International Communications in Heat and Mass Transfer

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1 International Communications in Heat and Mass Transfer 39 (2012) Contents lists available at SciVerse ScienceDirect International Communications in Heat and Mass Transfer journal homepage: Heat transfer performance for turbulent flow through a tube using double helical tape inserts M.M.K. Bhuiya a,b,, M.S.U. Chowdhury b, J.U. Ahamed b, M.J.H. Khan c, M.A.R. Sarkar d, M.A. Kalam a, H.H. Masjuki a, M. Shahabuddin a a Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia b Department of Mechanical Engineering, Chittagong University of Engineering and Technology (CUET), Chittagong-4349, Bangladesh c Department of Chemical Engineering, University of Malaya, Kuala Lumpur, Malaysia d Department of Mechanical Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh article info abstract Available online 30 April 2012 Keywords: Nusselt number Double helical tape insert Enhancement efficiency Friction factor Correlation The augmentation of heat transfer for turbulent fluid flow through a tube by using double helical tape inserts was investigated experimentally in the present work. The effects of insertion of the helical tape turbulators with different helix angles (9, 15, 21 and 28 ) on heat transfer and pressure drop in the tube for Reynolds number ranging from 22,000 to 51,000 were examined. Experimental results showed that the heat transfer and thermal performance of the inserted tube were significantly increased compared to those of the plain tube. The study showed the Nussselt number, friction factor as well as thermal enhancement efficiency were increased with decreasing helix angles under the same operating conditions. The results indicated that the Nusselt number and friction factor were increased up to 305% and 170%, respectively, than those over the plain tube while the maximum thermal performance was found to be 215% for using the double helical tape insert with helix angle 9 at high Reynolds number. Furthermore, correlations of the Nusselt number and friction factor were developed in terms of the helix angle (α), Reynolds number (Re) and Prandtl number (Pr) based on the experimental data Elsevier Ltd. All rights reserved. 1. Introduction The applications of heat transfer enhancement techniques can significantly increase the performance of heat exchanger, directing to the reduction of heat exchanger size as well as operating cost. Heat transfer enhancement has significant meanings for energy conservation and environmental problems. The induced swirl flow, potentially promoted fluid mixing, causes a thinner boundary layer and consequently, resulting in higher convective heat transfer rate. Swirl flow generators with different geometrical configurations are widely used to enhance the heat transfer rate in many engineering applications, for example, heat recovery processes, air conditioning and refrigeration systems, internal cooling of gas turbine blades, chemical reactors, thermal regenerators, gas-cooled reactors, food and dairy processes. Extensive studies have been performed to modify the helical tapes in order to improve their performance with regard to the typical one. Gul and Evin [1] experimentally studied the heat transfer and fluid Communicated by W.J. Minkowycz. Corresponding author at: Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia. address: mkamalcuet@yahoo.com (M.M.K. Bhuiya). friction characteristics for turbulent flow using short helical swirl generators with different helix angles of 30, 45 and 60 placed at the entrance of the test section for the Reynolds number range of 5000 to 30,000. Zohir et al. [2] investigated the heat transfer and pressure drop characteristics for turbulent flow in a sudden expansion pipe fitted with propeller type swirl generators for the Reynolds range between 7500 and 18,500 with several pitch ratios under a uniform heat flux condition. Eiamsa-ard and Promvonge [3] studied on the heat transfer characteristics in a tube fitted with helical screwtape with/without core-rod inserts. Kurtbas et al. [4] investigated the performances of heat transfer and pressure drop through a tube with different swirl generators for Reynlods number range of 10,000 to 35,000 under a constant heat flux condition. Sarkar et al. [5] experimentally studied the heat transfer in turbulent flow through a tube with wire-coil inserts. Sivashanmugam and Suresh [6] experimentally studied on the heat transfer and friction factor characteristics in a circular tube fitted with regularly spaced helical screw-tape inserts. Sivashanmugam and Suresh [7] also investigated the heat transfer and friction factor characteristics in a circular tube equipped with full length helical screw inserts of the different twist ratio with uniform heat flux under turbulent flow conditions. Promvonge [8] experimentally investigated on the thermal enhancement in a round tube with snail entry and coiled-wire inserts. Eiamsa-ard and Promvonge [9] experimentally /$ see front matter 2012 Elsevier Ltd. All rights reserved. doi: /j.icheatmasstransfer

