CHAPTER 6: THERMO-PHYSICAL PROPERTIES OF ZnO-ETHYLENE GLYCOL AND ZnO-ETHYLENE GLYCOL WATER NANOFLUIDS

Transcription

1 220 CHAPTER 6: THERMO-PHYSICAL PROPERTIES OF ZnO-ETHYLENE GLYCOL AND ZnO-ETHYLENE GLYCOL WATER NANOFLUIDS 6.1 INTRODUCTION It is challenging to achieve higher thermal conductivity and lower viscosity simultaneously in nanofluids. Introduction of solid nanoparticles in liquids eventually increases thermal conductivity [1 3] as well as viscosity and hence higher pumping power [1,4]. However, with the demonstration of the preparation of ZnO-propylene glycol (PG) nanofluids with lower viscosity and higher thermal conductivity than pure propylene glycol [5 7], this seems to have been overcome. Hence, it may be possible to prepare nanofluids of lower viscosity and higher thermal conductivity with ethylene glycol as base fluid also, since ethylene glycol (EG) is chemically similar to propylene glycol having hydrogen bonding networks. The properties of ethylene glycol based nanofluids have been widely studied [8 16] due to their use as coolants in automobiles. Sand-ethylene glycol-water dispersions prepared using stirred bead milling and ultrasonication showed thermal conductivity enhancement of above 20 % at a particle concentration of ~1.8 vol % [9]. Single-walled CNT inclusions were dispersed in ethylene glycol using bile salt as dispersant with 14.8 % enhancement in thermal conductivity at 0.2 vol % nanotube loading [13]. Thermal conductivity of Al2O3-ethylene glycol nanofluids have been studied over a wide temperature range ( K) using a liquid metal transient hot wire apparatus [8]. Viscosities of CuO-EG-water mixture nanofluids have been studied over a temperature range of -35 to 50 C [12]. The dispersions showed Newtonian flow behavior over the temperature range investigated [12].

2 221 ZnO-ethylene glycol nanofluids containing 5 vol % of nanoparticles, prepared by 3-hour ultrasonication yielded thermal conductivity enhancement of 26.5 % [10]. These nanofluids were non-newtonian (shear-thinning) at higher concentrations and Newtonian at lower concentrations [10]. Kole and Dey [11] prepared ZnO-EG nanofluids by extended ultrasonication and the optimum ultrasonication time was found to be 60 h. Approximately 40 % enhancement in thermal conductivity of ZnO-EG nanofluids was reported for particle volume concentration of 3.75 vol % [11]. Understanding the mechanism of interaction between nanomaterials and base fluid molecules could lead to nanofluid formulation methods, which result in well dispersed nanofluids. One such formulation method (sequential methid) has been reported (Chapter 5) which resulted in nanofluids with improved transport properties. Surfactant-free ZnO- Propylene glycol nanofluids prepared using ultrasonication had higher thermal conductivity [17] and lower viscosity compared to those of propylene glycol [5]. Formation of propylene glycol molecular layers over ZnO nanoparticles in ZnOpropylene glycol nanofluids was found to be responsible for higher thermal conductivity, while the disturbance to hydrogen bonding network of propylene glycol contributed to lowering of their viscosity in comparison to that of pure propylene glycol. Thus, a coolant with higher thermal conductivity and lower viscosity was prepared and the underlying mechanisms were understood [5,17]. With these findings, a new method of formulation of ZnO-PG-water nanofluids was developed in which, water was commixed with ZnO- PG dispersion instead of dispersing ZnO nanoparticles in PG-water mixture using the conventional method. This method aided in preservation of propylene glycol molecular layers over ZnO nanoparticles and foreclosed the interaction of ZnO nanoparticles and

3 222 water molecules. The proposed method allowed preparation of ZnO-PG-water nanofluids without any surfactants and with better transport properties [18]. However, viscosity reduction of ethylene glycol based nanofluids has not been evidenced in literature thus far. This chapter discusses the preparation, characterization of thermo-physical properties of ZnO- ethylene glycol (EG) and ZnO-EG-water nanofluids. ZnO-EG nanofluids were prepared using ultrasonication without using any surfactant. ZnO-EG-water nanofluids were prepared using sequential method as proposed in Chapter 5. Transport properties of ZnO-EG and ZnO-EG-water nanofluids were studied as a function of temperature and nanoparticle volume concentration. In addition, this chapter discusses the models that have been developed to predict the viscosity and thermal conductivity of ZnO-EG nanofluids and ZnO-EG-water nanofluids. 6.2 MATERIALS AND METHODS Materials Ethylene glycol was procured from Merck, India and was used as procured without any purification Synthesis of ZnO nanoparticles ZnO nanoparticles were synthesized using chemical precipitation method using Zinc nitrate hexahydrate as precursor at room temperature as described in Chapter Formulation of nanofluids ZnO-EG nanofluid of concentration 4 vol % was prepared by dispersing the synthesized ZnO nanoparticles in ethylene glycol by ultrasonication (Vibracell TM, Sonics, USA). Ultrasonication (130 W, 20 khz) was carried out until ZnO-EG dispersion attained

4 223 maximum thermal conductivity and minimum viscosity. Viscosity and thermal conductivity of ZnO-EG dispersions were measured at regular intervals of time during ultrasonication. ZnO-EG dispersions of different volume fractions were prepared by diluting 4 vol % ZnO-EG nanofluids with required volume of ethylene glycol. A hierarchical method (sequential method) demonstrated in Chapter 5 [18] has been utilized for the preparation of 2 vol % ZnO-EG-water nanofluids by mixing water with 4 vol % of ZnO-EG in equal volumes. The prepared nanofluids thus had a base fluid composition of 50 vol % water and 50 vol % ethylene glycol. Different concentrations of ZnO-EG-water nanofluids were prepared by further diluting 2 vol % ZnO-EG-water nanofluids with vol % ethylene glycol-water mixture Characterization of nanofluids Transport properties like viscosity and thermal conductivity were studied for the ZnO-EG and ZnO-EG-water nanofluids as a function of temperature and nanoparticle volume fraction. Thermal conductivity of nanofluids was measured using transient-hot wire method (Decagon devices, USA). KS-1 probe has been used for measurements. Temperature of the sample was maintained using a circulating water bath (TC-502, Brookfield Engineering, USA). Viscosities of nanofluids were measured using a rotational viscometer (LVDV-II + Pro, Brookfield Engineering, USA). S18 and S64 spindles were used for ZnO-EG at higher (27 to 140 C) and lower temperatures (10-20 C) respectively. S00 spindle was used for ZnO-EG-water nanofluids over the entire temperature range investigated. During the viscosity measurements, temperature of the ZnO-EG nanofluids and ZnO-EG-water nanofluids were maintained using a temperature controller (Thermosel, Brookfield

5 224 Engineering, USA) and constant temperature bath (TC-502, Brookfield Engineering, USA). Hydrodynamic particle size distribution of ZnO-EG dispersions was measured using dynamic light scattering technique (NanoZS, Malvern instruments, UK) as a function of ultrasonication time. Thermal conductivity, viscosity and hydrodynamic size distribution measurements were repeated atleast three times to ascertain the repeatability and reproducibility of measurements. Uncertainties in viscosity, thermal conductivity and average hydrodynamic size are taken as standard errors of their respective repeated measurements. Uncertainties in relative viscosity and thermal conductivity ratio were calculated using the following formula [19] U f N j1 U x j f x j (6.1) Figure 6.1. Schematic representation of procedures of preparation and study of thermophysical properties of and ZnO-EG and ZnO-EG-water nanofluids.

