AASCIT Journal of Nanoscience 2015; 1(4): Published online September 10, 2015 (

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1 AASCIT Journal of Nanoscience 2015; 1(4): Published online September 10, 2015 ( Heat Transfer Enhancement in a Circular Tube Fitted with Passive Technique as Twisted Tape Insert in Turbulent Flow Regime: A Review of the Recent Literature Laith J. Habeeb 1, Fouad A. Saleh 2, Bassim M. Maajel 2 1 Mechanical Engineering Department, University of Technology, Baghdad, Iraq 2 Mechanical Engineering Department, University of Almustansiriya, Baghdad, Iraq Keywords Heat Transfer Augmentation, Reynolds Number, Friction Factor, Nusselt Number, Constant Heat Flux, Twisted Tape, Heat Transfer Coefficient Received: August 15, 2015 Revised: August 27, 2015 Accepted: August 28, address laithhabeeb1974@gmail.com (L. J. Habeeb) Citation Laith J. Habeeb, Fouad A. Saleh, Bassim M. Maajel. Heat Transfer Enhancement in a Circular Tube Fitted with Passive Technique as Twisted Tape Insert in Turbulent Flow Regime: A Review of the Recent Literature. AASCIT Journal of Nanoscience. Vol. 1, No. 4, 2015, pp Abstract The increase in heat transfer rate in turbulent flow regime is more considerable than that in laminar flow regime. As a passive heat transfer technique, the turbulent flow can be produced by inserting different geometries in the circular pipe like twisted tapes which is used in various industries for their cost savings, lower maintenance requirements and the fact that they are easily set up. In fact, at higher Reynolds number the turbulent effects become a dominant factor over secondary flow. This paper introduces the principle of heat transfer enhancement of twisted tapes and reviews some of the experimental works done by researchers on this technique in recent years, also it focuses on the various methods or geometries used to produce turbulent geometries and its effect on the heat transfer rate. Insertion of such geometries may lead to increase the pressure drop and friction factor which lead to enhance the heat transfer characteristics. 1. Introduction The process of improving the heat transfer system performance is referred as the Heat transfer augmentation technique. The heat transfer rate can be enhanced by introducing the disturbance in the fluid flow but in process industries pumping power may increase significantly and in the end the pumping cost becomes high. This means that these types of tapes induce swirl flow causing a thinner boundary layer and consequently resulting in a higher heat transfer coefficient and Nusselt number due to repeated changes in the twisted tape geometry. Therefore to achieve the desired heat transfer rate in an existing heat exchange equipment s at an economic pumping power, large numbers of techniques have been made and an optimal design is needed to achieve the best heat transfer performance with less friction loss. s can be classified in sex categories: Typical twisted tapes, Varying length and width twisted tapes, s with alternate axis, Twisted tapes with modification in the peripheral region, s with modification on the surface, Multiple twisted tapes, s with a rod and varying spacer and Twisted tapes with wings. Hence this paper will focus more attention on these twisted tapes. 2. Literature Review The important investigations of twisted tape in turbulent flow are summarized in Table

2 44 Laith J. Habeeb et al.: Heat Transfer Enhancement in a Circular Tube Fitted with Passive Technique as Twisted Tape Insert in Turbulent Flow Regime: A Review of the Recent Literature (1). Heat transfer augmentation insert in turbulent flow is effective up to certain Reynolds number. More Reynolds number blocks the flow passage and increases the pressure drop.in turbulent flow, the dominant thermal resistance is limited to a thin viscous sublayer near the wall. The following section discusses the performance of twisted tape inserts in turbulent flow. A tube inserted with a twisted tape performs better than a plain tube, and a twisted tape with a lower twist ratio provides an improved heat transfer rate at a cost of increase in pressure drop for low Prandtl number fluids. This is because the thickness of the thermal boundary layer is small for a low Prandtl number fluid and a tighter twist ratio disturbs the entire thermal boundary layer, thereby increasing the heat transfer with increase in the pressure