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Enhancement the heat transfer process play an important role in increasing the
efficiency of the most important energy industry applications in heat transfer
including, power generation, chemical production, air conditioning, Transportation
and microelectronics, heating of circulating fluid in solar collector, heat transfer in
compact heat exchangers and many other industrial sectors associated with different
processes depends on the heating or cooling fluid inside tubes. During the last decades
many researchers have been experimentally, investigate the effects of nanofluid
technology and the tabulators' passive techniques on heat transfer enhancement and
pressure drop. Lazarus et al. [2009] experimentally investigated the effect of
convective heat transfer of de-ionized water with a low volume fraction =0.003% of copper oxide (CuO) nanoparticles to form nanofluid flows through copper tube under
laminar flow and heat flux conditions. The results has shown 8% enhancement for
convective heat transfer coefficient of the nanofluid even with a low volume
concentration of CuO nanoparticles. The heat transfer enhancement increased
considerably as the Reynolds number increased. They predicted a new correlation for
local Nusselt number variation along the flow direction of the nanofluid. Alimullah
Anwar [2014] studied experimentally the heat transfer augmentation and friction
factor characteristics through circular tube fitted with full-length helical screw insert
device for laminar and turbulent flow under constant heat flux condition. The results
showed that with this type of inserts a high swirl flow generates which increases the
convection heat transfer, thus Nusslet number increases as twisted ratio decreasing as
compared with plain tube. Akeel Abdullah [2011] studied experimentally the heat
transfer enhancement and pressure drop in turbulent flow of air for Reynolds number
range=5000 to 23000 in a horizontal circular tube under constant wall heat flux
condition fitted with combined conical-ring tabulators and a twisted-tape swirl
generator. It noticed from experimental results that temperature values increases along
the tube length and decrease as twist ratio decreases while the average Nusslet
number increases as Re increases and decreasing as twisted ratio decrease in case of
combined twisted tape and conical ring. The results showed a significant enhancement
in heat transfer process with conical ring tabulator than empty plain tube and much
better enhancement in case of combined twisted tape and conical ring. It's noticed that
the fanning friction factor decreases as Re increases and the values of friction factor
become higher when using conical ring in combined with twist tape than using
conical ring alone and especially at smaller twisted tapes ratio due to increase swirl
flow which leads to higher contact between secondary flow and tube wall. He also
predicted new empirical relationships for Nusslet number and friction factor for
combined conical ring and twisted tape. Esmaeilzadeh et al. [2014] studied
experimentally the characteristics of heat transfer and friction factor enhancement of
ɣ- /water nanofluid in laminar flow region flowing through uniform heated circular tube fitted with twisted tapes inserts with various thicknesses. They noticed
from results that the performance of convective heat transfer becomes better with the
addition of ɣ- /water nanofluid compared with water and the values of
convection heat transfer coefficient increases with increasing volume concentration
The effects of triangular cut twisted tape (TCTT) and the semicircular cut twisted tape
(SCTT) on the variation of average Nusselt number and friction factor are clarified in
figures (10) and (11). As shown, at the same operation condition the heat average
Nusselt number is much higher for TCTT by 4% and by 6% than SCTT and typical
twisted tape respectively and by 65% than for smooth tube case. Generally, the typical
twisted tape (TTT) generates only a swirling flow but in case of adding several cuts
along the twisted tape edge it will produces many local vortices at each cutting section
that provides an excellent mixing for the viscous boundary layers in all direction
along the tube leads to higher improvement in heat transfer enhancement. Also, the
heat transfer enhancement rate depends on the cutting shape which controls the
strength of vortex generated which mean the vortices preformed behind the triangular
cut is more stronger than those preformed through the semicircular cut. On the other
hand, the friction factor enhances more with TCTT by13 % than SCTT and by 27%
than TTT because the addition of these local vortices promotes an additional shear
stress due to increasing in flow mixing between the viscous boundary layers of fluid
at the tube wall and twisted tape.