6 225 Figure 6.1 shows the schematic representation of the study of formulation and thermophysical properties of ZnO-PG-water nanofluids. 6.3 RESULTS AND DISCUSSION Nanofluids with high dispersion quality are known to have superior transport properties [15,20,21]. The advantages of well dispersed nanofluids to be used as coolants are (i) higher surface area for heat transfer (ii) lower viscosity and lesser pumping power (iii) good colloidal stability due to the smaller size of aggregates [2]. Ultrasonication plays a critical role in the formulation of nanofluids by assisting in breakage of aggregates resulting in high quality dispersions. However, there exists an optimum ultrasonication time in the preparation of nanofluids [5 7,20,21]. In case of extending ultrasonication beyond the optimum ultrasonication time, re-aggregation may occur leading to formation of larger aggregates resulting in instability of the nanofluids [22]. Other than the physical and chemical properties of the dispersed phase of nanofluids, dispersion characteristics have significant influence on thermal conductivity, viscosity and stability of nanofluids. Hence, it is pertinent to determine optimum ultrasonication energy (or) time required to prepare nanofluids with superior dispersion characteristics, evidenced in terms of increased thermal conductivity and reduced viscosity. Figure 6.2(a) shows the influence of ultrasonication time on thermal conductivity and viscosity of 4 vol % ZnO-EG nanofluid. Thermal conductivity of ZnO-EG nanofluid (4 vol %) gradually increased with increasing ultrasonication time and saturated at about 30 h. Also, viscosity of the dispersion decreased with increasing sonication time.

7 Figure 6.2. Influence of ultrasonication time on (a) thermal conductivity and viscosity of 4 vol.% ZnO EG nanofluids at 27 C and (b) average hydrodynamic diameter of ZnOnanoparticles in ZnO EG nanofluids. 226

8 227 Figure 6.2(b) shows the influence of ultrasonication time on the average hydrodynamic diameter of the ZnO-EG dispersion, from which it is evident that the average hydrodynamic diameter decreased with ultrasonication time. As ultrasonication time increased, larger aggregates of ZnO nanoparticles had been broken up into smaller aggregates (Figure 6.2(b)). Formation of smaller aggregates resulted in increase in the fraction of heat conducting paths and decrease in the fraction of continuous phase with relatively higher thermal resistance. In other words, size reduction of aggregates or dispersion of nanoparticles resulted in increased surface area which resulted in increase in thermal conductivity. As ZnO-EG dispersion was ultrasonicated, larger aggregates were broken up progressively and hence the persistent increase in thermal conductivity. At about ~30 h, saturation in thermal conductivity increase was observed indicating that nanoparticles/clusters were well dispersed. Aggregate sizes in solid-liquid dispersions have profound influence on their viscosity. Greater the ratio of aggregate size to primary particle size, higher is the resistance to fluid flow [22,23] and hence higher viscosity. As ultrasonic processing progressed, viscosity decreased (Figure 6.2(a)) probably due to the reduction in the size of the aggregates Viscosity of dispersions Influence of nanoparticle concentration on viscosity ZnO-EG dispersions Viscosities of ZnO-EG nanofluids were independent of shear rate over a range of s -1 (Figure 6.3(a)) for the concentration range investigated ( vol %). This reveals that dispersion of ZnO nanoparticles did not alter the flow behavior significantly and nanofluids remained to be Newtonian.

9 Figure 6.3. (a) Influence of shear rate on viscosity of ZnO EG nanofluids of different concentrations and ethylene glycol and (b) influence of nanoparticle volume concentration on relative viscosity of ZnO EG nanofluids. 228

10 229 Relative viscosity (r=nf/b)-zno nanoparticle volume concentration relationship of ZnO-EG nanofluids (Figure 6.3(b)) shows that relative viscosity of ZnO-EG dispersions (averaged over shear rates in the range s -1 ) decreased with increasing nanoparticle loading. Viscosities and relative viscosities of ZnO-EG nanofluids showed a contradictive behavior with those reported for other nanofluid systems as well as ZnO- EG nanofluids in literature. Gallego et al. [24] reported increasing viscosity of ZnO-EG nanofluids with increasing nanoparticle concentration. However, those nanofluids were prepared by ultrasonication for a time period of 16 min only, which was much lower compared to the ultrasonication time used in this study. Yu et al. [10] dispersed ZnO nanoparticles of size nm in ethylene glycol using ultrasonic processing for 3 hour. ZnO-EG dispersions thus prepared had higher viscosities (~100 % increase) than base fluid and showed shear thinning characteristics at particle concentrations 3 vol %. Moosavi et al. [25] observed ~ 26 % enhancement in viscosity of ZnO-EG nanofluids for a very low particle concentration of 0.6 vol %, while using ammonium citrate as dispersant. From the nanofluid viscosity-zno nanoparticle concentration data of the above [10,24] and the shear-thinning nature of nanofluids in the work of Mossavi et al. [25], it appears that those nanoparticles exhibited high degree of aggregation. ZnO-EG nanofluids prepared by Kole and Dey [11] were ultrasonicated for an optimum ultrasonication time of 60 h and viscosities of ZnO-EG nanofluids ( 3.5 vol %) were very close to that of base fluid. This was attributed to the well-dispersed nature of ZnO nanoparticles in ethylene glycol. Hence the disparity observed in the viscosities of ZnO- EG nanofluids might be due to the differences in nanofluid formulation method

