at different
locations along test section of the duct. Reynolds
Nusselt number and friction factor values
were calculated by using the following relationships:
(10)
(11)
(12)
as a function of
smooth and roughened
It has been observed
that heat transfer coefficient increases consistently
mass flow rate
oncave roughened plate
(s/e=14, l/e=16, e/D=.03928655) have highest heat
other concave and
roughened plates as well as smooth plate.
Variation of Nusselt number as a function of Reynolds
smooth absorber plate is
Roughened plate (l/e=16,
highest Nusselt
number as compared all other concave and convex
roughened plates as well as smooth plate
turbulence caused by roughness el
laminar sub-layer [4]. Pressure drop increases with
rise of mass flow rate of air for all roughened
compared to smooth plate as shown in Fig. 1
noted that for entire range of mass flow rate of air,
pressure drop is highest in case of convex plate
roughened with (s/e=14, l/e=16, e/D=.03928655)
compared all other convex and concave roughened
plates as well as smooth plate. Variation of friction
factor as a function of Reynolds number for smooth
and roughened plate is shown in Fig. 1
that friction factor for roughened plate (s/e=12, l/e=10,
e/D=.03928655) has highest value as compare to
smooth plate and other roughened plates for all values
of Reynolds number, due to mixing of flow an
generation of secondary flows in flow regimes similar
to as reported by Saini and Saini (10).
Fig. 9: Variation of heat transfer coefficient
of mass flow rate of air for smooth and roughened
absorber plate.
Fig. 10: Variation of Nusselt number as a function of
Reynolds number for roughened and smooth
plate.
Fig.11: Variation of pressure drop as a function of
flow rate of air for roughened smooth absorber plate
ISSN 0976-3945
(3)
(6)
(8)
(9)
(10)
(12)
other concave and convex
roughened plates as well as smooth plate, due to
turbulence caused by roughness elements in the
ressure drop increases with
roughened plate as
smooth plate as shown in Fig. 11. It is
for entire range of mass flow rate of air,
pressure drop is highest in case of convex plate
roughened with (s/e=14, l/e=16, e/D=.03928655) as
compared all other convex and concave roughened
Variation of friction
factor as a function of Reynolds number for smooth
roughened plate is shown in Fig. 12. It is observed
that friction factor for roughened plate (s/e=12, l/e=10,
e/D=.03928655) has highest value as compare to
smooth plate and other roughened plates for all values
, due to mixing of flow and
generation of secondary flows in flow regimes similar
heat transfer coefficient as a function
smooth and roughened
Variation of Nusselt number as a function of
and smooth absorber
s a function of mass
roughened smooth absorber plate.
Pal et al., International Journal of Advanced Engineering Technology
Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86
Fig. 12: Variation of friction factor as a function of
Reynolds number for roughened smooth absorber plate
CONCLUSION
This paper presents an experimental investigation of
artificially roughened duct used in solar air heaters.
Variation of heat transfer and friction loss due to
artificial roughness (created by concave streamlined
dimples on absorber plate) has been investigated for
Reynolds numbers range of 8000–24000. It has been
observed that roughened absorber plate results into
higher heat transfer coefficient at the cost of
penalty. It is also noted that roughened plate (concave,
s/e=14, l/e=16, e/D=.03928655) has Nusselt number
and friction factor of the order of 2.87 times and
times as compared to smooth absorber plate
respectively.
ACKNOWLEDGEMENT
The author would like to express a deep sense of
gratitude and thanks to Er. Tarun Mahajan,
Professor and Head, Department of Mechanical
Engineering, Sri Sai College of Engineering
Technology (SSCET), Pathankot for their valuable
guidance, persistent encouragement and suggestions at
every stage of my dissertation. Without able guidance
and wise counseling, it would have been impossible to
complete the research work in this manner.
The constant encouragement received from Prof. J.S.
Kaler M.Tech Coordinator, Department of Mechanical
Engineering, and SSCET Pathankot has been of great
importance in carrying out the present research work.
The author also expresses gratitude to all other faculty
members of Mechanical Engineering Department,
SSCET Pathankot for their intellectual support
throughout the course of this work. The help rendered
by Sh. Balbir Singh, Workshop Superindent and other
workshop staff of SSCET Pathankot for conducting
experimentation is greatly acknowledged.
The author is highly grateful to Principal, Dr. Dayal
Chand SSCET Pathankot for providing moral support
and help in each aspect to carry out the present
research work. I am deeply indebted to my parents for
their best wishes and co-operation extended
throughout the work duration.
Finally, the author is indebted to all whosoever have
helped directly or indirectly in this dissertation
and for friendly stay at SSCET Pathankot.
REFERENCES 1. Kumar P., Singh S., Mahajan T., “Effect of Relative long
way length on Nusselt number and Friction factor of streamlined dimpled artificially roughened duct solar air
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2. Bhushan B., Singh R., “A review on methodology of artificial roughness used in duct of solar air heaters
Energy, 35 (2010) 202-212.
International Journal of Advanced Engineering Technology E-ISSN 0976
86-89
as a function of
roughened smooth absorber plate.
This paper presents an experimental investigation of
artificially roughened duct used in solar air heaters.
Variation of heat transfer and friction loss due to
concave streamlined
on absorber plate) has been investigated for
24000. It has been
observed that roughened absorber plate results into
higher heat transfer coefficient at the cost of frictional
It is also noted that roughened plate (concave,
s/e=14, l/e=16, e/D=.03928655) has Nusselt number
times and 2.13
imes as compared to smooth absorber plate
The author would like to express a deep sense of
gratitude and thanks to Er. Tarun Mahajan, Assistant
, Department of Mechanical
Engineering, Sri Sai College of Engineering
for their valuable
tent encouragement and suggestions at
every stage of my dissertation. Without able guidance
and wise counseling, it would have been impossible to
complete the research work in this manner.
The constant encouragement received from Prof. J.S.
ordinator, Department of Mechanical
Engineering, and SSCET Pathankot has been of great
importance in carrying out the present research work.
The author also expresses gratitude to all other faculty
members of Mechanical Engineering Department,
kot for their intellectual support
The help rendered
by Sh. Balbir Singh, Workshop Superindent and other
workshop staff of SSCET Pathankot for conducting
experimentation is greatly acknowledged.
eful to Principal, Dr. Dayal
Chand SSCET Pathankot for providing moral support
and help in each aspect to carry out the present
I am deeply indebted to my parents for
operation extended
Finally, the author is indebted to all whosoever have
dissertation work
and for friendly stay at SSCET Pathankot.
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