Effect of variable spacing on performance of plate heat exchanger using nanofluids

Energy 114 (2016) 1107e1119

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Effect of variable spacing on performance of plate heat exchanger using nanofluids Vikas Kumar a, *, Arun Kumar Tiwari b, Subrata Kumar Ghosh a a b

Department of Mechanical Engineering, Indian School of Mines, Dhanbad 826004, India Department of Mechanical Engineering, Institute of Engineering and Technology, GLA University, Mathura 281406, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 May 2016 Received in revised form 21 August 2016 Accepted 26 August 2016 Available online 1 September 2016

This paper presents details of an experimental investigation into the effect of different spacings (DX ¼ 2.5 mm, 5.0 mm, 7.5 mm and 10.0 mm) in plate heat exchanger (PHE) on the basis of its combined energetic and exergetic performance by using various nanofluids, i.e., TiO2, Al2O3, ZnO, CeO2, hybrid (CuþAl2O3), graphene nanoplate (GNP) and multi-walled carbon nanotube (MWCNT). On the basis of experiment data, various energetic and exergetic performance parameters have been evaluated and their inter-relationship has been discussed. The optimum heat transfer characteristics in the nanofluids and their exergetic performance have been found to be achieved with a spacing of DX ¼ 5.0 mm. Based on these data, it has been found that the MWCNT/water nanofluid, with a spacing of DX ¼ 5 mm in PHE, has the maximum heat transfer coefficient, which is 53% higher compared to water at 0.75 vol % (optimum). Nanofluids significantly improve heat transfer capacity with a nominal rise in pressure drop at 0.75 vol %. This study will help to understand the process of heat transfer augmentation by using various nanofluids in the PHE on the basis of energetic and exergetic performance of the system. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Nanofluids Plate heat exchanger (PHE) Spacing Exergy loss Exergetic efficiency

1. Introduction The phenomenon of heat transfer is common in many engineering applications which are carried out with the help of various appliances, including plate heat exchanger (PHE). A PHE consists of a number of thin corrugated plates. Each plate has port holes providing a passage for flowing fluids. The corners of the plates are sealed with gaskets to support the interplate channels directing fluids into alternate channels. The plates of a PHE are held together between a fixed frame plate and a moving pressure plate. They are clamped together by compression with a bolt which compresses the gaskets, thus, making a seal. The transferring of heat takes place through the plate between the channels. The objective behind corrugation of the plates is to enhance the turbulence, thereby increasing heat transfer and strengthening the plate pack [1,2]. The efficacy of heat transfer in a heat exchanging equipment greatly depends on the thermophysical properties of working fluids [3]. Due to inherent properties of high thermal conductivities, enhanced heat transfer coefficient (HTC) and a small penalty of

* Corresponding author. E-mail address: [email protected] (V. Kumar). http://dx.doi.org/10.1016/j.energy.2016.08.091 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

pressure drop, nanofluids have been widely used as coolant for the last two decades [4e9]. ~ ez et al. [10] studied designing of plate fin heat Picon-Nun exchanger (PFHE) with two different surfaces (Plain-fin and louvered fins), based on the volume performance index at various Reynolds numbers. The effect of the plate geometry (angles, depth and types of corrugation) of a PHE on heat transfer characteristics under turbulent conditions for different Reynolds numbers and Prandtl numbers was investigated by Khan et al. [11]. They found a significant effect of plate geometry on the heat transfer performance. Faizal and Ahmed [12] studied the process of optimum heat transfer by varying the spacing between the plates in PHE. Their findings revealed that the optimum heat transfer occurred at a minimum spacing for water-water stream in PHE. Abed et al. [13] carried out a numerical study on the heat transfer performance in corrugated trapezoidal PHE. They used various nanomaterials (Al2O3, SiO2, CuO and ZnO) with volume fractions 0e4.0% and particle diameter 20e80 nm under turbulent flow conditions. Their results established that the heat transfer rate increases with an increased volume fraction of nanofluid. However, the pressure drop increases with decreased diameter of nanoparticles. The performance of heat transfer of nanofluid (Al2O3/water) as a cooling medium in corrugated PHE was examined by Tiwari et al. [14,15]. Pandey and Neema [16] observed that addition of

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V. Kumar et al. / Energy 114 (2016) 1107e1119

