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Development and assessment of ceria–propylene glycol nanofluid as an alternative to propylene glycol for cooling applications
Accepted Manuscript Development and Assessment of Ceria-Propylene glycol Nanofluid as an Alternative to Propylene glycol for Cooling Applications M. Prabhakaran, S. Manikandan, K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan PII: DOI: Reference:
S1359-4311(16)30468-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.159 ATE 8027
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
24 November 2015 27 March 2016
Please cite this article as: M. Prabhakaran, S. Manikandan, K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan, Development and Assessment of Ceria-Propylene glycol Nanofluid as an Alternative to Propylene glycol for Cooling Applications, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.159
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Development and Assessment of Ceria-Propylene glycol Nanofluid as an Alternative to Propylene glycol for Cooling Applications M. Prabhakaran, S. Manikandan, K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan* Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401, India. ________________________________________________________________ * - Corresponding author Prof. K. S. Rajan Seshasayee Paper & Boards Chair Professor in Chemical Engineering, Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401 India Email: [email protected] Telephone: 919790377951 Fax: 91 4362 264120
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Abstract Spherical, crystalline ceria nanoparticles of 18-25 nm were synthesized from cerium nitrate precursor. The dispersion of as-synthesized ceria nanoparticles in propylene glycol was achieved through extended probe ultrasonication for 14 hours, leading to ceria – propylene glycol nanofluids. The influence of nanoparticle concentration (0-1 vol %) and temperature on viscosity and thermal conductivity of ceria – propylene glycol nanofluids were investigated. Our data indicate that the higher thermal conductivity enhancement at elevated temperatures (18.8 % at 80 °C for 1 vol % nanofluid) can be attributed to the particle clustering and Brownian-motion induced microconvection. Ceria nanoparticles interact with propylene glycol leading to disturbance in hydrogen bonding network prevalent in propylene glycol. This resulted in lower viscosity of 0.5 vol % and 1 vol % ceria – propylene glycol nanofluids than propylene glycol over a wide range of temperatures. The heat absorption by ceria – propylene glycol nanofluids under transient, natural convective heat trasnfer conditions increased with ceria nanoparticle concentration. Hence ceria-propylene glycol nanofluids are suitable for cooling applications.
Keywords Nanofluid, Ceria, Propylene glycol, Heat transfer, Natural convection, Transient.
Nomenclature Symbol
Meaning
Cp
Specific heat (J/kgK)
D in Eq. (5)
Fractal dimension
Da
Aggregate size (m)
Dp
Particle size (m)
2
g
Acceleration due to gravity (m/s2)
h
Heat transfer coefficient (W/m2K)
k
Thermal conductivity (W/mK)
kB
Boltzmann constant (=1.381x10-23) J/K
Q
Amount of heat transferred (W)
T
Temperature (°C)
t
Time (s)
uB
Brownian Velocity (m/s)
ut
Terminal settling velocity (m/s)
V
Volume (m3)
Greek symbols
Nanoparticle volume fraction
a
Aggregate volume fraction
Viscosity (mPas)
Density (kg/m3)
θ
angle of diffraction
Subscripts bf
Base fluid
BM
Brownian motion
f
Fluid
nf,ϕ
Nanofluid of a specified nanoparticle concentration ‘ϕ’
np
Nanoparticle
p
Particle
PG
Propylene glycol
3
r
Ratio
1. Introduction The role of heat transfer in energy generation and energy consumption systems is significant. For instance, the intensification of heat transfer in collectors of solar thermal power plants can result in higher collection efficiency [1], improved steam or vapour generation for a fixed collector area. Heat transfer intensification in an energy consuming system such as heat exchangers can provide higher heat duty for a fixed heat exchanger dimensions [2]. Heat transfer can also be intensified through use of heat transfer fluids with favourable properties. The thermal conductivity of conventional coolants can be improved by addition of nanoparticles and maintaining the nanoparticle-liquid system in a stable, colloidal form. Solid-liquid dispersions containing collodially stable nanoparticles (particle size < 100 nm) in common coolants (base fluid) are called nanofluids [3]. Metal oxides are compatible with common coolants such as water, ethylene glycol, propylene glycol, etc. and are preferred choice of nanomaterial for preparation of nanofluids. Cerium oxide (ceria) is one of the metal oxides whose potential for nanofluid preparation has been reported in a few works only [4–6]. For a fixed nanoparticle concentration, the thermal conductivity and viscosity of nanofluids can be tuned through control of nanoparticle aggregation. While particle clustering contributes to thermal conductivity enhancement in nanofluids containing larger aggregates, the contribution of Brownian-motion induced microconvection is reduced in such dispersions [7]. Nanofluids with larger aggregates are more viscous than those containing smaller aggregates at the same nanoparticle concentration [8,9]. Propylene glycol is a food grade coolant and hence heat transfer fluid of choice in solar water heaters and in food industries [10]. In the present work, preparation and measurement
4
of thermophysical properties of ceria – propylene glycol are being reported for the first time. The capability of ceria – propylene glycol nanofluids to absorb heat under natural convection conditions from a surface subjected to constant heat flux has also been investigated. Such a study would facilitate establishment of heat transfer performance – transport properties link for ceria-propylene glycol nanofluid and explore the utility of single-phase heat transfer correlations for prediction of performance of nanofluids.
