International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com
Computational Analysis of Different Nanofluids effect on Convective Heat Transfer Enhancement of Double Tube Helical Heat Exchanger
1 2 3 Saurabh Kumar , Neha Maheshwari , Dr. Brajesh Tripathi 1Ph.D, ME, UPTU, U.P. India 2 Mtech, ME, NIU, U.P. India 3 Associate Prof, ME, GBU, UP, India
Abstract discussed on the preparation methods for
The heat transfer enhancement and flow nanofluids [2-4]. A nanofluid is a fluid produced characteristics in double pipe helical coil heat by the distribution of nanoparticles with a typical exchanger using nanofluids are numerically size of less than 100 nm in a liquid. Nanofluids studied and researched by many researchers. have enhanced thermal properties. In an industry heat exchangers are Studies are carried out for SiO2, CuO, and Al2O3 nanofluid with different volume fractions (1-5%). broadly used and performance of heat The graphs are plotted for Nusselt number and exchangers can be improved by different Heat transfer coefficient for MIXTURE methods. Due to the compressed structure and MODEL using ANSYS FLUENT 15. The finite high heat transfer coefficient, helical coil heat volume method with k–ε standard turbulent exchangers find open use in industrial model was used to discritize the main governing applications [5] such as power generation, equations. Result showed that the value of nuclear industry, process plants, heat recovery Nusselt number for nanofluid is higher than that systems, food industry, refrigeration, Seban and McLaughlin [6] of the base fluid as water where the SiO2 nanofluid yields the best heat transfer experimentally investigated the heat transfer in coiled tubes for both laminar and turbulent enhancement followed by CuO and Al2O3. Enhancement in heat transfer is achieved by flows. . Prabhanjan et al. [7] investigated the increasing the Dean Number and volumetric heat transfer rates of a straight tube heat concentration. exchanger to that of a helically coiled heat Keywords: Nanofluid, FLUENT, Dean Number, exchanger. Wongwises et al. [8] studied the Nusselt number, Heat transfer coefficient condensation heat transfer in a tube-in-tube helical heat exchanger. 1. Introduction Thermal performance in helically coiled Cooling is one of the most considerable heat exchangers was experimentally and scientific challenges in the industrial area, which numerically investigated by Jayakumar et al. [9]. applies to many varied productions, counting It was studied that the constant wall flux microelectronics, transportation and boundary condition was a better hypothesis to manufacturing and technological developments. investigate the heat transfer inside the helical There is an urgent need for new and spanking coil than either constant wall temperature or new coolants with superior performance. The constant wall heat transfer coefficient boundary new concept of ‘nanofluids’ – heat transfer conditions. fluids consisting of suspended of nanoparticles – has been expected as a vision for these challenges [1].some review papers have ABBREVIATIONS
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International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com
Ac= flow area, m2 total surface heat flux= ׳Al2O3 =aluminium oxide q 2 As =surface area, m Q =heat transfer rate (kW) Cu =copper Re =Reynolds Number CuO =copper oxide Recr=Critical Reynolds number Cp =specific heat, J/kg-K St= Stanton Number CFD =computational fluid dynamics Thi=temperature of hot fluid at inlet, K dii=inner diameter of inner pipe, inch Tho=temperature of hot fluid at outlet, K dio=outer diameter of inner pipe inch Tci= temperature of cold fluid at inlet, K doi =inner diameter of outer pipe, inch Tco= temperature of cold fluid at outlet, K doo =outer diameter of outer pipe, inch Tw= wall temperature, K Dh = hydraulic diameter, mm Tf= fluid mean temperature, K Dn =diameter of nanofluid particles D = coil diameter, mm ux, uy and uz=velocity in x ,y, z directions De =Dean Number x, y, z coordinates HTF =Heat transfer fluid X, Y, Z body force in x, y, z directions h =heat transfer coefficient, W/m2k V= Average velocity of flow H =pitch of coil, mm Vs = wetted volume, m3 k =thermal conductivity, W/m-K K =turbulent kinetic energy Greek symbol L =length of pipe, m β=surface area density, m2/m3 m =mass flow rate, kg/s 3 n= number of turns ρ =density, Kg/m Nux =Local Nusselt Number Nu=Average Nusselt number μ=dynamic viscosity, Kg/ms Φ=Rayleigh dissipation factor P= Pressure, N/m2 2 3 Δp= Pressure drop ε =dissipation kinetic energy (m /s ) Pr= Prandtl Number φ =volume fraction (%) q= heat flux, W/m2 δ= Curvature Ratio
Rennie and Raghavan [10] studied had been done on the performance of other types numerically on the heat transfer characteristics of heat exchangers [11-13]. of a double tube heat exchanger. They showed In present work, the heat transfer that flow in the inner tube is the limiting factor performance of a double helical coiled heat of overall heat transfer coefficient of heat exchanger was investigated numerically. The exchanger and while stabilizing other end of the tubes act as the entry and exit of hot parameters. The overall heat transfer coefficient as well as cold fluid where the cold fluid flows will increase, Also there is a widespread in the outer pipe and the hot fluid flows in the research inner pipe of the double pipe helical coil heat exchanger.