nanoparticles (Al2O3) in a base fluid (water) improves the effectiveness of PHE, thereby decreasing the overall heat transfer coef et al. [17] experimentally compared the ficient (OHTC). Mare thermal performance of Al2O3/water and carbon nanotubes/water (CNTs/water) at low temperatures in a PHE. Their results revealed a significant enhancement in HTC with the aforementioned nanofluids. Khoshvaght-Aliabadi et al. [18] carried out a numerical investigation to study the effect of geometrical parameters on heat transfer characteristics of Cu/water nanofluid in a PHE with vortexgenerator channels. Pantzali et al. [19] studied both experimentally and numerically the effect of nanofluid (4 vol% CuO/water) as a working fluid in a PHE. They observed that nanofluid increases the rate of heat transfer more as compared to water. Ray et al. [20] investigated the performance of three types of nanofluid in a PHE both experimentally and numerically. They used Al2O3, CuO and SiO2 nanoparticles dispersed in a mixture of ethylene glycol (EG) and water. When measured, the mixture showed rise in HTC. Hence, less pumping power was required for a fixed amount of heat transfer in the PHE in each of the three cases. Khoshvaght-Aliabadi [21] studied the heat transfer and flow characteristics of sinusoidalcorrugated channel with different parameters (i.e. height/length of channel, length/amplitude of wave and phase shift) using Al2O3water (f ¼ 0e4.0 vol %) nanofluid at Re ¼ 6000e22000 numerically. Their results revealed that values of Nusselt number and friction factor were highly influenced by channel height and wave amplitude respectively. An experiment was conducted to study the HTC in Al2O3/EG and CuO/EG nanofluids in PHE and double pipe by Zamzamian et al. [22]. They found a significant rise in HTC ranging from 3.0%e49.0% when compared with base fluid. Jokar and O'Halloran [23] analyzed the heat transfer process in Alumina/ water (1.0e4.0 vol %) nanofluid by using computational fluid dynamic (CFD) under the laminar condition. Their results showed that the dispersion of nanoparticles in the base fluid caused reduction in the total heat transfer in the PHE. Khoshvaght-Aliabadi et al. [24] performed an experiment to study the thermal e hydraulic performance of different plate fin channels (plain/perforated/offset strip/louvered/wavy/vortex generator and pin) using CuO/water nanofluid. The HTC and pressure drop values were increased with weight concentration of tested nanofluid for all channels considered in their experimentation. However, the vortex generator channel provided the appropriate thermalehydraulic performance and reduced surface area. Khairul et al. [25] analyzed the heat transfer and exergy of CuO/ water (0.5%e1.5 vol %) nanofluid in PHE. In their experimentation, the HTC increased from 18.5%e27.2% while the exergy loss decreased by 24% due to the use of nanofluid. Kabeel et al. [26] studied the thermal effect of Al2O3 nanoparticles on PHE. Their results revealed a 13% rise in HTC for 4.0% vol. concentration of Al2O3/water nanofluid. Javadi et al. [27] compared the thermal properties and performance of heat transfer in various nanofluids (Al2O3, TiO2, and SiO2) with base fluid (liquid nitrogen) flowing in PHE. They found that the addition of nanomaterials in base fluid improved the thermal conductivity (TC). Moreover, the value of TC in Al2O3 and TiO2 was almost equal but higher than SiO2 nanomaterial. The HTC was found to be the least in SiO2 nanomaterials in their experimentation. Chen et al. [28] studied the effect of chevron angles in PHE on the performance of heat transfer by using lithium bromide (LiBr) solution and Al2O3 nanoparticles dispersed in LiBr solution at different mass flow rates. They demonstrated that the heat transfer characteristics were excellent at chevron angle 60 /60 in PHE as compared to other chevron angles for LiBr solution. The HTR, however, was augmented by about 3e8% with 3.0 vol % of LiBr/Al2O3 nanofluid in all cases. The effect of chevron angles (b ¼ 30 /30 , 60 /60 and 30 /60 ) on heat transfer