2. Materials and Methods 2.1 Preparation and characterization of ceria nanoparticles Ceria nanoparticles were prepared from cerium nitrate hexahydrate by wet chemical synthesis, as reported elsewhere [11]. The surface morphology of calcined powder was ascertained using a cold field emission scanning electron microscope (JSM 6701F, JEOL, Japan). The primary particle size was determined using transmission electron microscopy (JEM 2100F, JEOL, Japan). The powder X-ray diffraction patterns of the calcined powder were recorded using a X-ray diffractometer (D8Focus, Bruker, Germany). 2.2 Preparation of ceria – propylene glycol nanofluids The maximum concentration of ceria nanoparticles was restricted to 1 vol % owing to requirement of larger mass of nanoparticles due to higher density of ceria (7.65 g/cm3) [12]. A predetermined mass of ceria nanoparticles conforming to 1 vol % was added to propylene glycol, followed by ultrasonic exposure in a probe ultrasonicator (Vibra-cellTM, Sonics, USA). No surfactant was employed as metal oxide nanoparticles can be dispersed in propylene glycol without surface modification or encapsulation [13]. The thermal conductivity of 1 vol % dispersion was measured at periodic intervals of probe ultrasonication to determine the optimum ultrasonication time required. The probe ultrasonication was discontinued, after the thermal conductivity measurements between successive time periods of ultrasonication were
5
not significantly different. Ceria – propylene glycol of different nanoparticle concentrations (0.25 – 1 vol %) were prepared from 14-hours ultrasonicated stock of 1 vol % ceria – propylene glycol nanofluid through dilution with propylene glycol as required. 2.3 Characterization of ceria – propylene glycol nanofluids The hydrodynamic size distribution of ceria – propylene glycol nanofluids was determined by dynamic light scattering (Nano ZS, Malvern Instruments, UK). Thermal conductivities of propylene glycol and ceria – propylene glycol nanofluids were determined using transient hot-wire technique. A thermal conductivity meter (KD2 Pro, Decagon Devices, USA) was used for this purpose. The accuracy of thermal conductivity meter was ascertained by measuring the thermal conductivity of manufacturer-supplied test fluid (glycerine). The viscosity of propylene glycol and that of nanofluids were measured using a viscometer (LVDV-II+Pro, Brookfield Engineering, USA) using the S18 spindle. The accuracy of viscosity measurement was ascertained by comparing the measured value with the manufacturer-specified value for the standard (silicone oil). The deviations between the measured value and manufacturer specified value for thermal conductivity and viscosity were less than 2.8 % and 1.84 % respectively. The temperatures of propylene glycol and ceria propylene glycol nanofluids were maintained at desired temperature during the measurement of thermal conductivity in a constant temperature bath (TC-502, Brookfield Engineering, USA). The maintenance of temperature during the viscosity measurements was ensured through use of a thermosel (Thermosel®, Brookfield Engineering, USA). The measurements of viscosity and thermal conductivity were repeated at least thrice to ascertain reproducibility of transport property data. 2.4 Transient heat transfer studies The capability of ceria – propylene glycol nanofluids to absorb heat from a surface subjected to constant flux has been investigated. The nanofluids were taken in a cylindrical
6
container, whose outer surface was wound with an AC electrical heating coil followed by a layer of asbestos rope. The power supply to heating coil was controlled using a dimmerstat. The temperature of nanofluids was recorded at regular intervals using a ‘J’ type thermocouple connected to a digital temperature indicator. The schematic diagram of the experimental setup is shown in Fig. 1. The amount of heat absorbed by nanofluids was calculated from their initial (~ 30 °C), final temperature (~ 50 °C) and the time taken for increase in temperature from the initial to final values as detailed in section 3.4. 2.5 Uncertainty analysis The uncertainty analysis has been carried out taking into account of random errors during the measurement (Type A uncertainty). Hence, the uncertainties in measurements of thermal conductivity, viscosity and amount of heat absorbed were taken as the standard errors of the mean of respective measurements.
3. Results and Discussion 3.1 Characteristics of synthesized product The synthesized powder consisted of nanoparticles with primary particle size ranging between 18 nm and 25 nm, as evident from scanning electron micrograph (Fig. 2a) and transmission electron micrograph (Fig. 2b). The particles were nearly spherical and contained significant amount of aggregates (Fig. 2a). The powder X-ray diffraction data shown in Fig. 2c revealed the presence of crystalline ceria, as confirmed by comparison with JCPDS card No.81-0792. 3.2 Thermal Conductivity of Ceria - Propylene glycol nanofluids Nanoparticles tend to agglomerate when dispersed in liquids. In addition, ceria nanopowder too contained few aggregates as evidenced from scanning electron micrograph. Hence probe ultrasonication was utilized to achieve dispersion of ceria nanoparticles in
7