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International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com
In double tube helical heat exchangers, due to the curvature of tubes and application of The model used in this analysis is the centrifugal force on fluid flow, the secondary double tube helical heat exchanger (fig 1) shows flow motion is generated which improves heat schematic cross section of used model, it transfer coefficient considerably [14]. Dravid et consists of two tubes with two turns in which al.[15] projected the correlation for inner Nu at nanofluid flows through inner tube and water the Dean number (De) range of 50 to 2000. He flows through outer tube. Both fluids flow in the investigated the secondary flow motion same direction. Copper is used for tubes to produced by the curvature effects. He concluded maximize the heat transfer rate. The end of tubes that resultant centrifugal force makes heat acts as the entry and exit for hot as well as cold transfer coefficient greater than that of straight fluid .The temperature for nanofluid and water tube. are taken as 3480C and 2830C respectively. Studies are carried out in CFD tool for In the present study, velocity inlet the effects of several geometrical parameters on boundary condition is enforced at the inlet of heat transfer characteristics for water and the inner and outer tubes and pressure outlet effect of different nanofluids SiO2, CuO, and boundary condition is enforced at the outlet of Al2O3 with different volume fractions (1-5%).on inner and outer tubes. The flow is assumed to be heat transfer characteristics. Newtonian, turbulent and incompressible. As there are two walls in double tube helical heat Characteristics of helical coils exchanger so insulated outer wall condition is Stretching of the flow through the coils is used at outer wall and bilateral wall is used for the key point of helical coil heat exchanger inner wall for the heat transfer to be occurred on which produces the secondary flow due to the both sides. effect of centrifugal forces. For the analysis of flow in helical pipe, Dean (De) number is used defined as follows, 1/ 2 ρ Vdin d oi Dein = µ D
As turbulent flow is used in double tube helical heat exchangers, thus in this work, turbulent flow ranges were chosen for flow rates. Therefore, to know about the type of flow, the obtained Reynolds number can be compared with critical Reynolds number (Recr):
0.45 Recr = 2300[1+8.6(d / D) ]
Where δ is curvature ratio and defined as: d oi Fig. (1) Schematic cross section of used model D (double tube helical coil heat exchanger)
Table 1, 2, and 3 shows the dimensional 2. CFD Methodology parameters, properties of base fluid and 2.1 Geometry and Boundary Conditions
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International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com nanoparticle and Properties of copper Description Value Unit respectively. 3 3 Table 1: The dimensional parameters of helically Densit,y ρ (Kg/m ) 8978 Kg/m coiled tube heat exchanger Specific Heat Capacity, 381 J/Kg-K Cp (J/Kg K) Thermal Conductivity, Helically coiled tube Value (inch) 387.6 W/m-K K (W/m K) Inner diameter of inner 0.545 tube (dii) 2.2 Numerical Simulation Outer diameter of inner 0.625 tube (doi) In this study, commercial software Inner diameter of outer 0.785 ANSYS 15 is used to create the geometry and tube (dio) meshing of double tube helical coil heat Outer diameter of outer exchanger. MAPPED FACE MESH with Tetra 0.875 tube (doo) and Hexahedral cells is done for creating fine mesh. The number of nodes used in this analysis Pitch of coil 2 is 53710. A numerical analysis with standard (k- ε) k-epsilon model as a turbulent model is used Table 2: The thermo - physical properties of base to understand the flow characteristics. A mixture fluid and nano particles at T = 300k model is used for this analysis. Inlet and outlet are defined as velocity inlet and pressure outlet with insulated outer wall condition. Dean Thermo- number is used to calculate the input value. The physical Water Al O CuO SiO 2 3 2 coupled scheme with second order upwind is Properties used to deal with pressure-velocity coupling problem and to solve governing equations. The Density, ρ relaxation factor has been set to default values. 