2 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) Nomenclature A Surface area of tube [m 2 ] A x cross sectional area of test section [m 2 ] C p specific heat at constant pressure [J/(kg. C] D i tube inside diameter [m] d core-rod diameter [m] f friction factor, dimensionless h convective heat transfer co-efficient [W/m 2. C] k thermal conductivity [W/m. C] L tube length [m] L t tape length [m] _m mass flow rate [kg/s] p pitch length [m] ΔP pressure drop [N/m 2 ] P inlet pressure [N/m 2 ] P m blower power [W] Q heat transfer rate [W] q heat flux [W/m 2 ] T temperature [ C] V mean velocity [m/s] tape width [m] W d Greek symbols Α helix angle [deg] η enhancement efficiency, dimensionless ρ density [kg/m 3 ] Subscripts avg average b bulk i inlet o outlet p tape inserts pre predicted s plain w wall x local Dimensionless numbers Nu Nusselt number, dimensionless Pr Prandtl number, dimensionless Re Reynolds number, dimensionless Re p Equivalent Reynolds number for tube with helical tape inserts, dimensionless Re s Equivalent Reynolds number for plain tube, dimensionless studied on the enhancement of heat transfer in a tube with regularlyspaced helical tape swirl generators for the Reynolds number range of 2300 to 8800 and showed that the use of helical tapes led to a higher heat transfer rate over the plain tube. Ahamed et al. [10] experimentally studied the prediction of heat transfer in turbulent flow through a tube with perforated twisted-tape inserts and also developed a new correlation. Sreenivasulu and Prasad [11] numerically studied the flow and heat transfer characteristics in an annulus wrapped with a helical wire for constant heat flux boundary condition. Naphon and Suchana [12] experimentally studied the heat transfer enhancement and pressure drop of the horizontal concentric tube with twisted wire brush inserts. Wongcharee and Eiamsa-ard [13] investigated the influences of twisted tapes with alternate-axes and triangular, rectangular and trapezoidal wings on heat transfer, fluid friction and thermal performance characteristics in a tube. Karwa et al. [14] experimentally investigated the effect of relative roughness pitch and perforation of the spring roughness on heat transfer and friction factor for turbulent flow in an asymmetrically heated annular duct (radius ratio=0.39) with a heated tube having a spirally wound helical spring. Hasan and Sumathy [15] studied on the potential use of swirl flow generators in enhancing the thermal performance of solar air heaters. Pethkool et al. [16] experimentally investigated the augmentation of convective heat transfer in a single-phase turbulent flow using the helically corrugated tube. Bhuiya et al. [17] experimentally studied the heat transfer enhancement and developed new correlations for turbulent flow through a tube with triple helical tape inserts. Ibrahim [18] investigated the heat transfer and fluid friction characteristics in horizontal double pipe flat tubes with full legth helical screw inserts of different twist ratio and helical screw inserts of different spacer length. However, none of the research works were reported on heat transfer performance evaluation through a tube with double helical tape inserts. Two tapes of equal width and thickness were parallelly and helically wrapped very closely on a core rod with different helix angles, α. Tape of small thickness is easy to warp on a core rod in comparison to the tape of high thickness. Helically wrapped double tapes on a core rod were inserted into the plain tube with different helix angles, α. The main goal of the present work is to investigate the effect of swirling flow generated by the double helical tape inserts on heat transfer performance and friction factor in a tube heat exchanger. The empirical correlations of heat transfer and friction factor were also presented for predicting the heat transfer and friction factor, respectively, based on the experimental data. In this investigation, some assumptions were made in order to make easy experiments, comparison and analysis, which created some limitations in the actual results. These were: i. Inside diameter of the tube (D i ) was used instead of hydraulic diameter (D h )indefining Reynolds number (Re), Nusselt number (Nu) and friction factor (f). ii. All the fluid properties were calculated at fluid mean bulk temperature instead of film temperature and at the atmospheric pressure instead of local pressure in the test section which was slightly less than the atmospheric pressure. iii. Bulk temperature, T b was assumed to vary linearly along the length of the test section, whereas wall temperature, T w was varied non-linearly along the length of the tube. iv. Pressure drop was measured through the test section, and the blower power was calculated corresponding to the pressure drop in the test section. v. The heat transfer was considered only by forced convection from the inside wall of the tube to the fluid. However, there were points of contact between the inserts and the inside of the wall of the tube. Thus, there was the potential for heat transfer to occur through the inserts by conduction. 2. Data reduction equations Mass flow rate was calculated by, _m ¼ ρa x V i where, ρ is the density of air, A x is the cross-sectional area of test section and V i is the mean inlet velocity. ð1þ