drop, as discussed by Royds [1]. Seemawute and Eiamsa-ard [2] studied experimentally the effect of peripherally-cut twisted tape with alternate axis (PT- A) on the fluid flow and heat transfer enhancement characteristic in a uniform heat flux. Experiments were conducted using water as a testing fluid in a turbulent, tube where the Reynolds number was varied from 5000 to 20,000. Evidently, the heat transfer rates in the tube fitted with the PT-A, Peripherally-cut twisted tape (PT) and typical twisted tape (TT) are respectively enhanced up to 184%, 102% and 57% of that in the plain tube. In the present Reynolds number range tested, the PT-A, PT and TT offer the maximum thermal performances at constant pumping power of 1.25, 1.11 and 1.02, respectively. Murugesan et al. [3] investigated the effect of V-cut twisted tape insert on heat transfer, friction factor and thermal performance factor characteristics in a for three twist ratios (2.0, 4.4 and 6.0) and three different combinations of depth and width ratios (DR=0.34 and WR=0.43, DR=0.34 and WR=0.34, DR=0.43 and WR=0.34). The obtained results showed that the mean Nusselt number and the mean friction factor in the tube with V-cut twisted tape (VTT) increase with decreasing twist ratios, width ratios (WR) and increasing depth ratios (DR). Yangjun et al. [4] presented the configuration optimization of regul-arly spaced short-length twisted tape in a circular tube for turbulent heat transfer in air using computational fluid dynamics (CFD) modeling. The configuration parameters include the free space ratio, twist ratio and rotated angle. The results indicated that the larger rotated angle yields a higher heat transfer value and a greater flow resistance, whereas the smaller twist ratio results in better heat transfer performance using a twist ratio ranging from 2.5 to 8.0 except for a large rotated angle and a high Reynolds number. Halit and Veysel [5] investigated experimentally the flow friction and heat transfer behavior in a twisted tape swirl generator inserted tube. The twisted tapes were inserted separately from the tube wall. The effects of twist ratios (2, 2.5, 3, 3.5 and 4) and clearance ratios ( and ) were discussed in the range of Reynolds number from 5132 to 24,989. Uniform heat flux was applied to the tube wall. The air was used as a working fluid. The using of twisted tapes supplies considerable increase on heat transfer and pressure drop when compared with those from the plain tube. The Nusselt number increases with the decrease of clearance ratio and twist ratio, also increase of Reynolds number. The highest heat transfer enhancement is achieved as for clearance ratios= and twist ratio = 2 at Reynolds number of Consequently, the experi-mental results presented that the best operating regime of all investigated twisted tape swirl generator inserts is detected at low Reynolds number, leading to more compact heat exchanger. Behabadi et al. [6] investigated experimentally the heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long doubletube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation. Hata and masuzaki [7] investigated the twisted tapeinduced swirl flow heat transfer due to exponentially increasing heat inputs with various exponential periods and the twisted tape -induced pressure drop were systematically measured. The influence of y and Reynolds numbers based on swirl velocity, Re sw, on the twisted-tape-induced swirl flow heat transfer was investigated into details and the widely and precisely predictable correlation of the twistedtape-induced swirl flow heat transfer was derived based on the experimental data. The correlation can describe for the twisted-tape-induced swirl flow heat transfer for the wide ranges of twist ratios (y = ), mass velocities (G= kg/m 2 s) and Reynolds numbers based on swirl velocity (Re sw = to ) within 10 to +30 % difference. Thianpong et al. [8] investigated experimentally the friction and compound heat transfer behaviors in a dimpled tube fitted with a twisted tape swirl generator, using air as working fluid. The effects of the pitch and twist ratio on the average heat transfer coefficient and the pressure loss are determined in a with the fully developed flow for the Reynolds number in the range of 12,000 to 44,000. The experiments are performed using two dimpled tubes with different pitch ratios of dimpled surfaces (PR=0.7 and 1.0)