performance using different nanofluids (Al2O3/water and SiO2/ water) with f ¼ 0e1.0 vol % in a PHE was investigated experimentally by Elias et al. [29]. Their findings revealed the HTR, HTC, pressure drop and pumping power increased with volume concentration and these parameters were higher at b ¼ 60 /60 . Abed et al. [13] conducted a numerical investigation to study the heat transfer in trapezoidal PHE by using nanofluids (Al2O3, SiO2, CuO and ZnO) with f ¼ 0e4.0 vol % and dp ¼ 20e80 nm for turbulent flow. Their results revealed that as the f of nanofluid increases, dp decreases followed by increment in the Nusselt number and pressure drop. Sarafraz and Hormozi [30] conducted an experiment to investigate the heat transfer performance and pressure drop of multi-walled carbon nanotube (MWCNT) in a PHE under the following operating conditions: volume concentration (f ¼ 0.5e1.5 vol %) of MWCNT, inlet temperature of MWCNT (50e70  C) and Re (700e25000). Their experiment data demonstrated that the addition of the aforesaid nanofluid intensified the HTC with a little penalty in pressure drop. The TC also increased approximately from 21%e68%. Anoop et al. [31] studied the thermal performance of SiO2/water in shell, tube and PHE. In their experimentation, they found no anomalous rise in the value of HTC while using nanofluids. An investigation on the heat transfer and pressure drop characteristics of CeO2/water nanofluid in PHE was carried out by Tiwari et al. [32]. The authors observed a rise in HTC by 39% in the nanofluid as compared to water. A few other authors [33e36] also reviewed the application of various nanofluids in the heat exchangers. Askari et al. [37] examined thermal performance of MWCNTs and graphene nanofluids in cooling tower for counter flow arrangement. In their experimentation, the rise in TC was found to be 20% and 16% for MWNTs and graphene nanofluids, respectively, at 45  C. Huang et al. [38] conducted an experiment on the effect of hybrid nanofluid (HNF) on heat transfer and pressure drop characteristics in PHE. In their experimentation, they added MWCNTs/ water nanofluid in Al2O3/water nanofluid to prepare the HNF. After comparing the experiment results of HNF with Al2O3/water nanofluid and water, the value of HTC was reported to be higher in HNF than that of Al2O3/water nanofluid and water. The pressure drop in HNF was less than Al2O3/water nanofluid but higher than water. HNF of GNPeAg was investigated by Yarmand et al. [39]. They found that TC and viscosity of HNF (f ¼ 0.1 wt %) increased by 22.22% and 30%, respectively, as compared to base fluid at 40  C temperature. Suresh et al. [40] synthesized the HNF (Al2O3þCu/ water) with different volume concentration (0.1e2.0 vol %) by a two-step method to study its thermo-physical properties. The experiment data of TC showed an improvement of 12.11% for f ¼ 2.0 vol %. Nimmagadda and Venkatasubbaiah [41] numerically analyzed the heat transfer phenomenon in HNF (Al2O3 þ Ag/Water) in microchannel. They reported that while employing HNF, the heat transfer increased by 143% than base fluid. Madhesh et al. [42] investigated the heat transfer and rheological characteristics of hybrid nano-composite Cu-TiO2 (0.1%e2.0 vol %) in a tube and a tube heat exchanger. Their study revealed that the heat transfer rate and the overall HTC increase by 52% and 68%, respectively, up to a concentration of 1.0 vol % of hybrid nanocomposite. Till now, these authors are not aware of the availability of sufficient information regarding the effect of spacing DX between the PHE plates on the heat transfer characteristics and exergetic performance. However, the performance of PHE is affected by the spacing DX between the plates. Thus, the authors of the present article were motivated to conduct an experiment on this aspect. The aim of the present study has been to investigate the effect of spacing DX between the plates of PHE on HTR, HTC, OHTC, pressure drop ratio, pumping power ratio, exergy destruction and

V. Kumar et al. / Energy 114 (2016) 1107e1119

exergetic efficiency by applying TiO2, Al2O3, ZnO, CeO2, hybrid (CuþAl2O3), GNP, MWCNT, and nanoparticles dispersed in deionized water. Following investigations, the appropriate nanofluid was found from within a number of tested nanofluids for the maximum improvement in heat transfer performance. The thermal conductivity ratio, viscosity ratio, specific heat and density of the aforesaid nanofluids were also calculated systematically.

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Nuhw ¼ hhw Dh =k

(12)

. Prhw ¼ mhw cp;hw k

(13)

The heat transfer coefficient of nanofluids is:

2. Theoretical background

1 1 1 t ¼ þ þ U hhw hnf kw

The heat transfer performance, pressure drop, pumping power and exergetic analysis of working fluid were calculated by obtaining the experimentation readings. Reynolds numbers for hot fluid (water) and cold fluid (nanofluids) were:

where t ¼ thickness of plate ¼ 0.5 mm and kw ¼ thermal conductivity of plate material. The values of pressure drop, Dp for fluids were observed experimentally. The pumping power (Say 80% pump efficiency) for fluids was obtained by Ref. [43].

Rehw ¼ Ghw Dhw =mhw

(1)

PPumpingpower ¼

(2)

Assuming the ambient temperature (To) at 25  C, the exergy changes for the two fluids, based on the experimentation data are presented below: For hot water

.

Renf ¼ Gnf Dh mnf

The hydraulic diameter could be obtained by the following formula

Dh ¼ 2b=4

(3)

The channel mass velocity of hot water and nanofluid were estimated by Eq. (4) and (5)

 _ hw Ncp :b:Lw Ghw ¼ m Gnf

(4)

. _ nf Ncp :b:Lw ¼m

(5)

(14)

_ nf Dpnf m 0:8 rnf

(15)

" Exhw ¼ ð273:15 þ To ÞChw ln

DExhw ¼ Q hw  Exhw

(16)

(17)

For cold fluid (i.e. water or nanofluid), it is:

" Exnf ¼ ð273:15 þ To ÞCnf ln

  , Q hw ¼ m hw cp;hw Thw;in  Thw;out

(6)

DExnf ¼ Q nf  Exnf

  , Q nf ¼ m nf cp;nf Tnf;out  Tnf ;in

(7)

273:15 þ Tnf;out 273:15 þ Tnf;in

# (18)

The exergy change for cold fluid is as follows:

(19)

The total exergy loss is as follows:

The mean heat transfer rate was calculated as follows:

Q hw þQ nf 2

#

The exergy change for hot water is expressed as follows:

The number of pass for hot water and for nanofluid was taken as 10 and 9 in Eq. (4) and Eq. (5), respectively. The heat rejection and absorption by the fluids were calculated by Eq. (6) & Eq. (7), respectively.

