(Microsoft Word - \323\307\343\355 \346\321\343\322\35589-97) Al-Khwarizmi Engineering Journal Al-Khwarizmi Engineering Journal,Vol. 13, No. 4, P.P. 89- 97 (2017) Augmentation of Nanofluids Heat Transfer in a Circular Tube with Baffled Winged Twisted Swirl Generator Sami D. Salman* Ramzi Ata Abd Alsaheb ** *,**Department of Biochemical Engineering/ Al-khwarizmi College of Engineering/ University of Baghdad/ Iraq *E-mail: sami.albayati@gmail.com **E-mail: ramzi_eng_1981@yahoo.com (Received 14 June 2017; accepted 17 July 2017) https://doi.org/10.22153/kej.2017.07.006 Abstract This article introduces a numerical study on heat exchange and corrosion coefficients of Zinc–water nanofluid stream in a circular tube fitted with swirl generator utilizing CFD emulation. Different forms of swirl generator which have the following properties of plain twisted tape (PTT) and baffle wings twisted tape (BTT) embeds with various ratio of twisting (y = 2.93, 3.91 and 4.89), baffle inclination angles (β = 0°, - 30° and 30) joined with 1%, 1.5% and 2% volume fraction of ZnO nanofluid were utilized for simulation. The results demonstrated that the heat and friction coefficients conducted by these two forms of vortex generator raised with Reynolds number, twist ratio and baffle inclination angles decreases. Likewise, the results showed that the heat transfer rate raised with accretion of ZnO nanoparticle concentration. Furthermore, the maximum rate of heat transfer with significant intension in friction coefficient has been produced by baffle wings tape with ratio of twisting y=2.93 and baffle angle β= -30 with 2% volume fraction of ZnO nanofluid. Keywords: Baffled wings twisted tape, Nanofluid, Heat transfer augmentation, Friction factor, Uniform heat flux. 1. Introduction The thermal performance of heat exchangers can be essentially increased by utilizing heat transfer augmentation techniques. The augmentation techniques can be categorized three major kinds; the active technique, the passive technique and compound technique. The active techniques imposed an extrinsic power like; electrical field, surface vacillation, etc. The passive technique includes additives to gases or liquids, particular face geometries or swirl generator i.e; Tape inserts. However, the combined techniques can be created by utilizing at least two or more active and/or passive techniques. Different exploratory investigations on these augmentation techniques have been conducted using tape inserts [1-10], while numerical simulation utilizing CFD methods have been reported. Pathipakka G & Sivashanmugam [11] implemented numerical simulation on heat rate characteristics for Al2O3 nanofluid with helical inserts in circular tube using FLUENT software version 6.3.2. Various volume fractions (0.5%, 1.0%, 1.5%) of Al2O3 nanoparticles in water conjoined helical twist inserts with various ratio of twisting (y = 2.93, 3.9 and 4.89) were utilized for numerical simulation. The obtained results have been compared with data collected from literature with great assertion. Salman et al. [12] have been actualized theoretical study on the heat characteristics and friction factor in circular tube embedded tape with elliptical cut using CFD-FLUENT software version 6.3.26. Two forms of twisted tape have been used; Classical and elliptical cut tapes with Sami D. Salman Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 1- 10 (2017) 90 different ration of twisting (y=2.93 - 4.89) and various cut depth (w = 0.5 - 1.5 cm) with water as test liquid were utilized for numerical simulation. The results obtained demonstrate that the heat rate and the friction characteristics conducted by twist tape with elliptic cut have been augmented with Reynolds number increasing and decreasing ratio of twisting. Likewise, the result demonstrate that the higher heat transfer rate was obtained by tape of elliptical cut with w = 0.5 cm and y=2. 