HEAT TRANSFER AUGMENTATION OF LAMINAR NANOFLUID FLOW IN HORIZONTAL TUBE INSERTED WITH TWISTED TAPES

International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 3, May June 216, pp.225 239, Article ID: IJMET_7_3_21 Available online at http://www.iaeme.com/ijmet/issues.asp?jtype=ijmet&vtype=7&itype=3 Journal Impact Factor (216): 9.2286 (Calculated by GISI) www.jifactor.com ISSN Print: 976-634 and ISSN Online: 976-6359 IAEME Publication HEAT TRANSFER AUGMENTATION OF LAMINAR NANOFLUID FLOW IN HORIZONTAL TUBE INSERTED WITH TWISTED TAPES Prof. Dr. Qasim S. Mahdi Mechanical Engineering Department, College of Engineering, Al Mustansirya University, Iraq Noor AM. Mohammed Mechanical Engineering Department, College of Engineering, Al Mustansirya University, Iraq ABSTRACT In this paper, an experimental study on the heat transfer enhancement and friction factor characteristics for fully developed laminar CuO/distilled-water (DI-water) nanofluid flow through horizontal tube inserted with different geometries of twisted tapes under constant heat flux condition ranged from 4483 to 1 W/. =.8% and.35% volume concentrations of CuO nanoparticles are suspending in distilled water to prepare nanofluid. Twisted types made from copper material with twist ratios Y=2.6 and 5.3 twist ratios, thickness t=1 and 2mm and with semicircular and triangular cuts shape were used to study their effect on twisted tape performance. Results showed that both convective heat transfer in terms of average Nusselt number and friction factor have significantly increasing with inserting twisted tape with nanofluids as working fluid comparing with nanofluids or DI-water in smooth tube case and this enhancement increases as both and volume concentration increases. Triangular cut twisted tape (TCTT) at Y=2.6 and t=2mm with CuO nanofluid at =.35% showed the best preformance among the other twisted tapes on heat transfer enhancement where increased by 73% than smooth tube with DI-water, while friction factor increased by 62%. New Empirical correlations have been developed for both average Nusselt number and friction factor in the terms of the parameters mentioned above. Key words: Nanofluid, Twisted Tape, Heat Transfer Enhancement, Friction Losses. http://www.iaeme.com/ijmet/index.asp 225 editor@iaeme.com

Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed Cite this Article: Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed, Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes. International Journal of Mechanical Engineering and Technology, 7(3), 216, pp. 225 235. http://www.iaeme.com/currentissue.asp?jtype=ijmet&vtype=7&itype=3 1. INTRODUCTION Enhancement the heat transfer process play an important role in increasing the efficiency of the most important energy industry applications in heat transfer including, power generation, chemical production, air conditioning, Transportation and microelectronics, heating of circulating fluid in solar collector, heat transfer in compact heat exchangers and many other industrial sectors associated with different processes depends on the heating or cooling fluid inside tubes. During the last decades many researchers have been experimentally, investigate the effects of nanofluid technology and the tabulators' passive techniques on heat transfer enhancement and pressure drop. Lazarus et al. [29] experimentally investigated the effect of convective heat transfer of de-ionized water with a low volume fraction =.3% of copper oxide (CuO) nanoparticles to form nanofluid flows through copper tube under laminar flow and heat flux conditions. The results has shown 8% enhancement for convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement increased considerably as the Reynolds number increased. They predicted a new correlation for local Nusselt number variation along the flow direction of the nanofluid. Alimullah Anwar [214] studied experimentally the heat transfer augmentation and friction factor characteristics through circular tube fitted with full-length helical screw insert device for laminar and turbulent flow under constant heat flux condition. The results showed that with this type of inserts a high swirl flow generates which increases the convection heat transfer, thus Nusslet number increases as twisted ratio decreasing as compared with plain tube. Akeel Abdullah [211] studied experimentally the heat transfer enhancement and pressure drop in turbulent flow of air for Reynolds number range=5 to 23 in a horizontal circular tube under constant wall heat flux condition fitted with combined conical-ring tabulators and a twisted-tape swirl generator. It noticed from experimental results that temperature values increases along the tube length and decrease as twist ratio decreases while the average Nusslet number increases as Re increases and decreasing as twisted ratio decrease in case of combined twisted tape and conical ring. The results showed a significant enhancement in heat transfer process with conical ring tabulator than empty plain tube and much better enhancement in case of combined twisted tape and conical ring. It's noticed that the fanning friction factor decreases as Re increases and the values of friction factor become higher when using conical ring in combined with twist tape than using conical ring alone and especially at smaller twisted tapes ratio due to increase swirl flow which leads to higher contact between secondary flow and tube wall. He also predicted new empirical relationships for Nusslet number and friction factor for combined conical ring and twisted tape. Esmaeilzadeh et al. [214] studied experimentally the characteristics of heat transfer and friction factor enhancement of ɣ- /water nanofluid in laminar flow region flowing through uniform heated circular tube fitted with twisted tapes inserts with various thicknesses. They noticed from results that the performance of convective heat transfer becomes better with the addition of ɣ- /water nanofluid compared with water and the values of convection heat transfer coefficient increases with increasing volume concentration http://www.iaeme.com/ijmet/index.asp 226 editor@iaeme.com

Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes and becomes much higher when used in combined with twisted tape inserts, especially with the thicker twisted tape. They indicated that the friction factor values increases as twisted tape thickness increases when these values compared with the friction factor values for pure water or with nanofluid, thus the use of twisted tape inserts leads to larger surface contact and reduction the free flow area causes high speed swirl flow and increasing the pressure drop. Many researches attempt to improve the efficiency of heating systems and heat exchanger by selecting different, geometries, working fluids or operational mode and boundary condition. To overcome these problems, nanofluids and swirl flow devices insert are the best techniques used to reduce size and costs of heat exchangers and achieve a high heat transfer rate with minimum pumping power. Therefore, in this study the effect of changing the twist ratio, thickness and cutting shape of twisted tape with the flowing of CuO nanofluids at different volume concentrations through horizontal uniform heated tube will be investigated in order to get to the desired efficiency for heat transfer enhancement with less friction losses. 2. EXPERIMENTAL APPARATUAS AND PROCEDURE 2.1. Test Rig Description Straight copper tube with 14.2mm inner diameter,.9mm thickness and 1mm length was used as the test section. Ten thermocouples were soldered, five on the outer upper and five on outer lower surface tube along the test section in opposite position with an equal distance between them in order to increasing the temperatures readings accuracy. Thermocouples heads were well insulated. A.5mm thickness asbestos heat resistance insulation wrapped around the tube to electrically isolated it from the heater coils. Electrical heater coils with rating 1W, resistivity 4.9 ohm/meter, (2*.16) mm cross section and 32mm length are wound tightly around the tube to heated the test section with the desired heat flux by connecting it to a Variac voltage transformer that supply an electric AC power to regulate the input voltage across the heater coils to give a constant heat flux boundary condition along the test tube. The test tube covered with a layer of fire resistance asbestos insulation (3mm width and 5mm thickness) and another layer of fiberglass insulation with 5mm thickness to prevent heat losses. Two 4mm pressure taps inserted at the inlet and outlet of the test section. The test tube has an entrance length before section part and it's long enough to make sure that the flow is hydro dynamically fully developed when it's enter the heated section. Figure (1) shows a 3D schematic diagram of experimental test rig. Figure (2) shows a photograph for the test rig. 2.2. Twisted Tapes Geometries Twisted tapes were made from copper straight tape with length 1m and width 12mm. all the types and dimensions of twisted tapes used during the experimental work are demonstrated in table (1). Twisted tape manufacturing by clamping one end of the tape and twisting the other end carefully to reach to the desired twist ratio. The two different cutting shapes along the twisted tape edge has been extruded out by manufactures special pieces for each cut shape and then these tapes inserted inside the core tube along the test section by moving passage (flange) equipment at the end of the test tube. Figure (3) shows the geometrical details of twisted tapes. http://www.iaeme.com/ijmet/index.asp 227 editor@iaeme.com

Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed 2.3. Data Deduction Heat transfer calculation The electric power applied at the tube wall to achieve heating effect was determined by: where, I is the electric current d and V the electric voltage deliverers through heater coils. Assumed well-insulated outer surface tube (no heat losses) therefore the energy heat transfer to be absorb by the fluid: where, ṁ is the mass flow rate of water through the horizontal tube, C p the specific heat of water and are the inlet and outlet water temperatures for the test tube. In order to calculate the average heat transfer coefficient inside the tube the wellknown Newton s law of cooling used as follows, Holman and John [21]: where, is the average heat transfer coefficient inside tube, the surface area calculated from: and the mean fluid temperatures estimated by: surface temperature Sami.et al. [214]. then, the average inner Nusselt's number (Nu) calculated as: The Internal flow for heated tube is laminar fully developed flow and the Reynold number values are ranging from 29 to 2 and its estimate by the relation: ) The thermal resistant value across tube wall is too small thus, the inner surface temperature equal to the outer surface temperature [. Friction factor The friction factor coefficient ( ) which is related to the pressure drop ( ) across the heated tube length can be calculated by equation: where: http://www.iaeme.com/ijmet/index.asp 228 editor@iaeme.com

Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes where, is the average velocity inside the tube, is the Cross section area of tube. The thermal conductivity ( ), the dynamic viscosity ( ) and the density of base fluid are based on (mean bulk temperature). 3. RESULTS AND DISCUSSION Experimental results for local Nusselt number and friction factor for smooth tube were compared with the well-known Shah's correlation for the constant heat flux condition in tubes and Hagen-Poiseuille's equation for pressure drop in laminar flow respectively to sureness the accuracy of the experimental work results as shown in figures (4) and (5). The results showed logical agreement with the results from the mentioned equations. 3.1. Effect of Twist Ratios of Twisted Tape The variation of average Nusslet number and friction factor at different water Reynold's number for twisted tape ratios (Y=2.6 and 5.3) are clarified in figures (6) and (7). From the resulting curves, it can be realized that the enhancement of heat transfer in terms of average Nusselt number increases by decreasing the twist ratio and by increasing the value. The strength of swirling flow generated from the twist of the tape depends on the twist ratio thus the swirling flow generates from lower twist ratio is much higher than for higher twist ratio. Lowering the twist ratio of twisted tape generates a stronger swirling flow which increases the turbulent intensity of the main flow in order to improving the viscous boundary layer mixing near the inner tube wall to augment the heat transfer process. Nusselt number for the present study improved by 5.6% for Y=5.3 and by 55% for Y=2.6 at Re= 1923 than for smooth tube case. In basic case for smooth tube the friction factor decreases as Re increases due to the increasing in pressure drop, but it's found to be with inserting twisted tape there is a considerable augmented in friction losses and whenever the twist ratio decreased the friction factor increased and become much higher than for smooth tube at the same values for Re due to the increasing in shear forces near the tube wall. The friction factor for the present results increased by 38.47% for Y=2.6 and by 27% for Y=5.3 than those for smooth tube case. 3.2. The Effect of Twisted Tape Thickness Figure (8) clarify the variation of heat transfer enhancement in terms of average Nusselt number for twisted tapes thicknesses (t=1mm and 2mm) at different Reynold number. It's clear from resulting figures that the twisted tape thickness has a significant effect on the heat transfer process and as the tape thickness increases the Nusselt number increases. Increasing the tape thickness narrowing the swirling path flow that enhances the tangential velocities for better mixing to the viscous boundary layer near the tube wall region. In addition, the tape edge effect which acting like a fin dissipates heat from the inner tube surface to the working fluid thus increasing this area improving the convective heat transfer. Nusslet number increased by 62% for t=2mm and by 54% than t=1mm and for smooth tube respectively, while there was increasing in friction losses by 23% for twisted tape at t=2mm comparing to twisted tape at t=1mm as shown in figure (9). http://www.iaeme.com/ijmet/index.asp 229 editor@iaeme.com

Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed 3.3. The Cut Shape Effect of Twisted Tape The effects of triangular cut twisted tape (TCTT) and the semicircular cut twisted tape (SCTT) on the variation of average Nusselt number and friction factor are clarified in figures (1) and (11). As shown, at the same operation condition the heat average Nusselt number is much higher for TCTT by 4% and by 6% than SCTT and typical twisted tape respectively and by 65% than for smooth tube case. Generally, the typical twisted tape (TTT) generates only a swirling flow but in case of adding several cuts along the twisted tape edge it will produces many local vortices at each cutting section that provides an excellent mixing for the viscous boundary layers in all direction along the tube leads to higher improvement in heat transfer enhancement. Also, the heat transfer enhancement rate depends on the cutting shape which controls the strength of vortex generated which mean the vortices preformed behind the triangular cut is more stronger than those preformed through the semicircular cut. On the other hand, the friction factor enhances more with TCTT by13 % than SCTT and by 27% than TTT because the addition of these local vortices promotes an additional shear stress due to increasing in flow mixing between the viscous boundary layers of fluid at the tube wall and twisted tape. 3.4. Combined Effect of Twisted Tape and Nanofluid on Heat Transfer Figures (12) and (13) clarified the variation of average Nusslet number ( ) and friction factor of CuO nanofluid for =.8% and.35% volume concentrations, flowing at various s through the inserted tube with triangular cut twisted tape (TCTT) and typical twisted tape for 2.6 twist ratio and 1mm thickness. It's observed that the heat transfer enhancement reached the highest level during this study through the joint use of TCTT with CuO nanofluid. Where, the increased by 8% than TCTT without CuO nanofluid and by 22%, 21% than TTT with CuO nanofluid for the same operation condition and volume concentrations. Also, this enhancement increases with increasing both flow and nanoparticles volume concentration. Random motion of nanoparticles even at low volume concentration become more active in convective heat transfer and accelerated due to swirling flow and local vortices generated by the TTT and TCTT that provides perfect mixing for the viscous layer for the working fluid. The friction factor for TCTT with CuO nanofluid increased by 2% for =.8% and by 3% for =.35% than TCTT without CuO nanofluide and for the TTT with CuO nanofluid increased by 1.5% for =.8 and by 2.7% for =.35% than TTT without CuO nanofluid for the same operation conditions. The friction losses along the tested tube increases due to increasing in the turbulent intensity of the flow that accelerating nanoparticles motion through the swirling flow, which enhances shear stress forces near the inner tube wall. 4. DEVELOPING OF EMPIRICAL EQUATION The Nusselt number and friction factor experimental results have been correlated by the following equations: Nusselt number and friction factor correlation for Twist ratio: = 1.1 (16) = 868.8 (17) Valid for 451<Re< 21, 2.6<Y<5.3, 5.34<Pr<7.1. Nusselt number and friction factor correlation for TTT thickness: http://www.iaeme.com/ijmet/index.asp 23 editor@iaeme.com

Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes =.515 (18) = 294.43 (19) Valid for 451<Re< 21, 1mm <t< 2mm, 5.34<Pr<7.1. Nusselt number correlation for TCTT with nanofluids: =.943 (1+we/ (1+ (2) Valid for 286<Re< 1772, <de/ < 1.272, 5.34<Pr<7.1, <we/ < 1.363, < <.35. 5. CONCLUSIONS Generally observation, high heat transfer enhancement and pressure drop occurs with using the twisted tapes which is depends on the twist ratio, thickness and the cutting shape for twisted tapes. Twisted tape twist ratio and thickness have greater impact on heat transfer enhancement instead of adding different cutting shape on twisted tape body, where increased by 54% and 38% with increasing thickness and twist ratio, while increased by 27% with adding triangular cutting shape comparing with TTT with the same dimensions. Triangular cuts showed better performance in heat transfer enhancement than semicircular cuts which confirms that the fluid velocity accelerates more through sharp cuts. The CuO nanofluid give better performance on heat transfer enhancement when it's flowing through the inserted tube with twisted tape than the flowing in smooth tube and this performance become more efficient with increasing the nanoparticles volume concentration and. Where the highest value for was for TCTT with.35% volume concentration of CuO nanofluid. Insert set Typical twisted tape (TTT) Typical twisted tape (TTT) Typical twisted tape (TTT) Triangular cut twisted tape (TCTT) Table 1 Characteristic dimensions of the twisted tapes inserted tubes Revolution No. Thickness mm (t) Pitch(H) (Y*di) mm Width mm Twisted ratio (Y) Metal Cut dimension 3 1 37 12 2.6 Copper.. 3 2 37 12 2.6 Copper.. 15 1 75 12 5.3 Copper.. 3 2 37 12 2.6 Copper width cut (we=4mm) depth cut (de=3mm) Semicircular cut twisted tape (SCTT) 3 2 37 12 2.6 Copper Radius cutting (re=4mm) Table 2 Properties for the two types of the nanosized particle at temperature 25C Property Cuo Al 2 O 3 Cp(kJ/kg.K) 535 71 ρ(kg/m 3 ) 64 37 K(W/m.K) 69 46 α x1-7 (m 2 /s) 12 18 Di (nm) 4 8 http://www.iaeme.com/ijmet/index.asp 231 editor@iaeme.com

Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed Variac Manometer Thermometer Figure 1 3D schematic diagram of experimental test rig 1) Test section, 2) Water chiller system, 3) Insulated cold water tank, 4) flow meter, 5) Thermocouples, 6) Valves, 7) Variac, 8) Temperature, 9) water pump. Figure (2) Photograph for experimental test rig http://www.iaeme.com/ijmet/index.asp 232 editor@iaeme.com

Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes Figure 3 Twisted tapes types and geometries 25 2 Present Experimental Work Shah Equation [1978] q=6725 W/m 15 1 5 1 2 3 4 5 6 7 Figure 4 Comparison between experimental work and Shah Equation at =6725 W/, Re=944 http://www.iaeme.com/ijmet/index.asp 233 editor@iaeme.com

Average Nusselt number Friction factor Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed.25.2 Hagen-Poiseuille equation[1996] Present Experimental Work.15.1.5 5 1 15 2 25 Figure 5 Comparison between experimental work and Hagen equation at =6725 W/, Re=944 5 45 4 35 3 25 2 15 1 5 Tube with TTT at T.W=2.6 Tube with TTT at T.W=5.3 smooth Tube 5 1 15 2 25 Figure 6 Variation of Nusselt number with for different twist ratio at = 1 W/ http://www.iaeme.com/ijmet/index.asp 234 editor@iaeme.com

Average Nusselt number Friction factore Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes.35.3.25 Tube with TTT at T.W=2.6 Tube with TTT at T.w=5.3 smooth Tube.2.15.1.5 5 1 15 2 25 Figure 7 Variation of friction factor with for twisted tapes at different twist ratios at = 1 W/ 6 5 Smooth Tube Tube with TTT at thickness =1mm Tube with TTT at thickness =2mm 4 3 2 1 5 1 15 2 25 Figure 8 Variation of Nusselt number with for twisted tapes at different thicknesses at = 1 W/ http://www.iaeme.com/ijmet/index.asp 235 editor@iaeme.com