Q mean ¼

273:15 þ Thw;in 273:15 þ Thw;out

Ex ¼ DExhw  DExnf

(20)

The exergetic efficiency can be calculated by using the following equation:

(8)

The experimental overall heat transfer coefficient was calculated from Eq. (9)

U¼

Q mean A  qmin

(9)

where qmin, the logarithmic mean temperature, was obtained by Eq. (10)



qmin

   Thw;in  Tnf;out  Thw;out  Tnf;in ! ¼ ln

(10)

Thw;in Tnf;out Thw;out Tnf;in

The heat transfer coefficient of the hot fluid for b ¼ 60 /60 was obtained by the following equations [43] respectively:

Nuhw ¼ 0:306  Rehw 0:529  Prhw 0:33

(11)

Nusselt and Prandtl numbers were computed by Eq. (12) and (13)

Fig. 1. Comparison of experimental relative thermal conductivity with different existing models for different nanofluids.

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Fig. 2. Variation of Relative thermal conductivity with respect to volume concentration for different nanofluids.

hII ¼

DExnf  100 DExhw

(21)

3. Materials and method 3.1. Nanofluid preparation Various types of water-based nanofluid were prepared by using commercially available nanomaterials, such as TiO2, Al2O3, ZnO, CeO2, hybrid (CuþAl2O3), GNP and MWCNT procured from Alfa Aesar. The underlying reasons for choosing the oxide based nanofluids (TiO2, Al2O3, ZnO and CeO2) are its better stability and lower price relatively other nanofluids. Among the aforesaid nanofluids, CeO2 has highest thermal conductivity followed by ZnO, Al2O3 and TiO2. Therefore, in this research TiO2, Al2O3, ZnO, and CeO2 nanoparticles were used in the base fluid water to prepare the nanofluids. Moreover, the objective of selecting the hybrid composite is to achieve the physicochemical properties of its constituent nanomaterial. Generally, the mono nanoparticle does not have all favorable properties (thermal properties/rheological properties) required for practical application. Hence, the hybrid nanofluid is chosen in this experimentation for comparing the thermophysical properties to that of mono nanofluids. Further, GNP and MWCNT are selected due to its exceptional thermophysical properties. The concentration of nanofluids ranging from 0.5 to 2.0 vol % was used in this experimentation. In this experimentation, 0.75 vol % was found as optimum volume concentration. The samples of nanofluid were subjected to continuous ultrasonic vibration for 6 h with the help of a vibrator (Toshiba, India) to ensure uniform dispersion and stable suspension. The surfactant, Cetyl Trimethyl Ammonium Bromide, was added beforehand to the nanofluid by the manufacturer to ensure proper dispersion and better stability without altering the thermophysical properties of nanofluid. The concentration of applied surfactant is 0.01%.

thermophysical properties before conducting the experiment. The thermophysical properties of the TiO2/water, Al2O3/water, ZnO/ water, CeO2/water, hybrid, GNP and MWCNT nanofluids include TC, viscosity, specific heat and density. These thermophysical properties were measured for 0.5, 0.75, 1.0 and 1.25 vol % at a temperature of 35  C. A KD2 pro-thermal analyzer (Decagon Company), based on transient hot wire technique, was used for determining the values of thermal conductivity of nanofluids. Measurements of viscosity of the aforesaid nanofluids were done by using the LVDV-IIþPro Brookfield digital viscometer (cone and plate) with a computercontrolled temperature bath to set the nanofluid temperature at different degrees. The differential scanning calorimetry (Setaram C80D) was used to measure specific heat of nanofluids. 3.2.1. Thermal conductivity Fig. 1 exemplifies the comparison of relative TC with different existing models [32,44,45] in the literature for tested volume concentration of nanofluids at 35  C. As seen, the experimental data

3.2. Systematic measurement of thermophysical properties The work first necessitated systematic measurements of all

Fig. 3. Comparison of experimental relative viscosity with different existing models for different nanofluids.

V. Kumar et al. / Energy 114 (2016) 1107e1119

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Fig. 4. Variation of Relative viscosity with respect to volume concentration for different nanofluids.

for relative TC of TiO2/water and CeO2/water nanofluid is almost good matching with the results reported by Corcione [45] and Karimi et al. [44]. However, the results calculated by other investigators [32,45] overestimates to some extent the experimental results of Al2O3/water and ZnO/water nanofluid. Fig. 2 depicts the changes of relative TC (knf/kbf) with different volume concentrations (0.5, 0.75, 1.0 and 1.25%) of different nanofluids at 35  C temperature. It was observed that the relative TC of nanofluids increases with the increasing volume concentration of nanoparticles due to the formation of clusters caused by aggregation of nanoparticles. As observed, the MWCNT nanofluid showed the highest value of relative TC among the nanofluids considered in this experimentation. During experiments, the enhancement in relative TC of the MWCNT nanofluid was observed to the tune of 11.42%, 14.02%, 19.74% and 22.67% for 0.5, 0.75, 1.0 and 1.25 vol % respectively, at 35  C temperature with respect to the base fluid (water). 3.2.2. Viscosity Fig. 3 depicts the comparison of experimental relative viscosity