996.5 3600 6500 2220 -5 (Kg/m3) The normalized residual values are set to 10 for all the variables to converge the solution. The governing differential equations for the Specific fluid flow are given by Continuity equation, Heat, Cp 4181 765 533 495.2 Navier Stokes equation and energy conservation (J/Kg K) equation. These can be written in the following Thermal form [15]: Conductivit 0.010 36 17.65 13 y, K (W/m 3 2.2.1 Continuity Equation: K) ∂(ρu) ∂(ρv) ∂(ρw) + + = 0 (2.1) Viscosity, μ 0.001 ∂x ∂y ∂z - - - (mpa s) 003
Table 3: Properties of copper 2.2.2 Navier Stokes equation:
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International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com
∂u ∂u ∂u ∂p 1 ∂ ∂u ∂v ∂w 2 The empirical constants for the turbulence model are ρu + v + w = ρX − + µ + + + µ∇ u ∂x ∂y ∂z ∂x 3 ∂x ∂x ∂y ∂z arrived by comprehensive data fitting for a wide (2.2) range of turbulent flow.
∂v ∂v ∂v ∂p 1 ∂ ∂u ∂v ∂w 2 Cµ = 0.09, Cε1 =1.47, Cε 2 =1.92, σk =1.0, σε =1.3 ρu + v + w = ρY − + µ + + + µ∇ v ∂x ∂y ∂z ∂y 3 ∂y ∂x ∂y ∂z (2.9) (2.3) The governing differential equation for solid domain
∂w ∂w ∂w ∂p 1 ∂ ∂u ∂v ∂w 2 is only the Energy equation which is given by; ρu + v + w = ρZ − + µ + + + µ∇ w ∂x ∂y ∂z ∂z 3 ∂z ∂x ∂y ∂z 2 (2.4) ∇ T = 0 (2.10) Heat transfer coefficient is obtained by equating the 2.2.3 Energy Equation: conduction heat transfer to the convection heat transfer; ∂T ∂T ∂T ∂p ∂p ∂p 2 ρC p u + v + w = u + v + w + k∇ T + µφ ∂x ∂y ∂z ∂x ∂y ∂z qcond = qconv (2.11) (2.5) Where, Local Nusselt number is given by;
2 2 2 2 2 2 Nu = hD / k (2.12) ∂u ∂v ∂w ∂u ∂v ∂v ∂w ∂w ∂u X φ = 2 + + + + + + + + ∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ x y z y x z y x z 2 3. Result and discussion 2 ∂u ∂v ∂w − + + 3 ∂x ∂y ∂z For the helically coiled heat exchanger,
the heat is also transferred due to the effect of 2.2.4 Turbulent kinetic energy (k) equation: secondary flow. In this work, the effects of several geometrical parameters and different ∂ ∂ µ ∂k nanoparticles with different volume fractions on (ρku ) = µ + t + G − ρε (2.6) ∂ i ∂ σ ∂ k heat transfer characteristics were studied. xi x j k x j
2.2.5 Turbulent kinetic energy dissipation (ε) 3.1 Effect of curvature ratio (δ) equation: Curvature ratio defined by δ (doi / D) is
∂ ∂ µ ∂ε 2 the most important parameter for the analysis of (ρεu ) = µ + t + C ε G + C ρ ε i 1ε ( k ) k 2ε ( k ) helical heat exchanger. Dean Number includes ∂xi ∂x j σ ε ∂x j curvature ratio and both parameters d and D (2.7) oi can change curvature ratio. Here, to avoid the In the above equations, Gk represents the generation variation of Dean Number in annulus, just outer of turbulent kinetic energy due to mean velocity diameter of the tube was changed. In this figure gradients, σk and σε are effective Prandtl numbers for (2) it is observed that increasing of curvature turbulent kinetic energy and rate of dissipation, ratio makes reducing of heat transfer coefficient respectively; C1ε and C2ε are constants and µ is the t significantly. eddy viscosity and is modeled as As the tube thickness increases, the heat
2 transfer rate decreases due to the increasing µt = (ρCµ k ) / ε resistance offered by the (2.8)
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International Journal of Scientific Engineering and Applied Science (IJSEAS) - Volume-1, Issue-4, June 2015 ISSN: 2395-3470 www.ijseas.com tube. 25000 4120 20000 water 4115 15000 Al2O3-water 4110 CuO-water h (W/m-k) 10000 4105 SiO2-water Nusselt no.(Nu) 5000