11 230 (ultrasonication time/use of surfactant), morphological characteristics of ZnO nanopowder used, etc. which ultimately resulted in different aggregate size distributions. Intermolecular as well as intramolecular hydrogen bonding persists in ethylene glycol. Metal oxide nanoparticles are known to have hydroxyl groups on their surface when dispersed in polar liquids [26]. Hydroxyl groups engaged in intramolecular and intermolecular hydrogen bonding between ethylene glycol molecules are likely to form hydrogen bonds with hydroxyl groups on nanoparticles surface. Hence, the hydrogen bonding network of ethylene glycol is reorganized. At higher nanoparticle concentration, the number of ZnO nanoparticles interacting with ethylene glycol molecules was higher and hence, perturbations to the hydrogen bonding network of ethylene glycol were enhanced leading to decrease in viscosity with increasing nanoparticle concentration. For ZnO-PG nanofluids (as discussed in chapter 4) and as well as for other metal oxidepropylene glycol based nanofluids [5 7,27], the rheological behavior was similar to that of ZnO-EG nanofluids. CuO-PG, ZnO-PG, Fe2O3-PG, sand-pg, Mn0.43Fe2.57O4-PG nanofluids had viscosities lesser than propylene glycol and perturbations in hydrogen bonding network was identified as the rationale behind the unusual rheological behavior of propylene glycol based nanofluids ZnO-EG-water nanofluids The influence of nanoparticle concentration on relative viscosity of ZnO-EG-water nanofluids (Figure 6.4) show decreased viscosities at higher particle concentration. It may be recalled that the ZnO-EG-water nanofluids were prepared by addition of water to

12 231 ZnO-ethylene glycol dispersion, which had lower viscosity than ethylene glycol due to disturbance in hydrogen bonding network. By adding water to ZnO-EG dispersion, viscosity reduction has been maintained by preserving ethylene glycol molecular layers over ZnO nanoparticles and thus avoiding the direct contact of ZnO nanoparticles and water molecules. Figure 6.4. Influence of nanoparticle volume concentration on relative viscosity of ZnO EG water nanofluids.

13 232 It is known that the direct contact of surfactant-free ZnO nanoparticles with water molecules promote their aggregation [18,22,28] and increase dispersion viscosity. Hence, through prevention of direct contact between surfactant-free ZnO nanoparticles & water and preservation of ZnO nanoparticles-ethylene glycol interactions, % reduction in viscosity has been obtained for 2 vol % ZnO-EG-water nanofluid. The percentage reduction in viscosity of 2 vol % ZnO-EG-water nanofluids (17 %) is comparable to that of 2 vol % ZnO-EG nanofluid (~20 %) Influence of temperature on viscosity ZnO-EG nanofluids As coolants, nanofluids will be subjected to temperature variations. Hence, it becomes pertinent to investigate rheological behavior of nanofluids under thermal loads. Influence of temperature on viscosity of ZnO-EG nanofluids has been studied over a wide temperature range of C (Figure 6.5(a) & Figure 6.5 (b)). Viscosity of ZnO-EG nanofluids decreased in an asymptotic manner with increasing temperature similar to that of ethylene glycol, the base fluid (Figure 6.5(a)). Viscosity variation of nanofluids and the base fluid with temperature could be fitted into power law as follows AT B (6.2) Value of B signifies the temperature dependency of viscosity of the fluids. B value decreased with increasing concentration with pure ethylene glycol having the highest B value. This shows that the addition of nanoparticles had reduced the temperature dependency of ZnO-EG nanofluids.

14 Figure 6.5. Influence of temperature on viscosity of ZnO EG nanofluids and ethylene glycol (a) C and (b) C. 233

15 234 The decrease in viscosity of liquids with increasing temperatures is due to decrease in the magnitude of intermolecular attractive forces such as hydrogen bonds [29,30]. With reduction in intermolecular forces of attraction at higher temperatures, their influence on viscosity is also reduced. Hence at higher temperatures, the perturbation of hydrogen bonds does not proportionately lead to reduction in nanofluid viscosity, while the contribution of nanoparticles to increased viscous dissipation is temperature-independent. Hence, viscosity of ZnO-EG nanofluid shows lower temperature dependence than pure ethylene glycol, with lowest temperature dependence observed at the highest nanoparticle concentration (4 vol %) investigated. Figure 6.6. Influence of temperature on relative viscosity of ZnO EG nanofluids at different concentrations.

16 235 In order to further evaluate the effect of addition of nanoparticles on nanofluid viscosity, the relative viscosity of nanofluids was calculated as follows: Bnf Anf T nanofluid r (6.3) Bbf base fluid Abf T Figure 6.6 shows the relationship between relative viscosity and temperature at different nanoparticle concentrations. From Figure 6.6, it can be observed that relative viscosity increases with increasing temperature, which implies that decrease in viscosity of nanofluids with temperature is lower than that of base fluid. Highest reduction in viscosity of ZnO-EG nanofluids has been observed at the lowest temperature investigated. For instance, reduction in viscosity of 4 and 2 vol % ZnO-EG nanofluids at 10 C are 66 % and 45 % respectively. At lower temperatures, movement of liquid molecules is dominated by intermolecular forces, which have higher influence over liquid viscosities at lower temperatures. At higher temperatures, intermolecular forces between molecules begin to diminish and the movement of liquid molecules is controlled by their translational energy. Since the addition of ZnO nanoparticles brings out reduction of dispersion viscosity by disturbing hydrogen bonding network, viscosity reduction is more pronounced at lower temperatures at which liquid viscosities depend on intermolecular forces than that at higher temperatures at which influence of intermolecular forces on liquid viscosity is negligible. Another striking observation from viscosity measurements is that relative viscosities of ZnO-EG nanofluids were lower than unity up to temperature of 110 C. This shows that pumping power required to circulate ZnO-EG nanofluids will be lower than that required for pure ethylene glycol up to 110 C. As long as the temperature is below 110 C,

17 236 relative viscosity of 4 vol % nanofluid is lower than that of 2 vol % whereas at 140 C, the relative viscosity of 4 vol % is higher than that of 2 vol %. To get better understanding, variation of relative viscosity with nanoparticle concentration at different temperatures has been plotted in Figure 6.7. The slope of the curve gradually changes from negative to positive as the temperature is increased from 10 to 140 C. Figure 6.7. Influence of nanoparticle volume concentration on relative viscosities of ZnO EG nanofluids at different temperatures.

18 Viscosity model A new empirical model was developed to predict the viscosity of ZnO-EG nanofluids over a concentration and temperature range of 0-4 vol % and C respectively. This model takes into account of increase in viscosity due to particle addition, decrease in viscosity due to disturbance in intermolecular forces due to nanoparticle addition and the effect of temperature. Relative viscosity of ZnO-EG nanofluids can be expressed as follows: B1 A T (6.4) r nf b 1 A1 & B1 being functions of nanoparticle concentration, the expressions for A1 & B1 were obtained upon regression analysis of r-t-nanoparticle volume concentration data are as follows: 2 3 A (6.5) B 10. (6.6) Therefore, (6.4) becomes r T (6.7) Eq. (6.7) predicts the relative viscosities of ZnO-EG nanofluids over a concentration and temperature range of 0-2 vol % and C respectively. The model fits about 39 experimental data points with mean & maximum error of 4.66 % & 7.63 % and relative standard deviation of 6.13 %.