3 820 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) In the test section, the velocity of air was obtained from, V ¼ _m ρ b A x where, ρ b is the density at bulk fluid temperature. Pressure drop at any axial location x, was calculated by the following equation, ΔP ¼ P i P ðxþ where, P i is the inlet pressure, and P (x) is the pressure at any axial location x. Friction factor based on inside diameter was obtained by, ΔP f ¼ ð4þ L D i ρ b V 2 2 where, L is the length of the tube, and D i is the inner diameter of the tube. Blower power was calculated by, ð2þ ð3þ q ¼ Q A And the heat flux rate was calculated by, where, A is the internal surface area of the tube. The bulk fluid temperature was obtained by, ð T b ¼ T 0 þ T i Þ 2 All the fluid thermophysical properties were calculated at the average of the inlet and outlet bulk temperatures, T b. The average wall temperature was calculated from, T wavg ¼ T w ð9þ 8 where, T w is the wall temperature of the tube. The average wall temperature was calculated from 8 points of wall temperatures lined between the inlet and the exit of the test section. The average heat transfer co-efficient was obtained from, q h ¼ ð10þ T wavg T b ð7þ ð8þ ΔP _m P m ¼ ρ b ð5þ The Nusselt number was calculated according to the following way, Heat transfer rate was calculated from, Q ¼ _mc p ðt 0 T i Þ ð6þ where, C p is the specific heat of air, T i and T o are the inlet and outlet temperatures of air, respectively. In the experiments, the heat equilibrium test showed that the heat absorbed by the air (Q) was within 5% lower than the heat supplied by electrical winding, this was due to the heat leakag from the tube wall. Nu ¼ hd i k where, k is the thermal conductivity of air. 3. Experimental setup ð11þ The experimental setup used for the present study was same as that used in Bhuiya et al. [17] for triple helical tape inserts and was explained in this section. The experimental facility consists of an inlet section, a test section, an air supply system (Electric blower) and a heating arrangement. The schematic diagram of the experimental facility is shown in 1. Inlet section 7. Voltmeter 13. Flexible pipe 2. Traversing pitot tube 8. Temperature controller 14. Diffuser 3. Inclined tube manometer 9. Heater on off lamp 15. Blower 4. U-tube manometer 10. Pressure tappings 16. Motor 5. Variable voltage Transformer 11. Thermocouples 17. Traversing thermocouple 6. Ammeter 12. Flow control valve Fig. 1. Schematic diagram of the experimental facility.

4 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) Fig. 1. The tube-shaped inlet section (533 mm long) was made integral to avoid any flow disturbances upstream of the test section to get fully developed flow in the test section as well. The inlet section shape of the experimental setup was made as per suggestions of Owner and Pankhurst [19] to avoid separation and stratification of the flow. Geometry of test section fitted with the double helical tape insert with a core-rod and geometric parameters of the double helical tape insert are shown in Fig. 2(a) and (b), respectively. The plain tube (test section) was made of brass having 70 mm inside diameter, 90 mm outside diameter and 1500 mm in length. The tapes were made of mild steel and were tightly wrapped on a circular rod of diameter 12 mm for different helix angles 9, 15, 21 and 28 with corresponding pitches of 600 mm, 770 mm, 1035 mm and 1500 mm, respectively. The geometric dimensions of double helical tapes were; length of each tape, L t =1500 mm, width of each tape, W d =26.5 mm and thickness of each tape, t=3 mm. The tapes were in turn inserted into the plain tube. Nichrome wire (resistance 1.2 Ω/m) was used as an electric heater to heat the test section at constant heat flux condition. Nichrome wire was spirally wounded uniformly around the tube. The terminals of the Nichrome wire heating coil were connected to the variac transformer. The electrical output power was controlled by a variac transformer to obtain a constant heat flux condition throughout the entire test section. The outer surface of the test section was well insulated to minimize the heat leaks to the surroundings. Eight K-type thermocouples were tapped along the tube wall for monitoring the local temperatures of the tube surface. Air inlet and outlet temperatures were also measured with the help of K-type thermocouples. The pressure tappings were made at the inlet of the test section as well as at eight axial locations of the test section. U-tube manometers at an inclination of 30º were fitted with the pressure tappings. The heat transfer and pressure drop experiments were carried out individually. The heat transfer experiment was performed under a constant heat flux condition. In contrast, the pressure drop (friction) test was conducted under an isothermal