with the results reported by various researchers [32,45e47] published in the literature previously. From this figure, it is observed that the experimental results of the relative viscosity of Al2O3/ water, ZnO/water and CeO2/water nanofluids are quite matching with the results reported by Corcione [45] and Tiwari et al. [32]. However, for TiO2/water nanofluid, the measured value is significantly higher than those calculated by other researchers [32,45e47] in the present experimentation. These deviations in relative viscosity probably due to the differences in the production procedure, since the samples are procured from different agencies. The relative viscosity (mnf/mbf) of different nanofluids in terms of volume concentration is illustrated in Fig. 4. In the present experimentation, viscosity of nanofluids was measured at 35  C. As seen, relative viscosity of the nanofluids goes up linearly/non-linearly with their volume concentration because of hydrodynamic interaction among the particles. As observed, the MWCNT nanofluid showed the lowest value of relative viscosity among the nanofluids considered in this experimentation. The scale of enhancement in relative viscosity of MWCNT nanofluid was observed at 2.01%, 3.70%, 6.31% and 14.4% for 0.5, 0.75, 1.0 and 1.25 vol % at 35  C

Fig. 5. Variation of specific heat against volume concentration for different nanofluids.

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Fig. 6. Variation of density with respect to volume concentration for different nanofluids.

temperature as compared to the base fluid. 3.2.3. Specific heat The literature survey reveals that the specific heat of nanofluid is lesser than its base fluid. The variation of experimental measured specific heat against the volume concentration of tested nanofluids is depicted in Fig. 5. As the volume concentration increases, the specific heat of the nanofluids are found to decreases depicted in Fig. 5. This happens due to the enhancement in thermal diffusivity.

The similar results are reported in the literature by numerous researchers [46,48e52]. MWCNT exhibited the peak value of the specific heat. Thus it could be beneficial to use MWCNT nanofluid among the tested nanofluids as a coolant in PHE for cost effective and clean heat transfer process in this experimentation. 3.2.4. Density The variation of density of tested nanofluids is presented in Fig. 6. The measured value of density, at a temperature of 35  C, is

Fig. 7. Schematic of experimental setup.

V. Kumar et al. / Energy 114 (2016) 1107e1119

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Fig. 8. Photograph of experimental setup.

growing up with the volume concentration of nanofluids. The MWCNT nanofluid exhibits the highest density among the tested nanofluids for referred volume concentration. As instance, the amount of enhancement in density of MWCNT nanofluid for 0.75 vol % of nanoparticles in comparison with TiO2/water, Al2O3/ water, ZnO/water, CeO2/water, hybrid and GNP are calculated as 3.43%, 2.76%, 2.19%, 2.09%, 1.71% and 0.95% respectively.

3.4. Error analysis and measurement In the present experimentation, the uncertainties in parameters were computed by methods proposed by Moffat [54]. The total numbers of uncertainties in performance parameters have been tabulated in Table 2.

4. Results and discussion 3.3. Experiment setup and procedure The schematic and photograph of the experimental set up are depicted in Figs. 7 and 8 respectively. It consists of two fluid loops viz. cold fluid loop and hot fluid loop. The cold fluid loop incorporated four components namely nanofluid tank (25 L volume capacity), gear pump, gate valve and Coriollis flow meter. Here, the nanofluids played a role of coolant. Similarly, the hot fluid loop incorporated a DM water tank (25 L volume capacity) with 4 kW heater, a hot fluid pump, a gate valve and a Coriollis flow meter. Water was the working fluid in this loop. There was provision for insulation to minimize the heat loss. However, The details of procedure of the experimentation were similar to our previous study [53]. The details of plate geometry are given in Table 1. The desired inlet temperatures were 20  C and 50  C for nanofluid and hot DM water, respectively. In this experimentation, the volume flow rate of nanofluids and hot DM water were kept at 3 lpm.

Table 1 Geometrical parameters of corrugated plate. S.N.

Parameter

Value

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Lw (Plate width inside gasket) Lv (Vertical distance between centers of ports) Lh (Horizontal distance between centers of ports) Dp (Port diameter) Np (Number of plates) b (Mean channel spacing) Gap between two consecutive plate Pc (Corrugation pitch) p (Plate pitch) Gasket width Gasket thickness b (Chevron angle) Plate thickness Plate pack length

180 mm 357 mm 60 mm 35 mm 20 2.4 mm 2.4 mm 14.2 mm 2.9 mm 7.4 mm 0.37 mm 60 /60 0.5 mm 58 mm