19 ZnO-EG-water nanofluids Figure 6.8(a) shows the influence of temperature on viscosity of ZnO-EG-water nanofluids and EG-water (base fluid) over a temperature range of C. Viscosity of ZnO-EG-water nanofluids decreased with increasing temperature exponentially and could be fitted to power law as ZnO-EG nanofluids (Eq. (6.2)). B values (from Eq. (6.2)) of EG-water, 1 vol % ZnO-EG-water and 2 vol % ZnO-EGwater were found to be , and respectively. Similar to ZnO-EG nanofluids, B values decreased with increasing nanoparticle concentration. Value of B in Eq. (6.2) is an indication of dependency of viscosity of the fluids on temperature. Viscosity of ethylene glycol-water mixture (base fluid) showed higher temperature dependency compared to nanofluids, which might be due to the higher magnitude of intermolecular forces that existed in the base fluid. Relative viscosities of ZnO-EG-water nanofluids have been calculated using Eq. (6.3) and have been plotted against temperature (Figure 6.8(b)). Relative viscosities were less than one over the entire temperature range investigated (10-55 C). Relative viscosity of 2 vol % nanofluid was lower than that of 1 vol % nanofluids at all temperatures.

20 Figure 6.8. (a) Influence of temperature on viscosity of ZnO EG-water nanofluids and ethylene glycol and (b) influence of temperature on relative viscosity of ZnO EG-water nanofluids at different concentrations 239

21 Viscosity model Relative viscosity of ZnO-EG-water nanofluids can be expressed as follows: B1 A T (6.8) r nf b 1 A1 & B1 being functions of nanoparticle concentration, the expressions for A1 & B1 were obtained upon regression analysis of r-t-nanoparticle volume concentration data are as follows: 2 A (6.9) B 2. (6.10) 1 18 Therefore (6.8) becomes r T (6.11) Eq. (6.11) predicts the relative viscosities of ZnO-EG-water nanofluids over a concentration and temperature range of 0-2 vol % and C respectively. The model fits about 20 experimental data points with mean & maximum error of 9.46 E-04 % & 1.6 E-03 % and relative standard deviation of 1.2 E-03 % Thermal conductivity of dispersions ZnO-EG nanofluids Influence of nanoparticle volume concentration Figure 6.9(a) shows the variation of thermal conductivity of ZnO-EG nanofluids with nanoparticle volume concentration at 27 C.

22 Figure 6.9. (a) Influence of nanoparticle volume concentration on thermal conductivity of ZnO EG nanofluids at 27 C and (b) Influence of nanoparticle volume concentration on thermal conductivity ratio of ZnO EG nanofluids and comparison with literature. 241

23 242 Linear variation of thermal conductivity with nanoparticle concentration depicts the well dispersed nature of nanoparticles or nanoclusters in ZnO-EG nanofluids. About 33.4 % increase in thermal conductivity has been observed for 4 vol % ZnO-EG nanofluids. Thermal conductivity ratios of ZnO-EG nanofluids prepared in this study had relatively higher thermal conductivity ratios than that of other researchers [10,24,25] except that of Kole and Dey [11] (Figure 6.9(b)). The differences in thermal conductivity of ZnO-EG nanofluids can be attributed to the difference in morphology of the nanomaterial used and ultrasonic processing. ZnO-EG nanofluids prepared by Kole and Dey [11] being sonicated for prolonged time (>60 h) had smaller sizes of aggregates (<100 nm) resulting in higher thermal conductivity than that of the present study (sonication time 30 h). From the experimental thermal conductivity data, thermal conductivity ratio-nanoparticle volume concentration relationship (at 27 C) can be expressed as k r (6.12) Hamilton-Crosser model of thermal conductivity of dispersions for spherical particles is as follows: k r 1 3 (6.13) Comparing Eqs. (6.12) & (6.13), it can be observed that slope of the Eq. (6.12) is about 2.64 times greater than that of Hamilton-Crosser equation (Eq. (6.13)). Anomalous enhancement in thermal conductivity of nanofluids is due to nanoscale phenomena occurring in the colloidal solid-liquid dispersions. Microconvection induced by Brownian motion of nanoparticles, ordered arrangement of liquid molecules over solid

24 243 nanoparticles (liquid layering), particle clustering and ballistic heat transport [31] have been identified as the major mechanisms responsible for anomalous enhancement of thermal conductivity. Contribution of each of these mechanisms to thermal conductivity enhancement of nanofluids depend on hydrodynamic size distribution of solid particles, physical and chemical nature of base fluid, temperature, interaction between dispersed and continuous phase, etc. Hence, contribution of each of the nanoscale mechanisms to thermal conductivity enhancement of nanofluids differs for each nanofluid system. Liquid layering of propylene glycol molecules over ZnO nanoparticles was found to influence thermal conductivity enhancement in ZnO-PG nanofluids and its unique temperature dependence [17]. As propylene glycol and ethylene glycol have similarity in chemical nature, the course of thermal conductivity change with change in temperature can be explained by the liquid layering of ethylene glycol molecules on ZnO nanoparticles surface Influence of temperature Figure 6.10 shows the influence of temperature on thermal conductivity of ZnO-EG nanofluids and ethylene glycol, the base fluid. Highest thermal conductivity enhancement was achieved at the lowest temperature investigated, 10 C (Figure 6.10). The trend of variation of thermal conductivity of ZnO-EG nanofluids with temperature could be divided into two sections, (i) a temperature range (10-30 C), within which nanofluid thermal conductivity decreased with temperature and (ii) a temperature range (30-60 C), within which nanofluid thermal conductivity was temperature-independent.

