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What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
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Original Research Paper
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What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?
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S. Anitha a,1, Tiju Thomas b,c,2, V. Parthiban d,3, M. Pichumani a,⇑
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a
Department of Nanoscience and Technology, Sri Ramakrishna Engineering College, Coimbatore, Tamil Nadu, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai, Tamil Nadu, India c DST Solar Energy Harnessing Center – An Energy Consortium, Indian Institute of Technology, Chennai, Tamil Nadu, India d School of Advanced Sciences, Vellore Institute of Technology, Chennai, Tamil Nadu, India b
a r t i c l e
i n f o
Article history: Received 22 April 2019 Received in revised form 24 August 2019 Accepted 18 September 2019 Available online xxxx Keywords: Single pass shell and tube heat exchanger Hybrid nanofluid Heat transfer performance Nanoparticle concentration Nanoparticle proportion
a b s t r a c t Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al2O3 – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al2O3/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Ultrahigh cooling performance of equipment is one of the important needs of several industrial sectors (automobile, marine, food processing, chemical and mechanical unit etc.). With the help of heat exchanger, overheating of equipment can be prevented. The heat exchanger enables transfer of heat from one medium to another. It uses fluids like water, organic liquids (e.g. ethylene gly-
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⇑ Corresponding author at: Department of Nanoscience & Technology, Sri Ramakrishna Engineering College, Vattamalaipalayam, Coimbatore 641022, Tamil Nadu, India. E-mail addresses: [email protected] (S. Anitha), [email protected] (T. Thomas), [email protected] (V. Parthiban), [email protected] (M. Pichumani). 1 Department of Nanoscience & Technology, Sri Ramakrishna Engineering College, Vattamalaipalayam, Coimbatore 641022, Tamil Nadu, India. 2 Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India. 3 School of Advanced Sciences, Vellore Institute of Technology, Vandalur-Kelambakkam Road, Chennai 600127, Tamil Nadu, India.
col, tri ethylene glycol), engine oil and bio fluid as coolants. However, the heat transfer performance (HTP) of these coolants is most often not adequate to meet the rather challenging requirements of industrial cooling. Subsequently, industries face equipment degradation and production losses. In general, higher heat transfer performance of the operating coolant results in better HTP of the heat exchanger. Heat transfer performance of the operating coolant can be increased by optimizing its thermal conductivity. A case in point (among others) is the work by Masudha et al. [1], wherein a rather small amount nano sized particles dispersed in water resulted in substantial improvement in thermal conductivity. The addition of nanoparticles in coolant (e.g. water, organic liquids, engine oil, bio fluid) was first labeled as nanofluid by Choi in 1995 [2]. Since 1995, HTP of nanofluids has been investigated [3–10]. It is now known that the HTP of nanofluid is influenced by nanoparticle shape, size, and volume concentration (in the case for many engineering applications). The influence of nanoparticle diameter for effective HTP of nanofluid along with different geometrical structure of the tube
https://doi.org/10.1016/j.apt.2019.09.018 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Nomenclature Cp D g h K l n NTU Nu q q00 Ra Re T u; v ; w U VG Dp DT
heat capacity, J/kg K diameter of the tube, mm gravity force, m/s2 heat transfer coefficient, W/m2 K thermal conductivity, W/m K length, mm shape factor, – Number of Transfer Units Nusselt number, – heat transfer rate, W Heat flux, W/m2 Rayleigh number, – Reynolds Number, – temperature, °C (or) K velocity components, m/s overall heat transfer coefficient, W/m2 K viscosity grade, – pressure drop, Pa temperature difference, K
f hnf in k m nf p s s1 s2 w
Fluid hybrid nanofluid Inlet nanoparticle phase in Eqs. (1)–(5) mixture theory in Eqs. (1)–(6) nanofluid secondary phase nanoparticle (solid particle) nanoparticle (Al2O3) nanoparticle (Cu) wall
Greek symbols q density, kg/m3 c kinematic viscosity, m2/s k mean free path, – b thermal expansion coefficient, 1/K a thermal diffusivity, m2/s u volume concentration, % l viscosity, kg/m s
Subscripts bf base fluid dr drift component in Eqs. (3)–(6)
77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114
(where nanofluid passes in) is examined [11,12]. The convection HTP of Ag/water nanofluid in a helical coil is numerically and experimentally studied by Siamak et al. [13]. They showed that performance of helical coils can be increased due to the usage of nanofluid. As volume concentration of nanoparticle increases, the heat transfer performance of nanofluid improves [14]. But, Mohammed et al. [15] indicated that, this improvement is not monotonic. For example, they stated that once nanoparticle volume concentration exceeds 5%, the HTP of nanofluid reduces to same as HTP of water. Hence the careful optimization of the nanofluid volume concentration is essential to achieve industrial goals like effective heat transfer of nanofluids, improve the life of heat exchanger, and prevent equipment degradation etc. Mehdi et al. [16] investigated thermal and hydraulic characteristics of a spiral heat exchanger using a new biological nanofluid. They used functionalized graphene nanoplatelets as nanofluid. They reported that, the importance of nanoparticle is seen at higher Reynolds number of nanofluid. Triple-tube heat exchanger that equipped with inserted ribs is numerically investigated [17]. The overall heat transfer coefficient, effectiveness and heat transfer rate of the heat exchanger are presented. They recommended to use triple tube heat exchanger with the smaller rib height and lower rip pitch with the highest nanoparticle concentration for the effective energy efficiency. In addition, heat transfer performance of graphene nanoplatelets decorated with silver nanoparticles is investigated [18]. Alsabery et al. [19] investigated mixed convection in a double rid-driven wavy cavity that involving solid circular cylinder. They have shown that, variation of the moving walls direction allows to increase the average Nusselt number. Furthermore, the effects of non-equilibrium model on natural convection in a nanofluid filled in wavy-walled porous cavity is numerically studied [20]. This study provides the impact of governing parameters such as Brownian motion Reynolds number, Prandtl number, Nusselt number etc., on the fluid flow of temperature distribution. The entropy generation in a heat exchanger with the effects of particle distribution is evaluated [21]. They have shown that by increasing the concentration of nanoparticle, thermal entropy generation of the
nanofluid decreases. Mixed convection and entropy generation in a wavy walled cavity filled with nanofluid is studied numerically [22]. Motivated by academic efforts, the US DOE’s since 2008 has been exploring nanofluid coolants for effective technology translation. In fact, many high performing nanofluids (in lab scale) do not perform well when they are being employed in industries. In particular, the stability of the nanofluid suspension, achieving target thermal conductivity and low viscosity are required during operation in the industry. Achieving this combination of properties is clearly challenging and has been a limiting factor in large scale deployment of nanofluid. This is primarily due to the instability of nanofluids. For a case, Wen and Ding [23] studied the stability of Cu-water and thermal conductivity of Al2O3 –water nanofluid. They reported that after 30 h, Cu-water nanofluid clustered slightly; interestingly, the Al2O3 –water nanofluid remained stable for over a week. Nowadays, such kind of challenges is faced by the suspension of more than one type of nanoparticle in base fluid (e.g. water, organic liquids, engine oil, bio fluid). This suspended fluid is known as ‘‘hybrid nanofluid”. HTP of Al2O3 /water with the small addition of Cu and MWCNTs, respectively, was studied by [24,25]. In both cases, this small addition of higher thermal conductive nanoparticle (Cu and MWCNTs, respectively) makes Al2O3/water as remarkable heat transfer performing fluid. A detail survey on recent research on nanofluid employed in various heat exchangers is reviewed by Mehdi et al. [26]. They have concluded that shell and tube heat exchanger is the most utilized heat exchangers in many industries. Also, in future hybrid nanofluid can be used as promising nanofluid for heat transfer augmentation in heat exchangers. Minea [27] observed the enhancement in HTP about 241% in Nusselt number at Reynolds number 22,000 in a tube length of 1.75 m. Al2O3, TiO2, SiO2 nanoparticles are dispersed in water is used as nanofluid in the work. Also, a new Nusselt number is correlated as a function of Reynolds number, Prandtl number and nanoparticle volume concentration. Due to the significant advantage of hybrid nanofluid, many researchers exploring hybrid nanofluids for the application of micro sink, mini sink, double pipe
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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heat exchanger, plate heat exchanger, shell and tube heat exchanger, coil heat exchangers [28–33,40–42]. Shell and tube heat exchanger (STHE) is widely used in industries like hydraulic power packs, transmissions, marine, and food industry and so on. Also, the simple design of shell and tube heat exchangers (STHE) makes it an ideal cooling solution in a variety of applications. The application of nanofluid for the optimal design of STHE is studied [34] and a new design of STHE is proposed by them. Likewise there are numerous studies reported the HTP of nanofluid. For illustration, nanofluids like c- Al2O3 /water and TiO2/water [35], CuO/water and TiO2 /water [36], c - AlOOH (Boehmite alumina) [37] and hybrid nanofluid such as Al2O3-MWCNT, Al2O3 –Ag, Al2O3 –Cu, Al2O3 –TiO2 [38] is used as coolant in STHE. A review work on thermophysical properties and HTP of hybrid nanofluid is reported [39]. They stated that even though many researchers worked on hybrid nanofluids, the deep understanding in mixing ratio (nanoparticle proportion), nanoparticle volume concentration and the stability of hybrid nanofluid is yet to be investigated. Therefore, it is important to investigate the influence of nanoparticle volume concentration and proportion on heat transfer performance of hybrid nanofluid. In this work, optimization of nanoparticle volume concentration and proportion that dominates HTP of Al2O3 – Cu/water hybrid nanofluid in heat exchanger is studied. Significance of hybrid nanofluid as an effective coolant can be visualized by comparing the HTP of nanofluids and water. A real world heat exchanger problem is mathematically modelled with the datum collected from heat exchanger fabrication industry. The results are discussed in terms of the Reynolds number (Re), Rayleigh number (Ra) and Nusselt number (Nu), Overall heat transfer coefficient, Number of Transfer Unit (NTU) and Effectiveness. The impact of thermophysical properties of Al2O3/water, Cu/water nanofluid and Al2O3 – Cu /water hybrid nanofluid are presented.