3 AASCIT Journal of Nanoscience 2015; 1(4): and three twisted tapes with three different twist ratios (y/w=3, 5, and 7). Experiments using plain tube and dimpled tube acting alone are also carried out for comparison. The experimental results reveal that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube. It is also found that the heat transfer coefficient and friction factor in the combined devices increase as the pitch ratio (PR) and twist ratio (y/w) decrease. In addition, an empirical correlation based on the experimental results of the present study is sufficiently accurate for prediction the heat transfer (Nu) and friction factor (f) behaviors. Chang et al. [9] experimentally examined the turbulent heat transfer in a swinging tube with a serrated twist tape insert under seagoing condi-tions. This swirl tube swings about two orthogonal axes under single and compound rolling and pitching oscillations. Synergistic effects of compound rolling and pitching oscillations with either harmonic or non-harmonic rhythms improve heat transfer performances. Buoyancy effects in the swinging swirl tube elevate local Nu, but are weakened as the relative strength of swinging force increases. A set of heat transfer correlations is derived that permits the individual and interactive effects of single and compound swinging force effects with buoyancy interactions on the developed flow heat transfer value (Nu) to be quantified. Smith Eiamsa-ard et al. [10] presented an experimental study on the mean Nusselt number, friction factor and enhancement efficiency charact-eristics in a round tube with short-length twisted tape insert under unifo-rm wall heat flux boundary conditions. In the experiments, measured data are taken at Reynolds numbers in a turbulent region with air as the test fluid. The full-length twisted tape is inserted into the tested tube at a single twist ratio of y/w = 4.0 while the shortlength tapes mounted at the entry test section are used at several tape length ratios (LR = l s / l f ) of 0.29, 0.43, 0.57 and 1.0 (full-length tape). The experimental result indicates that the short-length tapes of LR = 0.29, 0.43 and 0.57 perform lower heat transfer and friction factor values than the fulllength tape around 14 %, 9.5 % and 6.7 %; and 21 %, 15.3 % and 10.5 %, respectively. Hong et al. [11] studied Pressure drop and compound heat transfer characteristics of a converging-diverging tube with evenly spaced twisted-tapes (CD-T tube) have been investigated experimentally. Swirl was generated by evenly spaced twisted-tape elements which vary in twist ratio and rotation angle. Space ratio also has an important effect on the characteristics. For Comparison, experiments in a smooth and a converging diverging (CD) tube without twisted-tapes were carried out. The results show that the twisted tape with twist ratio y =4.72 and rotation angle θ =1800 has the best performance among the four types of twisted-tapes presented in this paper. Eiamsa-ard et al. [12] presented a comparative investigation of enhanced heat transfer and pressure loss by insertion of single twisted tape, full-length dual and regularly spaced dual twisted tapes as swirl generators, in a round tube under axially uniform wall heat flux (UHF) conditions. The investigation encompassed the Reynolds number based on the inlet tube diameter (0) ranging from 4000 to 19,000. The experiments are performed using single twisted tapes and full-length dual twisted tapes with three different twist ratios (y/w =3.0, 4.0 and 5.0) and also regularly-spaced dual twisted tapes with three different space ratios (S/D = 0.75, 1.5 and 2.25). The effects of major parameters on heat transfer and friction factor are discussed and the results from both single and dual twisted tape inserts are compared with those from the plain tube. Sivashanmugam and Sunduram [13] studied experimentally the improvement in performance of a double pipe heat exchanger fitted with twisted tape as a turbulence promoter with twist ratios of , 8.54, 5.882, 4.95, and under constant heat flux using water as working fluid.a maximum percentage gain of 44.7 % in energy transfer rate was obtained for the twisted tape of twist ratio For all twist ratios, the gain decreases with the Reynolds number and becomes constant for Reynolds numbers greater than 3,000. The smaller the twist ratio is, the larger the gain in energy for a specific Reynolds