25 244 Figure Influence of temperature on thermal conductivity of ZnO EG nanofluids. Ordered arrangement of liquid molecules over solid surfaces have higher thermal conductivity compared to bulk liquid, since the ordered liquid molecular structure mimics the atomic arrangement in crystalline solids with high thermal conductivity. The thickness of such ordered layers greatly influences thermal conductivity enhancement of nanofluids and have been shown to increase with decreasing temperature [17]. The thickness of ethylene glycol molecular layer increased with decreasing temperature and the maximum thermal conductivity enhancement was observed at the lowest temperature (10 C). It can also be observed from Figure 6.10 that the magnitude of the slope of the curve (10-30 C) increases with increasing concentration. Thermal conductivity increase

26 245 at 10 C for 4 vol %, 2 vol % and 1 vol % are 55%, 36 % and 14 % respectively. The volume fraction of liquid layers contributing to thermal conductivity enhancement is higher at higher concentrations owing to the presence of larger number of nanoparticles & accompanying liquid layers. In Chapter 4, a simplified form of Yu and Choi s model given below, was used to predict thermal conductivity enhancement of ZnO-PG nanofluids at lower temperatures. knf k r (6.14) k bf In the above equation, is the volume fraction, is the ratio of thickness of liquid layer to the radius of nanoparticle and knf and kbf are the thermal conductivity of nanofluid and thermal conductivity of base fluid respectively. Since temperature dependency of thermal conductivity of ZnO-EG nanofluids followed the same trend as ZnO-PG nanofluids, simplified Yu and Choi s model [17] was used to predict thermal conductivity enhancement of ZnO-EG nanofluids. By fitting the experimental thermal conductivity data of ZnO-EG nanofluids in Eq. (6.14), values at 10, 20, 27, 30 C were calculated to be , , 0.407, and respectively. Decreasing values with increasing temperature showed that the thickness of liquid layers surrounding nanoparticles decreased with increasing temperature. Substituting the temperature-dependent values in Eq. (6.14), the experimental data for thermal conductivity ratio for about 20 points were predicted with mean & maximum error of 0.8 % & 2.7 % and RSD of 0.97 %. Eq. (6.14) encompassed temperature range of C and nanoparticle concentration range of 0-4 vol %.

27 246 Other than liquid layering, Brownian motion of nanoparticles and particle clustering are two important mechanisms responsible for thermal conductivity enhancement in nanofluids [31]. Increase in thermal conductivity with increasing temperature is attributed to the Brownian motion of particles due to their higher energy at higher temperatures and micro-convection induced by Brownian motion of particles [32 35]. No significant difference in thermal conductivity was observed over the temperature range of C. Since viscosity of the dispersion is relatively low in this temperature range, an increase in thermal conductivity due to Brownian motion of particles is expected. However, no such increase in thermal conductivity was observed. Hence, we postulate that the increase in thermal conductivity due to Brownian motion of particles might have been offset by the decrease in thermal conductivity due to decrease in thickness of liquid layers in accordance with decreasing at higher temperatures ZnO-EG-water nanofluids Influence of nanoparticle volume concentration Figure 6.11 shows the influence of nanoparticle concentration on thermal conductivity ratio of ZnO-EG-water nanofluids. The thermal conductivity was found to increase with nanoparticle concentration in a linear fashion, which indicates absence of significant particle-particle interactions. The solvation layers of ethylene glycol on ZnO nanoparticles surface has been preserved in ZnO-EG-water nanofluids also, as evidenced by negligible particle-particle interaction and colloidal stability. If water molecules are in contact with surfactant-free ZnO nanoparticles, agglomeration of nanoparticles, instability of the dispersion and deterioration of transport properties become inevitable.

28 247 However, they have been eliminated by the use of simple hierarchical method of preparation of ZnO-EG-water nanofluids. Figure Influence of nanoparticle volume concentration on thermal conductivity of ZnO EG water nanofluids at 27 C. Thermal conductivity ratio-nanoparticle volume concentration relationship of ZnO-EGwater nanofluids at 27 C is as follows: k r (6.15)

29 248 From Eqs. (6.12) & (6.15), it is evident that the thermal conductivity enhancements of ZnO-EG and ZnO-EG-water nanofluids are comparable. This might be yet another testimony to the preservation of layers of ethylene glycol over ZnO nanoparticles. Figure Influence of temperature on thermal conductivity of 2 vol.% ZnO EG water nanofluids. The influence of temperature on thermal conductivity of ZnO-EG-water nanofluids (2 vol %) is shown in Figure Temperature-independent behavior of ZnO-EG-water nanofluids can be attributed to the combined influence of liquid layering and Brownian

30 249 motion. As temperature increases, thickness of liquid layers decreases and hence thermal conductivity decreases. This is offset by Brownian motion due to lower dispersion viscosity. Hence, these two phenomena counterbalanced each other resulting in temperature-independent thermal conductivity of ZnO-EG-water nanofluids Comparison with literature data ZnO-EG nanofluid Table 6.1 shows the comparison of thermal conductivity ratio and relative viscosity of ZnO-EG nanofluid (present study) with other metal oxide-eg nanofluids. The thermal conductivity ratio of ZnO-EG nanofluid is comparable (or) higher than that reported in literature (Table 6.1). However, viscosity of metal oxide-eg nanofluids reported in literature were greater than that of the base fluid (r>1). The difference in the viscosity and thermal conductivity data can be attributed to the difference in the formulation methods. The viscosity of ZnO-EG nanofluids prepared by Kole and Dey [36] was closer to that of base fluid due to prolonged ultrasonication and the well dispersed nature of the nanofluid. Table 6.1. Comparison of thermal conductivity ratio and relative viscosity of ZnO-EG nanofluid (present study) with other metal oxide-eg nanofluids. Nanofluid system Al2O3-EG [37] Nanoparticle concentration Thermal conductivity ratio % enhancement in thermal conductivity per unit volume of nanoparticle concentration in case of linearity of k- l i hi Relative viscosity 3.99 vol %

31 250 Al2O3-EG [38] Al2O3-EG [39] Al2O3-EG [40] Al2O3-EG [8] Al2O3-EG [41] CeO2-EG [42] Co3O4-EG [43] Co3O4-EG [43] CuO-EG [44] CuO-EG [38] CuO-EG [45] Mg(OH)2- EG [46] MgO-EG [40] SiO2-EG [40] TiO2-EG [15] TiO2-EG [40] TiO2-EG [41] 4.3 vol % vol % vol % # vol % vol % 1.27 ** vol % vol % vol % vol % vol% vol % 1.06 ** - 2 vol % vol % vol % # vol % vol % # vol % 1.16 ** 1.30 TiO2-EG 1.8 vol %

32 251 [47] ZnO-EG [40] ZnO-EG [36] ZnO-EG [10] ZnO-EG [24] ZnO-EG [48] ZnO-EG [49] ZnO EG (present study) 5 vol % # vol % ~1 5 vol % vol % wt.% vol % vol % % enhancement in thermal conductivity per unit volume of nanoparticle concentration in case of linearity of k- relationship could not be estimated due to **-non-linear k- relationship #-thermal conductivity data available for a single nanoparticle concentration alone ZnO-EG-water nanofluid Table 6.2 shows the comparison of thermal conductivity ratio and relative viscosity of ZnO-EG-water nanofluid (present study) with other metal oxide-eg-water nanofluids. The thermal conductivity ratio of ZnO-EG-water nanofluid is comparable (or) higher than that reported in literature (Table 6.2). The thermal conductivity and viscosity of ZnO-EG-water nanofluids were found to be superior to other metal oxide-eg-water systems reported in literature, which can be attributed to the favorable interactions between ZnO nanoparticles and ethylene glycol and preservation of ethylene glycol