93. Salman et al. [13] announced a numerical investigation on heat transfer augmentation in a uniform heat-fluxed tube fitted with V-cut twisted tape in laminar stream of water utilizing FLUENT software. The simulated results demonstrate that the greatest value of heat transfer improvement was acquired by V-cut tape with cut tape with y=2. 93 w = 0.5 cm model. Salman et al. [14] additionally proposed numerical simulation of the heat rate characteristics in a circular tube with twisted tape under steady state heat flux condition using FLUENT software. Plain tapes and baffled tapes with various ratio of twisting (y = 2.93, 3.91, and 4.89) and various angles of inclination (β= 0°, -30°, and 30°) were used for simulation. The results coordinated with the correlations excerpted from literatures for a plain tube within ± 8% and ± 7 % deviation for Nusselt and friction factors, respectively. Additionally, the results demonstrate that baffled tapes with ratio of twisting y=2.93 and inclination angle β= -30° produced the greatest heat augmentation. The present research reports a numerical study for heat transfer improvement in a tube with baffled winged twist tape (BTT) with various ratio of twisting (y = 2.93, 3.91, and 4.89) and various baffle inclination angle (β= 0°, -30°, and 30) conjugated with 1%, 1.5% and 2% volume fraction of ZnO nanofluid are accounted. The result acquired offered about 10% improvement for Nusselt number with noteworthy increments in friction characteristics compared with plain twisted tape. 2. Technical Specification 2.1. Baffled Winged Tape Model Baffled winged twist tape (BTT) model is demonstrated in Figure 1. Aluminium tape of 2.45 cm width and 0.08 cm thickness is consistently twisting over different lengths of 7.5, 10, and 12.5 cm to create different ratio of twisting (y = 2.93, 3.91 and 4.89). The ratio of twisting `y' characterized as a fraction of single full twist (360°) length to tape width. Stainless steel tube with diameter of 2.54 cm and length of 180 cm was utilized as test sample, Water and ZnO nanoparticles (dp=20 nm) have been chosen as test liquid. The thermo-physical characteristics of substances used for simulation are appeared in Table 1 and Table 2. Table 1, Water and ZnO nanofluids Thermo-physical characteristics [15], [16]. Liquid Density (Kg/m3 ) Specific heat (J/Kg K) Thermal Conductivity (W/m°C) Viscosity (N s/m2) Water 997.2 4180 0.6096 0.0009632 ZnO 5600 495.2 13 – Water + 1% ZnO 1043 3982.35 0.6450 0.001 Water + 1.5% ZnO 1066 3889.83 0.6526 0.0011 Water + 2% ZnO 1089 3801.23 0.6605 0.00106 Table 2, Material Thermophysical characteristics. Material Density (Kg/m3) Thermal conductivity (W/m K) Specific heat (J/Kg K) Aluminium 2719 202.4 871 Steel 8030 16.27 502.48 Sami D. Salman Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 1- 10 (2017) 91 Fig. 1. (a) Baffle winged twist tape with baffle inclination angle (β= 0º) (b) Baffle winged twist tape with baffle inclination angle (β= 30º). 2.2. Nanofluids Thermophysical Properties Nanofluids thermophysical properties were utilized as a part of this research using equations below [15]: ( ) (1 ) ( ) ( ) n f f n p C p C p C pρ φ ρ φ ρ= − + ...(1) (1 ) n f f np ρ φ ρ φρ= − + ...(2) Where (ρCp)f and (ρCp)np are heat capacity of the base liquid and the nanoparticles, respectivly. ρf and ρnp are the densities of the base liquid and the nanoparticles, φ nanoparticle volume fraction The effective thermal conductivity estimated using correlations below [15]: ( 2 ) 2 ( ) ( 2 ) ( ) np f f np S tatic f np f f np K K K K K K K K K K φ φ  + − − =   + + +   ...