number. K. Kalyani Radha et al. [14] carried out experimental investigations for the augmentation of turbulent flow heat transfer in a horizontal tube by means of varying width twisted tape inserts with air as the working fluid. To reduce excessive pressure drops associated with full width twisted tape inserts, with less corresponding reduction in heat transfer oefficients, introduce a varying width twisted tapes having range from 10 mm to 22 mm. Experiments were carried out for plain tube with/without twisted tape insert at constant wall heat flux and different mass flow rates. The twisted tapes use are of three different twist ratios (3, 4 and 5) each with five different widths (26-full width, 22, 18, 14 and 10 mm) respectively. The Reynolds number varied from 6000 to Both pressure drop and heat transfer coefficient are calculated and the results are compared with those of plain tube. It was found that the enhancement of heat transfer with twisted tape inserts as compared to plain tube varied from 36 to 48 % for full width (26 mm) and 33 to 39 % for reduced width (22 mm) inserts. Jin Shuen Liou et al. [15] studied and measured experimentally the axial heat transfer distributions and the pressure drop coefficients of the tube fitted with a broken twisted tape of twist ratio 1, 1.5, 2 and 2.5 is performed in the Re range of ,000. This type of broken twisted tape is newly invented without previous investigations available. Local Nusselt numbers and mean Fanning friction factors in the tube fitted with the broken twisted tape increase as the twist ratio decreases. Heat transfer coefficients, mean Fanning friction factors and thermal performance factors in the tube fitted with the broken twisted tape are, respectively, augmented to , and times of those in the tube fitted with the smooth twisted tape. Yamada et al. [16] studied experimentally the heat transfer performance of a crossflow shell-and-tube heat exchanger for

4 46 Laith J. Habeeb et al.: Heat Transfer Enhancement in a Circular Tube Fitted with Passive Technique as Twisted Tape Insert in Turbulent Flow Regime: A Review of the Recent Literature high-temperature use in which heat transfer is augmented by the use of wall radiation in both shell and tube sides. Radiation plates are inserted in the shell side, and twisted cross-tapes in the tube side. Overall heat transfer coefficients are measured to be about a maximum 80 percent larger than those without radiation, where the inlet temperatures of the hot gas range up to 800 C, while those of the cold gas are about room temperature. Analytical results agree well with experimental results, and an approximate calculation procedure is found to be simple and accurate enough for practical use. Gupte and Date [17] evaluated semi-empirically the heat transfer coefficient and friction factor for twisted tape generated swirl flow in an annulus. Results have been obtained for radius ratios of 0.41 and 0.61 and twist ratios of, 5.302, 5.038, and The increase in pressure drop and heat transfer rates obtained are comparable to those reported for twisted tape generated swirl flow in tubes. Also, for the same heat transfer rates the pumping power requirements compare favorably with those for empty annuli. The analytical predictions based on the principle of superposition of pressure drops and analogy between heat and momentum transfer have yielded excellent predictions for y = and but somewhat poor agreement at y = Algifri and Bharadwaj [18] presentd an analytical study of the heat transfer characteristics in decaying turbulent swirl flow generated by short twisted-tapes placed at the entrance of the test section. The expressions were obtained by having a series solution of the swirl equation deduced from the Navier-Stokes equations with the aid of an order of magnitude analysis. The approach is found to be in good agreement with the available experimental data, with an average deviation of 5% and a maximum of 15%. An augmentation in heat transfer as high as 80% is obtained. Kumar and Prasad [19] investigated experimentally the performa-nce of a solar water heater with twisted tape inserts having twist pitch to tube diameter ratio ranging from 3-12 for varying mass flow rates. Within the range of investigated parameters, the heat transfer in the twisted tape insert collectors has been found to increase by %, whereas the pressure drop increased by %, as