33 252 layers over ZnO nanoparticles surface achieved through sequential method of nanofluid formulation. Table 6.2. Comparison of thermal conductivity ratio and relative viscosity of ZnO-EG nanofluid (present study) with other metal oxide-eg nanofluids. Nanofluid system Al2O3-EG-water (40:60) [50] Al2O3-EG-water (50:50) [39] Al2O3-EG-water (60:40) [50] Al2O3-EG-water (50 % w/w) (d=10 nm) [37] Al2O3-EG-water (50 % w/w) (d=50 nm) [37] Al2O3-EG-water (50 % v/v) [51] Al2O3-EG-water (45 % v/v) [52] CuO-EG-water (60 % w/w) [12] Fe3O4-EG-water (40:60) [53] Fe3O4-EG-water (60:40) [53] Nanoparticle concentration Thermal conductivity ratio % enhancement in thermal conductivity in case of linearity of k- relationship Relative viscosity 1.5 vol % vol % vol % vol % vol % vol % ** vol % vol % vol % vol %

34 253 Mn0.43Fe2.57O4- EG-water [54] Sand-EG-water [9] TiO2-EG-water (40:60) [55] TiO2-EG-water (50:50) [55] ZnO-EG-water (present study) 1 vol % vol % vol % vol % vol % **-% enhancement in thermal conductivity per unit volume of nanoparticle concentration in case of linearity of k- relationship could not be estimated due to non-linear k- relationship 6.4 CONCLUSIONS Spherical ZnO nanoparticles with a size range of nm were dispersed in ethylene glycol by probe ultrasonication without any surfactant. ZnO-EG-water nanofluids were prepared by hierarchical formulation strategy (sequential method), which preserved ethylene glycol molecular layers over ZnO nanoparticles surface. The influence of nanoparticle concentration and temperature on thermal conductivity and viscosity of ZnO-EG and ZnO-EG-water nanofluids revealed increased thermal conductivity and reduced viscosity. ZnO-EG and ZnO-EG-water nanofluids showed 33.4 % and % enhancements in thermal conductivity and 39.2 % and % reduction in viscosity at particle volume concentrations of 4 and 2 vol % respectively. Thickness of ethylene glycol layers of ZnO nanoparticles increased at lower temperatures resulting in higher thermal conductivity under such conditions. Liquid layering of molecules contributed to thermal conductivity enhancements in ZnO-EG nanofluids, while liquid layering as well

35 254 as Brownian motion to temperature independent thermal conductivity of ZnO-EG-water nanofluids. Empirical models developed predicted the transport properties of the nanofluids well. 6.5 REFERENCES 1. Das SK, Choi SUS, Patel HE. Heat Transfer in Nanofluids A Review. Heat Transf Eng 2006;27: Wang X, Mujumdar AS. Heat transfer characteristics of nanofluids: a review. Int J Therm Sci 2007;46: Ozerinc S, Kakac S, Yazıcıoglu AG. Enhanced thermal conductivity of nanofluids: a state-of-the-art review. Microfluid Nanofluidics 2010;8: Mahbubul IM, Saidur R, Amalina MA. Latest developments on the viscosity of nanofluids. Int J Heat Mass Transf 2012;55: Suganthi KS, Anusha N, Rajan KS. Low viscous ZnO propylene glycol nanofluid: a potential coolant candidate. J Nanoparticle Res 2013;15: Aishwarya V, Suganthi KS, Rajan KS. Transport properties of nano manganese ferrite propylene glycol dispersion (nanofluids): new observations and discussion. J Nanoparticle Res 2013;15: Suganthi KS, Radhakrishnan AK, Anusha N, Rajan KS. Influence of Nanoparticle Concentration on Thermo-Physical Properties of CuO-Propylene Glycol Nanofluids. J Nanosci Nanotechnol 2014;14: Beck MP, Sun T, Teja AS. The thermal conductivity of alumina nanoparticles dispersed in ethylene glycol. Fluid Phase Equilib 2007;260: Manikandan S, Karthikeyan N, Silambarasan M, Suganthi KS, Rajan KS. Preparation and Characterization of sub-micron dispersions of sand in ethylene glycol-water mixture. Brazilian J Chem Eng 2012;29: Yu W, Xie H, Chen L, Li Y. Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid. Thermochim Acta 2009;491: Kole M, Dey TK. Effect of prolonged ultrasonication on the thermal conductivity of ZnO ethylene glycol nanofluids. Thermochim Acta 2012;535:58 65.

36 Namburu PK, Kulkarni DP, Misra D, Das DK. Viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water mixture. Exp Therm Fluid Sci 2007;32: Harish S, Ishikawa K, Einarsson E, Aikawa S, Chiashi S, Shiomi J, et al. Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions. Int J Heat Mass Transf 2012;55: Chopkar M, Kumar S, Bhandari DR, Das PK, Manna I. Development and characterization of Al2Cu and Ag2Al nanoparticle dispersed water and ethylene glycol based nanofluid. Mater Sci Eng B 2007;139: Sonawane SS, Khedkar RS, Wasewar KL. Effect of sonication time on enhancement of effective thermal conductivity of nano TiO2 water, ethylene glycol, and paraffin oil nanofluids and models comparisons. J Exp Nanosci 2015;10: Yiamsawas T, Mahian O, Dalkilic AS, Kaewnai S, Wongwises S. Experimental studies on the viscosity of TiO2 and Al2O3 nanoparticles suspended in a mixture of ethylene glycol and water for high temperature applications. Appl Energy 2013;111: Suganthi KS, Parthasarathy M, Rajan KS. Liquid-layering induced, temperaturedependent thermal conductivity enhancement in ZnO propylene glycol nanofluid. Chem Phys Lett 2013; : Suganthi KS, Rajan KS. A formulation strategy for preparation of ZnO Propylene glycol water nanofluids with improved transport properties. Int J Heat Mass Transf 2014;71: Sardarabadi M, Passandideh-Fard M, Zeinali Heris S. Experimental investigation of the effects of silica/water nanofluid on PV/T (photovoltaic thermal units). Energy 2014;66: Ruan B, Jacobi AM. Ultrasonication effects on thermal and rheological properties of carbon nanotube suspensions. Nanoscale Res Lett 2012;7: Hong T-K, Yang H-S, Choi CJ. Study of the enhanced thermal conductivity of Fe nanofluids. J Appl Phys 2005;97: Suganthi KS, Rajan KS. Temperature induced changes in ZnO water nanofluid: Zeta potential, size distribution and viscosity profiles. Int J Heat Mass Transf 2012;55: Kole M, Dey TKK. Effect of aggregation on the viscosity of copper oxide-gear oil nanofluids. Int J Therm Sci 2011;50:

37 Pastoriza-Gallego MJ, Lugo L, Cabaleiro D, Legido JLL, Piñeiro MMM. Thermophysical profile of ethylene glycol-based ZnO nanofluids. J Chem Thermodyn 2014;73: Moosavi M, Goharshadi EK, Youssefi A. Fabrication, characterization, and measurement of some physicochemical properties of ZnO nanofluids. Int J Heat Fluid Flow 2010;31: Degen A, Kosec M. Effect of ph and impurities on the surface charge of zinc oxide in aqueous solution. J Eur Ceram Soc 2000;20: Shylaja A, Manikandan S, Suganthi KS, Rajan KS. Preparation and Thermo- Physical Properties of Fe2O3-Propylene Glycol Nanofluids. J Nanosci Nanotechnol 2014;15: Witharana S, Palabiyik I, Musina Z, Ding Y. Stability of glycol nanofluids The theory and experiment. Powder Technol 2013;239: Green DW, Perry RH. Perry s Chemical Engineers Handbook. New York: McGraw-Hill Professional; Rashin MN, Hemalatha J. Viscosity studies on novel copper oxide coconut oil nanofluid. Exp Therm Fluid Sci 2013;48: Keblinski P, Phillpot SR, Choi SUS, Eastman JA. Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids). Int J Heat Mass Transf 2002;45: Rohini Priya K, Suganthi KS, Rajan KS. Transport properties of ultra-low concentration CuO water nanofluids containing non-spherical nanoparticles. Int J Heat Mass Transf 2012;55: Jang SP, Choi SUS. Role of Brownian motion in the enhanced thermal conductivity of nanofluids. Appl Phys Lett 2004;84: Li CH, Peterson GP. Experimental investigation of temperature and volume fraction variations on the effective thermal conductivity of nanoparticle suspensions (nanofluids). J Appl Phys 2006;99: Das SK, Putra N, Thiesen P, Roetzel W. Temperature Dependence of Thermal Conductivity Enhancement for Nanofluids. J Heat Transfer 2003;125: Kole M, Dey TK. Thermophysical and pool boiling characteristics of ZnOethylene glycol nanofluids. Int J Therm Sci 2012;62:61 70.

38 Beck MP, Yuan Y, Warrier P, Teja AS. The thermal conductivity of alumina nanofluids in water, ethylene glycol, and ethylene glycol + water mixtures. J Nanoparticle Res 2010;12: Lee, S, Choi, S.U.S, Li, S, Eastman J. Measuring Thermal Conductivity of Fluids Containing Oxide Nanoparticles. J Heat Transfer 1999;121: Zakaria I, Azmi WH, Mohamed WANW, Mamat R, Naja G. Experimental Investigation of Thermal Conductivity and Electrical Conductivity of Al2O3 Nanofluid in Water - Ethylene Glycol Mixture for Proton Exchange Membrane Fuel Cell Application. Int Commun Heat Mass Transf 2015;61: Xie H, Yu W, Chen W. MgO nanofluids : higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles. J Exp Nanosci 2010;5: Longo GA, Zilio C. Experimental Measurements of Thermophysical Properties of Al2O3 and TiO2 Ethylene Glycol. Int J Thermophys 2013;34: Mary EEJ, Suganthi KS, Manikandan S, Anusha S, Rajan KS. Cerium oxide ethylene glycol nanofluids with improved transport properties: Preparation and elucidation of mechanism. J Taiwan Inst Chem Eng Mariano A, Pastoriza-Gallego MJ, Lugo L, Mussari L, Piñeiro MM. Co3O4 ethylene glycol-based nanofluids: Thermal conductivity, viscosity and high pressure density. Int J Heat Mass Transf 2015;85: Liu M, Lin MC, Wang C. Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale Res Lett 2011;6: Kwak K, Kim C. Viscosity and thermal conductivity of copper oxide nanofluid dispersed in ethylene glycol. Korea-Australia Rheol J 2005;17: Esfe MH, Saedodin S, Asadi A, Karimipour A. Thermal conductivity and viscosity of Mg(OH)2 -ethylene glycol nanofluids. J Therm Anal Calorim 2015 (article in press). 47. Chen H, Ding Y, He Y, Tan C. Rheological behaviour of ethylene glycol based titania nanofluids. Chem Phys Lett 2007;444: Li H, Wang L, He Y, Hu Y, Zhu J, Jiang B. Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids. Appl Therm Eng 2014:1 6.

39 Lee G-J, Kim CK, Lee MK, Rhee CK, Kim S, Kim C. Thermal conductivity enhancement of ZnO nanofluid using a one-step physical method. Thermochim Acta 2012;542: Syam Sundar L, Venkata Ramana E, Singh MK, Sousa ACM. Thermal conductivity and viscosity of stabilized ethylene glycol and water mixture Al2O3 nanofluids for heat transfer applications: An experimental study. Int Commun Heat Mass Transf 2014;56: Mojarrad MS, Keshavarz A, Ziabasharhagh M, Raznahan MM. Experimental investigation on heat transfer enhancement of alumina/water and alumina/water ethylene glycol nanofluids in thermally developing laminar flow. Exp Therm Fluid Sci 2014;53: Yu W, Xie H, Li Y, Chen L, Wang Q. Experimental investigation on the heat transfer properties of Al2O3 nanofluids using the mixture of ethylene glycol and water as base fluid. Powder Technol 2012;230: Sundar LS, Singh MK, Sousa ACM. Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid. Int Commun Heat Mass Transf 2013;49: Vishnu Vardhan P, Suganthi KS, Manikandan S, Rajan KS. Nanoparticle Clustering Influences Rheology and Thermal Conductivity of Nano-Manganese Ferrite Dispersions in Ethylene Glycol and Ethylene Glycol-Water Mixture. Nanosci Nanotechnol Lett 2014;6: Reddy MCS, Rao VV. Experimental studies on thermal conductivity of blends of ethylene glycol-water-based TiO2 nanofluids. Int Commun Heat Mass Transf 2013;46:31 6.