propylene glycol. The effect of probe ultrasonication time on thermal conductivity of 1 vol % ceria-propylene glycol nanofluids is shown in Fig. 3. Thermal conductivity of 1 vol % ceriapropylene glycol nanofluids was found to increase with ultrasonication time till about 12 hours, beyond which no further increase in thermal conductivity was observed. The influence of concentration of ceria nanoparticles on thermal conductivity of ceria – propylene glycol nanofluids at 27 °C indicates linear increase in their thermal conductivity with ceria concentration (Fig. 4). The thermal conductivity of 1 vol % ceria-propylene glycol nanofluid was higher than that of propylene glycol by 10.7 %. The thermal conductivity enhancement for 1 vol % nanofluid is comparable to those already reported for optimally probe ultrasonicated 1 vol % metal oxide – propylene glycol nanofluids such as ZnOpropylene glycol (12.5 %, [14]), MgO-propylene glycol (10.8%, [13]), sand – propylene glycol (11.5%, [15]), Fe2O3-propylene glycol (10.5%, [16]) nanofluids. The hydrodynamic size distribution of 1 vol % ceria – propylene glycol nanofluids revealed the average particle size to be 70±1.8 nm. The average hydrodynamic size is about 3 times the primary particle size, which indicates that nanofluid contained colloidally-stable aggregates. In the absence of net attractive forces, the ratio of Brownian velocity (uB) to terminal settling velocity (ut) of aggregates provides an indication about the colloidal stability of dispersion. Higher the Brownian-to-settling velocity ratio (uB/ut), better is the colloidal stability. Brownian velocity (uB) and terminal settling velocity (ut) of aggregates in ceriaproylene glycol nanofluid were calculated using the following formulae [17]: uB
ut
2k B T D p2
p
(1)
f gD p2
(2)
18
Brownian-to-terminal settling velocity ratio (uB/ut) was estimated to be 30,000, which is high
8
enough to ensure that the random motion of particles overcomes settling of particles by gravity. This was confirmed visually as there was no phase separation between nanoparticles and the base fluid. The nanofluids were stable for more than a month despite repeated heating and cooling. Thermal conductivity of 1 vol % ceria – propylene glycol nanofluid varied very little with temperature (27 – 80 °C), with minimum and maximum thermal conductivities within this temperature range differing from average thermal conductivity in this temperature range by less than 1% (Fig. 5). The thermal conductivity of propylene glycol was found to decrease with temperature when heated above 50 °C (Fig. 5). However, the thermal conductivity of 1 vol % ceria-propylene glycol nanofluid did not decrease. Hence thermal conductivity ratio, defined as the ratio of thermal conductivity of 1 vol % ceria-propylene glycol nanofluid to propylene glycol, was found to increase with temperature (Fig. 6). The thermal conductivity ratio increased from 1.107 at 27 °C to 1.188 at 80 °C, corresponding to 10.7% and 18.8% enhancements in thermal conductivity at 27 and 80 °C respectively. Such higher thermal conductivity ratio at higher temperatures has been reported for several water-based nanofluids systems [18,19] attributed to increased Brownian motion at elevated temperatures, enabled by higher thermal energy and lower liquid viscosity. The thermal conductivity enhancement for 1 vol % ceria-propylene glycol nanofluid at the lowest temperature investigated (27 °C) is 10.7 %, which is greater than the thermal conductivity enhancement (3 %) predicted by the simplified Hamilton-Crosser model. The nanofluid viscosity at 27 °C was measured to be 30.53 cP, considered high enough to reduce the impact of Brownian motion on thermal conductivity enhancement. Therefore, the fact that the actual higher thermal conductivity enhancement even at 27 °C was found to be higher than that predicted by simplified Hamilton-Crosser model points out to the role of other possible mechanisms in thermal conductivity modulation of 1 vol % ceria-propylene glycol
9
nanofluid. It may be recalled that the ceria-propylene glycol nanofluid contained stable aggregates whose diameter was three times the primary particle size. These stable aggregates form longer path of heat conduction than those with individual particles [20]. The effective volume fraction of aggregates (ϕa) may be related to aggregate size (Da), primary particle size (Dp) and volume fraction of nanoparticles (ϕ) as [21]:
a Da D p
3 D
(3)
The value of fractal dimension (D) ranges from 2 to 2.2 for rate-limited aggregation [21]. The simplified Hamilton-Crosser model can be modified to account for the effect of particle clustering on thermal conductivity ratio (kr) by replacing the nanoparticle volume fraction (ϕ) by the effective volume fraction of aggregates (ϕa) as follows [22]:
k r 1 3a
(4)
Eliminating ‘ϕa’ from Eq. (4) using Eq. (3) leads to D k r 1 3 a D p
3 D
(5)
It may be recalled that the average aggregate and primary particle sizes were 70 and 23 nm respectively. Substituting the values of ‘Da/Dp’ and D (fractal dimension) as 3 and 2 respectively in Eq. (5), results in
k r 1 9
(6)
Eq. (6) accounts for thermal conductivity enhancement due to nanoparticle addition (effective-medium approximation) and particle clustering. For a fixed nanoparticle concentration, the influence of temperature on Brownian motioninduced thermal conductivity enhancement can be expressed as [19,23]: k r , BM f T
(7) 10
Taking into account of role of particle clustering and Brownian motion on thermal conductivity enhancement, thermal conductivity ratio – temperature relationship for 1 vol % ceria-propylene glycol nanofluid may be expressed as:
k r 1 9 f T
(8)
An expression for f(T) was obtained as follows through regression analysis of kr-T data in accordance with the form of Eq. (8):
f T 6.067 10 5 T 1.66
(9)
Combining Eq. (8) and Eq. (9)
k r 1 9 6.067 10 5 T 1.66 27 T 40 C
(10)