(3) 4 5 1 0 ( , ) 2 B ro w n ian f f n p np K T K C p f T R = × ρ φβ φ ρ ...(4) e f f S tatic B ro w n ian K K K= + ...(5) Where β = 9.881(100ϕ)− 0.9446 for ZnO nanoparticle k=Boltzmann constant 2 3 2 3 0 ( ,T) (2.8217 10 3.917 10 )( ) ( 3.669 10 3.391123 10 ) T f T − − − − = × + × + − × − ×φ φ φ The effective viscosity estimated using the correlation below [16]: 1.03 0.3 (1 34.87 ( / ) ) f eff dp df − = − × × µ µ φ …(6) 6 fo M df N πρ   =     …(7) Where ρfo is the density of the base fluid at temperature To=293 K. M is the molecular mass of base fluid. N is the Avogadro number. 3. Data Structures and Numerical Analysis 3.1. Geometry Grid Creation The geometry of baffle winged twist tape as exhibited in Figure 2 was generated by GAMBIT which exported to ANSYS-FLUENT 16 for simulation. This geometry model consists of a plain tube with length of 1800mm and 25.54 mm diameter. Plain twist tape with various ratio of twisting was created by twisting consistently piece of width equal to 25.54 mm which utilized by alternative turns edge of 360º for a length of 75, 100, and 125mm to produce the following twist ratios 2.93, 3.91 and 4.89. Though, the baffle winged twist tape geometry of with ratio of twisting ( y= 2.93) produced by using Cuboids of 25.54 mm width in the broad at a twist angle of 360º and 75 mm length. The desired volume formed for simulation produced by subtract the baffle winged twist tape shape from the plain tube shape and the obtained model exported to the ANSYS-FLUENT 16 for meshing and simulation. Edge meshing is utilized using specific intervals, while the front surface was meshed by tetrahedral and pave mesh method, then the meshed surface swept through whole volume utilizing T grids form. The boundary conditions of the desired model including; model walls, inlet, outlet, and type of liquid (water) were characterized for CFD simulation. Fig. 2. Baffle winged twist tape grids with angle of inclination (β= 30º). Sami D. Salman Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 1- 10 (2017) 92 3.2. Mathematical Model 3.2.1. Hypothesis The suspension nanoparticles in water assumed as single phase fluid under thermal equilibrium.This assumption will precisely define the nanofluid behaviour in engineering cases. The case scrutinized for steady state and laminar flow conditions in three dimension using governing equations with numerical values illustrated in Table 3. 3.2.2 Equation of Mass Conservation. .( ) m p S t ρυ ∂ + ∇ = ∂ r ..(8) 3.2.3 Equation of momentum Conservation ( . ) . ij p g F t υ ρ υ υ ρ τ ∂ + ∇ = −∇ + + ∇ + ∂ ur r ur …(9) 3.2.4 Equation of Energy Conservation. { }( ) .{ ( )} . ( . )eff h eff i E E S K T h t ∂ +∇ + = ∇ ∇ − ∂ + ∑ r r r ρ ρ υ ρ ρ τ υ …(10) Table 3, Numerical values used for simulation. 4. Result and Discussion 4.1. Model Validation and Grid Independence Test The independence of grid was investigated to evaluate mesh sizes influence on the recreated results about; five mesh work volumes were taken into account (401226, 532338, 656404, 727890 and 838278) for Re = 2000 which gives different values of Nusselt number with percentage deviation up to 0.3%. Subsequently, mesh work volume domain of 656404 was performed for plain tube and water as test fluid validates the model versus literature Stephan correlations [17]. Figures 3 and 4 demonstrate the variety of Nusselt number and friction coefficient with Reynolds number. Evidently, the results sensibly concurred well with these correlations. 