40 259 CHAPTER 7: TESTING OF ZnO-GLYCOL AND ZnO-GLYCOL-WATER BASED NANOFLUIDS FOR HEAT TRANSFER APPLICATIONS 7.1 INTRODUCTION Nanofluids have emerged as a new class of heat transfer fluids due to their enhanced thermal conductivity [1 4]. The dispersion of nanoparticles in coolants such as water, aqueous glycol solutions, etc. alters their thermal conductivity, viscosity, density and specific heat. Extensive research has been carried out on the influence of particle shape [5 7], size [8 10], nanoparticle concentration [11 20], temperature [21 29] on thermophysical properties of nanofluids [25 27,30]. Several theoretical and empirical models have been developed for prediction of thermal conductivity and viscosity enhancement in nanofluids. In addition, heat transfer performance of nanofluids have been studied experimentally [4,31 34] and numerically [35 38]. Considerable experimental data exist showcasing better heat transfer capabilities of nanofluids, in comparison to their respective base fluids under constant heat flux [39 45] and constant temperature [46,47] boundary conditions. Critical heat flux in pool boiling is enhanced with the use of nanofluids [48 51]. While thermal conductivity enhancement in nanofluids renders them as preferred choice for thermal management, considerable viscosity increase and possible reduction in specific heat require attention. In certain cases, though the thermal conductivity increased with increasing nanoparticle concentration, the heat transfer coefficient exhibited a maxima at an intermediate nanoparticle concentration [52 54]. Hence, an optimum nanoparticle concentration was found to exist at which the heat transfer coefficient was

41 260 maximum [52 54]. Nanoparticle aggregation at higher nanoparticle concentrations was found to be detrimental for heat transfer [34]. Most of the experiments on heat transfer in nanofluids have been performed with waterbased nanofluids [39,40,43 47]. Despite the importance of glycols as coolants, there are only relatively few heat transfer experiments using glycol-based nanofluids [55] including those using ZnO-EG [51]. It is desirable to have higher thermal conductivity enhancement than viscosity enhancement (i.e.) kr>r, in order to achieve overall energy savings using nanofluids. ZnO-glycol nanofluids (present study) had higher thermal conductivity and lower viscosity compared to the base fluids and kr/r ratios of ZnO-glycol nanofluids were found to be greater than 1 (Table 7.1), which highlights the potential of ZnO glycol nanofluids for cooling applications. Table 7.1. Thermal conductivity and viscosity modulation of ZnO-glycol and ZnOglycol-water nanofluids. Nanofluid system (2 vol %) Viscosity reduction (%) Thermal conductivity enhancement (%) k r / r ZnO-PG ZnO-EG ZnO-PG-water ZnO-EG-water

42 261 To access the true potential of nanofluids realized by their enhanced thermo-physical properties, their heat transfer performance needs to be evaluated under conditions in which forced convection is absent. Hence, evaluation of heat transfer performance at nonflow conditions will highlight the effect of improved transport properties of nanofluids alone on the heat absorption behavior of nanofluids. It is pertinent to note that majority of heat transfer studies on nanofluids have been carried out under forced convection. Heat transfer fluids are used for heat removal under conditions of constant temperature or constant heat flux. Most of the heat sources in industrial and electronic cooling are constant heat flux sources. Condensation heat transfer without subcooling occurs at constant temperature. The commercial heat transfer fluids employed in collectors of two-fluid loop solar water heating systems contain propylene glycol (PG) as the main constituent [56]. Water is added to such PG based heat transfer fluids and utilized as heat transfer fluids [56]. Hence, PG water mixture finds application in collectors of solar thermal systems for heating. Therefore, improving the transport properties of PG water mixture may benefit solar thermal applications. This section discusses the transient heat transfer performance of ZnO-glycol nanofluids & ZnO-glycol-water nanofluids under constant heat flux and constant temperature boundary conditions. In addition, the ability of ZnO propylene glycol water nanofluids for solar heat absorption, when used inside a metallic container has been investigated.

43 MATERIALS AND METHODS Testing of ZnO-glycol-water & ZnO-glycol-water nanofluids for heat absorption In this chapter, transient heat absorption behavior of ZnO PG water nanofluids was investigated using three different heat transfer scenarios, maintaining the nanofluid in unagitated/unstirred conditions Transient heat transfer under constant heat flux boundary condition The schematic representation of the experimental setup used for transient heat transfer under constant heat flux condition is shown in Figure 7.1(a). A cylindrical stainless steel sample holder, heating filament wound around the sample holder and a dimmerstat to regulate power supply constitute the experimental setup for constant heat flux transient heat transfer studies. The sample holder was insulated to avoid heat losses. In a typical constant heat flux experiment, a constant volume of the test fluid was placed in the sample holder. A constant AC power supply (10 V) was supplied via the nichrome wire wound around the sample reservoir. The increase in test fluid temperature was measured at periodic intervals using a J-type thermocouple over a time period of 20 min. The actual power supplied to the test section was 2 W, which was calculated using Q actual V I Transient heat transfer under constant temperature boundary condition Thermal energy storage systems use organic phase change materials (PCM) like paraffin wax, stearic acid, etc. involving several cycles of charging and discharging. Phase change temperature of organic PCM like paraffin wax lies in the range of C. Heat transfer fluids are used to discharge thermal energy from the thermal energy storage systems at constant temperature (50-60 C) utilizing the latent heat of fusion. In the present study, experiments were designed to study the performance of ZnO-glycol & ZnO-glycol-water

44 263 nanofluids as heat transfer fluids for discharging of thermal energy storage using a constant temperature bath as simulated thermal energy storage discharge system. A sample holder, a temperature controlled oil bath and a J-type thermocouple connected to a temperature indicator comprise the constant bath temperature experimental setup. In a typical constant bath temperature experiment, sample container with a constant volume of test fluid was immersed in oil bath (TC-502, Brookfield Engineering, USA) maintained at a constant temperature (60 C). Temperature increase of the test fluid was measured using a J-type thermocouple for a period of 20 min. Standard tolerance of J- type thermocouple used for temperature measurements is ±0.75 %. The test section was visually inspected for the presence of particles after transferring the nanofluids from the test section. There were no such particle settling testifying the colloidal stability of the nanofluids. The schematic representation of the experimental setup used for this experiment is shown in Figure 7.1(b) Transient heat transfer in solar collector A constant volume of nanofluid/base fluid was placed in a cylindrical aluminium sample container. The sample container was irradiated with simulated sunlight. Halogen lamps were used in the solar simulator for irradiation. The temperature of test fluid was measured at regular intervals of time using a J-type thermocouple. The schematic drawing of the experimental setup is shown in Figure 7.1(c).

45 264

46 265 Figure 7.1. Experimental setup of (a) transient heat transfer under constant heat flux boundary condition (b) transient heat transfer under constant temperature boundary condition (c) transient heat transfer in solar collector. The specifications of the measuring systems/equipment used in the experiments are provided in Table 7.2. Table 7.2. Specifications of the measuring systems/equipment used in the experiments. Equipment Stainless steel sample holder (Constant heat flux boundary condition) Specifications Height 5.2 cm, inner diameter 1.9 cm, outer diameter 2.1 cm Aluminium sample holder (Constant Height 7 cm, inner diameter 1.9 cm, outer