3.3 Viscosity of Ceria - Propylene glycol nanofluids The influence of temperature and nanoparticle concentration on the viscosity of nanofluids prepared from 14 hour-ultrasonicated stock is shown in Fig. 7. It is clear from Fig. 7 that the viscosity of 0.5 vol % and 1 vol % ceria-propylene glycol nanofluid decreased with temperature, qualitatively matching the viscosity-temperature relationship for propylene glycol (base fluid). The qualitative similarity between viscosity-temperature profiles of ceriapropylene glycol nanofluid and propylene glycol indicates that the magnitude of intermolecular attractive forces decreased in nanofluids as well over the entire temperature and concentration ranges investigated. It may also be observed from Fig. 7 that the viscosities of 0.5 vol % and 1 vol % ceria-propylene glycol nanofluids were lower than that of propylene glycol in the temperature range of 30-70 °C. It is widely believed and reported that viscosity of nanofluid is greater than that of the liquid in which the nanoparticles are dispersed (base fluid). In nanofluid systems with no chemical interactions between nanoparticles and liquid, the viscosity of nanofluid can be estimated using the viscosity models [24] that account for increased viscous dissipation due to nanoparticles. However when metal oxide nanoparticles 11
are dispersed in strongly-hydrogen bonded liquids, strong interactions originate between liquid and nanoparticles on particles' surface. These interactions seem to interfere with the hydrogen bonding network of liquid, leading to reduction in viscosity [15,25], as confirmed by FTIR spectra (data not shown to maintain brevity). Hence the viscosity change caused by the addition of nanoparticles is attributable to both the increased viscous dissipation that causes viscosity increase and the disturbance to hydrogen bonding network in base fluid that causes viscosity reduction. 3.4 Heat transfer performance of ceria – propylene glycol nanofluids The temporal variation of temperature of ceria – propylene glycol nanofluids of different concentrations is shown in Fig. 8. The rate of change of temperature of 1 vol % ceria – propylene glycol nanofluids was the highest among different nanofluid concentrations investigated as evident from the slope of temperature – time data (Fig. 8). The quantity of heat transferred to nanofluids or heat absorbed by nanofluids was calculated from the initial and final temperature as follows: Qnf Vnf C p ,nf nf
dT dt
(11)
The product of specific heat and density of nanofluid was estimated using the following equation [26]: C p,nf nf C p, PG PG 1 C p ,np np
(12)
The density and specific heat of propylene glycol are 1040 kg/m3 [27] and 2500 J/kgK [28] respectively, while those of ceria are 7650 kg/m3 [12] and 390 J/kgK [29]. Hence with increase in nanoparticle concentration, the volumetric specific heat (nf cp,nf) of ceria – propylene glycol nanofluid increased due to higher volumetric specific heat of particles. The higher slope (dT/dt) of temperature – time data and higher volumetric specific heat (nf cp,nf) at higher nanoparticle concentration resulted in improved heat absorption for 1
12
vol% ceria – propylene glycol nanofluid when compared to those of 0.5 and 0.25 vol% ceria – propylene glycol nanofluid as shown in Fig. 9. The amount of heat absorbed by nanofluids is related to nanoparticle volume fraction as Qnf , 17.65 0.8785
Qnf , 0.8785
(13)
17.65 1 0.8785
(14)
From Eq. (14), ϕ→0, Qnf,0 = 0.8785. Therefore, Eq. (14) can be re-written as Qnf , Qnf ,0
20.09 1
(15)
From Eq. (15), it may be understood that the addition of ceria nanoparticles to propylene glycol at the concentration of 1 vol % would result in ~20 % enhancement in quantity of heat absorbed. Under the experimental conditions employed in the present study, the predominant mode of heat transfer is natural convection. The natural convective heat transfer coefficient is related to specific heat, density, viscosity and thermal conductivity as [30]:
C p , nf nf2 hnf nf k nf
n
k nf
(16) The value of ‘n’ is 0.25 for conditions of the present study [30]. The quantity of heat absorbed by nanofluids (Qnf,ϕ) is proportional to natural convective heat transfer coefficient (hnf). Therefore,
Qnf , hnf
(17)
Combining Eqs. (16) & (17), the ratio of heat absorbed by nanofluids to heat absorbed by nanoparticle-free fluid (base fluid) may be calculated as:
13
Qnf , Qnf , 0
The
C 2 p , nf nf k nf nf
0.25
k nf C p , nf , 0 nf2 , 0 nf ,0 k nf ,0
0.25 k nf ,0
(18)
Qnf ,1% Qnf ,0.5% and ratios calculated using Eq. (18) were found to be 1.096 and 1.191. Qnf ,0 Qnf ,0
These ratios were closer to the ones determined from heat transfer experiments (1.10 and 1.201 for 0.5 vol % and 1 vol % ceria – propylene glycol nanofluids respectively) in accordance with Eq. (15). Hence it may be concluded that the improvement in thermophysical properties of ceria – propylene glycol nanofluids were reflected in their heat absorption performance under natural convective conditions.
4. Conclusions Ceria – propylene glycol nanofluid with high colloidal stability was prepared by dispersing surfactant-free ceria nanoparticles of 18-25 nm diameter in propylene glycol using probe ultrasonication for 14 hours. The thermal conductivity of 1 vol % ceria – propylene glycol nanofluid was higher than that of propylene glycol by 10.7 % and 18.8 % at temperatures of 27 °C and 80 °C respectively. Brownian motion-induced microconvection and particle clustering were found to be major contributors for thermal conductivity enhancement of ceria – propylene glycol nanofluid. The viscosities of 1 vol % and 0.5 vol % ceria – propylene glycol nanofluids were lower than that of pure propylene glycol at temperatures below 80 °C, due to ceria nanoparticles-induced perturbations in intermolecular hydrogen bonding of propylene glycol. The improved thermophysical properties (higher volumetric specific heat, higher thermal conductivity and lower viscosity) of 1 vol % ceria – propylene glycol nanofluid resulted in ~ 20 % improvement in quantity of heat absorption under transient, natural-convective constant wall heat flux conditions. 14
Acknowledgements This work was supported by (i) PG teaching grant No: SR/NM/PG-16/2007 of Nano Mission Council, Department of Science & Technology (DST), India (ii) Grant No: SR/FT/ET061/2008, DST, India (iii) INSPIRE fellowship (Reg Nos: IF110312, IF130529) of Department of Science and Technology (DST), India and (iv) Research & Modernization Project #1, SASTRA University, India.