The present experiment has been described in three Sections. The first section concentrates on the validation of experimentation and find out the optimum volume concentration of nanofluids for further analysis. The second section discusses the heat transfer characteristics and pumping power at optimum volume concentration of nanofluids. The third section exclusively offers a glimpse of exergetic performances of PHE by using the aforesaid nanofluid to determine the requisite nanofluid for operating condition. The most appropriate nanofluid with optimum volume concentration was chosen from among the tested nanofluids to achieve the maximum heat transfer performance of PHE at optimal spacing. This investigation focussed on the effect of spacing DX between the plate of PHE on the average HTR, HTC, OHTC, pressure drop ratio, pumping power ratio, exergy destruction and exergetic efficiency, i.e., second law efficiency for various nanofluids. First, the experiment was carried out with the DM water before evaluating the heat transfer of nanofluids so that experimentation could be validated. To achieve this validation, the heat transfer coefficients of hot and cold fluid, are estimated by using the Eq. (11). Table 2 Uncertainties in performance parameters. Parameters

Uncertainty (%)

Temperatures To (Ambient temperature) Volume flow rate hh (HTC for water) hnf (HTC for nanofluid) q min (LMTD) Dp (Pressure drop) Dh (Hydraulic diameter) Length knf (Thermal conductivity of nanofluid) m nf (Viscosity of nanofluid) r nf (Density of nanofluid) cp,nf (Specific heat of nanofluid)

±0.13 ±0.11 ±2.5 ±2.1 ±2.1 ±3.5 ±4.0 ±2.1 ±0.12 ±3.0 ±2.2 ±3.5 ±3.2

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V. Kumar et al. / Energy 114 (2016) 1107e1119

f > 0.75 vol %.This divulges that the use of nanofluids with f > 0.75 vol % should be avoided, as they will not provide any

Fig. 9. Comparison between experimental and theoretical value for overall heat transfer coefficient of water.

Then, Eq. (14) is utilized to compute the theoretical OHTC. In the same way, using experimental data, Eq. (9) predicted the experimental OHTC. The experimental OHTCs are in good agreement with the theoretical ones for DM water, at a particular spacing i.e. DX ¼ 5.0 mm as depicted in Fig. 9, which qualifies the experimental results for validation. Now, the experiment was carried out to study optimum volume concentrations of tested nanofluids with fixed spacing (i.e. DX ¼ 5.0 mm) and volume flow rates (i.e. 3 lpm) for both nanofluids and hot water. The variation of overall heat transfer coefficient (OHTC) with varying volume concentration is depicted in Fig. 10 at given operating conditions. As observed, the OHTC is increasing with volume concentration up to 0.75 vol % and decreasing further with volume concentration more than 0.75 vol %. This observation divulged that the OHTCs are optimum at f ¼ 0.75 vol % for all tested nanofluids for spacing DX ¼ 5.0 mm. Moreover, the OHTC of nanofluids decreases with volume concentration of nanoparticles

contribution towards heat transfer resulting higher pressure drop caused by higher viscosity. The MWCNT deserved for the highest OHTC among the tested nanofluids. However, the enhancement in OHTC for MWCNT can be observed to the tune of 35.58%, 30.67%, 24.68%, 18.55%, 11.70% and 5.0% as compared to TiO2/water, Al2O3/ water, ZnO/water, CeO2/water, hybrid and GNP respectively, at f ¼ 0.75 vol % for spacing DX ¼ 5.0 mm. Since the above observation revealed that the maximum enhancements in OHTCs were found at f ¼ 0.75 vol % for all tested nanofluids. Hence, for the present experimentation, the nanofluid concentration of 0.75 vol % was considered to be optimum for further analyzing the heat transfer characteristics and exergetic performance with varying the spacing DX. Moreover, From the available literature, it has been also observed that a number of authors [14,25,32,55,56] chose nanofluids of less than 1.0% volume concentration for their experimentations. The plot of average HTR (Qavg) against the spacing (DX) between the plates is shown in Fig. 11. It is clear from the plot that the average value of HTR is the highest for MWCNT nanofluid due to its extraordinary thermophysical properties and the lowest with water. In this experimentation, the optimum spacing between the plates was found to occur at DX ¼ 5 mm. Thus, it can be concluded that the HTR increases with an increase in the value of DX up to the optimum spacing and then it decreases as depicted in Fig. 11. This happens due to a narrow passage disabling turbulence, resulting in improper mixing of fluids for spacing DX ¼ 2.5 mm. On the contrary, the intensity of turbulence remains low despite achieving proper mixing of fluids for spacing DX ¼ 7.5 mm and DX ¼ 10 mm. Thus, it is recommended that spacing beyond the optimum value (DX¼ 5 mm) should be avoided as it is not beneficial for the HTR. The rise in average HTR was 6.72%, 12.81%, 15.91%, 18.68%, 28.54%, 36.12% and 44.63% for TiO2/water, Al2O3/water, ZnO/water, CeO2/ water, hybrid, GNP and MWCNT respectively at f ¼ 0.75 vol% when compared with water at DX ¼ 2.5 mm. This enhancement in average HTR, however, is the maximum at DX ¼ 5.0 mm because it generates adequate turbulence enabling proper mixing of fluids. This enhances the HTR by 11.21%, 16.28%, 19.52%, 22.38%, 30.86%, 37.87% and 45.96% for TiO2/water, Al2O3/water, ZnO/water, CeO2/ water, hybrid, GNP and MWCNT respectively, at f ¼ 0.75 vol% as

Fig. 10. The overall heat transfer coefficient, U, at varying volume concentration.