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1100 mm and 1390 mm respectively. ISO VG 68 OIL and coolant enters shell and tube respectively at 75 °C and 30 °C. The heat transfers from ISO VG 68 OIL to the coolant. Either water or Cuwater nanofluid or Al2O3–water nanofluid or Al2O3–Cu/water hybrid nanofluid is used as coolant in this investigation. The average diameter of Al2O3 and Cu nanoparticle is 42 nm. The thermophysical properties of ISO VG 68 OIL, water, nanoparticles are described in Table 1. The heat flux applied for initialization is 1000 W/m2. The two fluids dynamically exchange heat between each other through the wall there after. The exchanger is designed with the datum (Ref. Table 2) collected from heat exchanger fabrication industry.
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2.2. Governing equations
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To construct governing equations, the following assumptions are made: (1) both heat transfer and fluid flow in STHE are in three-dimensional form, (2) the coolant enclosed in the tube is unsteady, laminar, incompressible and Newtonian, (3) fluid phase and nanoparticle(s) phase are considered as a multiphase system, (4) the physical properties of fluid phase and nanoparticle phase are temperature independent, (5) nanoparticles are spherical, it is uniformly distributed and dispersed in water. The mixture theory is adopted to simulate the hybrid nanofluid and nanofluid flow. The governing equations for the multiphase system can be written as reported in [28]. Continuity equation
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@ ! ðq Þ þ r: qm v m ¼ 0 @t m
ð1Þ
! Here v m , qm is the mass-average velocity and mixture density respectively given by;
!
Pn
n X uk qk ! vk ; qm ¼ uk q k qm k¼1
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2. Problem formulation
vm ¼
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2.1. Model description
uk is the volume concentration of phases k.
Shell and tube heat exchanger (STHE) (Fig. 1(a)) is a type of heat exchangers that used in oil refineries and other large chemical processes. We consider single pass STHE with shell and tube length of
Momentum equation The momentum equation for the mixture model can be obtained by summing the individual momentum equations for all the phases. It can be expressed as
188 189 190
k¼1
ð2Þ
Fig. 1. (a) Geometry of the problem (b) Schematic diagram of the tube with boundary conditions.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Table 1 Thermophysical properties of water, OIL, copper, and alumina nanoparticles [45]. Properties
Water
ISO VG 68 OIL
Cu
Al2O3
q(kg/m )
998.2 4182 0.6 0.001003 21 105 147 105
865 2062 0.097 68 – 5.43 108
8933 703 400 – 1.67 105 116 105
3970 765 40 – 0.85 105 1.31 105
3
C p (J/kg k) K (W/m K) l (kg/m s) b(1/K) a (m2/s)
Table 2 Dimension of shell and tube heat exchanger. Heat exchanger type Application Shell fluid(ISO VG 68 OIL) Tube fluid - Coolant(Either Hybrid nanofluid or nanofluid or water) Shell length Tube length Shell inner diameter Shell outer diameter Tube inner diameter Tube outer diameter
229
231 232 233 234
235
237 238
239
Shell Shell Tube Tube
inlet outlet inlet outlet
STHE with single pass. Hydraulic oil heat exchanger. Hot oil (75 °C) Cold oil Cold fluid (30 °C) Hot fluid 1100 mm 1390 mm 43.2 mm 60 mm 33.726 mm 48 mm
@ ! qm v m þ r qm ! v m! vm @t h i ! ! ! ! T ¼ rp þ r lm r v m þ r v m þ qm g þ F ! n X uk q k ! v dr;k ! v dr;k þr
ð3Þ
k¼1
! ! where n; F , lm , v dr;k is the number of phases, the body force, viscosity of the mixture, drift velocity for the secondary phase k ! respectively. lm and v dr;k is defined as follows
lm ¼
n X
uk lk ; ! v dr;k ¼ ! vk! vm
ð4Þ
Energy equation
¼ r:ðkeff rT Þ þ SE
248
P Here keff is the effective thermal conductivity ð uk ðkk þ kt ÞÞ, kt is the turbulent thermal conductivity that is defined in the case of turbulent modelling. SE denotes the volumetric heat source (where ever applicable). Volume concentration equation for the secondary phase The volume concentration equation for secondary phase p can be obtained as, [28]
251
r up qp ! v m ¼ r up qp ! v dr;p
244 245 246 247
249
257 258
2.3. Boundary conditions
259
The following boundary conditions are used to solve the system of Eqs. (1)–(6). At shell inlet and tube inlet, uniform velocity and
253 254 255 256
260
271
The governing equations with the associated boundary conditions are numerically solved by finite volume method. Finite volume method converts the governing equations to a set of algebraic equations that can be solved numerically. The differential equation is discretized using the control volume technique. In discretization methods, a second upwind scheme is selected for the momentum and energy equations. These discretized algebraic equations are sequentially solved in the physical domain. The SIMPLE procedure is used to couple the velocity and pressure terms. This is an iterative solution procedure where the computation is initialized by guessing the pressure field. The temperature is calculated by solving the energy equation.
272
3.1. Code validation
284
In order to validate the numerical results for solving heat transfer in a single pass shell and tube heat exchanger, a comparison with the experimental results for pure water is undertaken. Fig. 2 compares the Nusselt number obtained from this numerical result with those of Zhaoqin experimental results [43] at an inlet temperature of 283.15 K (10 °C) and Reynolds number varies from 170 to 900. They carried out an experimental study on Cu-water nanofluid in a cylindrical tube. The numerical results show good agreement with the experimental results.
285
3.2. Grid convergence study
294
In this work, grid study is taken for analyzing the velocity and pressure components against the length of the tube. Water at 30 °C enters the tube shown in Fig. 1(b) availing the properties listed in Table 1. The grid study was evaluated for three different
295
ð6Þ
The symbols used in governing equations are defined in the nomenclature.q; l; K; C p ; bare the density, viscosity, thermal conductivity, heat capacity and thermal expansion coefficient of fluid (nanofluid/hybrid nanofluid) respectively. The variation in thermophysical properties of nanofluid and hybrid nanofluid are discussed in Section 4.
252
3. Numerical procedure
262 263 264 265 266 267 268 269 270
273 274 275 276 277 278 279 280 281 282 283
286 287 288 289 290 291 292 293
!
ð5Þ
241
243
261
k¼1
n n X @ X ðuk qk Ek Þ þ r: uk ! v k ðqk Ek þ pÞ @t k¼1 k¼1
242
temperature are stated. ISO VG 68 OIL enters the shell at 75 °C; the coolant enters the tube at 30 °C. The no-slip condition is considered at the inner wall (which is the surface in contact with fluid) of tube and shell. At the shell outlet and tube outlet, the adiabatic wall temperature is considered. To analyze the HTP of the fluidsolid interface, the two-phase mixture model is considered. The heat flux is calculated by q} ¼ k @T . Where K is the thermal con@x ductivity of water/nanofluid/hybrid nanofluid. @T is the temperature difference between tube inlet and outlet. At the shell and tube outlet, a static pressure pgauge ¼ 0 is specified.
Fig. 2. Variation of Nusselt number against Reynolds number.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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cases of mesh size as shown in Fig. 3. In addition, Table 3 lists shell outlet temperature profiles for various grid sizes. Obviously, the grid 80*20*20 is chosen for further study, since any considerable changes are not seen in the temperature profiles by increasing the grid in x axis.
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4. Thermophysical properties
305
The thermophysical properties of nanofluid, hybrid nanofluid such as thermal conductivity, viscosity, density and specific heat capacity are calculated using the equations given in Table 4. These properties are estimated for the nanoparticle concentration in the range of 1–20%. Thermophysical properties are considered as temperature independent. Here f ; nf ; hnf denotes the properties of water, nanofluid and hybrid nanofluid respectively. S1 , S2 respectively denotes alumina and copper nanoparticle. In hybrid nanofluid,uS denotes the total volume concentration of alumina and copper nanoparticle (i.e.uS1 þ uS2 Þ. Numerical calculations of thermophysical properties (density, thermal conductivity viscosity, specific heat capacity) of Al2O3Cu/water hybrid nanofluid, Cu/water nanofluid and Al2O3/water nanofluid with various nanoparticle volume concentration is presented in Fig. 4(a)–(d). Thermophysical properties of coolant
306 307 308 309 310 311 312 313 314 315 316 317 318 319 320
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(nanofluid and hybrid nanofluid) are indeed important to estimate in order to understand the HTP. As shown in Fig. 4(a), the density of hybrid nanofluid is the average of Al2O3/water nanofluid and Cu/ water nanofluid. This shows the dominance of alumina nanoparticles in hybrid nanofluid. From Fig. 4(b) and (c), it is seen that thermal conductivity and viscosity of hybrid nanofluids are higher than nanofluids. The viscosity of hybrid nanofluid increases with stronger intermolecular forces and makes more convection. This in turns improves heat transfer rate of hybrid nanofluid. On the other hand, the specific heat capacity of hybrid nanofluid is lower than Al2O3/ water nanofluid and Cu/water nanofluid [15] (Fig. 4(d)). According to the kinetic theory of relation on thermal conductivity, mean free path is inversely proportional to the heat capacity K hnf ¼ ðC p Þhnf V hnf khnf , k 1 ¼ ðC p Þhnf . Consequently, as volume concentra-
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tion of nanoparticle increases, the specific heat capacity of nanofluid decreases.