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Research Highlights
Stable ceria – propylene glycol nanofluids prepared by probe ultrasonication
These nanofluids possess higher thermal conductivity & volumetric specific heat
Particle clustering & Brownian motion contribute to thermal conductivity increase
1 vol % ceria-PG viscosity lower than PG viscosity at temperatures <80 °C
Heat absorption by nanofluids increase with nanoparticle concentration
24
S1359-4311(16)30468-9 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.159 ATE 8027
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
24 November 2015 27 March 2016
Please cite this article as: M. Prabhakaran, S. Manikandan, K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan, Development and Assessment of Ceria-Propylene glycol Nanofluid as an Alternative to Propylene glycol for Cooling Applications, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.159
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Development and Assessment of Ceria-Propylene glycol Nanofluid as an Alternative to Propylene glycol for Cooling Applications M. Prabhakaran, S. Manikandan, K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan* Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401, India. ________________________________________________________________ * - Corresponding author Prof. K. S. Rajan Seshasayee Paper & Boards Chair Professor in Chemical Engineering, Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401 India Email: [email protected] Telephone: 919790377951 Fax: 91 4362 264120
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Abstract Spherical, crystalline ceria nanoparticles of 18-25 nm were synthesized from cerium nitrate precursor. The dispersion of as-synthesized ceria nanoparticles in propylene glycol was achieved through extended probe ultrasonication for 14 hours, leading to ceria – propylene glycol nanofluids. The influence of nanoparticle concentration (0-1 vol %) and temperature on viscosity and thermal conductivity of ceria – propylene glycol nanofluids were investigated. Our data indicate that the higher thermal conductivity enhancement at elevated temperatures (18.8 % at 80 °C for 1 vol % nanofluid) can be attributed to the particle clustering and Brownian-motion induced microconvection. Ceria nanoparticles interact with propylene glycol leading to disturbance in hydrogen bonding network prevalent in propylene glycol. This resulted in lower viscosity of 0.5 vol % and 1 vol % ceria – propylene glycol nanofluids than propylene glycol over a wide range of temperatures. The heat absorption by ceria – propylene glycol nanofluids under transient, natural convective heat trasnfer conditions increased with ceria nanoparticle concentration. Hence ceria-propylene glycol nanofluids are suitable for cooling applications.
Keywords Nanofluid, Ceria, Propylene glycol, Heat transfer, Natural convection, Transient.
Nomenclature Symbol
Meaning
Cp
Specific heat (J/kgK)
D in Eq. (5)
Fractal dimension
Da
Aggregate size (m)
Dp
Particle size (m)
2
g
Acceleration due to gravity (m/s2)
h
Heat transfer coefficient (W/m2K)
k
Thermal conductivity (W/mK)
kB
Boltzmann constant (=1.381x10-23) J/K
Q
Amount of heat transferred (W)
T
Temperature (°C)
t
Time (s)
uB
Brownian Velocity (m/s)
ut
Terminal settling velocity (m/s)
V
Volume (m3)
Greek symbols
Nanoparticle volume fraction
a
Aggregate volume fraction
Viscosity (mPas)
Density (kg/m3)
θ
angle of diffraction
Subscripts bf
Base fluid
BM
Brownian motion
f
Fluid
nf,ϕ
Nanofluid of a specified nanoparticle concentration ‘ϕ’
np
Nanoparticle
p
Particle
PG
Propylene glycol
3
r
Ratio
1. Introduction The role of heat transfer in energy generation and energy consumption systems is significant. For instance, the intensification of heat transfer in collectors of solar thermal power plants can result in higher collection efficiency [1], improved steam or vapour generation for a fixed collector area. Heat transfer intensification in an energy consuming system such as heat exchangers can provide higher heat duty for a fixed heat exchanger dimensions [2]. Heat transfer can also be intensified through use of heat transfer fluids with favourable properties. The thermal conductivity of conventional coolants can be improved by addition of nanoparticles and maintaining the nanoparticle-liquid system in a stable, colloidal form. Solid-liquid dispersions containing collodially stable nanoparticles (particle size < 100 nm) in common coolants (base fluid) are called nanofluids [3]. Metal oxides are compatible with common coolants such as water, ethylene glycol, propylene glycol, etc. and are preferred choice of nanomaterial for preparation of nanofluids. Cerium oxide (ceria) is one of the metal oxides whose potential for nanofluid preparation has been reported in a few works only [4–6]. For a fixed nanoparticle concentration, the thermal conductivity and viscosity of nanofluids can be tuned through control of nanoparticle aggregation. While particle clustering contributes to thermal conductivity enhancement in nanofluids containing larger aggregates, the contribution of Brownian-motion induced microconvection is reduced in such dispersions [7]. Nanofluids with larger aggregates are more viscous than those containing smaller aggregates at the same nanoparticle concentration [8,9]. Propylene glycol is a food grade coolant and hence heat transfer fluid of choice in solar water heaters and in food industries [10]. In the present work, preparation and measurement
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of thermophysical properties of ceria – propylene glycol are being reported for the first time. The capability of ceria – propylene glycol nanofluids to absorb heat under natural convection conditions from a surface subjected to constant heat flux has also been investigated. Such a study would facilitate establishment of heat transfer performance – transport properties link for ceria-propylene glycol nanofluid and explore the utility of single-phase heat transfer correlations for prediction of performance of nanofluids.