V. Kumar et al. / Energy 114 (2016) 1107e1119

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Fig. 11. The average heat transfer rate, Qavg, at varying DX.

compared with water under given operating conditions. Moreover, the rise in HTR has been found to be 7.85%, 13.3%, 18.14%, 21.88%, 29.58%, 35.35%, 43.97% and 7.36%, 12.09%, 17.22%,20.83%, 28.41%, 34.94% and 42.75% for TiO2/water, Al2O3/water, ZnO/water, CeO2/ water, hybrid, GNP and MWCNT respectively, at f ¼ 0.75 vol% as compared with water for DX ¼ 7.5 mm and DX ¼ 10 mm, respectively. Fig. 12 displays the changes in HTC with spacing (DX) between the plates. As observed, the value of HTC is the highest for MWCNT nanofluid and the lowest with water for all values of DX considered in this experimentation. This happens due to rising of MWCNT thermal conductivity rather than water. The maximum rise in HTC could be observed at DX ¼ 5 mm as compared to other values of DX. These enhancements were recorded at 13.91%, 19.17%, 21.58%, 24.11%, 31.11%, 39.93%, and 53.05% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT respectively, at f ¼ 0.75 vol% with respect to water. Thus, MWCNT recorded the maximum rise (~53%) in HTC for spacing of DX ¼ 5 mm at

f ¼ 0.75 vol %. The OHTC is the measurement of resistance encountered during heat transfer. As depicted in Fig. 13, the value of OHTC is the highest for MWCNT nanofluid and the lowest with water for all values of DX. It was found that the OHTC increases with an increase in the value of DX up to the optimum spacing (DX ¼ 5.0 mm) and then it decreases. This can probably be witnessed due to the generation of sufficient turbulence and proper mixing of the fluids at spacing DX ¼ 5.0 mm. The rise in OHTC is to the tune of 9.09%, 16.81%, 19.28%, 23.41%, 37.11%, 45.45% and 52.86% for TiO2/water, Al2O3/ water, CeO2/water, ZnO/water, hybrid, GNP and MWCNT respectively, at f ¼ 0.75 vol% as compared to water for DX ¼ 2.5 mm. On the other hand, the rise in OHTC for optimum spacing (DX ¼ 5.0 mm), is 9.64%, 13.75%, 19.22%, 25.38%, 33.08%, 41.58% and 48.65% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT at f ¼ 0.75 vol% with respect to water. Further, the rise in OHTC can be observed to the tune of 8.64%, 11.64%, 15.66%, 19.67%, 28.21%, 33.48%, 42.22% and 11.41%, 14.17%, 18.32%,

Fig. 12. The heat transfer coefficient, h, at varying DX.

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Fig. 13. The overall heat transfer coefficient, U, at varying DX.

22.47%, 29.59%, 37.11, 42.53% for TiO2/water, Al2O3/water, ZnO/ water, CeO2/water, hybrid, GNP and MWCNT respectively at f ¼ 0.75 vol% as compared to water for DX ¼ 7.5 mm and DX ¼ 10 mm, respectively. Thus, the maximum enhancement (~49%) in OHTC for spacing DX ¼ 5 mm at f ¼ 0.75 vol % can be witnessed in case of MWCNT. The comparison of the pressure drop ratio of nanofluids (f ¼ 0.75 vol %.) with the spacing DX is shown in Fig 14. As depicted in Fig 14, the maximum pressure drop ratio is for TiO2/water and the minimum for MWCNT. It was found to be the lowest for spacing DX ¼ 5 mm for the given nanofluids in this experimentation. With respect to the base fluid (water), the pressure drop ratios were 5.39%, 4.57%, 3.90%, 3.21%, 1.81%, 1.28% and 0.53% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT respectively, for spacing DX ¼ 5 mm at f ¼ 0.75 vol%. Thus, MWCNT registered the lowest value of pressure drop ratio among the given

nanofluids in this experimentation. The pumping power ratio versus the spacing DX for the given nanofluids (f ¼ 0.75 vol %) has been plotted in Fig. 15. The rise in pumping power ratios with respect to base fluid (water) were recorded at 10.66%, 9.79%, 9.09%, 8.37%, 6.91%, 6.34% and 5.56% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT respectively, for spacing DX ¼ 5 mm at f ¼ 0.75 vol%. This implies that the use of MWCNT nanofluid as a coolant is beneficial as evident in the least value of pumping power ratio in this experimentation. The effect of spacing DX on the exergy destruction of various nanofluids is depicted in Fig. 16. It is demonstrated that water causes the highest exergy destruction as compared to nanofluids for all cases of spacing DX. It can be clearly understood from Fig. 16 that the value of exergy destruction is the lowest for spacing DX ¼ 5 mm in case of all given nanofluids. The reduction in the

Fig. 14. The comparison of pressure drop ratio of nanofluids with the spacing DX.