335
5. Data processing [17]
337
5.1. Overall heat transfer coefficient (U)
338
Overall heat transfer coefficient of heat exchanger can be calculated by the following equation:
339
hnf
q U¼ AðDTÞLMTD
ð7Þ
here A represents the heat transfer area, and ðDTÞLMTD is the logarithmic mean temperature difference which is obtained as follows:
DT LMTD
DT 2 DT 1 ¼ ln ðDT 2 =DT 1 Þ
ð8Þ
Grid size
Tout (°C)@Shell
40 * 20 * 20 60 * 20 * 20 80 * 20 * 20 100 * 20 * 20 120 * 20 * 20
52.176 53.524 54.125 54.265 54.395
q
340
341 343 344 345
346 348
353 354
357
qmax
K hnf K bf
¼
K S þðn1ÞK f ðn1ÞuS ðK f K S Þ ðK bf K S2 Þ K bf 2 ; ¼ 1K S þðn1ÞK f þu ðK1f K S Þ 1 S1 ðK bf K S2 Þ K f 1 1 ¼ ð1 uS Þ qC p nf þ uS1 ðqC p ÞS1 þ uS2 ðqC p ÞS2
K S2 þðn1ÞK f ðn1ÞuS
ðC p Þhnf
K S2 þðn1ÞK bf þuS
364 365 366 367 368
ð11Þ
K nf Kf
þ uS ðqC p ÞS
336
361 363
K
f
334
ð10Þ
qhnf ¼ uS1 qS1 þ uS2 qS2 þ ð1 uS Þqnf lhnf ¼ lnf ð1 þ 2:5ðuS1 þ uS2 ÞÞ
¼ ð1 uS Þ qC p
333
_ h cp;h ðT h;i T h;o Þ qh ¼ m
qnf ¼ uS qS þ ð1 uS Þqf lnf ¼ lf ð1 þ 2:5uS Þ
nf
332
358 360
q l
Cp
331
ð9Þ
Hybrid Nanofluid
Cp
330
qc ¼ m_ c cp;c ðT c;o T c;i Þ
e¼
329
356
The effectiveness of the heat exchanger is calculated as below:
328
Heat transfer rate for the cold fluid (coolants) and hot fluid (ISO VG 68 OIL) are calculated via Eqs. (9) and (10), respectively.
5.3. Effectiveness (e)
K S þðn1ÞK f ðn1ÞuS ðK f K S Þ K S þðn1ÞK f þuS ðK f K S Þ
327
355
Nanofluid
326
5.2. Heat transfer rate (qÞ
Properties
¼
325
352
Table 4 Thermophysical properties of nanofluid and hybrid nanofluid [44,45].
324
h; i and h; o represent inlet and outlet temperature of hot fluid, respectively. c; i andc; o represent inlet and outlet temperature of cold fluid, respectively.
_ h denote mass flow rate for the coolant and ISO VG 68 OIL m_ c and m respectively. Heat transfer rates for the coolant and oil are equal, and have a negligible difference in this study. It indicates that qc ¼ qh ¼ q:
Table 3 Shell outlet temperature profiles for various grid sizes.
323
349 351
in which DT 1 ¼ T h;i T c;o ; DT 2 ¼ T h;o T c;i
Fig. 3. Effects of velocity and pressure of the water against length of the tube.
322
2
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Fig. 4. Variation in thermophysical properties for nanoparticle volume concentration varies from 1% to 20% (a) density (b) thermal conductivity (c) viscosity (d) heat capacity. Unlike density, thermal conductivity and viscosity, heat capacity decreases with increasing nanoparticle volume concentration [15]. 373 374
375 377 378 379
380 382 383 384
385 388 387 389 391
where qmax denotes maximum possible heat transfer rate which is calculated from Eq. (12)
qmax ¼ C min ðT h;i T c;i Þ
ð12Þ
where C min is the minimum heat capacity rate and it is determined as follows:
C min ¼ min½C h C c
ð13Þ
where C h and C c indicate the heat capacity rates of hot fluid and cold fluid, respectively:
C h ¼ m_ h cp;h
ð14Þ
C c ¼ m_ c cp;c
ð15Þ
392
5.4. Number of transfer Units (NTU)
393 394
The parameter NTU is calculated in terms of minimum heat capacity rate as follows:
398 397
NTU ¼
395
UA C min
ð16Þ
399
6. Results and discussion
400
In this work, influence of nanoparticle volume concentration and proportion on heat transfer performance of Al2O3 – Cu /water
401
hybrid nanofluid in a single pass shell and tube heat exchanger (STHE) is analyzed. The comprehensive comparison between heat transfer performance (HTP) of water, nanofluids (Al2O3/water, Cu/water) and hybrid nanofluid (Al2O3 – Cu /water) is obtained. Mixture theory is adopted to simulate the hybrid nanofluid and nanofluid flow. The range of nanoparticle volume concentration, Rayleigh number, Reynolds number used in this work is 0% u 20%,103 Ra 106 ; 800 Re 2400 respectively. Fig. 5 compares the HTP of hybrid nanofluid, nanofluid in terms of shell & tube outlet temperature. Effect of shell outlet temperature against Reynolds number for water is shown in the inset. Lower shell outlet temperature is an indicator for higher HTP of the coolant. In all the cases, hybrid nanofluid exhibits higher HTP compared to its counterparts, due to the fact that the hybrid nanofluid incorporates both the characteristics of alumina and copper nanoparticle. In addition, hybrid nanofluid overcomes individual drawbacks of nanofluid like low thermal conductivity and low stability. Therefore, the combination of higher thermal conductive (Cu nanoparticle) and higher stability (Al2O3 nanoparticle) nanoparticle is preferable for effective HTP of a hybrid nanofluid. In addition, as the Reynolds number increases, the HTP of coolants (hybrid nanofluid and nanofluids) diminish for any concentration of coolants. Although some studies used hybrid nanofluid for heat transfer application [27–35,42], the optimization of nanoparticle concentration and proportion for HTP is yet to interrogate. Therefore, vari-
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Optimization of nanoparticles proportion (mixing ratio) is essential wherein a case two different nanoparticles dispersed in a base fluid. As can be noticed in Fig. 6(b), at 90(Al2O3): 10 (Cu) proportion, HTP of hybrid nanofluid is higher compares to other proportions. Therefore, in the combination of higher thermal conductive (Cu nanoparticle) and higher stability nanoparticle (Al2O3 nanoparticle), stability nanoparticle should dominate the thermal conductive nanoparticle. Therefore, we can say that both nanoparticle volume concentration and proportion dominates the HTP of hybrid nanofluid provided with appropriate optimization. The HTP of hybrid nanofluid are discussed in terms of governing parameters namely; Rayleigh number, Nusselt number, Reynolds number, heat transfer coefficient in Figs. 7–11. Fig. 7 shows the variation in shell outlet temperature in terms of Rayleigh number for water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/
Fig. 5. Effect of shell and tube outlet temperature against Reynolds number for Al2O3–Cu/water hybrid nanofluid, Cu/water nanofluid, and Al2O3/water nanofluid at 20% nanoparticle volume concentration with 90:10 (Al2O3:Cu) are presented. Effect of shell outlet temperature against Reynolds number for water is shown in inset.
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ation in shell outlet temperature for different nanoparticle concentration and proportion of Al2O3 – Cu/water hybrid nanofluid is studied in this work. It is reported in Fig. 6. Fig. 6 shows the effect of shell outlet temperature against Reynolds number for Al2O3-Cu/water hybrid nanofluid for various nanoparticle (a) concentrations (b) proportions. The nanoparticle volume concentration varies from 1% to 20% and proportion varies from 50(Al2O3): 50 (Cu) to 90(Al2O3): 10 (Cu). Fig. 6(a) illustrates that, as nanoparticle volume concentration increases, shell outlet temperature decreases. It indicates the effective HTP of hybrid nanofluid in heat exchanger. In addition, 20 vol% hybrid nanofluid shows effective HTP compare to other volume concentration. It is due to the term that thermal conductivity of Al2O3 – Cu/water hybrid nanofluid is higher than Cu/water and Al2O3/water (Ref. Fig. 4(b)). It leads to increase thermal performance of Al2O3 – Cu/ water hybrid nanofluid.
Fig. 7. Shell outlet temperature in terms of Rayleigh number for water, nanofluid and hybrid nanofluid. The Rayleigh number varies from 102 to 108. It is seen that HTP of hybrid nanofluid is greater than nanofluid and water at any given Rayleigh number.
Fig. 6. Effects of shell outlet temperature against Reynolds number for various nanoparticle (a) concentration (b) proportion of Al2O3-Cu/water hybrid nanofluid. The nanoparticle concentration varies from 1% to 20%. As nanoparticle volume concentration increases, HTP of hybrid nanofluid increases. At 90 (Al2O3): 10 (Cu) nanoparticle proportion, notable HTP of hybrid nanofluid is attained.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Fig. 8. (Color online) Temperature distribution in shell wall (a) Water (b) Al2O3-Cu/water hybrid nanofluid at 20% nanoparticle volume concentration and 90:10 nanoparticle proportion.
Fig. 9. Effects (a) Heat transfer Coefficient (b) Dimensionless Nusselt number for Al2O3/water, Cu/water nanofluids and Al2O3-Cu/water hybrid nanofluid at different nanoparticle volume concentration. Heat transfer coefficient and Nusselt number of coolants increases with increase in nanoparticle volume concentration. Also, heat transfer coefficient of hybrid nanofluid is higher compared to nanofluids.