Curvature ratio Curvature 4100 0 4095 5837 6500 7000 7500 0.20833 0.225 0.2333 Dean no.(De) Heat transfer coefficient
Fig. (2) Variation of Heat transfer coefficient Fig. (3) Variation of the effect of different with respect to curvature ratio nanofluids under the range of Dean Number on the average Nusselt Number 3.2 Effect of different nanoparticles: 3.3 Influence of Dean Number (De) : In this section, three different types of For the analysis of flow in helical tube, Dean nanoparticles: Al2O3, CuO, and SiO2 with water Number is used. The values of Nusselt Number as a base fluid are used. The graph is plotted for different Dean Number are shown in fig (4) between Nusselt Number and Dean Number for and these values increases with increase in Dean different nanoparticles at 4 % volume fraction Number. As Dean Number is directly related to and 20 nm particle sizes. It can be seen from the the velocity of the flow. At higher flow rates, fig. (3) that for different nanofluids, Nusselt mixing fluctuations intensifies due to the Number increases with the increase in Dean dispersion effects and chaotic movement of the Number. These results also indicate that all the nanoparticles and results in increased heat four types of nanofluids are comparatively richer transfer coefficient. In this way the Nusselt in heat transfer rate than the pure water because Number will also increase. all the nanofluids possess higher Nusselt number compared to pure water. It is clearly seen that SiO2 – water nanofluid shows the best nanofluid 25000 and posses the higher Nusselt Number compared 20000 to pure water. It is clear that the SiO2 nanofluid 15000 Nu has the highest value, followed by CuO and 10000 Al2O3.
Nusselt No. (Nu) 5000 0 5837 6500 7000 7500 Dean Number (De)
Fig. (4) Variation of the Nusselt number with respect to Dean Number
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3.4 Effect of different nanoparticle volume the coil diameter (doi), torsional behavior fraction approaches towards linear behavior. In this section, the graph is plotted • Heat transfer coefficient increases with between Nusselt Number and Dean Number at increase in Dean Number for convective different volume fraction range (0.01-0.05). It is heat transfer. Because at higher flow clear from the fig. (5) that by adding low volume rates, the dispersion effects and chaotic fraction of nanoparticle with 20 nm particle size movement of the nanoparticles intensifies to the water as base fluid, enhancement in the mixing fluctuations and causes Nusselt Number can be achieved. Because increase in heat transfer coefficient. larger surface area for molecular collisions is Nanofluid containing small amount of achieved by the presence of nanoparticle and nanoparticles have substantially higher heat higher momentum is achieved by increasing transfer coefficient than those of base fluids. concentration of nanoparticles. This momentum And it increases with increase in particle volume transfers thermal energy more efficiently to the fraction. Because increase in the nanoparticle coolant and enhances the heat absorption of it, volume fractions intensifies the interaction and causing its temperature to increase. It is clear collision of nanoparticles that the Nusselt number increases with increased value of volume fraction. 5. References
25000 [1] D. Wu, H. Zhu, L. Wang, L. Liua, Critical 20000 V.F=0.01 issues in nanofluids preparation, characterization V.F=0.02 15000 and thermal conductivity, Curr. Nanosci. 5 V.F=0.03 10000 (2009) 103–112. V.F=0.04 [2] A. Ghadimi, R. Saidur, H.S.C.Metselaar, A 5000 V.F=0.05 review of nanofluid stability properties and
Average Nusselt No.(Nu) Nusselt Average 0 5837 6500 7000 7500 characterization in stationary conditions, Int. J. Dean no.(De) Heat Mass Transf. 54 (17) (2011) 4051–4068.