condition without turning on the heater. Flow of air through the test section was measured with the help of a traversing pitot tube. The traversing pitot tube was fitted at a distance of 4D i from the inlet section according to Owner and Pankhurst [19]. Arithmetic mean method was applied to determine the position of the traversing pitot tube for determination of the mean velocity of air. The experiments were conducted for the Reynolds number ranging from 22,000 to 51,000. The uncertainties in the experimental measurements were determined by using the method introduced by Kline and McClintock [20]. The uncertainty calculation method used the calculation of derivatives of the desired variables with respect to the individual experimental quantities and applied with the known uncertainties. Consequently, the maximum uncertainties of the non-dimensional parameters were found ±1.6% for velocity, ±6.5% for heat transfer rate, ±6.5% for Nusselt number and ±3.8% for friction factor. 4. Results and discussion 4.1. Validation test of the plain tube results The results obtained from present experiments on heat transfer and friction factor characteristics of the plain tube were verified in terms of Nusselt number and friction factor. The Nusselt number and friction factor data obtained from the present plain tube were validated with those from the proposed correlations by Gnielinski [21] and Petukhov [22] for the Nusselt number and friction factor as shown in Fig. 3(a) and (b), respectively. The results obtained from the present plain tube were agreed well with those from the proposed correlations within ±5% and ±4% deviations for the Nusselt number and friction factor, respectively. These results revealed the accuracy of the present experimental facility and used measurement technique. The correlations obtained from the present plain tube results for Nusselt number and friction factor, respectively, were given as follows: Nu ¼ 0:0077Re 0:893 Pr 0:33 ð12þ Fig. 2. (a) Geometry of test section fitted with double helical tape insert with a core-rod, and (b) geometric parameters of the double helical tape insert.

5 822 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) a Experimental Gnielinski correlation a Helix angle, α = 9 (deg.) Helix angle, α = 15 (deg.) Helix angle, α = 21 (deg.) Helix angle, α = 28 (deg.) Plain tube Nusselt number, Nu Nusselt number, Nu b Friction factor, f Experimental Petukhov correlation b Heat flux, q (Watt/m 2 ) Helix angle, α = 9 (deg.) Helix angle, α = 15 (deg.) Helix angle, α = 21 (deg.) Helix angle, α = 28 (deg.) Plain tube f ¼ 0:687Re 0: Heat transfer characteristics Fig. 3. Verification of (a) Nusselt number, and (b) friction factor for the plain tube. ð13þ Fig. 4(a) shows the relationship between the Nusselt number and Reynolds number of the helical tape inserted tubes with different helix angles. It was shown from Fig. 4(a) that for all cases, the Nusselt number increased with increasing Reynolds number. This was attributed due to the increase of turbulent intensity as Reynolds number was increased, which led to an amplification of convective heat transfer. The influence of using the double helical tape swirl generators on the heat transfer rate was significant for all the Reynolds number. At the comparable Reynolds number, the Nusselt numbers for the tube equipped with double helical tape inserts were considerably higher than those of the plain tube. In general, the double helical tape inserts generated swirl flow or secondary flow offering a longer flowing path of fluid flow through the tube; intensive mixing of fluid and pressure gradient might be created along the radial direction. The boundary layer along the tube wall would be thinner with the increased of radial swirl and pressure resulting in more heat flow through the fluid. Furthermore, the swirl enhanced the flow turbulence, which led to even better convection heat transfer. 400 Fig. 4. Relationship between (a) Nusselt number and Reynolds number, (b) heat flux and Reynolds number for the tube with double helical tape inserts. From the experimental results, it could be observed that the heat transfer rate increased with decreasing helix angle of the tape inserts. It was depicted from Fig. 4(a) that the Nusselt number for a given Reynolds number was higher at the lower helix angle (α) of the tape inserts which indicated the enhanced heat transfer rate. This could be explained by the fact that at lower helix angle (α), stronger swirl intensity was generated, which led to more efficient interruption of boundary layer along the flow path. Over the range studied, the helical tape insert with smaller helix angle (α=9 ) provided the higher heat transfer rate than those for the tube with higher helix angles. According to the experimental results, the Nusselt numbers of the tube with double helical tape inserts were % higher over the plain tube. Fig. 4(b) exhibits the relationship between heat flux and Reynolds number of the tube with double helical tape inserts. It could be noted from Fig. 4(b), the heat flux increased with increasing Reynolds number. As expected from Fig. 4(b), the heat fluxes obtained from the tube with inserts were significantly higher than those of the plain tube. This was because the plain tube had been a lower wetted perimeter, and less contact area with the working fluid compared to that of the tube with inserts and its ability to transfer heat was low. Considering the effect of helix angles on the heat transfer rate, it was depicted that the heat flux rate at the lower helix angle was