Fig. 10. Pressure drop variation against Reynolds number for water, Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid. Larger pressure drop is noticed for Al2O3-Cu/water hybrid nanofluid.
water hybrid nanofluid. The Rayleigh number varies from 102 to 108. Reynolds number, nanoparticle volume concentration and proportion are fixed as 844.4, 20%, and 90:10 respectively. At low Rayleigh number the heat transfer performance of all coolants are notable. The temperature (75 °C) of the shell fluid is decreases to 61.27 °C in the case of water penetrated as a coolant. In the same case, it is noted as 50.78 °C, 48.16 °C, 47.24 °C for Al2O3/water nanofluid, Cu/water nanofluid, and Al2O3-Cu/water hybrid nanofluid respectively. For further analysis, the temperature distribution of the shell wall is shown in Fig. 8. (a) Water (b) Al2O3-Cu/ water hybrid nanofluid. The temperature distribution in shell wall is shown in Fig. 8. As seen in Fig. 8, the temperature gradient near the inlet is high. The inlet temperature of shell is 348.15 K (75 °C). This temperature decreases to 334.15 K (61 °C), 318.15 K (45 °C) for water and hybrid nanofluid is used as coolant respectively. Hitherto, HTP of coolants are analyzed with shell side performance, the following results will be discuss HTP of coolants by tube side (Ref Fig. 2(b)). The effect of nanoparticle volume concentration on heat transfer coefficient for Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid is compared in Fig. 9 (a). The heat transfer coefficient is calculated using: [16]
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Fig. 11. (a) Heat transfer rate (b) Histogram of overall heat transfer coefficient of water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid in terms of Reynolds number. Figure (a) shows that heat transfer rate enhances as Reynolds number increases. Further, hybrid nanofluid shows highest heat transfer rate compare to nanofluid and water. In figure (b), it is noticed that over all heat transfer coefficient is an increasing function of Reynolds number. Overall heat transfer coefficient of hybrid nanofluid is higher than its counterparts.
00
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500
502
h¼
q Tw Tf
ð17Þ
00
where q ; T w ; and T f represent the average heat flux between two fluids, average temperature of the wall, and bulk temperature of the considered fluid (nanofluid or hybrid nanofluid). From Fig. 9(a), it is observed that hybrid nanofluid (20 vol% with 90:10 proportions) shows higher heat transfer coefficient than nanofluids. This is due to the inverse relationship between T w T f and heat transfer coefficient. It is noted that T w T f decreases as nanoparticle volume concentration increases. At Re = 844.4, the heat transfer coefficient of Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid are 1903.33, 3500, 4554 W/ m2K. It is observed that the percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% higher than Cu/water nanofluid. The effect of nanoparticle volume concentration on dimensionless Nusselt number for Al2O3-Cu/water hybrid nanofluid, Cu/water, Al2O3/water, and is compared in Fig. 9 (b). Nusselt number is calculated using: 00
Nu ¼
q D KðT w T f Þ
ð18Þ 00
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In the above expression, q ; T w ; and T f represent the heat flux, average temperature of the wall, and bulk temperature of the considered fluid (nanofluid or hybrid nanofluid). From Fig. 9(b) it is noted that as the particle concentration increases, the Nusselt umber increases. About 40% increase in Nu is observed when hybrid nanofluid is used as coolant rather than nanofluids. This is because of the enhanced thermal conductivity of hybrid nanofluid in comparison with other nanofluids. Only a few studies have reported the effect of pressure drop and friction factor of hybrid nanofluid [46,47]. Pressure drop in STHE for various coolants (Water, Al2O3/water, Cu/water, and Al2O3Cu/water hybrid nanofluid) against Reynolds number is shown in Fig. 10. The pressure drop is calculated using [43]:
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Dp ¼
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516
519 520 521 522 523
64qReu2 D3
increases, there is more velocity near the boundary layer of the pipe causes change in flow pattern in the fluid flow in the pipe. These variations in the flow make the pressure drop initially increasing and then decreasing and again increasing. This non-monotonic variation in Dp is due to larger viscosity of hybrid nanofluid than nanofluid as indicated in Fig. 4(c). Increase in pressure drop of hybrid nanofluid proves that the hybrid nanofluid is the better HTP fluid compared to its counterparts as discussed from previous results. Table 5 lists the values of Number of Transfer Units (NTU) at Reynolds number varying from 800 to 2400 for water, nanofluids and hybrid nanofluid. As compared to overall heat transfer, NTU also increases as Reynolds number increases. At Re = 2321.5, the increment in NTU between water and hybrid nanofluid is close to 75%. Hybrid nanofluid shows highest NTU when compared to their other counterparts. Fig. 11 shows (a) heat transfer rate (b) histogram of overall heat transfer coefficient of heat exchanger for water, Al2O3/water, Cu/ water and Al2O3-Cu/water hybrid nanofluid employed as a coolant. Heat transfer rate of all coolants increases as Reynolds number increases. The range of Reynolds number varies from 800 to 2400. This trend is reported [16]. In addition, heat transfer rate of nanofluid and hybrid nanofluid is higher than that of water. This is because of the influence of volume concentration of nanoparticles and thermophysical properties of nanofluid and hybrid nanofluid. There by, the heat transfer rate for hybrid nanofluid increases almost 78% by increasing the Reynolds number from 844.4 to 2321.54. Fig. 11(b) presents the histogram of overall heat transfer coefficient for water, Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid in terms of Reynolds number. Hybrid nanofluid shows highest overall heat transfer coefficient (U) than water
Table 5 NTU values of water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid at different Re. Re
Water
844.4 1147.3 1425.3 1719.9 2026.9 2321.5
3.43 3.71 4.01 4.12 4.26 4.33
ð19Þ
where q, u is the density and inlet velocity of respective coolant, D is the diameter of the tube. Fig. 10 depicts that the pressure drop linearly rises with the increases in Reynolds number. But a sharp decrease is noted at the critical point of Reynolds number (flow changes from laminar to turbulent). As the Reynolds number
Nanofluid Al2O3/water
Cu/water
3.68 4.65 5.23 5.77 6.01 6.08
3.86 5.15 5.81 6.38 6.76 6.90
Hybrid nanofluid Al2O3-Cu/water 4.16 5.43 6.45 6.72 7.39 7.57
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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The heat transfer rate (q) enhances with increase in Reynolds number. Further, the heat transfer rate for hybrid nanofluid increases almost 78% by increasing the Reynolds number from 844.4 to 2321.54. At any Reynolds number, the effectiveness of single pass shell and tube heat exchanger employing hybrid nanofluid is high. Nanoparticle volume concentration and proportion dominate the heat transfer performance of hybrid nanofluid in heat exchanger.
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Fig. 12. Effectiveness of Shell and Tube Heat Exchanger (STHE) in terms of Reynolds number for water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid. Effective heat transfer performance is attained when hybrid nanofluid is employed as coolant. Further, as Reynolds number increases, effectiveness of heat exchanger increases.
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and nanofluids. As evident from Eq. (7), overall heat transfer coefficient depends on heat transfer rate (q), Area (A), and logarithmic temperature difference (DT LMTD ). This increment is due to the increase in heat transfer rate. Fig. 12 shows the effectiveness of Shell and Tube Heat Exchanger (STHE) for various coolants. Interestingly, it is noted that STHE shows higher effectiveness when hybrid nanofluid used as a coolant. At any Reynolds number, the effectiveness of nanofluids and hybrid nanofluid is higher than that of water. Effectiveness of heat exchanger depends on heat capacity rate of cold and hot fluid (see Eq. (11)). Also, it is noticed that the difference between effectiveness of heat exchanger increases as Reynolds number increases. As a result, addition of nanoparticles to the base fluid (water) enhances the heat transfer performance of heat exchanger.
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7. Conclusions
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There are ongoing efforts, to translate lab scale nanofluids to industries. However, scaling up is considered non trivial and is an ongoing effort worldwide. Al2O3-Cu/water system is considered one of the most promising for industry scale deployment; hence it is used in this study. Governing equations are written based on unsteady, three-dimensional flow of an incompressible viscous hybrid nanofluid in shell and tube heat exchanger. A no slip assumption is made at the inner walls of the exchanger. At shell and tube outlet, adiabatic wall temperature is considered. The model studied here is found to be relevant for various Al2O3: Cu ratios. This numerical investigation shows conclusions relevant to current usage of nanofluids in shell and tube heat exchangers. The salient outcomes are:
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The existence of nanoparticle in the base fluid flowing in STHE have huge enhancement in the heat transfer coefficient and Nusselt number as well as in the performance of heat exchanger. Optimization of nanoparticle volume concentration and proportion for effective heat transfer performance of hybrid nanofluid is done. The pressure drop increases with increase in Reynolds number and a remarkable decrease is noticed at the critical point of Reynolds number. An increase in the volume concentration of nanoparticles, improves the heat transfer performance of hybrid nanofluid system.
Acknowledgements
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We acknowledge The Management, Sri Ramakrishna Engineering College, Coimbatore, India. This work is supported by DST-WOS (A) the project no. SR/WOSA/PM-86/2017. This work is partly supported by Government of India- DST INSPIRE project 04/2013/000209. Tiju Thomas (TT) would like to thank the Department of Science and Technology of India for support through the project nos. DST FILE NO. YSS/2015/001712 and DST 11-IFA-PH07 and DST FILE No. DST/TMDSERI/UB/1(C). TT also thankful for the Nanoelectronics Network for Research and Application (NNetRA) project via the Ministry of Electronics & information Technology. Finally, we highly acknowledge Universal Heat Exchanger, Coimbatore, India for the support.
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Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
2
Original Research Paper
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What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?