V. Kumar et al. / Energy 114 (2016) 1107e1119

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Fig. 15. The comparison of pumping power ratio of nanofluids with the spacing DX.

Fig. 16. Exergy destruction variation of nanofluids with the spacing DX.

value of exergy destruction, however, was recorded at 11.64%, 18.54%, 26.05%, 35.79%, 45.05%, 62.59% and 75.91% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT respectively, as compared to water for spacing DX ¼ 5 mm at f ¼ 0.75 vol%. Fig. 17 shows results of the exergetic efficiency (hII) variation with the spacing DX for different nanofluids in PHE. As seen in Fig. 17, one can easily understand that the maximum value of exergetic efficiency is attained at spacing DX ¼ 5 mm for the aforementioned nanofluids in PHE. The MWCNT, however, recorded the maximum value of exergetic efficiency among the referred nanofluids while the minimum value of exergetic efficiency was recorded with water in this experimentation. The underlying reason for highest exergetic efficiency of MWCNT is it's the lowest change in exergy destruction, among the tested nanofluids. The data from experiment reveal that the values of exergetic efficiency are 7.56%, 12.58%, 18.66%, 26.70%, 41.45%, 50.26% and 58.14% for TiO2/water, Al2O3/water, ZnO/water, CeO2/water, hybrid, GNP and

MWCNT respectively, with DX ¼ 5 mm at f ¼ 0.75 vol%.

respect

to

water

for

spacing

5. Conclusion The present study mainly focussed on the effect of the variable spacing on the performance of a PHE by using different kinds of nanofluids. The spacing between the plates (DX ¼ 2.5, 5.0, 7.5 and 10 mm) has been varied for given operating conditions by using different nanomaterials, viz., TiO2, Al2O3, ZnO, CeO2, hybrid, graphene nanoplate (GNP) and MWCNT dispersed in deionized water as a base fluid. The volume concentration of tested nanofluids was 0.75 vol %. The thermophysical properties of tested nanofluids, including, TC, viscosity, specific heat and density were assessed systematically. The salient experimental findings are pointed out as follows:

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Fig. 17. Exergetic efficiency variation with the spacing DX for different nanofluids.

 MWCNT has the most outstanding thermophysical properties among the tested nanofluids.  When particle volume concentration reaches 0.75 vol %, OHTC exhibits optimum response.  The spacing DX ¼ 5.0 mm is considered to be optimal for the maximum heat transfer.  The spacing beyond the optimum value (DX ¼ 5 mm) should be avoided as no significant rise in HTR was observed in this experimentation.  The maximum HTR, HTC and the OHTC were enhanced by 46%, 53.05% and 49.0% respectively, for MWCNT (f ~ 0.75 vol %.) amongst tested nanofluids atDX ¼ 5 mm when compared with water under given operating conditions.  The pressure drop ratios and pumping power ratios of tested nanofluids were greater than base fluid (water). The lowest pressure drop ratio and pumping power ratio were recorded with MWCNT for spacing DX ¼ 5 mm at f ¼ 0.75 vol %. The respective values of the aforesaid ratios were found to be 0.53% and 5.56% more than that of the base fluid for MWCNT. Thus, MWCNT nanofluid qualified as the best coolant amongst the tested nanofluids in this experimentation.  The exergy destruction was the lowest for nanofluids as compared to water. The MWCNT nanofluid exhibited the minimum exergy destruction amongst the tested nanofluids for spacing DX ¼ 5 mm at f ¼ 0.75 vol %. The reduction in the value of exergy destruction, however, was 75.91% for MWCNT nanofluid as compared to water.  The maximum and the minimum value of exergetic efficiency was recorded with MWCNT nanofluid and water, respectively. The values of exergetic efficiency were found to be 7.56%, 12.58%, 18.66%, 26.70%, 41.45%, 50.26% and 58.14% for TiO2/water, Al2O3/ water, ZnO/water, CeO2/water, hybrid, GNP and MWCNT nanofluid, respectively, with respect to water for spacing DX ¼ 5 mm at f ¼ 0.75 vol%.

dp Dh G k kw Q Re To PHE HNF MWCNT GNP HTC HTR OHTC Ex

diameter of nanoparticle(nm) hydraulic diameter (m) channel mass velocity (kg/m2 s) thermal conductivity (W/m K) thermal conductivity of stainless steel material (W/m K) heat transfer rate (W) Reynolds number Ambient temperature ( C) plate heat exchanger hybrid nanofluid multiwalled carbon nanotubes graphene nanoplate heat transfer coefficient heat transfer rate overall heat transfer coefficient Exergy destruction (Watt)

Subscripts hw hot water in inlet condition max maximum out outlet condition nf nanofluid h heat transfer coefficient (W/m2 K) Greek symbols b Chevron angle (degree) q min logarithmic mean temperature ( C) f volume concentration (%) 4 enlargement factor h II exergetic efficiency (%) r density (kg/m3) m dynamic viscosity (mPa s) References

Nomenclature A b cp

area of plate (m2) mean channel spacing (m) specific heat (J/kg K)

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