2. Materials and Methods 2.1 Preparation and characterization of ceria nanoparticles Ceria nanoparticles were prepared from cerium nitrate hexahydrate by wet chemical synthesis, as reported elsewhere [11]. The surface morphology of calcined powder was ascertained using a cold field emission scanning electron microscope (JSM 6701F, JEOL, Japan). The primary particle size was determined using transmission electron microscopy (JEM 2100F, JEOL, Japan). The powder X-ray diffraction patterns of the calcined powder were recorded using a X-ray diffractometer (D8Focus, Bruker, Germany). 2.2 Preparation of ceria – propylene glycol nanofluids The maximum concentration of ceria nanoparticles was restricted to 1 vol % owing to requirement of larger mass of nanoparticles due to higher density of ceria (7.65 g/cm3) [12]. A predetermined mass of ceria nanoparticles conforming to 1 vol % was added to propylene glycol, followed by ultrasonic exposure in a probe ultrasonicator (Vibra-cellTM, Sonics, USA). No surfactant was employed as metal oxide nanoparticles can be dispersed in propylene glycol without surface modification or encapsulation [13]. The thermal conductivity of 1 vol % dispersion was measured at periodic intervals of probe ultrasonication to determine the optimum ultrasonication time required. The probe ultrasonication was discontinued, after the thermal conductivity measurements between successive time periods of ultrasonication were
5
not significantly different. Ceria – propylene glycol of different nanoparticle concentrations (0.25 – 1 vol %) were prepared from 14-hours ultrasonicated stock of 1 vol % ceria – propylene glycol nanofluid through dilution with propylene glycol as required. 2.3 Characterization of ceria – propylene glycol nanofluids The hydrodynamic size distribution of ceria – propylene glycol nanofluids was determined by dynamic light scattering (Nano ZS, Malvern Instruments, UK). Thermal conductivities of propylene glycol and ceria – propylene glycol nanofluids were determined using transient hot-wire technique. A thermal conductivity meter (KD2 Pro, Decagon Devices, USA) was used for this purpose. The accuracy of thermal conductivity meter was ascertained by measuring the thermal conductivity of manufacturer-supplied test fluid (glycerine). The viscosity of propylene glycol and that of nanofluids were measured using a viscometer (LVDV-II+Pro, Brookfield Engineering, USA) using the S18 spindle. The accuracy of viscosity measurement was ascertained by comparing the measured value with the manufacturer-specified value for the standard (silicone oil). The deviations between the measured value and manufacturer specified value for thermal conductivity and viscosity were less than 2.8 % and 1.84 % respectively. The temperatures of propylene glycol and ceria propylene glycol nanofluids were maintained at desired temperature during the measurement of thermal conductivity in a constant temperature bath (TC-502, Brookfield Engineering, USA). The maintenance of temperature during the viscosity measurements was ensured through use of a thermosel (Thermosel®, Brookfield Engineering, USA). The measurements of viscosity and thermal conductivity were repeated at least thrice to ascertain reproducibility of transport property data. 2.4 Transient heat transfer studies The capability of ceria – propylene glycol nanofluids to absorb heat from a surface subjected to constant flux has been investigated. The nanofluids were taken in a cylindrical
6
container, whose outer surface was wound with an AC electrical heating coil followed by a layer of asbestos rope. The power supply to heating coil was controlled using a dimmerstat. The temperature of nanofluids was recorded at regular intervals using a ‘J’ type thermocouple connected to a digital temperature indicator. The schematic diagram of the experimental setup is shown in Fig. 1. The amount of heat absorbed by nanofluids was calculated from their initial (~ 30 °C), final temperature (~ 50 °C) and the time taken for increase in temperature from the initial to final values as detailed in section 3.4. 2.5 Uncertainty analysis The uncertainty analysis has been carried out taking into account of random errors during the measurement (Type A uncertainty). Hence, the uncertainties in measurements of thermal conductivity, viscosity and amount of heat absorbed were taken as the standard errors of the mean of respective measurements.
3. Results and Discussion 3.1 Characteristics of synthesized product The synthesized powder consisted of nanoparticles with primary particle size ranging between 18 nm and 25 nm, as evident from scanning electron micrograph (Fig. 2a) and transmission electron micrograph (Fig. 2b). The particles were nearly spherical and contained significant amount of aggregates (Fig. 2a). The powder X-ray diffraction data shown in Fig. 2c revealed the presence of crystalline ceria, as confirmed by comparison with JCPDS card No.81-0792. 3.2 Thermal Conductivity of Ceria - Propylene glycol nanofluids Nanoparticles tend to agglomerate when dispersed in liquids. In addition, ceria nanopowder too contained few aggregates as evidenced from scanning electron micrograph. Hence probe ultrasonication was utilized to achieve dispersion of ceria nanoparticles in
