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EXPERIMENTAL STUDY OF HEAT TRANSFER ENHANCEMENT IN A PIPE USING TWISTED TAPES

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EXPERIMENTAL STUDY OF HEAT TRANSFER ENHANCEMENT IN A PIPE USING TWISTED TAPES Powered By Docstoc
					 International Journal of JOURNAL OF MECHANICAL ENGINEERING
INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 –
 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
                         AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                     IJMET
Volume 4, Issue 2, March - April (2013), pp. 100-111
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)                 ©IAEME
www.jifactor.com




   EXPERIMENTAL STUDY OF HEAT TRANSFER ENHANCEMENT IN
         A PIPE USING TWISTED TAPES AND WIRE COILS

                            Dr. A. G. Matani1, Swapnil A. Dahake2
       1
       Associate Professor, Mechanical Engineering Department, Government College of
                             Engineering, Amravati, (M.S.) India
   2           nd
     M.Tech. [2 Year Thermal Engineering], Government College of Engineering, Amravati,
                                        (M.S.) India.



  ABSTRACT

          In the proposed work, the heat transfer enhancement using twisted tapes and wire coil
  on pressure drop, Nusselt number (Nu), friction factor (f) and thermal enhancement index ( )
  are experimentally determined. The twisted tapes are used as swirl flow generators while wire
  coil along with twisted tapes used as co-swirl flow generators in a test section. The tests are
  conducted using the twisted tape with three different twist ratios (y/w = 3.5, 2.66 and 2.25)
  and wire coil along with twisted tapes, pitch ratio of 1.17 & 0.88 for Reynolds numbers range
  between 5000 and 18,000 under uniform heat flux conditions. Also double twist generating
  counter swirl are compared. The experiments using the twisted tape and with wire coil
  performed under similar operation test conditions, for comparison. The experimental results
  indicate that the tube with the various inserts provides considerable improvement of the heat
  transfer rate over the plain tube. The experimental results demonstrate that friction factor (f)
  and thermal enhancement index ( ) increase with decreasing twist ratio (y/w) and Reynolds
  number. The results also show that the wire coils along with twisted tapes are more efficient
  than the twisted tapes for heat transfer enhancement.

  1. INTRODUCTION

         Research is going on to investigate the level of heat transfer enhancement that can
  achieved by forced convection in which air is flow inside horizontal pipe. The comparison
  between bare tube and enhanced tube configuration are made on the basis of forced
  convection with air instabilities. These types of inserts increase the heat transfer coefficient

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with respect to smooth tube. Differences among the enhanced configuration are also
determined to observe which of them most stable and unstable one is. These inserts induced
swirl flow heat transfer due to exponentially increasing heat input with exponential periods.
         In the last decades, significant effort has been made to develop heat transfer
enhancement techniques in order to improve the overall performance of heat exchangers. The
interest in these techniques is closely tied to energy prices and, it is expected that the heat
transfer enhancement field will go through a new growth phase with the present increase in
energy cost. Although there is need to develop novel technologies, experimental work on the
older ones is still necessary. The knowledge of its performance shows a large degree of
uncertainty which makes their industrial implementation difficult [6]. The efficiency of heat
transfer equipment is essential in energy conservation. Furthermore, a more efficient heat
exchanger can reduce the size of the heat exchanger, thus reducing the costs associated with
both material and manufacturing of the heat exchanger [2]. Heat transfer enhancement
technology has been widely applied to heat exchanger applications in refrigeration,
automobile, process industries etc. There have been numerous attempts to reduce the size and
cost of heat exchangers. Hence, there have been continuous attempts to improve the
efficiency of heat exchangers by various methods. One of the best methods to achieve this is
the use of augmented heat transfer surfaces. Improved heat transfer can make heat exchangers
smaller and more energy efficient. The tube insert technology is one of the most common
heat transfer enhancement technologies for shell and tube heat exchangers.
         Tube side enhancement techniques can be classified according to the following
criteria: (1) additional devices which are incorporated into a plain round tube (twisted tapes,
wire coils) and (2) non-plain round tube techniques such as surface modification of a plain
tube (corrugated and dimpled tubes) [5] or manufacturing of special tube geometries
(internally finned tubes) [6]. The dominant literature study usually mentions five types: wire
coils, twisted tapes, extended surface devices, mesh inserts and displaced elements [7]. The
main advantage of these types in respect to other enhancement techniques such as the
artificial roughness by mechanical deformation or internal fin types is that they allow an easy
installation in an existing smooth-tube heat exchanger. The various single and double twisted
tapes are also used to generate swirl/co-swirl [9]. Due to its low cost, the insert devices which
are most frequently used in engineering applications are wire coils and twisted tapes.
         Heat transfer enhancement techniques have been extensively developed to improve
the thermal performance of heat exchanger systems with a view to reducing the size and cost
of the systems. Swirl/vortex flow is the one of the enhancement techniques widely applied to
heating/cooling systems in many engineering applications. The vortex flows can be classified
into two types: continuous swirl and decaying swirl flows. The former represents the swirling
motion that persists over the entire length of the duct for example helical/twisted tape and
coiled wires inserts while the latter means the swirl created at the duct entrance and then
decays along the flow path such as the tangential injection, the rib/baffle and the winglet
vortex generators [10]. Swirl flow has been used in a wide range of applications from various
engineering areas such as chemical and mechanical mixing and separation devices,
combustion chambers, turbo machinery to pollution control devices. It is commonly known
that the swirl flow enhances the heat transfer mainly due to the increased velocity in the swirl
tube and the circulation of the fluid by forced convection [4].