4.2. Influence of Twist Ratio Varieties of Nusselt and friction coefficients versus Reynolds number with plain twisted tape existence of are demonstrated in Figures 5 and 6. . It’s clearly noted that Nusselt and friction coefficients for minimum twist ratios were greater than those obtained from maximum ratio (y). This implied that the minimum twist ratio prompts greater tangential combination between the tube wall and swirl streams. 4.3. Influence of Tape Configuration Variety of Nusselt and the friction coefficients versus Reynolds number with plain twist tape (y = 2.93) and baffle winged twist tape that has similar twist ratio (y=2.93, 3.91 and 4.89) and baffle inclination angles (β= 0º, -30º and 30º) is appeared in Figures 7 and 8. It’s discovered that the friction coefficient and Nusselt number for a tube with baffle winged twisted tape (y= 2.93) and baffle inclination angle (β= -30º) was greater than the other twisted tapes. The baffle winged twist tape promote an extra liquid turbulences close to tube wall and vortices beyond the baffle winged shape which destructed the thermal boundary layers and generating supercar stream mixing between the liquid and heating surface Reynolds Number (Re) Heat flux (W/m2) 240 350 475 600 710 840 950 1075 1200 2100 782.92 1565.85 2348.78 3131.71 3914.63 4697.56 5480.5 6263.42 7046.35 7829.27 Ahmed N. Al-Khazraji Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 89- 97 (2017) 93 1 6 11 0 500 1000 1500 2000 2500 3000 N u ss e lt N u m b e r ( N u ) Reynolds Number (Re) Simulated Nusselt Number Literature Nusselt Number Fig. 3. Numerical Nusselt Number vs literature correlations for Plain tube. Fig. 4. Numerical Friction factor vs literature correlations for Plain tube. 0 25 50 75 0 1000 2000 3000 N u ss e lt N u m b e r (N u ) Reynolds Number (Re) PTT (y=4.89) PTT (y=3.91) PTT (y=2.93) Fig. 5. The Influence of twist ratio on Nusselt Numbers. 0 0.1 0.2 0.3 0.4 0.5 0 1000 2000 3000 F ri ct io n F a ct o r (f ) Reynolds Number (Re) PTT (y=4.89) PTT (y=3.91) PTT (y=2.93) Fig. 6. The Influence of twist ratio on friction factor. Ahmed N. Al-Khazraji Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 89- 97 (2017) 94 0 15 30 45 60 75 0 1000 2000 3000 N u s s e lt N u m b e r (N u ) Reynolds Number (Re) PTT (y=2.93) BTT (y=2.93 & β=0°) BTT (y= 2.93 & β= -30°) BTT (y=2.93 & β= 30°) Fig. 7. The Influence of tape configuration on Nusselt number. 0 0.15 0.3 0.45 0.6 0.75 0 1000 2000 3000 F ri ct io n F a ct o r (f ) Reynolds Number (Re) PTT (y=2.93) BTT (y=2.93 & β=0 °) BTT (y=2.93 & β= -30 °) BTT (y=2.93 & β= 30°) Fig. 8. The Influence of tape configuration on fiction factor. 4.4. Influence of Tape Configuration with Nanofluds The impact of Baffle winged tape with ratio of twisting y=2.93 and baffle inclination angle (β= - 30º) with various volume fraction of ZnO nanofluid on Nusselt number and friction coefficient are illustrated in Figures 9 and 10. It is plainly noted from Figure 8 that the Nusselt number well improved with increments of nanoparticles concentration. This implies that the nanoparticles concentration enhanced the random movement of nanoparticles which improves fluid flow thermal dispersion as well and vortices behind the baffle winged configuration. Figure 9 demonstrates the friction coefficient variation with the Reynolds number for various nanoparticles concentration. It’s unmistakably noticed that the surface shear stress raised with the nanoparticles concentration expansion. 0 10 20 30 40 50 60 70 80 0 500 1000 1500 2000 2500 N u ss e lt N u m b e r (N u ) Reynolds Number (Re) BTT (y=2.93 & β= 30° ) BTT (y=2.93 & β= 30° with 1% ZnO) BTT (y=2.93 & β= 30° with 1.5% Zn O) BTT (y=2.93 & β= 30° with 2% ZnO) Fig. 9. The Influence of tape configuration with Nanofluid concentration on Nusselt number. 