6 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) higher than those from higher ones across the range of Reynolds number. The heat fluxes for the tube with double helical tape inserts were 21 60% higher in comparison to those of the plain tube Fluid flow characteristics Effect of the helix angles of the tube fitted with double helical tape inserts on the friction factor characteristics at different Reynolds number is exhibited in Fig. 5(a). It could be shown from Fig. 5(a) that the friction factor was in the similar trend both for the plain tube and the tube with inserts. The friction factor of the tape inserted tube gradually decreased with increasing Reynolds number. At a particular Reynolds number, the tube fitted with double helical tape inserts led to higher friction factors over those of the plain tube. This was because of the flow blockage, larger contact surface areas, the act caused by the swirl flow and the dissipation of dynamic pressure of the fluid due to high viscosity loss near the tube wall. Moreover, the pressure loss had a high possibility to occur by the interaction of the pressure forces with inertial forces in the boundary layer. As shown in Fig. 5(a), the friction factor tended was increased with decreasing helix angle. This could be attributed to the use of double helical tape insert with lower helix angle which caused stronger swirl flow or turbulence flow and long residence time in the tube. Over the range investigated, the friction factors for the tube fitted with double helical tape inserts were % higher compared to those of the plain tube values. It could be illustrated from Fig. 5(b) that the blower power of the tube equipped with double helical tape inserts decreased at low Reynolds numbers due to weak swirling flow but increased substantially at higher values of Reynolds number. It could be shown from Fig. 5(b) that the required blower powers obatined from the tube with inserts were higher than those of the plain tube at a particular Reynolds number. The required blower powers of the tube fitted with double helical tape inserts were % higher compared to those of the plain tube values Correlations for prediction of heat ttransfer and friction factor The correlations were developed for the turbulent flow region in a wide range of Reynolds number 22,000 to 51,000. The correlations developed for Nusselt number and friction factor obtained from the present experimental results of the tube fitted with double helical tapes could be written in terms of helix angle (α), Reynolds number (Re) and Prandtl number (Pr) in Eqs. (14) and (15), respectively. n o Nu pre ¼ 0:0139ðtanαÞ þ 0:0541ðtanαÞ 0:0614ðtanαÞþ0:0165 : 7:616 tan α Re f ð Þ3 þ6:8146ðtan αþ 1:3614ðtan αþþ1:087g :Pr 0:33 ð14þ n o f pre ¼ 977:25ðtan αþ :7ðtan αþ 2 þ 368:53ðtan αþ 8:2096 : 1:7004 tan α Re f ð Þ3 þ3:0528ðtan αþ 2 1:2033ðtan αþ 0:4745g ð15þ The Nusselt numbers and friction factors values predicted from the above correlations Eqs. (14) and (15) were compared with the experimental values, and the comparisons were shown in Fig. 6(a) and (b), respectively. From Fig. 6(a) and (b), it could be noted that the Nusselt numbers and friction factors values obtained from the predicted correlations Eqs. (14) and (15) agreed well with the experimental values for all the investigated cases within +3% to 4% and +3% to 4% deviations of the proposed correlations, respectively Performance evaluation In order to appraise the heat transfer augmentation performance of four double helical tape inserts with plain tube, a constant blower power comparison was made, in the present work. Bergles et al. [23] and Webb [24] proposed several performance criteria to evaluate the thermohydraulic performance of the enhanced techniques. The performance was evaluated on the basis of constant blower power. According to constant blower power evaluation criteria [24]: _V ΔP ¼ V _ ΔP ð16þ s p and the relationship between the friction factor and Reynolds number could be expressed as: fre 3 ¼ s fre3 ð17þ p Re s ¼ Re p 1 f 3 p f s ð18þ The enhancement efficiency (η) at constant blower power is the ratio of the convective heat transfer coefficient of the tube with double a Friction factor, f b Blower power, P m (Watt) Helix angle, α = 9 (deg.) Helix angle, α = 15 (deg.) Helix angle, α = 21 (deg.) Helix angle, α = 28 (deg.) Plain tube Helix angle, α = 9 (deg.) Helix angle, α = 15 (deg.) Helix angle, α = 21 (deg.) Helix angle, α = 28 (deg.) Plain tube 0 Fig. 5. Variation of (a) friction factor with Reynolds number, and (b) blower power with Reynolds number for different double helical tape inserts.