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propylene glycol. The effect of probe ultrasonication time on thermal conductivity of 1 vol % ceria-propylene glycol nanofluids is shown in Fig. 3. Thermal conductivity of 1 vol % ceriapropylene glycol nanofluids was found to increase with ultrasonication time till about 12 hours, beyond which no further increase in thermal conductivity was observed. The influence of concentration of ceria nanoparticles on thermal conductivity of ceria – propylene glycol nanofluids at 27 °C indicates linear increase in their thermal conductivity with ceria concentration (Fig. 4). The thermal conductivity of 1 vol % ceria-propylene glycol nanofluid was higher than that of propylene glycol by 10.7 %. The thermal conductivity enhancement for 1 vol % nanofluid is comparable to those already reported for optimally probe ultrasonicated 1 vol % metal oxide – propylene glycol nanofluids such as ZnOpropylene glycol (12.5 %, [14]), MgO-propylene glycol (10.8%, [13]), sand – propylene glycol (11.5%, [15]), Fe2O3-propylene glycol (10.5%, [16]) nanofluids. The hydrodynamic size distribution of 1 vol % ceria – propylene glycol nanofluids revealed the average particle size to be 70±1.8 nm. The average hydrodynamic size is about 3 times the primary particle size, which indicates that nanofluid contained colloidally-stable aggregates. In the absence of net attractive forces, the ratio of Brownian velocity (uB) to terminal settling velocity (ut) of aggregates provides an indication about the colloidal stability of dispersion. Higher the Brownian-to-settling velocity ratio (uB/ut), better is the colloidal stability. Brownian velocity (uB) and terminal settling velocity (ut) of aggregates in ceriaproylene glycol nanofluid were calculated using the following formulae [17]: uB
ut
2k B T D p2
p
(1)
f gD p2
(2)
18
Brownian-to-terminal settling velocity ratio (uB/ut) was estimated to be 30,000, which is high
8
enough to ensure that the random motion of particles overcomes settling of particles by gravity. This was confirmed visually as there was no phase separation between nanoparticles and the base fluid. The nanofluids were stable for more than a month despite repeated heating and cooling. Thermal conductivity of 1 vol % ceria – propylene glycol nanofluid varied very little with temperature (27 – 80 °C), with minimum and maximum thermal conductivities within this temperature range differing from average thermal conductivity in this temperature range by less than 1% (Fig. 5). The thermal conductivity of propylene glycol was found to decrease with temperature when heated above 50 °C (Fig. 5). However, the thermal conductivity of 1 vol % ceria-propylene glycol nanofluid did not decrease. Hence thermal conductivity ratio, defined as the ratio of thermal conductivity of 1 vol % ceria-propylene glycol nanofluid to propylene glycol, was found to increase with temperature (Fig. 6). The thermal conductivity ratio increased from 1.107 at 27 °C to 1.188 at 80 °C, corresponding to 10.7% and 18.8% enhancements in thermal conductivity at 27 and 80 °C respectively. Such higher thermal conductivity ratio at higher temperatures has been reported for several water-based nanofluids systems [18,19] attributed to increased Brownian motion at elevated temperatures, enabled by higher thermal energy and lower liquid viscosity. The thermal conductivity enhancement for 1 vol % ceria-propylene glycol nanofluid at the lowest temperature investigated (27 °C) is 10.7 %, which is greater than the thermal conductivity enhancement (3 %) predicted by the simplified Hamilton-Crosser model. The nanofluid viscosity at 27 °C was measured to be 30.53 cP, considered high enough to reduce the impact of Brownian motion on thermal conductivity enhancement. Therefore, the fact that the actual higher thermal conductivity enhancement even at 27 °C was found to be higher than that predicted by simplified Hamilton-Crosser model points out to the role of other possible mechanisms in thermal conductivity modulation of 1 vol % ceria-propylene glycol
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nanofluid. It may be recalled that the ceria-propylene glycol nanofluid contained stable aggregates whose diameter was three times the primary particle size. These stable aggregates form longer path of heat conduction than those with individual particles [20]. The effective volume fraction of aggregates (ϕa) may be related to aggregate size (Da), primary particle size (Dp) and volume fraction of nanoparticles (ϕ) as [21]:
a Da D p
3 D
(3)
The value of fractal dimension (D) ranges from 2 to 2.2 for rate-limited aggregation [21]. The simplified Hamilton-Crosser model can be modified to account for the effect of particle clustering on thermal conductivity ratio (kr) by replacing the nanoparticle volume fraction (ϕ) by the effective volume fraction of aggregates (ϕa) as follows [22]:
k r 1 3a
(4)
Eliminating ‘ϕa’ from Eq. (4) using Eq. (3) leads to D k r 1 3 a D p
3 D
(5)
It may be recalled that the average aggregate and primary particle sizes were 70 and 23 nm respectively. Substituting the values of ‘Da/Dp’ and D (fractal dimension) as 3 and 2 respectively in Eq. (5), results in
k r 1 9
(6)
Eq. (6) accounts for thermal conductivity enhancement due to nanoparticle addition (effective-medium approximation) and particle clustering. For a fixed nanoparticle concentration, the influence of temperature on Brownian motioninduced thermal conductivity enhancement can be expressed as [19,23]: k r , BM f T
(7) 10
Taking into account of role of particle clustering and Brownian motion on thermal conductivity enhancement, thermal conductivity ratio – temperature relationship for 1 vol % ceria-propylene glycol nanofluid may be expressed as:
k r 1 9 f T
(8)
An expression for f(T) was obtained as follows through regression analysis of kr-T data in accordance with the form of Eq. (8):
f T 6.067 10 5 T 1.66
(9)
Combining Eq. (8) and Eq. (9)
k r 1 9 6.067 10 5 T 1.66 27 T 40 C
(10)
3.3 Viscosity of Ceria - Propylene glycol nanofluids The influence of temperature and nanoparticle concentration on the viscosity of nanofluids prepared from 14 hour-ultrasonicated stock is shown in Fig. 7. It is clear from Fig. 7 that the viscosity of 0.5 vol % and 1 vol % ceria-propylene glycol nanofluid decreased with temperature, qualitatively matching the viscosity-temperature relationship for propylene glycol (base fluid). The qualitative similarity between viscosity-temperature profiles of ceriapropylene glycol nanofluid and propylene glycol indicates that the magnitude of intermolecular attractive forces decreased in nanofluids as well over the entire temperature and concentration ranges investigated. It may also be observed from Fig. 7 that the viscosities of 0.5 vol % and 1 vol % ceria-propylene glycol nanofluids were lower than that of propylene glycol in the temperature range of 30-70 °C. It is widely believed and reported that viscosity of nanofluid is greater than that of the liquid in which the nanoparticles are dispersed (base fluid). In nanofluid systems with no chemical interactions between nanoparticles and liquid, the viscosity of nanofluid can be estimated using the viscosity models [24] that account for increased viscous dissipation due to nanoparticles. However when metal oxide nanoparticles 11