8
S. Anitha a,1, Tiju Thomas b,c,2, V. Parthiban d,3, M. Pichumani a,⇑
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a
Department of Nanoscience and Technology, Sri Ramakrishna Engineering College, Coimbatore, Tamil Nadu, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Chennai, Tamil Nadu, India c DST Solar Energy Harnessing Center – An Energy Consortium, Indian Institute of Technology, Chennai, Tamil Nadu, India d School of Advanced Sciences, Vellore Institute of Technology, Chennai, Tamil Nadu, India b
a r t i c l e
i n f o
Article history: Received 22 April 2019 Received in revised form 24 August 2019 Accepted 18 September 2019 Available online xxxx Keywords: Single pass shell and tube heat exchanger Hybrid nanofluid Heat transfer performance Nanoparticle concentration Nanoparticle proportion
a b s t r a c t Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al2O3 – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al2O3/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Ultrahigh cooling performance of equipment is one of the important needs of several industrial sectors (automobile, marine, food processing, chemical and mechanical unit etc.). With the help of heat exchanger, overheating of equipment can be prevented. The heat exchanger enables transfer of heat from one medium to another. It uses fluids like water, organic liquids (e.g. ethylene gly-
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⇑ Corresponding author at: Department of Nanoscience & Technology, Sri Ramakrishna Engineering College, Vattamalaipalayam, Coimbatore 641022, Tamil Nadu, India. E-mail addresses: [email protected] (S. Anitha), [email protected] (T. Thomas), [email protected] (V. Parthiban), [email protected] (M. Pichumani). 1 Department of Nanoscience & Technology, Sri Ramakrishna Engineering College, Vattamalaipalayam, Coimbatore 641022, Tamil Nadu, India. 2 Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India. 3 School of Advanced Sciences, Vellore Institute of Technology, Vandalur-Kelambakkam Road, Chennai 600127, Tamil Nadu, India.
col, tri ethylene glycol), engine oil and bio fluid as coolants. However, the heat transfer performance (HTP) of these coolants is most often not adequate to meet the rather challenging requirements of industrial cooling. Subsequently, industries face equipment degradation and production losses. In general, higher heat transfer performance of the operating coolant results in better HTP of the heat exchanger. Heat transfer performance of the operating coolant can be increased by optimizing its thermal conductivity. A case in point (among others) is the work by Masudha et al. [1], wherein a rather small amount nano sized particles dispersed in water resulted in substantial improvement in thermal conductivity. The addition of nanoparticles in coolant (e.g. water, organic liquids, engine oil, bio fluid) was first labeled as nanofluid by Choi in 1995 [2]. Since 1995, HTP of nanofluids has been investigated [3–10]. It is now known that the HTP of nanofluid is influenced by nanoparticle shape, size, and volume concentration (in the case for many engineering applications). The influence of nanoparticle diameter for effective HTP of nanofluid along with different geometrical structure of the tube
https://doi.org/10.1016/j.apt.2019.09.018 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Nomenclature Cp D g h K l n NTU Nu q q00 Ra Re T u; v ; w U VG Dp DT
heat capacity, J/kg K diameter of the tube, mm gravity force, m/s2 heat transfer coefficient, W/m2 K thermal conductivity, W/m K length, mm shape factor, – Number of Transfer Units Nusselt number, – heat transfer rate, W Heat flux, W/m2 Rayleigh number, – Reynolds Number, – temperature, °C (or) K velocity components, m/s overall heat transfer coefficient, W/m2 K viscosity grade, – pressure drop, Pa temperature difference, K
f hnf in k m nf p s s1 s2 w
Fluid hybrid nanofluid Inlet nanoparticle phase in Eqs. (1)–(5) mixture theory in Eqs. (1)–(6) nanofluid secondary phase nanoparticle (solid particle) nanoparticle (Al2O3) nanoparticle (Cu) wall
Greek symbols q density, kg/m3 c kinematic viscosity, m2/s k mean free path, – b thermal expansion coefficient, 1/K a thermal diffusivity, m2/s u volume concentration, % l viscosity, kg/m s
Subscripts bf base fluid dr drift component in Eqs. (3)–(6)
77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114
(where nanofluid passes in) is examined [11,12]. The convection HTP of Ag/water nanofluid in a helical coil is numerically and experimentally studied by Siamak et al. [13]. They showed that performance of helical coils can be increased due to the usage of nanofluid. As volume concentration of nanoparticle increases, the heat transfer performance of nanofluid improves [14]. But, Mohammed et al. [15] indicated that, this improvement is not monotonic. For example, they stated that once nanoparticle volume concentration exceeds 5%, the HTP of nanofluid reduces to same as HTP of water. Hence the careful optimization of the nanofluid volume concentration is essential to achieve industrial goals like effective heat transfer of nanofluids, improve the life of heat exchanger, and prevent equipment degradation etc. Mehdi et al. [16] investigated thermal and hydraulic characteristics of a spiral heat exchanger using a new biological nanofluid. They used functionalized graphene nanoplatelets as nanofluid. They reported that, the importance of nanoparticle is seen at higher Reynolds number of nanofluid. Triple-tube heat exchanger that equipped with inserted ribs is numerically investigated [17]. The overall heat transfer coefficient, effectiveness and heat transfer rate of the heat exchanger are presented. They recommended to use triple tube heat exchanger with the smaller rib height and lower rip pitch with the highest nanoparticle concentration for the effective energy efficiency. In addition, heat transfer performance of graphene nanoplatelets decorated with silver nanoparticles is investigated [18]. Alsabery et al. [19] investigated mixed convection in a double rid-driven wavy cavity that involving solid circular cylinder. They have shown that, variation of the moving walls direction allows to increase the average Nusselt number. Furthermore, the effects of non-equilibrium model on natural convection in a nanofluid filled in wavy-walled porous cavity is numerically studied [20]. This study provides the impact of governing parameters such as Brownian motion Reynolds number, Prandtl number, Nusselt number etc., on the fluid flow of temperature distribution. The entropy generation in a heat exchanger with the effects of particle distribution is evaluated [21]. They have shown that by increasing the concentration of nanoparticle, thermal entropy generation of the
nanofluid decreases. Mixed convection and entropy generation in a wavy walled cavity filled with nanofluid is studied numerically [22]. Motivated by academic efforts, the US DOE’s since 2008 has been exploring nanofluid coolants for effective technology translation. In fact, many high performing nanofluids (in lab scale) do not perform well when they are being employed in industries. In particular, the stability of the nanofluid suspension, achieving target thermal conductivity and low viscosity are required during operation in the industry. Achieving this combination of properties is clearly challenging and has been a limiting factor in large scale deployment of nanofluid. This is primarily due to the instability of nanofluids. For a case, Wen and Ding [23] studied the stability of Cu-water and thermal conductivity of Al2O3 –water nanofluid. They reported that after 30 h, Cu-water nanofluid clustered slightly; interestingly, the Al2O3 –water nanofluid remained stable for over a week. Nowadays, such kind of challenges is faced by the suspension of more than one type of nanoparticle in base fluid (e.g. water, organic liquids, engine oil, bio fluid). This suspended fluid is known as ‘‘hybrid nanofluid”. HTP of Al2O3 /water with the small addition of Cu and MWCNTs, respectively, was studied by [24,25]. In both cases, this small addition of higher thermal conductive nanoparticle (Cu and MWCNTs, respectively) makes Al2O3/water as remarkable heat transfer performing fluid. A detail survey on recent research on nanofluid employed in various heat exchangers is reviewed by Mehdi et al. [26]. They have concluded that shell and tube heat exchanger is the most utilized heat exchangers in many industries. Also, in future hybrid nanofluid can be used as promising nanofluid for heat transfer augmentation in heat exchangers. Minea [27] observed the enhancement in HTP about 241% in Nusselt number at Reynolds number 22,000 in a tube length of 1.75 m. Al2O3, TiO2, SiO2 nanoparticles are dispersed in water is used as nanofluid in the work. Also, a new Nusselt number is correlated as a function of Reynolds number, Prandtl number and nanoparticle volume concentration. Due to the significant advantage of hybrid nanofluid, many researchers exploring hybrid nanofluids for the application of micro sink, mini sink, double pipe
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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heat exchanger, plate heat exchanger, shell and tube heat exchanger, coil heat exchangers [28–33,40–42]. Shell and tube heat exchanger (STHE) is widely used in industries like hydraulic power packs, transmissions, marine, and food industry and so on. Also, the simple design of shell and tube heat exchangers (STHE) makes it an ideal cooling solution in a variety of applications. The application of nanofluid for the optimal design of STHE is studied [34] and a new design of STHE is proposed by them. Likewise there are numerous studies reported the HTP of nanofluid. For illustration, nanofluids like c- Al2O3 /water and TiO2/water [35], CuO/water and TiO2 /water [36], c - AlOOH (Boehmite alumina) [37] and hybrid nanofluid such as Al2O3-MWCNT, Al2O3 –Ag, Al2O3 –Cu, Al2O3 –TiO2 [38] is used as coolant in STHE. A review work on thermophysical properties and HTP of hybrid nanofluid is reported [39]. They stated that even though many researchers worked on hybrid nanofluids, the deep understanding in mixing ratio (nanoparticle proportion), nanoparticle volume concentration and the stability of hybrid nanofluid is yet to be investigated. Therefore, it is important to investigate the influence of nanoparticle volume concentration and proportion on heat transfer performance of hybrid nanofluid. In this work, optimization of nanoparticle volume concentration and proportion that dominates HTP of Al2O3 – Cu/water hybrid nanofluid in heat exchanger is studied. Significance of hybrid nanofluid as an effective coolant can be visualized by comparing the HTP of nanofluids and water. A real world heat exchanger problem is mathematically modelled with the datum collected from heat exchanger fabrication industry. The results are discussed in terms of the Reynolds number (Re), Rayleigh number (Ra) and Nusselt number (Nu), Overall heat transfer coefficient, Number of Transfer Unit (NTU) and Effectiveness. The impact of thermophysical properties of Al2O3/water, Cu/water nanofluid and Al2O3 – Cu /water hybrid nanofluid are presented.