7 824 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) helical tape insert (h p )totheplaintube(h s ) which could be written as follows: η ¼ h p pp ¼ Nu p h s Nu s pp ð19þ The variation of enhancement efficiency with Reynolds number is plotted for all the cases in Fig. 7. The enhancement efficiency increased with increasing Reynolds number for all the investigated cases. As depicted from Fig. 7 that the tube with inserts at higher Reynolds number provided higher thermal efficiency. A performance analysis was performed to evaluate the net energy gain of all the tested inserts based on the constant blower power. It was found efficient from an energy point of view by applying all of these tested inserts as η was greater than unity. The enhancement efficiency was higher at the lower helix angles, α and lower at the higher helix angles, α. The highest enhancement efficiency was obtained for the tube with double helical tape insert of helix angle, α=9 and the value was Fig. 7. Variation of enhancement efficiency (η) with Reynolds number. a Predicted Nusselt number, Nupre b Predicted friction factor, fpre % - 4% Experimental Nusselt number, Nu % - 4% Experimental friction factor, f Fig. 6. Comparison between (a) predicted and experimental Nusselt numbers, and (b) predicted and experimental friction factors for the tube with double helical tape inserts. 5. Conclusion The heat transfer enhancement, thermal performance and friction factor characteristics of double helical tape inserted tube were investigated experimentally. The experiments were performed for the tube fitted with double helical tapes with different helix angles (9, 15, 21 and 28 ). The use of double helical tape inserts provided significant augmentation of heat transfer with the corresponding increase in friction factor. It was found that the Nusselt number, friction factor and thermal enhancement efficiency increased with decreasing helix angles. Based on the experimental results, key findings of this study could be summarized as follows: The Nusselt number obtained for the tube with double helical tape inserts was 305% higher in comparison to those of the plain tube values. Helical tape insert of helix angle 9 showed the highest heat flux among the inserts tested and was around 60% higher the value of the plain tube. The friction factor for the tube equipped with double helical tape inserts was 170% higher, whereas the blower power was 160% higher than those of the plain tube values. The maximum thermal enhancement efficiency (η) of 215% was obtained for the double helical tape inserted tube with helix angle 9. The empirical correlations were developed in the present study which predicted the results of the Nusselt number and friction factor. The maximum deviations between the predicted results and experimental results for Nusselt number and friction factor were found to be +3 to 4% and +3 to 4%, respectively. Acknowledgements Authors are grateful to the authority of Bangladesh University of Engineering and Technology (BUET) for their financial support in this work. The Chittagong University of Engineering and Technology (CUET) authority is highly acknowledged for necessary assistance to do this research. The authors would like to acknowledge the High Impact Research (HIRG) project, University of Malaya, for funding the project. The research has been carried out under the project UM.C/625/1/HIR/022.