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2. TWISTED TAPES & WIRE COIL TO GENERATE SWIRL

        The knowledge of heat transfer in a short circular tube with the twisted-tape insert
tube is important to investigate the influence of the twist ratio of twisted tape on enhancement
of critical heat flux. A lot of the experimental investigations were conducted with the heat
transfer characteristics of the swirl flow. It was shown that the twisted-tape insert tube
provided considerable enhancement of turbulent heat transfer for heating/cooling of fluid.
The twisted-tape insert tubes are encountered in a number of important engineering and
science systems such as plasma facing components in fusion experimental facilities, high
power laser systems and etc. We believe that the enhancement of heat transfer for the twisted-
tape insert tube would be due to reduction of viscous sub-layer thickness on heated surface of
test tube with an increase in liquid flow velocity from straight flow to swirl one even at a
fixed mass velocity.
        Wire coil inserts are devices whose reliability and durability are widely contrasted. In
extreme applications such as the tube-side of fuel pyrotubular boilers with great fouling
problems and with high variations in temperature that produce great dilatations, wires are
used without any problem. This is a cheap enhancement technique and it is completely viable
for many industrial applications. This fact hinders a widespread use of wire coil inserts in
industrial heat exchangers.
        The twisted tapes are made of mild steel and have tape width (w) of 10 mm, 15 mm &
20mm shown in Figure 1, tape thickness (d) of 0.8 mm, and tape length (l) of 900 mm. All
tapes were prepared with different twist ratios, y/w = 3.5, 2.66 and 2.25 respectively where
twist ratio is defined as twist length (l) to tape width (w). The double twisted tapes are
manufactured by combining two single twisted tapes, which generate counter swirl in pipe
shown in Figure 2. On the other hand, to avoid an additional friction in the system that might
be caused the thicker tape. To produce the twisted tape, one end of a straight tape was
clamped while another end was carefully twisted to ensure a desired twist length. Another
insert, wire coil is made up of aluminum wire of 2 mm diameter having pitch of 30mm & 40
mm shown in Figure 3. These twisted tapes are fixed one by one inside the pipe having wire
coil to generate co-swirl.




                   Figure 1: Single twisted tapes (y/w = 3.5, 2.66 & 2.25)


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                         Figure 2: Double twisted tape (y/w = 2.66)




                              Figure 3: Wire coil (p/d = 0.88)


3. TEST SECTION

        The test section is surrounded by nichrome heating wire, which is wrapped around the
test section with a pitch distance of 5 mm. This pitch is good enough to provide a nearly
uniform heating on the outer surface of the test section tube. The heating wire was powered
by a variable AC power supply. The overall electrical power added to the heating section, Q,
                                            (0–200                               (0–2
was calculated by measuring the voltage (0 200 V) and the electrical current (0 A). To
control the convection losses from the test section and other components, foam insulation and
glass wool used. Four thermocouples are to be embedded on the test section to measure
surface temperature of pipe and two thermocouples are placed in air stream at entrance and
exist of test section to measure air temperature. To avoid floating voltage effects, the
thermocouple bead is insulated from the electrically heated tube wall surface with a very thin
sheet of mica between the thermocouple and the tube surface so as not to be effected from
      icity.                                                  set
electricity. Fig. 2 shows the schematic view of experimental set-up.