3
1100 mm and 1390 mm respectively. ISO VG 68 OIL and coolant enters shell and tube respectively at 75 °C and 30 °C. The heat transfers from ISO VG 68 OIL to the coolant. Either water or Cuwater nanofluid or Al2O3–water nanofluid or Al2O3–Cu/water hybrid nanofluid is used as coolant in this investigation. The average diameter of Al2O3 and Cu nanoparticle is 42 nm. The thermophysical properties of ISO VG 68 OIL, water, nanoparticles are described in Table 1. The heat flux applied for initialization is 1000 W/m2. The two fluids dynamically exchange heat between each other through the wall there after. The exchanger is designed with the datum (Ref. Table 2) collected from heat exchanger fabrication industry.
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2.2. Governing equations
203
To construct governing equations, the following assumptions are made: (1) both heat transfer and fluid flow in STHE are in three-dimensional form, (2) the coolant enclosed in the tube is unsteady, laminar, incompressible and Newtonian, (3) fluid phase and nanoparticle(s) phase are considered as a multiphase system, (4) the physical properties of fluid phase and nanoparticle phase are temperature independent, (5) nanoparticles are spherical, it is uniformly distributed and dispersed in water. The mixture theory is adopted to simulate the hybrid nanofluid and nanofluid flow. The governing equations for the multiphase system can be written as reported in [28]. Continuity equation
204
@ ! ðq Þ þ r: qm v m ¼ 0 @t m
ð1Þ
! Here v m , qm is the mass-average velocity and mixture density respectively given by;
!
Pn
n X uk qk ! vk ; qm ¼ uk q k qm k¼1
186
2. Problem formulation
vm ¼
187
2.1. Model description
uk is the volume concentration of phases k.
Shell and tube heat exchanger (STHE) (Fig. 1(a)) is a type of heat exchangers that used in oil refineries and other large chemical processes. We consider single pass STHE with shell and tube length of
Momentum equation The momentum equation for the mixture model can be obtained by summing the individual momentum equations for all the phases. It can be expressed as
188 189 190
k¼1
ð2Þ
Fig. 1. (a) Geometry of the problem (b) Schematic diagram of the tube with boundary conditions.
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Table 1 Thermophysical properties of water, OIL, copper, and alumina nanoparticles [45]. Properties
Water
ISO VG 68 OIL
Cu
Al2O3
q(kg/m )
998.2 4182 0.6 0.001003 21 105 147 105
865 2062 0.097 68 – 5.43 108
8933 703 400 – 1.67 105 116 105
3970 765 40 – 0.85 105 1.31 105
3
C p (J/kg k) K (W/m K) l (kg/m s) b(1/K) a (m2/s)
Table 2 Dimension of shell and tube heat exchanger. Heat exchanger type Application Shell fluid(ISO VG 68 OIL) Tube fluid - Coolant(Either Hybrid nanofluid or nanofluid or water) Shell length Tube length Shell inner diameter Shell outer diameter Tube inner diameter Tube outer diameter
229
231 232 233 234
235
237 238
239
Shell Shell Tube Tube
inlet outlet inlet outlet
STHE with single pass. Hydraulic oil heat exchanger. Hot oil (75 °C) Cold oil Cold fluid (30 °C) Hot fluid 1100 mm 1390 mm 43.2 mm 60 mm 33.726 mm 48 mm
@ ! qm v m þ r qm ! v m! vm @t h i ! ! ! ! T ¼ rp þ r lm r v m þ r v m þ qm g þ F ! n X uk q k ! v dr;k ! v dr;k þr
ð3Þ
k¼1
! ! where n; F , lm , v dr;k is the number of phases, the body force, viscosity of the mixture, drift velocity for the secondary phase k ! respectively. lm and v dr;k is defined as follows
lm ¼
n X
uk lk ; ! v dr;k ¼ ! vk! vm
ð4Þ
Energy equation
¼ r:ðkeff rT Þ þ SE
248
P Here keff is the effective thermal conductivity ð uk ðkk þ kt ÞÞ, kt is the turbulent thermal conductivity that is defined in the case of turbulent modelling. SE denotes the volumetric heat source (where ever applicable). Volume concentration equation for the secondary phase The volume concentration equation for secondary phase p can be obtained as, [28]
251
r up qp ! v m ¼ r up qp ! v dr;p
244 245 246 247
249
257 258
2.3. Boundary conditions
259
The following boundary conditions are used to solve the system of Eqs. (1)–(6). At shell inlet and tube inlet, uniform velocity and
253 254 255 256
260
271
The governing equations with the associated boundary conditions are numerically solved by finite volume method. Finite volume method converts the governing equations to a set of algebraic equations that can be solved numerically. The differential equation is discretized using the control volume technique. In discretization methods, a second upwind scheme is selected for the momentum and energy equations. These discretized algebraic equations are sequentially solved in the physical domain. The SIMPLE procedure is used to couple the velocity and pressure terms. This is an iterative solution procedure where the computation is initialized by guessing the pressure field. The temperature is calculated by solving the energy equation.
272
3.1. Code validation
284
In order to validate the numerical results for solving heat transfer in a single pass shell and tube heat exchanger, a comparison with the experimental results for pure water is undertaken. Fig. 2 compares the Nusselt number obtained from this numerical result with those of Zhaoqin experimental results [43] at an inlet temperature of 283.15 K (10 °C) and Reynolds number varies from 170 to 900. They carried out an experimental study on Cu-water nanofluid in a cylindrical tube. The numerical results show good agreement with the experimental results.
285
3.2. Grid convergence study
294
In this work, grid study is taken for analyzing the velocity and pressure components against the length of the tube. Water at 30 °C enters the tube shown in Fig. 1(b) availing the properties listed in Table 1. The grid study was evaluated for three different
295
ð6Þ
The symbols used in governing equations are defined in the nomenclature.q; l; K; C p ; bare the density, viscosity, thermal conductivity, heat capacity and thermal expansion coefficient of fluid (nanofluid/hybrid nanofluid) respectively. The variation in thermophysical properties of nanofluid and hybrid nanofluid are discussed in Section 4.
252
3. Numerical procedure
262 263 264 265 266 267 268 269 270
273 274 275 276 277 278 279 280 281 282 283
286 287 288 289 290 291 292 293
!
ð5Þ
241
243
261
k¼1
n n X @ X ðuk qk Ek Þ þ r: uk ! v k ðqk Ek þ pÞ @t k¼1 k¼1
242
temperature are stated. ISO VG 68 OIL enters the shell at 75 °C; the coolant enters the tube at 30 °C. The no-slip condition is considered at the inner wall (which is the surface in contact with fluid) of tube and shell. At the shell outlet and tube outlet, the adiabatic wall temperature is considered. To analyze the HTP of the fluidsolid interface, the two-phase mixture model is considered. The heat flux is calculated by q} ¼ k @T . Where K is the thermal con@x ductivity of water/nanofluid/hybrid nanofluid. @T is the temperature difference between tube inlet and outlet. At the shell and tube outlet, a static pressure pgauge ¼ 0 is specified.
Fig. 2. Variation of Nusselt number against Reynolds number.
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cases of mesh size as shown in Fig. 3. In addition, Table 3 lists shell outlet temperature profiles for various grid sizes. Obviously, the grid 80*20*20 is chosen for further study, since any considerable changes are not seen in the temperature profiles by increasing the grid in x axis.
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4. Thermophysical properties
305
The thermophysical properties of nanofluid, hybrid nanofluid such as thermal conductivity, viscosity, density and specific heat capacity are calculated using the equations given in Table 4. These properties are estimated for the nanoparticle concentration in the range of 1–20%. Thermophysical properties are considered as temperature independent. Here f ; nf ; hnf denotes the properties of water, nanofluid and hybrid nanofluid respectively. S1 , S2 respectively denotes alumina and copper nanoparticle. In hybrid nanofluid,uS denotes the total volume concentration of alumina and copper nanoparticle (i.e.uS1 þ uS2 Þ. Numerical calculations of thermophysical properties (density, thermal conductivity viscosity, specific heat capacity) of Al2O3Cu/water hybrid nanofluid, Cu/water nanofluid and Al2O3/water nanofluid with various nanoparticle volume concentration is presented in Fig. 4(a)–(d). Thermophysical properties of coolant
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(nanofluid and hybrid nanofluid) are indeed important to estimate in order to understand the HTP. As shown in Fig. 4(a), the density of hybrid nanofluid is the average of Al2O3/water nanofluid and Cu/ water nanofluid. This shows the dominance of alumina nanoparticles in hybrid nanofluid. From Fig. 4(b) and (c), it is seen that thermal conductivity and viscosity of hybrid nanofluids are higher than nanofluids. The viscosity of hybrid nanofluid increases with stronger intermolecular forces and makes more convection. This in turns improves heat transfer rate of hybrid nanofluid. On the other hand, the specific heat capacity of hybrid nanofluid is lower than Al2O3/ water nanofluid and Cu/water nanofluid [15] (Fig. 4(d)). According to the kinetic theory of relation on thermal conductivity, mean free path is inversely proportional to the heat capacity K hnf ¼ ðC p Þhnf V hnf khnf , k 1 ¼ ðC p Þhnf . Consequently, as volume concentra-
321
tion of nanoparticle increases, the specific heat capacity of nanofluid decreases.