compared to plane collectors. The effect of twisted-tape geometry, flow Reynolds number and intensity of solar radiation on the thermal performance of the solar water heater has been presented. It has been found that the twisted-tape collectors perform remarkably better in the lower range of flow Reynolds number (Re =12,000), beyond which the increase in thermal performance is monotonous. Saha et al. [20] reported the results of an experimental investigation of turbulent flow heat transfer and pressure drop characteristics in a fitted with regularly spaced twisted-tape elements connected by thin orcular rods. The characteristics are governed by Reynolds number, Prandtl number, twist ratio, space ratio, and rod-to-tube dmmeter ratio. Correlations for friction factor and Nusselt number are also reported. It is shown that on the basis of both constant pumping power and constant heat duty, regularly spaced twisted-tape elements do not perform better than fulllength twisted tapes because the swirl breaks down inbetween the spacing of a regularly twisted tape. Rao and Sastri [21] carried out experimental investigation for friction factor and heat transfer coefficient in the case of a rotating tube with a twisted-tape insert for heat transfer augmentation. The data obtained were compared with existing data for a stationary tube with a twisted-tape insert. It has been observed that the enhancement in heat transfer offsets the rise in friction factor due to rotation, with respect to a plain tube under stationary conditions. A correlation has been proposed for the data obtained. Naumov et al. [22] used a mathematical model for an adiabatic cross-section to study the pulsed asymmetric heating of pipes by an external quasi-stationary heat flux with the high power density, q m 10 MW/m 2, and with the heat removal by a coolant from the parts of wetted pipe cross-section perimeter in the surface boiling regime, as well as under forced turbulent heat removal without phase transitions in the coolant. The obtained relationships are illustrated with the corresponding calculation for a cooper pipe, within a collector, with inserted twisted tape-heated by a pulse of critical heat flux from the directed ion beam, characteristic for the prescribed mode of operation under plasma heating by injection in T-15 tokamak. Hijikata et al. [23] investigated experimentally the characteristics of flow and heat transfer for a circular pipe in which a pair of twisted tapes were inserted to enhance the convective and radiative heat transfer. From the measurement of the three dimensional velocity field, it was clarified that a secondary flow was induced by the present promoter, heat transfer performance was enhanced about three times compared with that of a smooth pipe, and a remarkable increase of the local heat transfer rate was observed near the impinging region of the secondary flow. The performance of the present promoter was evaluated based on the total surface area for heat transfer under the same conditions of flow rate, pressure loss, and heat load. They also observed that the radiation between the pipe wall and the twisted tape increases the heat transfer rate by about 50 percent. S. Gulia and A. Parinam [24] carried out numerical study in with turbulent flow and found heat transfer rates for the tube with twisted tape inserted for y/w=4. The twisted tapes with length ratio LR=0.29,0.43,0.57 and 1, heat transfer rate increases 15%,18.8%,22.6% and 31% more than plain tube (fluid-air).the maximum thermal perfor-mance for using LR=0.29, 0.43, 0.57 and 1 is found to be 1.23, 1.3, 1.32 and A. K. Burse et al. [25] carried out experimental investigation to understand influence of twisted tape of different twist ratios on heat transfer characteristics in a. The work focuses on experimental investigation of heat transfer & friction factor characteristics of horizontal circular pipe by the means of insert, with air as a working fluid. Nusselt number & friction factor obtained experimentally were validated against those obtained from theoretical correlations. The twisted tapes used are of three