are dispersed in strongly-hydrogen bonded liquids, strong interactions originate between liquid and nanoparticles on particles' surface. These interactions seem to interfere with the hydrogen bonding network of liquid, leading to reduction in viscosity [15,25], as confirmed by FTIR spectra (data not shown to maintain brevity). Hence the viscosity change caused by the addition of nanoparticles is attributable to both the increased viscous dissipation that causes viscosity increase and the disturbance to hydrogen bonding network in base fluid that causes viscosity reduction. 3.4 Heat transfer performance of ceria – propylene glycol nanofluids The temporal variation of temperature of ceria – propylene glycol nanofluids of different concentrations is shown in Fig. 8. The rate of change of temperature of 1 vol % ceria – propylene glycol nanofluids was the highest among different nanofluid concentrations investigated as evident from the slope of temperature – time data (Fig. 8). The quantity of heat transferred to nanofluids or heat absorbed by nanofluids was calculated from the initial and final temperature as follows: Qnf Vnf C p ,nf nf
dT dt
(11)
The product of specific heat and density of nanofluid was estimated using the following equation [26]: C p,nf nf C p, PG PG 1 C p ,np np
(12)
The density and specific heat of propylene glycol are 1040 kg/m3 [27] and 2500 J/kgK [28] respectively, while those of ceria are 7650 kg/m3 [12] and 390 J/kgK [29]. Hence with increase in nanoparticle concentration, the volumetric specific heat (nf cp,nf) of ceria – propylene glycol nanofluid increased due to higher volumetric specific heat of particles. The higher slope (dT/dt) of temperature – time data and higher volumetric specific heat (nf cp,nf) at higher nanoparticle concentration resulted in improved heat absorption for 1
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vol% ceria – propylene glycol nanofluid when compared to those of 0.5 and 0.25 vol% ceria – propylene glycol nanofluid as shown in Fig. 9. The amount of heat absorbed by nanofluids is related to nanoparticle volume fraction as Qnf , 17.65 0.8785
Qnf , 0.8785
(13)
17.65 1 0.8785
(14)
From Eq. (14), ϕ→0, Qnf,0 = 0.8785. Therefore, Eq. (14) can be re-written as Qnf , Qnf ,0
20.09 1
(15)
From Eq. (15), it may be understood that the addition of ceria nanoparticles to propylene glycol at the concentration of 1 vol % would result in ~20 % enhancement in quantity of heat absorbed. Under the experimental conditions employed in the present study, the predominant mode of heat transfer is natural convection. The natural convective heat transfer coefficient is related to specific heat, density, viscosity and thermal conductivity as [30]:
C p , nf nf2 hnf nf k nf
n
k nf
(16) The value of ‘n’ is 0.25 for conditions of the present study [30]. The quantity of heat absorbed by nanofluids (Qnf,ϕ) is proportional to natural convective heat transfer coefficient (hnf). Therefore,
Qnf , hnf
(17)
Combining Eqs. (16) & (17), the ratio of heat absorbed by nanofluids to heat absorbed by nanoparticle-free fluid (base fluid) may be calculated as:
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Qnf , Qnf , 0
The
C 2 p , nf nf k nf nf
0.25
k nf C p , nf , 0 nf2 , 0 nf ,0 k nf ,0
0.25 k nf ,0
(18)
Qnf ,1% Qnf ,0.5% and ratios calculated using Eq. (18) were found to be 1.096 and 1.191. Qnf ,0 Qnf ,0
These ratios were closer to the ones determined from heat transfer experiments (1.10 and 1.201 for 0.5 vol % and 1 vol % ceria – propylene glycol nanofluids respectively) in accordance with Eq. (15). Hence it may be concluded that the improvement in thermophysical properties of ceria – propylene glycol nanofluids were reflected in their heat absorption performance under natural convective conditions.
4. Conclusions Ceria – propylene glycol nanofluid with high colloidal stability was prepared by dispersing surfactant-free ceria nanoparticles of 18-25 nm diameter in propylene glycol using probe ultrasonication for 14 hours. The thermal conductivity of 1 vol % ceria – propylene glycol nanofluid was higher than that of propylene glycol by 10.7 % and 18.8 % at temperatures of 27 °C and 80 °C respectively. Brownian motion-induced microconvection and particle clustering were found to be major contributors for thermal conductivity enhancement of ceria – propylene glycol nanofluid. The viscosities of 1 vol % and 0.5 vol % ceria – propylene glycol nanofluids were lower than that of pure propylene glycol at temperatures below 80 °C, due to ceria nanoparticles-induced perturbations in intermolecular hydrogen bonding of propylene glycol. The improved thermophysical properties (higher volumetric specific heat, higher thermal conductivity and lower viscosity) of 1 vol % ceria – propylene glycol nanofluid resulted in ~ 20 % improvement in quantity of heat absorption under transient, natural-convective constant wall heat flux conditions. 14
Acknowledgements This work was supported by (i) PG teaching grant No: SR/NM/PG-16/2007 of Nano Mission Council, Department of Science & Technology (DST), India (ii) Grant No: SR/FT/ET061/2008, DST, India (iii) INSPIRE fellowship (Reg Nos: IF110312, IF130529) of Department of Science and Technology (DST), India and (iv) Research & Modernization Project #1, SASTRA University, India.
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Research Highlights
Stable ceria – propylene glycol nanofluids prepared by probe ultrasonication
These nanofluids possess higher thermal conductivity & volumetric specific heat
Particle clustering & Brownian motion contribute to thermal conductivity increase
1 vol % ceria-PG viscosity lower than PG viscosity at temperatures <80 °C
Heat absorption by nanofluids increase with nanoparticle concentration
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