335
5. Data processing [17]
337
5.1. Overall heat transfer coefficient (U)
338
Overall heat transfer coefficient of heat exchanger can be calculated by the following equation:
339
hnf
q U¼ AðDTÞLMTD
ð7Þ
here A represents the heat transfer area, and ðDTÞLMTD is the logarithmic mean temperature difference which is obtained as follows:
DT LMTD
DT 2 DT 1 ¼ ln ðDT 2 =DT 1 Þ
ð8Þ
Grid size
Tout (°C)@Shell
40 * 20 * 20 60 * 20 * 20 80 * 20 * 20 100 * 20 * 20 120 * 20 * 20
52.176 53.524 54.125 54.265 54.395
q
340
341 343 344 345
346 348
353 354
357
qmax
K hnf K bf
¼
K S þðn1ÞK f ðn1ÞuS ðK f K S Þ ðK bf K S2 Þ K bf 2 ; ¼ 1K S þðn1ÞK f þu ðK1f K S Þ 1 S1 ðK bf K S2 Þ K f 1 1 ¼ ð1 uS Þ qC p nf þ uS1 ðqC p ÞS1 þ uS2 ðqC p ÞS2
K S2 þðn1ÞK f ðn1ÞuS
ðC p Þhnf
K S2 þðn1ÞK bf þuS
364 365 366 367 368
ð11Þ
K nf Kf
þ uS ðqC p ÞS
336
361 363
K
f
334
ð10Þ
qhnf ¼ uS1 qS1 þ uS2 qS2 þ ð1 uS Þqnf lhnf ¼ lnf ð1 þ 2:5ðuS1 þ uS2 ÞÞ
¼ ð1 uS Þ qC p
333
_ h cp;h ðT h;i T h;o Þ qh ¼ m
qnf ¼ uS qS þ ð1 uS Þqf lnf ¼ lf ð1 þ 2:5uS Þ
nf
332
358 360
q l
Cp
331
ð9Þ
Hybrid Nanofluid
Cp
330
qc ¼ m_ c cp;c ðT c;o T c;i Þ
e¼
329
356
The effectiveness of the heat exchanger is calculated as below:
328
Heat transfer rate for the cold fluid (coolants) and hot fluid (ISO VG 68 OIL) are calculated via Eqs. (9) and (10), respectively.
5.3. Effectiveness (e)
K S þðn1ÞK f ðn1ÞuS ðK f K S Þ K S þðn1ÞK f þuS ðK f K S Þ
327
355
Nanofluid
326
5.2. Heat transfer rate (qÞ
Properties
¼
325
352
Table 4 Thermophysical properties of nanofluid and hybrid nanofluid [44,45].
324
h; i and h; o represent inlet and outlet temperature of hot fluid, respectively. c; i andc; o represent inlet and outlet temperature of cold fluid, respectively.
_ h denote mass flow rate for the coolant and ISO VG 68 OIL m_ c and m respectively. Heat transfer rates for the coolant and oil are equal, and have a negligible difference in this study. It indicates that qc ¼ qh ¼ q:
Table 3 Shell outlet temperature profiles for various grid sizes.
323
349 351
in which DT 1 ¼ T h;i T c;o ; DT 2 ¼ T h;o T c;i
Fig. 3. Effects of velocity and pressure of the water against length of the tube.
322
2
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Fig. 4. Variation in thermophysical properties for nanoparticle volume concentration varies from 1% to 20% (a) density (b) thermal conductivity (c) viscosity (d) heat capacity. Unlike density, thermal conductivity and viscosity, heat capacity decreases with increasing nanoparticle volume concentration [15]. 373 374
375 377 378 379
380 382 383 384
385 388 387 389 391
where qmax denotes maximum possible heat transfer rate which is calculated from Eq. (12)
qmax ¼ C min ðT h;i T c;i Þ
ð12Þ
where C min is the minimum heat capacity rate and it is determined as follows:
C min ¼ min½C h C c
ð13Þ
where C h and C c indicate the heat capacity rates of hot fluid and cold fluid, respectively:
C h ¼ m_ h cp;h
ð14Þ
C c ¼ m_ c cp;c
ð15Þ
392
5.4. Number of transfer Units (NTU)
393 394
The parameter NTU is calculated in terms of minimum heat capacity rate as follows:
398 397
NTU ¼
395
UA C min
ð16Þ
399
6. Results and discussion
400
In this work, influence of nanoparticle volume concentration and proportion on heat transfer performance of Al2O3 – Cu /water
401
hybrid nanofluid in a single pass shell and tube heat exchanger (STHE) is analyzed. The comprehensive comparison between heat transfer performance (HTP) of water, nanofluids (Al2O3/water, Cu/water) and hybrid nanofluid (Al2O3 – Cu /water) is obtained. Mixture theory is adopted to simulate the hybrid nanofluid and nanofluid flow. The range of nanoparticle volume concentration, Rayleigh number, Reynolds number used in this work is 0% u 20%,103 Ra 106 ; 800 Re 2400 respectively. Fig. 5 compares the HTP of hybrid nanofluid, nanofluid in terms of shell & tube outlet temperature. Effect of shell outlet temperature against Reynolds number for water is shown in the inset. Lower shell outlet temperature is an indicator for higher HTP of the coolant. In all the cases, hybrid nanofluid exhibits higher HTP compared to its counterparts, due to the fact that the hybrid nanofluid incorporates both the characteristics of alumina and copper nanoparticle. In addition, hybrid nanofluid overcomes individual drawbacks of nanofluid like low thermal conductivity and low stability. Therefore, the combination of higher thermal conductive (Cu nanoparticle) and higher stability (Al2O3 nanoparticle) nanoparticle is preferable for effective HTP of a hybrid nanofluid. In addition, as the Reynolds number increases, the HTP of coolants (hybrid nanofluid and nanofluids) diminish for any concentration of coolants. Although some studies used hybrid nanofluid for heat transfer application [27–35,42], the optimization of nanoparticle concentration and proportion for HTP is yet to interrogate. Therefore, vari-
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Optimization of nanoparticles proportion (mixing ratio) is essential wherein a case two different nanoparticles dispersed in a base fluid. As can be noticed in Fig. 6(b), at 90(Al2O3): 10 (Cu) proportion, HTP of hybrid nanofluid is higher compares to other proportions. Therefore, in the combination of higher thermal conductive (Cu nanoparticle) and higher stability nanoparticle (Al2O3 nanoparticle), stability nanoparticle should dominate the thermal conductive nanoparticle. Therefore, we can say that both nanoparticle volume concentration and proportion dominates the HTP of hybrid nanofluid provided with appropriate optimization. The HTP of hybrid nanofluid are discussed in terms of governing parameters namely; Rayleigh number, Nusselt number, Reynolds number, heat transfer coefficient in Figs. 7–11. Fig. 7 shows the variation in shell outlet temperature in terms of Rayleigh number for water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/
Fig. 5. Effect of shell and tube outlet temperature against Reynolds number for Al2O3–Cu/water hybrid nanofluid, Cu/water nanofluid, and Al2O3/water nanofluid at 20% nanoparticle volume concentration with 90:10 (Al2O3:Cu) are presented. Effect of shell outlet temperature against Reynolds number for water is shown in inset.
428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443
ation in shell outlet temperature for different nanoparticle concentration and proportion of Al2O3 – Cu/water hybrid nanofluid is studied in this work. It is reported in Fig. 6. Fig. 6 shows the effect of shell outlet temperature against Reynolds number for Al2O3-Cu/water hybrid nanofluid for various nanoparticle (a) concentrations (b) proportions. The nanoparticle volume concentration varies from 1% to 20% and proportion varies from 50(Al2O3): 50 (Cu) to 90(Al2O3): 10 (Cu). Fig. 6(a) illustrates that, as nanoparticle volume concentration increases, shell outlet temperature decreases. It indicates the effective HTP of hybrid nanofluid in heat exchanger. In addition, 20 vol% hybrid nanofluid shows effective HTP compare to other volume concentration. It is due to the term that thermal conductivity of Al2O3 – Cu/water hybrid nanofluid is higher than Cu/water and Al2O3/water (Ref. Fig. 4(b)). It leads to increase thermal performance of Al2O3 – Cu/ water hybrid nanofluid.
Fig. 7. Shell outlet temperature in terms of Rayleigh number for water, nanofluid and hybrid nanofluid. The Rayleigh number varies from 102 to 108. It is seen that HTP of hybrid nanofluid is greater than nanofluid and water at any given Rayleigh number.
Fig. 6. Effects of shell outlet temperature against Reynolds number for various nanoparticle (a) concentration (b) proportion of Al2O3-Cu/water hybrid nanofluid. The nanoparticle concentration varies from 1% to 20%. As nanoparticle volume concentration increases, HTP of hybrid nanofluid increases. At 90 (Al2O3): 10 (Cu) nanoparticle proportion, notable HTP of hybrid nanofluid is attained.
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Fig. 8. (Color online) Temperature distribution in shell wall (a) Water (b) Al2O3-Cu/water hybrid nanofluid at 20% nanoparticle volume concentration and 90:10 nanoparticle proportion.
Fig. 9. Effects (a) Heat transfer Coefficient (b) Dimensionless Nusselt number for Al2O3/water, Cu/water nanofluids and Al2O3-Cu/water hybrid nanofluid at different nanoparticle volume concentration. Heat transfer coefficient and Nusselt number of coolants increases with increase in nanoparticle volume concentration. Also, heat transfer coefficient of hybrid nanofluid is higher compared to nanofluids.