8 M.M.K. Bhuiya et al. / International Communications in Heat and Mass Transfer 39 (2012) References [1] H. Gul, D. Evin, Heat transfer enhancement in circular tubes using helical swirl generator insert at the entrance, International Journal of Thermal Sciences 46 (2007) [2] A.E. Zohir, A.A.A. Aziz, M.A. Habib, Heat transfer characteristics in a sudden expansion pipe equipped with swirl generators, International Journal of Heat and Fluid Flow 32 (2011) [3] S. Eiamsa-ard, P. Promvonge, Heat transfer characteristics in a tube fitted with helical screw-tape with/without core-rod inserts, International Communications in Heat and Mass Transfer 34 (2007) [4] I. Kurtbas, F. Gulcimen, A. Akbulut, D. Buran, Heat transfer augmentation by swirl generators inserted into a tube with constant heat flux, International Communications in Heat and Mass Transfer 36 (2009) [5] M.A.R. Sarkar, M.Z. Islam, M.A. Islam, Heat transfer in turbulent flow through tube with wire-coil inserts, Journal of Enhanced Heat Transfer 12 (4) (2005) [6] P. Sivashanmugam, S. Suresh, Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with regularly spaced helical screw-tape inserts, Applied Thermal Engineering 27 (2007) [7] P. Sivashanmugam, S. Suresh, Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts, Chemical Engineering and Processing 46 (2007) [8] P. Promvonge, Thermal enhancement in a round tube with snail entry and coiled wire inserts, International Communications in Heat and Mass Transfer 35 (2008) [9] S. Eiamsa-ard, P. Promvonge, Enhancement of heat transfer in a tube with regularly-spaced helical tape swirl generators, Solar Energy 78 (2005) [10] J.U. Ahamed, M.A. Wazed, S. Ahmed, Y. Nukman, T.M.Y.S. Tuan Ya, M.A.R. Sarkar, Enhancement and prediction of heat transfer rate in turbulent flow through tube with perforated twisted tape inserts: a new correlation, Journal of Heat Transfer 133 (041903) (2011) 1 9. [11] T. Sreenivasulu, B.V.S.S.S. Prasad, Flow and heat transfer characteristics in an annulus wrapped with a helical wire, International Journal of Thermal Sciences 48 (2009) [12] P. Naphon, T. Suchana, Heat transfer enhancement and pressure drop of the horizontal concentric tube with twisted wire brush inserts, International Communications in Heat and Mass Transfer 38 (2011) [13] K. Wongcharee, S. Eiamsa-ard, Heat transfer enhancement by twisted tapes with alternate-axes and triangualr, rectangular and trapezoidal wings, Chemical Engineering and Processing 50 (2011) [14] R. Karwa, B.K. Maheshwari, P.K. Sailesn, Experimental study of heat transfer and friction in annular ducts with a heated tube having a spirally wound helical spring, Journal of Enhanced Heat Transfer 17 (1) (2010) [15] M.A. Hasan, K. Sumathy, Study on the potential use of swirl flow generators in enhancing the thermal performance of solar air heaters, International Journal of Ambient Energy 30 (4) (2009) [16] S. Pethkool, S. Eiamsa-ard, S. Kwankaomeng, P. Promvonge, Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube, International Communications in Heat and Mass Transfer 38 (2011) [17] M.M.K. Bhuiya, J.U. Ahamed, M.S.U. Chowdhury, M.A.R. Sarkar, B. Salam, R. Saidur, H.H. Masjuki, M.A. Kalam, Heat transfer enhancement and development of correlation for turbulent flow through a tube with triple helical tape inserts, International Communications in Heat and Mass Transfer 39 (2012) [18] E.Z. Ibrahim, Augmentation of laminar flow and heat transfer in flat tubes by means of helical screw-tape inserts, Energy Conversion and Management 52 (2011) [19] E. Owner, R.C. Pankhurst, The Measurement of air flow, Fifth Edition Pergamon Press, 1977 (in SI units). [20] S.J. Kline, F.A. McClintock, Describing uncertainties in single-sample experiments, Mechanical Engineering 75 (1) (1953) 3 8. [21] V. Gnielinski, New equations for heat and mass transfer in turbulent pipe and channel flow, International Chemical Engineering 16 (2) (1976) [22] F. Incropera, P.D. Dewitt, Introduction to heat transfer, 3rd edition John Wiley and Sons Inc., 1996 [23] A.E. Bergles, A.R. Blumenkrantz, J. Toberek, Performance evaluation criteria for enhanced heat transfer surfaces, Journal of Heat Transfer 2 (1974) [24] R.L. Webb, Performance evaluation criteria for use of enhanced heat transfer surfaces in heat exchanger design, International Journal of Heat and Mass Transfer 24 (1981)