Fig. (5) The effect of different volume fraction [3] Yu. Wei, Huaqing Xie, A review on under the range of Dean Number on the average nanofluids: preparation, stability mechanisms, Nusselt Number and applications, J. Nanomater. 2012 (2012) 1– 17. 4. Conclusion [4] P. Naphon, S. Wongwises, A review of flow Numerical simulation has been carried out and heat transfer characteristics in for double tube helical coil heat exchanger curved tubes, J. Renew. Sustain. Energy Rev. 10 subjected to insulated outer wall condition using (2006) 463–490. ANSYS FLUENT 15. The graphs are plotted for [5] R.A. Seban, E.F. McLaughlin, Heat transfer Nusselt number and heat transfer coefficient for in tube coils with laminar and turbulent flow, the water and nanofluids in which flow condition Int. J. Heat Mass Transf. 6 (1963) 387–395. is taken as turbulent. Following are the outcomes [6] D.G. Prabhanjan, G.S.V. Raghavan, T.J. of the above study; Rennie, Comparison of heat transfer rates • It is observed that heat transfer between a straight tube heat exchanger and a coefficient decreases with increasing helically coiled heat exchanger, Int. Commun. curvature ratio. Because by increasing Heat Mass Transfer 29 185-191, 2002.
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[7] S.Wongwises ,M. Polsongkram, “Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube- in-tube heat exchanger,” Int. J.Heat Mass Transfer 29 4386-4398, 2006. [8] J.S. Jayakumar, S.M. Mahajani, J.C. Mandal, Kannan N. Iyer, P.K. Vijayan, CFD analysis of single-phase flows inside helically coiled tubes, J. Comput. Chem. Eng. 34 (2010) 430–446. [9] Rennie T.J, Raghavan G.S.V. (2002) “Laminar parallel flow in a tube-in-tube helical heat exchange”; AIC2002 Meeting CSAE/SCGR Program, Saskatchwan.14-17. [10] Y. Vermahmoudi S.M. Peyghambarzadeh S.H. Hashemabadi, M. Naraki , “Experimental investigation on heat transfer performance of Fe2O3/water nanofluid in an air-finned heat exchanger,” European Journal of Mechanics - B/Fluids 44, 32–41,2014. [11] Rohit S. Khedkar, Shriram S. Sonawane, Kailas L. Wasewar, “Heat transfer study on concentric tube heat exchanger using TiO2– water based nanofluid,” International Communications in Heat and Mass Transfer,57, 163-169,2014. [12] M.M. Elias, I.M. Shahrul, I.M. Mahbubul, R. Saidur, N.A. Rahim, “Effect of different nanoparticle shapes on shell and tube heat exchanger using different baffle angles and operated with nanofluid,” International Journal of Heat and Mass Transfer 70, 289-297 , 2014 [13] Mir Hatef Seyyedvalilu and S.F.Ranjbar, “The Effect of Geometrical Parameters on Heat Transfer and Hydro Dynamical Characteristics of Helical Exchanger”. International Journal of Recent advances in Mechanical Engineering (IJMECH) Vol.4, No.1, 2015. [14] Thesis , Satyabrata Kanungo, “Numerical Analysis To Optimize The Heat Transfer Rate Of Tube-In-Tube Helical Coil Heat Exchanger” .National Institute Of Technology, Raourkela Odissa, 2014.
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