                               Figure 4: Experimental set-up


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4. DATA REDUCTION

        In the experiments, the heat transfer rate in the tube is taken into account under a
uniform heat flux (UHF) condition by using air as the test fluid. The heat transfer given by
the hot surface to fluid (i.e. air) at any Reynolds number is
Rate at which air is heated
 Qa = mCp (To - Ti)
The convection heat transfer from the test section is given as
Qc = hAs(Ts - Tm)
At steady state condition, the heat transfer is assumed to be equal to the heat loss from the
test section that can be drawn as
Qa = Qc
The mean temperature of the fluid in the test tube is given by
Tm = (To + Ti) / 2
Ts is the mean temperature of surface wall temperature of the test tube. The average wall
temperature is calculated from 4 points of local wall temperatures lined between the inlet and
the exit of the test tube. The average heat transfer coefficient (h) and the mean Nusselt
number (Nu) are estimated by
h = mCp (To - Ti) / hA(Ts - Tm)

The Nusselt number in terms of average heat transfer coefficient is defined as
Nu = hD / k
The Reynolds number is written as
Re = ρUD / µ
The experiment pressure losses, ∆p across the test tube are arranged in non-dimensional form
by using the following equation




in which U is mean velocity in the test tube and L is the test tube length. All of thermo-
physical properties of the air are determined at the overall mean air temperature (Tm).

5. EXPERIMENTAL RESULTS AND DISCUSSIONS

        In this section, the pressure drop, friction factor characteristics, heat transfer and
thermal enhancement index in a tube fitted with twisted tapes, double twisted tape and wire
coil (counter/co-swirl tape) are presented.
        The experiments are performed in the range of Reynolds number between 5000 and
18,000. The results obtained for the tube fitted with the single twisted tapes (ST), wire coil
and the empty tube are used as the reference data for the performance evaluation of the
modified tubes.


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5.1 Heat transfer result

        Experimental results of the Nusselt number (Nu) in plain tubes combined with a
twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d =
1.17, 0.88) are presented in Figure 5. The Nusselt numbers for the plain tube acting alone are
also plotted for comparison. The data show that the Nusselt number (therefore, the heat
transfer coefficient) increases with increasing Reynolds number for the conventional
turbulent tube flow. This is the most likely caused by a stronger turbulence and better contact
between fluid and heating wall. It is noted that the increasing Nusselt number in the plain
tube in common with a twisted tapes and wire coil is caused by the generating of pressure
gradient along the radial direction, and this leads to redeveloping of thermal/hydrodynamic
boundary layer. The higher increase of the Nusselt number in this style of both turbulence
and swirl flows is a consequence of the higher reduction of boundary layer thickness and
increase of resultant velocity.
        The variations of Nusselt number with Reynolds number for single and double twisted
tapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 6 and with wire coil of pitch
ratio (p/d = 0.88) shown in Figure 7. Nusselt number increases with the decrease of twist ratio
and the increase of Reynolds number. The highest Nusselt number is achieved for twist ratio
(y/w = 2.25) and pitch ratio (p/d =0.88).
        However, the heat transfer enhancement by twisted tapes is less efficient than that
offered by wire coil indicated by the lower Nusselt number depending on operating condition.


                                                                                 smooth tube
        90
                                                                                 STT(y/w)=3.5
        80
                                                                                 STT(y/w)=2.6
        70                                                                       6
                                                                                 STT(y/w)=2.2
        60                                                                       5
                                                                                 DTT(y/w)=3.5
   Nu




        50                                                                       DTT(y/w)=2.6
                                                                                 6
        40                                                                       WC(p/d)=1.1
                                                                                 7
        30

        20
         5000     7000      9000      11000     13000     15000     17000

                                          Re


Figure 5: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66)
and wire coil (p/d = 1.17, 0.88)