0 0.1 0.2 0.3 0.4 0.5 0 1000 2000 3000 F r ic ti o n F a c to r ( f) Reynolds Number (Re) BTT (y=2.93 & β= 30°) BTT (y=2.93 & β= 30° with 1% ZnO) BTT (y=2.93 & β= 30° with 1.5% ZnO) BTT (y=2.93 & β= 30° with 2% ZnO) Fig. 10. The Influence of tape configuration with Nanofluid concentration on friction factor. Ahmed N. Al-Khazraji Al-Khwarizmi Engineering Journal, Vol. 13, No. 4, P.P. 89- 97 (2017) 95 5. Conclusion Numerical simulations for rate of heat transfer in a tube with classical and baffle winged tapes as well 1-2% volume fraction of ZnO nanofluid was completed utilizing by ANSYS-FLUENT version 16. The results obtained were matched with literature correlations for validation with the inconsistency up to ±10% for friction characteristics and ± 8% for the Nusselt number. The results demonstrate that the Nusselt number expanded with the expansion of nanoparticles concentration, Reynolds number and twist ration as well baffle inclination angles diminishes. Likewise, the tape with y=2.93 and baffle inclination angles (β= -30) donated more prevailing than those of (β= 0°,30°) for various Reynolds number. In addition, the tape with y=2.93 and baffle inclination angles (β= -30) combined with 2% ZnO nanofluid offers almost 10% improvements in Nusselt number values with considerable increments in friction coefficient than those obtained by classical tape. Nomenclature Cp Fluid Heat Capacity, Joule/kg.K dp Size of Nanoparticle , nm E Energy, Joule F Force, Newton f Friction factor g Acceleration of gravity, m/s 2 keff Thermal conductivity (Effective), Watt/m.K m mass flow, kg/sec Re Reynolds numbe, dimensionless Nu Nusselt number, dimensionless p Pressure, N/m2 Sm Mass accumulation, Kg Sh Energy accumulation, J T Temperature. K. v Velocity, m/sec y Ratio of twisting, dimensionless Greek Letters ρ Density Kg/m3 eff τ uuur Shear stress, Newton/m 2 . Acknowledgment We wish to specially acknowledge the assistance offered by the University of Baghdad AL Khwarizmi College of Engineering for carrying our this investigation 6. References [1] M. Chandrasekar, S. Suresh, A.C. 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Preußer, “Wärmeübergang und maximale Wärmestromdichte beim Behältersieden binärer und ternärer Flüssigkeitsgemische”, Chemie Ingenieur Technik, vol. 51, Issue 1, pp. 37, 1979. )2017( 89-97، صفحة 4د، العد13ددسية المجلجلة الخوارزمي الهنم سامي داود سلمان 97 بصفيحة ملتوية ذات األجنحة مجهزفي أنبوب دائري تحسين األنتقال الحراري للموائع النانوية لتوليد الدوامات **عبد الصاحب رمزي عطا * مي داود سلمانسا العراق /جامعة بغداد /كلية الهندسة الخوارزمي /قسم الهندسة الكيميائية األحيائية*،** sami.albayati@gmail.com :البريد اإللكتروني* ramzi_eng_1981@yahoo.com**البريد االلكتروني: الخالصة د بمولد دوامة يقدم هذا البحث دراسة عددية عن التبادل الحراري ومعامالت األحتكاك لتدفق المائع النانوي من الزنك والماء في أنبوب دائري مزو : الصفيحة الملتوية الكالسيكية،تيةوقد استخدمت أشكال مختلفة من مولد الدوامات التي لها الخصائص اال باستخدام ديناميك الموائع الحاسوبي. الجسيمات النانوية مع ) β = 0°, - 30° , 30°( للجناحميل بزوايا ، و)y = 2.93, 3.91 and 4.89الصفيحة الملتوية ذات األجنحة وبنسبة التواء ( هذين الشكلين من يت لأظهرت النتائج أن معامالت الحرارة واالحتكاك التي أجرقد و .للمحاكاة (%2 ,1.5,%1)كأوكسيد الزنك وبتركيز حجمي يصل الى قد أظهرت النتائج أن معدل انتقال الحرارة الجناح للصفيحة، كما ميل التواء الصفيحة وزواية نسبة ونقصان عدد رينولدز، ات قد أزدادت بزيادة دوامالمولد في معامل االحتكاك تم زيادة ملحوظة عالوة على ذلك، فإن الحد األقصى لمعدل انتقال الحرارة مع وكسيد الزنك.ألالجسيمات النانوية تركيز أزداد بزيادة مع تركيز حجمي ألوكسيد الزنك النانوي (β = - 30°) ميل تصل الى زاويةوب (y=2.93)ذات األجنحة وبنسبة التواء الصفيحة الملتوية الحصول عليها . ٢يصل الى %