Average Nusselt number Friction factor Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed.4.35.3 Smooth Tube Tube with TTT at thickness =1mm Tube with TTT at thickness =2mm.25.2.15.1.5 5 1 15 2 25 Figure 9 Variation of friction factor with for different thicknesses at = 1 W/ 7 6 5 4 3 2 1 Tube with TTT Tube with SCTT Tube with TCTT 5 1 15 2 25 Figure 1 Variation of Nusselt number with for different cut shape of twisted tapes at = 1 W/ http://www.iaeme.com/ijmet/index.asp 236 editor@iaeme.com

Average Nusselt number Friction factor Heat Transfer Augmentation of Laminar Nanofluid flow in Horizontal Tube Inserted with Twisted Tapes.45.4.35 Tube with TTT Tube with SCTT Tube with TCTT.3.25.2.15.1.5 5 1 15 2 25 Figure 11 Variation of friction factor with for different cut shape of twisted tape at = 1 W/ 7 6 5 4 3 2 1 TCTT with DI-water TTT with DI-water TCTT with CUO.8% TTT with CUO.8% TCTT with CUO.35% TTT with CUO.35% 5 1 15 2 25 Figure 12 Variation of Nusselt number with different volume concentrations of CuO nanaofluid with TTT and TCTT http://www.iaeme.com/ijmet/index.asp 237 editor@iaeme.com

Friction factor Prof. Dr. Qasim S. Mahdi and Noor AM.Mohammed.45.4.35.3.25 TTT with DI-water TCTT with DI-water TTT with CUO.8% TCTT with CUO.8% TTT with CUO.35% TCTT with CUO.35%.2.15.1.5 5 1 15 2 25 Figure 13 Variation of friction factor with different volume concentrations of CUO nanofluid with TTT and TCTT REFERENCES [1] Akeel Abdullah Mohammed, Heat Transfer and Pressure Drop Characteristics of Turbulent Flow in a Tube Fitted with Conical Ring and Twisted Tape Inserts, Eng. & Tech. Journal, 29 (2), 211. [2] Alimullah Anwar Md., Effect on heat transfer of laminar flow in a tube insertion of twisted tape, Shri Chatrapati Shivajiraje College of Engineering, Kiran Suresh Patil, (214). [3] Esmaeilzadeh E., H. Almohammadi, A. Nokhosteen, A. Motezaker, A.N. Omrani, Study on heat transfer and friction factor characteristics of Al2O3/water through circular tube with twisted tape inserts with different thicknesses, International Journal of Thermal Sciences, 82, pp.72 83, 214. [4] Holman J. P. and John Lloyd, Heat Transfer, Southern Methodist University, McGraw-Hill Series in Mechanical Engineering, (21). [5] Keblinski P., Jeffery A.Eastman, David G. Cahill, Nanofluids for thermal transport, Materials Today, 8 (6):36 44, (22). [6] Lazarus Godson Asirvatham, Nandigana Vishal, Senthil Kumar angatharan and Dhasan Mohan Lal, Experimental study on forced convective heat transfer with low volume fraction of CuO/Water nanofluid, Journal Energies 2, pp.97 119, ISSN 1996-173, (29). [7] Liu.S, M.Sakr, A comprehensive review on passive heat transfer enhancements in pipe exchangers, Journal ElSEVIER, Renewable and Sustainable Energy Reviews, 19, pp.64 81, (213). [8] Radu Cazan and Cyrus K. Aidun, Experimental investigation of the swirling flow and the helical vortices induced by a twisted tape inside a circular pipe, Atlanta Georia, PHYSICS OF FLUIDS, 21, (29). [9] Rashidi.F, N. Mosavari Nezamabad, Experimental Investigation of Convective Heat Transfer Coefficient of CNTs Nanofluid under Constant Heat Flux, Proceedings of the World Congress on Engineering, Vol III, (211) http://www.iaeme.com/ijmet/index.asp 238 editor@iaeme.com

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