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                                                                           STT(p/d)=1.17,(y/w)=
        110
                                                                           3.5
        100                                                                STT(p/d)=1.17,(y/w)=
                                                                           2.66
            90                                                             STT(p/d)=1.17,(y/w)=
                                                                           2.25
            80                                                             DTT(p/d)=1.17,(y/w)=
                                                                           3.5
            70                                                             DTT(p/d)=1.17,(y/w)=
   Nu




                                                                           2.66
            60

            50

            40

            30
             5000                 10000                   15000
                                          Re



Figure 6: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17)

                                                                           STT(p/d)=0.88,(y/w)=3.
   120
                                                                           5
   110                                                                     STT(p/d)=0.88,(y/w)=2.
                                                                           66
   100                                                                     STT(p/d)=0.88,(y/w)=2.
                                                                           25
       90                                                                  DTT(p/d)=0.88,(y/w)=3.
       80                                                                  5
                                                                           DTT(p/d)=0.88,(y/w)=2.
       70
  Nu




                                                                           66
       60
       50
       40
       30
        5000        7000   9000     11000      13000      15000   17000
                                               Re

        Figure 7: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)

5.2 Friction factor results

    Experimental results of the friction factor (f) characteristics in plain tubes combined with
a twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66) and wire coil (p/d
= 1.17, 0.88) are presented in Figure 5. The friction factors of the plain tube acting alone are
also plotted for comparison. Figure shows the influence of a plain tube combined with a
twisted tape and wire coil on pressure loss, which indicates the friction in a heat exchanger.

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    The relationship between pressure loss in terms of friction factor and Reynolds number
for the tube with twisted tape (STT & DTT) and wire coil inserted and also for the plain tube
is presented in Figure 5. It is found that using twisted tape and wire coil gives higher friction
factor values than those from the plain tube as expected. The friction factor decreases with
the increase of twist ratio and Reynolds number.
    The variations of friction factor with Reynolds number for single and double twisted
tapes with wire coil of pitch ratio (p/d = 1.17) shown in Figure 9 and with wire coil of pitch
ratio (p/d = 0.88) shown in Figure 10. The highest friction factor (or pressure loss) is obtained
in case p/d = 0.88 & y/w = 2.66.

                              0.3                                                                          smooth tube

                                                                                                           STT
                          0.25                                                                             (y/w)=3.5
                                                                                                           STT
        Friction factor (f)




                              0.2                                                                          (y/w)=2.66
                                                                                                           STT
                          0.15                                                                             (y/w)=2.25
                                                                                                           DTT
                                                                                                           (y/w)=3.5
                              0.1                                                                          DTT
                                                                                                           (y/w)=2.66
                          0.05

                                0
                                 5000      7000     9000        11000         13000      15000     17000
                                                                 Re

 Figure 8: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66)
                              and wire coil (p/d = 1.17, 0.88)


                               0.5                                                                 STT
                                                                                                   (p/d)=1.17,(y/w)=3.5
                              0.45                                                                 STT
                                                                                                   (p/d)=1.17,(y/w)=2.66
                               0.4
                                                                                                   STT
                              0.35                                                                 (p/d)=1.17,(y/w)=2.25
  Friction factor (f)




                                                                                                   DTT
                               0.3                                                                 (p/d)=1.17,(y/w)=3.5
                              0.25

                               0.2

                              0.15

                               0.1
                                    5000   7000   9000     11000    13000        15000     17000
                                                           Re

Figure 9: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17)


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                                                                                            STT
                          0.6                                                               (p/d)=0.88,(y/w)=3.5
                         0.55                                                               STT
                                                                                            (p/d)=0.88,(y/w)=2.66
                          0.5                                                               STT
                         0.45                                                               (p/d)=0.88,(y/w)=2.25
                                                                                            DTT
   Friction factor (f)




                          0.4                                                               (p/d)=0.88,(y/w)=3.5
                                                                                            DTT
                         0.35                                                               (p/d)=0.88,(y/w)=2.66
                          0.3
                         0.25
                          0.2
                         0.15
                          0.1
                            5000      7000     9000    11000     13000    15000    17000

                                                       Re


                         Figure 10: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)


5.3 Performance factor results

       Siva Rama Krishna, Govardhan Pathipaka, P. Sivashanmugam [3] proposed a
performance evaluation analysis for the same pumping power and this method used for
present study, the performance ratio is defined as