3.4. Combined Effect of Twisted Tape and Nanofluid on Heat Transfer
Figures (12) and (13) clarified the variation of average Nusslet number ( ) and
friction factor of CuO nanofluid for = 0.08% and 0.35% volume concentrations, flowing at various Reynold numbers through the inserted tube with triangular cut
twisted tape (TCTT) and typical twisted tape for 2.6 twist ratio and 1mm thickness.
It's observed that the heat transfer enhancement reached the highest level during this
study through the joint use of TCTT with CuO nanofluid. Where, the increased by
8% than TCTT without CuO nanofluid and by 22%, 21% than TTT with CuO
nanofluid for the same operation condition and volume concentrations. Also, this
enhancement increases with increasing both Reynold number flow and nanoparticles
volume concentration. Random motion of nanoparticles even at low volume
concentration become more active in convective heat transfer and accelerated due to
swirling flow and local vortices generated by the TTT and TCTT that provides perfect
mixing for the viscous layer for the working fluid. The friction factor for TCTT with
CuO nanofluid increased by 2% for = 0.08% and by 3% for = 0.35% than TCTT
without CuO nanofluide and for the TTT with CuO nanofluid increased by 1.5% for
=0.08 and by 2.7% for =0.35% than TTT without CuO nanofluid for the same operation conditions. The friction losses along the tested tube increases due to
increasing in the turbulent intensity of the flow that accelerating nanoparticles motion
through the swirling flow, which enhances shear stress forces near the inner tube wall.
4. DEVELOPING OF EMPIRICAL EQUATION
The Nusselt number and friction factor experimental results have been correlated by
the following equations:
Nusselt number and friction factor correlation for Twist ratio:
= 1.1 (16)
= 868.8 (17)
Valid for 451<Re< 2100, 2.6<Y<5.3, 5.34<Pr<7.1.
Nusselt number and friction factor correlation for TTT thickness:
Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with
[10] Sami D. salman, Abdul Amir H.Kadhm, Mohd S.Takriff, and Abu Bakar
Mohamad, Heat transfer enhancement of laminar nanofluids flow in circular tube
fitted with parabolic-cut twisted tape inserts, Hindawi Publishing Corporation,
The Scientific World Journal, ID 543231, (2014).
[11] Shah R.K., A.L. London, Laminar Flow Forced Convection in Ducts, Supplement
1 to Advances in Heat Transfer, Academic Press, New York, (1978).
[12] Kavitha T, Rajendran A, Durairajan A, Shanmugam A, Heat Transfer
Enhancement Using Nano Fluids And Innovative Methods - An Overview.
International Journal of Mechanical Engineering and Technology, 3(2), 2012,
pp. 769–782.
[13] Sunil Jamra, Pravin Kumar Singh and Pankaj Dubey, Experimental Analysis of
Heat Transfer Enhancementin Circular Double Tube Heat Exchanger Using
Inserts. International Journal of Mechanical Engineering and Technology, 3(3),
2012, pp. 306–314.
[14] Qasim S. Mahdi and Ali Abdulridha Hussein, Enhancement of Heat Transfer In
Shell And Tube Heat Exchanger with Tabulator and Nanofluid. International
Journal of Mechanical Engineering and Technology, 7(3), 2016, pp. 125–138.
[15] Xuan Y., Roetzel W., Conceptions for heat transfer correlation of nanofluids,
International Journal of Heat and Mass Transfer, 43, pp 3701–3707, (2000).
NOMENCLATURE
Cross section area, Average wall temperature,
Surface area, Mean bulk fluid temperature, Specific heat, kJ/kg.K Inlet fluid temperature,
de Cutting depth, m Inner surface tube temperature, Inside tube diameter, m Outer surface tube temperature, Outer tube diameter, m Average inlet velocity, m/sec
Friction factor V Electric volte, Voltage H Pitch, m w Width, m
Inside heat transfer coefficient, W/ . we Cutting width, m I Electric current, Amp Y Twist ratio
K Thermal conductivity, W/m.K Z Axial distance, m L Length, m Viscosity, kg/m.sec Mass flow rate, kg/sec Density, kg/
Nu Nusselt number Volume concentration of nanofluid