5 AASCIT Journal of Nanoscience 2015; 1(4): different twist ratios such as 1.78, 2.32 and Initially experiment was carried out for plain tube at constant wall heat flux & different mass flow rates of air. Secondly experiment was carried out with insertions in a tube for the Table (1). Summary of important investigations of twisted tape in turbulent flow. same working conditions as that of plain tube. The experimental results obtained were compared with those obtained from plain tube. Authors Royds [1] Seemawute and Eiamsa-ard [2] Murugesan et al. [3] Yangjun et al. [4] Halit and Veysel [5] Behabadi et al. [6] Hata and masuzaki [7] Thianpong et al. [8] Chang et al. [9] Smith Eiamsaard et al. [10] Hong et al. [11] Eiamsa-ard et al. [12] Sivashanmugam and Sunduram [13] K. Kalyani Radha et al. [14] Jin Shuen Liou et al. [15] Working fluid (5000 < Re < 20,000) (2000 < Re < 12,000) Air (10,000 < Re < 20,200) Air (5132 < Re < 24,989) Refrigerant HFC-134a ( < Re < ) Air (12,000 < Re < 44,000) Air (5,000 < Re < 25,000) Air (4,000 < Re < 20,000) Air (3400 < Re < 20,000) Air (4000 < Re < 19,000) Air (6000 < Re < 13,500) Air (1000 < Re < 40,000) Configuration of twisted tape Full length & full length with tighter twist ratio twisted tape Peripherally-cut twisted tape with alternate axis (y = 3.0) V- cut twisted tapes (y = 2.0, 4.4 and 6.0) Regularly spaced short length twisted tape (2.5 y 8.0) inserts placed separately from the tube wall Peripherally-cut twisted tape (y = 6, 9, 12 and 15) Peripherally-cut twisted tape (2.39 y 4.45) (y = 3, 5, and 7) Serated Short-length twisted tape (y = 4.0) (y = 2.22 and 4.72) Full-length dual and regularly spaced dual twisted tapes (y = 3, 4, and 5) (4.149 y ) Varying width twisted tape (y = 3, 4 and 5) Broken twisted tape (y = 1, 1.5,2 and 2.5) Type of investigation Numerical study in double-tube counter-flow heat exchanger dimpled tube a convergingdiverging tube Observations Tighter twist ratio results in greater heat transfer and pressure drop. In turbulent flow, short length twisted tape is more effective than full lengh twisted tape. The heat transfer rates in the tube fitted with the peripherally-cut twisted tape with alternate axis and typical twisted tape are respectively enhanced up to 184%and 57% of that in the plain tube. The mean Nu and the mean friction factor in the tube with V-cut twisted tape increase with decreasing twist ratios, width ratios and increasing depth ratios. The larger rotated angle yields a higher heat transfer value and a greater flow resistance. The smaller twist ratio results in better heat transfer performance using a twist ratio ranging from 2.5 to 8.0. The Nu number increases with the decrease of clearance ratio and twist ratio, also increase of Re number. The best operating regime of all investigated twisted tape swirl generator inserts is detected at low Reynolds number. Decrease in twist ratio increase the heat transfer rate. Pressure drope increases with reduction in twist raio. The values of P become linearly higher with an increase in the Red, and also influence of twist ratio and Re based on swirl velocity, Resw. Overall enhancement ratio increases with tighter twist ratio and decreases with increase in Reynolds number. Heat transfer and friction factor increases with twist vratio decreases. Dimpled tube with twisted tape gives higher heat transfer coefficient than twisted tape. Synergic effect of rolling and pitching oscillations improve heat transfer rate. Serrated tube cause swirl motion and oscillation to the flow and increases the thermal performance factor. The short -length twisted tape perform lower heat transfer and friction factor values than the full-length tape. The twisted tape with twist ratio y =4.72 and rotation angle θ =1800 has the best perform-ance among the four types of twisted tapes presented in this paper. The heat transfer of the tube with dual twisted tapes is higher than that of the plain tube with / without single twisted tape insert. For both single twisted tape and full-length dual twisted tapes, (Nu) and friction factor (f) tend to increase with decreasing twist ratio. The smaller the twist ratio is, the larger the gain in energy for a specific Re. A maximum percentage gain in energy tran-sfer rate was obtained for y =4.14. The enhancement of heat transfer with twisted tape inserts as compared to plain tube varied from 36 to 48% for full width (26mm) and 33 to 39% for reduced width (22 mm) inserts. Local Nu number and mean fanning friction factors increase as the twist ratio decreases. Yamada et al. [16] Wall radiation shell and tube heat Hot cross tapes in flue way of water heater play important role in heat transfer to tube wall.