        The performance analysis was made by above Eq. and the results are shown in Figure
11, 12, 13. In the present work, a thermal performance factor is evaluated since the factor is
an important parameter indicating the potential of a twisted tape for practical applications.
The thermal evaluation is considered under constant pumping power for each twisted tape
with respect to the case without twisted tape (plain tube). The thermal performance factors
for single twisted tape, double twisted tape and wire coil are presented in Figure 11.
Apparently, a thermal performance factor decreases with increasing Reynolds number for all
tape inserts. A larger pressure loss at a higher Reynolds number is responsible for the
mentioned result. This suggests that the geometry of wire coil is more appropriate for
practical use than the others in the view point of energy as well as operating cost savings.




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                                                                                                               STT (y/w)=3.5
                         1.35
                                                                                                               STT
                          1.3                                                                                  (y/w)=2.66
                                                                                                               STT
                         1.25                                                                                  (y/w)=2.25
    Performance factor




                                                                                                               DTT (y/w)=3.5
                          1.2
                                                                                                               DTT
                         1.15
                                                                                                               (y/w)=2.66
                                                                                                               WC(p/d)=1.17
                          1.1

                         1.05

                            1

                         0.95
                                5000    7000     9000      11000         13000        15000     17000

                                                                Re


Figure 11: Single twisted tape (y/w = 3.5, 2.66, 2.25), double twisted tape (y/w = 3.5, 2.66)
                              and wire coil (p/d = 1.17, 0.88)




                                                                                                        STT
                         1.65                                                                           (p/d)=1.17,(y/w)=3.5
                                                                                                        STT
                         1.55                                                                           (p/d)=1.17,(y/w)=2.66
                                                                                                        STT
                                                                                                        (p/d)=1.17,(y/w)=2.25
                         1.45                                                                           DTT
  Performance factor




                                                                                                        (p/d)=1.17,(y/w)=3.5
                         1.35                                                                           DTT
                                                                                                        (p/d)=1.17,(y/w)=2.66
                         1.25

                         1.15

                         1.05

                         0.95
                            5000       7000    9000     11000   13000         15000     17000
                                                                   Re


 Figure 12: Single and double twisted tapes along with wire coil of pitch ratio (p/d = 1.17)




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                       1.75                                                                 STT
                                                                                            (p/d)=0.88,(y/w)=3.5
                       1.65                                                                 STT
                       1.55                                                                 (p/d)=0.88,(y/w)=2.66
  Performance factor



                                                                                            STT
                       1.45                                                                 (p/d)=0.88,(y/w)=2.25
                                                                                            DTT
                       1.35                                                                 (p/d)=0.88,(y/w)=3.5
                       1.25
                       1.15
                       1.05
                       0.95
                          5000                 10000                  15000
                                                       Re

                       Figure 13: Single and double twisted tapes with wire coil of pitch ratio (p/d = 0.88)

6. CONCLUSIONS

       Thermal characteristics in a tube fitted with twisted-tapes in co-swirl arrangement with
wire coil are presented in the present study. The work has been conducted in the turbulent flow
regime, Reynolds number from 5000 to 18,000 using air as the test fluid. The findings of the
work can be drawn as follows:

                         For the inserted tube, the pressure drop tends to increase with the rise in mass flow rate
                         while the friction factor and performance factor give the opposite trends.
                         The compound enhancement devices of the tube and the co-swirl show a considerable
                         improvement of heat transfer rate and thermal performance relative to the smooth tube
                         acting alone, depending on twist ratios.
                         The co-swirl tube yields higher friction factor and performance factor than the smooth
                         tube at low Reynolds number.
                         The various inserts in pipe mixes the bulk flow well and therefore performs better in
                         laminar flow.
                         The result also shows twisted tape insert is more effective, if no pressure drop penalty is
                         considered. Twisted tape in turbulent flow is effective up to a certain Reynolds number
                         range.
                         Swirl flow heat transfer is higher than non swirling flow.

NOMENCLATURE
d     wire diameter
f     Friction factor
      Performance factor
Nu    Nusselt number
p     pitch
p/d   pitch ratio
Re    Reynolds number
w     Width of tape
y     Twist length
(y/w) twist ratio


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14. Er. Pardeep Kumar, Manoj Sain and Shweta Tripathi, “Enhancement of Heat Transfer using
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