Fig. 10. Pressure drop variation against Reynolds number for water, Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid. Larger pressure drop is noticed for Al2O3-Cu/water hybrid nanofluid.
water hybrid nanofluid. The Rayleigh number varies from 102 to 108. Reynolds number, nanoparticle volume concentration and proportion are fixed as 844.4, 20%, and 90:10 respectively. At low Rayleigh number the heat transfer performance of all coolants are notable. The temperature (75 °C) of the shell fluid is decreases to 61.27 °C in the case of water penetrated as a coolant. In the same case, it is noted as 50.78 °C, 48.16 °C, 47.24 °C for Al2O3/water nanofluid, Cu/water nanofluid, and Al2O3-Cu/water hybrid nanofluid respectively. For further analysis, the temperature distribution of the shell wall is shown in Fig. 8. (a) Water (b) Al2O3-Cu/ water hybrid nanofluid. The temperature distribution in shell wall is shown in Fig. 8. As seen in Fig. 8, the temperature gradient near the inlet is high. The inlet temperature of shell is 348.15 K (75 °C). This temperature decreases to 334.15 K (61 °C), 318.15 K (45 °C) for water and hybrid nanofluid is used as coolant respectively. Hitherto, HTP of coolants are analyzed with shell side performance, the following results will be discuss HTP of coolants by tube side (Ref Fig. 2(b)). The effect of nanoparticle volume concentration on heat transfer coefficient for Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid is compared in Fig. 9 (a). The heat transfer coefficient is calculated using: [16]
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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Fig. 11. (a) Heat transfer rate (b) Histogram of overall heat transfer coefficient of water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid in terms of Reynolds number. Figure (a) shows that heat transfer rate enhances as Reynolds number increases. Further, hybrid nanofluid shows highest heat transfer rate compare to nanofluid and water. In figure (b), it is noticed that over all heat transfer coefficient is an increasing function of Reynolds number. Overall heat transfer coefficient of hybrid nanofluid is higher than its counterparts.
00
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500
502
h¼
q Tw Tf
ð17Þ
00
where q ; T w ; and T f represent the average heat flux between two fluids, average temperature of the wall, and bulk temperature of the considered fluid (nanofluid or hybrid nanofluid). From Fig. 9(a), it is observed that hybrid nanofluid (20 vol% with 90:10 proportions) shows higher heat transfer coefficient than nanofluids. This is due to the inverse relationship between T w T f and heat transfer coefficient. It is noted that T w T f decreases as nanoparticle volume concentration increases. At Re = 844.4, the heat transfer coefficient of Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid are 1903.33, 3500, 4554 W/ m2K. It is observed that the percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% higher than Cu/water nanofluid. The effect of nanoparticle volume concentration on dimensionless Nusselt number for Al2O3-Cu/water hybrid nanofluid, Cu/water, Al2O3/water, and is compared in Fig. 9 (b). Nusselt number is calculated using: 00
Nu ¼
q D KðT w T f Þ
ð18Þ 00
515
In the above expression, q ; T w ; and T f represent the heat flux, average temperature of the wall, and bulk temperature of the considered fluid (nanofluid or hybrid nanofluid). From Fig. 9(b) it is noted that as the particle concentration increases, the Nusselt umber increases. About 40% increase in Nu is observed when hybrid nanofluid is used as coolant rather than nanofluids. This is because of the enhanced thermal conductivity of hybrid nanofluid in comparison with other nanofluids. Only a few studies have reported the effect of pressure drop and friction factor of hybrid nanofluid [46,47]. Pressure drop in STHE for various coolants (Water, Al2O3/water, Cu/water, and Al2O3Cu/water hybrid nanofluid) against Reynolds number is shown in Fig. 10. The pressure drop is calculated using [43]:
518
Dp ¼
503 504 505 506 507 508 509 510 511 512 513 514
516
519 520 521 522 523
64qReu2 D3
increases, there is more velocity near the boundary layer of the pipe causes change in flow pattern in the fluid flow in the pipe. These variations in the flow make the pressure drop initially increasing and then decreasing and again increasing. This non-monotonic variation in Dp is due to larger viscosity of hybrid nanofluid than nanofluid as indicated in Fig. 4(c). Increase in pressure drop of hybrid nanofluid proves that the hybrid nanofluid is the better HTP fluid compared to its counterparts as discussed from previous results. Table 5 lists the values of Number of Transfer Units (NTU) at Reynolds number varying from 800 to 2400 for water, nanofluids and hybrid nanofluid. As compared to overall heat transfer, NTU also increases as Reynolds number increases. At Re = 2321.5, the increment in NTU between water and hybrid nanofluid is close to 75%. Hybrid nanofluid shows highest NTU when compared to their other counterparts. Fig. 11 shows (a) heat transfer rate (b) histogram of overall heat transfer coefficient of heat exchanger for water, Al2O3/water, Cu/ water and Al2O3-Cu/water hybrid nanofluid employed as a coolant. Heat transfer rate of all coolants increases as Reynolds number increases. The range of Reynolds number varies from 800 to 2400. This trend is reported [16]. In addition, heat transfer rate of nanofluid and hybrid nanofluid is higher than that of water. This is because of the influence of volume concentration of nanoparticles and thermophysical properties of nanofluid and hybrid nanofluid. There by, the heat transfer rate for hybrid nanofluid increases almost 78% by increasing the Reynolds number from 844.4 to 2321.54. Fig. 11(b) presents the histogram of overall heat transfer coefficient for water, Al2O3/water, Cu/water and Al2O3-Cu/water hybrid nanofluid in terms of Reynolds number. Hybrid nanofluid shows highest overall heat transfer coefficient (U) than water
Table 5 NTU values of water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid at different Re. Re
Water
844.4 1147.3 1425.3 1719.9 2026.9 2321.5
3.43 3.71 4.01 4.12 4.26 4.33
ð19Þ
where q, u is the density and inlet velocity of respective coolant, D is the diameter of the tube. Fig. 10 depicts that the pressure drop linearly rises with the increases in Reynolds number. But a sharp decrease is noted at the critical point of Reynolds number (flow changes from laminar to turbulent). As the Reynolds number
Nanofluid Al2O3/water
Cu/water
3.68 4.65 5.23 5.77 6.01 6.08
3.86 5.15 5.81 6.38 6.76 6.90
Hybrid nanofluid Al2O3-Cu/water 4.16 5.43 6.45 6.72 7.39 7.57
Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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The heat transfer rate (q) enhances with increase in Reynolds number. Further, the heat transfer rate for hybrid nanofluid increases almost 78% by increasing the Reynolds number from 844.4 to 2321.54. At any Reynolds number, the effectiveness of single pass shell and tube heat exchanger employing hybrid nanofluid is high. Nanoparticle volume concentration and proportion dominate the heat transfer performance of hybrid nanofluid in heat exchanger.
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Fig. 12. Effectiveness of Shell and Tube Heat Exchanger (STHE) in terms of Reynolds number for water, Al2O3/water nanofluid, Cu/water nanofluid, Al2O3-Cu/water hybrid nanofluid. Effective heat transfer performance is attained when hybrid nanofluid is employed as coolant. Further, as Reynolds number increases, effectiveness of heat exchanger increases.
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and nanofluids. As evident from Eq. (7), overall heat transfer coefficient depends on heat transfer rate (q), Area (A), and logarithmic temperature difference (DT LMTD ). This increment is due to the increase in heat transfer rate. Fig. 12 shows the effectiveness of Shell and Tube Heat Exchanger (STHE) for various coolants. Interestingly, it is noted that STHE shows higher effectiveness when hybrid nanofluid used as a coolant. At any Reynolds number, the effectiveness of nanofluids and hybrid nanofluid is higher than that of water. Effectiveness of heat exchanger depends on heat capacity rate of cold and hot fluid (see Eq. (11)). Also, it is noticed that the difference between effectiveness of heat exchanger increases as Reynolds number increases. As a result, addition of nanoparticles to the base fluid (water) enhances the heat transfer performance of heat exchanger.
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7. Conclusions
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There are ongoing efforts, to translate lab scale nanofluids to industries. However, scaling up is considered non trivial and is an ongoing effort worldwide. Al2O3-Cu/water system is considered one of the most promising for industry scale deployment; hence it is used in this study. Governing equations are written based on unsteady, three-dimensional flow of an incompressible viscous hybrid nanofluid in shell and tube heat exchanger. A no slip assumption is made at the inner walls of the exchanger. At shell and tube outlet, adiabatic wall temperature is considered. The model studied here is found to be relevant for various Al2O3: Cu ratios. This numerical investigation shows conclusions relevant to current usage of nanofluids in shell and tube heat exchangers. The salient outcomes are:
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The existence of nanoparticle in the base fluid flowing in STHE have huge enhancement in the heat transfer coefficient and Nusselt number as well as in the performance of heat exchanger. Optimization of nanoparticle volume concentration and proportion for effective heat transfer performance of hybrid nanofluid is done. The pressure drop increases with increase in Reynolds number and a remarkable decrease is noticed at the critical point of Reynolds number. An increase in the volume concentration of nanoparticles, improves the heat transfer performance of hybrid nanofluid system.
Acknowledgements
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We acknowledge The Management, Sri Ramakrishna Engineering College, Coimbatore, India. This work is supported by DST-WOS (A) the project no. SR/WOSA/PM-86/2017. This work is partly supported by Government of India- DST INSPIRE project 04/2013/000209. Tiju Thomas (TT) would like to thank the Department of Science and Technology of India for support through the project nos. DST FILE NO. YSS/2015/001712 and DST 11-IFA-PH07 and DST FILE No. DST/TMDSERI/UB/1(C). TT also thankful for the Nanoelectronics Network for Research and Application (NNetRA) project via the Ministry of Electronics & information Technology. Finally, we highly acknowledge Universal Heat Exchanger, Coimbatore, India for the support.
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Please cite this article as: S. Anitha, T. Thomas, V. Parthiban et al., What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.09.018
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