6 48 Laith J. Habeeb et al.: Heat Transfer Enhancement in a Circular Tube Fitted with Passive Technique as Twisted Tape Insert in Turbulent Flow Regime: A Review of the Recent Literature Authors Working fluid Configuration of twisted tape Type of investigation exchanger Observations Gupte and Date [17] Algifri and Bharadwaj [18] Kumar and Prasad [19] Saha et al. [20] Rao and Sastri [21] Naumov et al. [22] Hijikata et al. [23] S. Gulia and A. Parinam [24] A. K. Burse et al. [25] Air (2 104 Re ) (4000 Re 21,000) (5000 Re 43,000) (4 < Pr < 5.5) Air Air (8000 < Re < 20,000) (y =2.659, 5.302, and ) Short-length twisted tape (y = 4.0) (3 y 12) Regularly spaced twisted-tape (3.18 y ) Pair of twisted tape (y =1.78, 2.32 and 2.77) Numerical study in annulus Analytical study in Experiment in solar water heater Experimental study in rotating tube Experiment in vertical tube test section Numerical study in Semi-empirically evaluated friction and heat transfer data for tape-generated swirl flow in annulus. Presented series of solutions to governing equations for shortlength twisted tape generated decaying swirl flow. The heat transfer in the twisted tape insert collectors has been found to increase by %, whereas the pressure drop increased by %, as compared to plane collectors. On basis of constant heat flux regularly spaced tape does not perform better than full-length twisted tape. Correlations for friction factor and Nusselt number are also reported. Enhancement of heat transfer offsets rise in friction factor due to rotation. Used mathematical model for adiabatic cross-section to study pulsed asymmetric heating of pipe by external heat flux and calculated thermophysical parameters of cooled pipes containing tapes within collector of seviated ions in T-15 tokamak injection system. Radiation between pipe wall and tape increase heat transfer rate by 50 per cent. Heat transfer rate with length ratio LR=0.29 and 0.43 increases 15 % and 18.8 % more than plain tube. Nu number increased with increase in Re number. of y = 1.78 yielded the highest value of Nu. 3. Conclusion This paper review the technique of enhancing the heat transfer rate in a for turbulent flow by insert of twisted tape. The vast literature is available on twisted tape insert in a flow to improve the performance of heat exchanger. In this paper review has considered mainly for heat transfer and pressure drop characteristics experimentally and numerically investigated for various geometry of twisted tape. Heat transfer rate increase due to insertion of twisted tape turbulator in a is one of the most effective approach. Turbulator is a device that turns laminar flow into a turbulant flow. Turbulant flow can be desired on parts of the surface of an aircraft wing or in industrial application such as heat exchanger and the mixing of fluids. The inserted twisted tape generates swirling flow and increase turbulance intensity which is major influencing factor for heat transfer enhancement. From this Review work we conclude that: 1. inserts performance depends fluid properties such as Prandtle number.if the Prandtle number is high (Pr > 35) twisted tape will not provide good thermo hydrodynamic performance. 2. Variation of Nusselt number with Reynolds number in the tube fitted with typical twisted tape. Nusselt number increase with increasing Reynolds number. 3. s are effective in turbulent flow in certain Reynolds number but not over a wide Reynolds number range. 4. The Nusselt number, Reynolds number, Prandtl number, pressure drop and friction factor depend on the geometrics of the twisted tape with different twist ratio, pitch ratio, tape width, space ratio, phase angle etc. Heat transfer rate with typical twisted tape is higher than that of plain tube. 5. For less number of turn i.e. lower twist ratio performance achieved is better compare to higher twist ratio. Modified twisted tape insert in turbulent flow improved heat transfer characteristics for certain range of Reynolds number. 6. inserts are more widely applied and have been preferred in recent past than other technique probably because methods such as extended surface inserts suffer from a relatively high cost and mesh inserts suffers from high pressure drop and fouling problems. 7. Increase in friction and pressure drop in turbulent flows create dominant effect on heat transfer through the tube. 8. There is limited work on minimum twist ratio for better heat transfer performance. 9. Heat transfer rate will increases with increase contact surface area of twisted tape insert in the tube with fluid. The summary after review show that for modified twisted tape geometry the heat transfer rate is higher with reasonable pressure loss and friction factor for turbulent flow. These conclusions are very useful for the application of heat transfer enhancement in heat exchanger networks.

7 AASCIT Journal of Nanoscience 2015; 1(4): References [1] Royds, R., "Heat Transmission by Radiation, Conduction and Convection", 1st edition, 1921, pp (Constable and Camp Limited, London). 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