CHAPTER-4 EVALUATION OF NANOFLUIDS PROPERTIES

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1 CHAPTER-4 EVALUATION OF NANOFLUIDS PROPERTIES Nanofluids are prepared in different percentages of volume fractions (0.2, 0.4, 0.6, 0.8 and 1) of Al 2 O 3 nanoparticles in water to obtain the properties as discussed in the previous chapter. The thermo physical properties of the prepared nanofluids are estimated through various experiments. Thermal conductivity and Specific heat, Viscosity and Density are measured using Guarded hot plate apparatus, Redwood viscometer-1 and Specific gravity bottle method respectively MEASUREMENT OF THERMAL CONDUCTIVITY Thermal conductivity of the nanofluids is estimated using Guarded hot plate method apparatus for liquids (Fig. 4.1). Calibration of the apparatus is done by comparison with the values of water (A2.1.). Fig. 4.1 Guarded hot plate method apparatus (Make: ARB Educational equipment, India) The apparatus consists of guarded hot plate assembly and control unit. After introducing the sample into the apparatus, three heating coils and thermocouple belt are connected to the apparatus. Initially three sets of dimmer stats and temperature indicators are set to zero. A power supply of 30 V to the main heater and 10 V to ring guard and top guard is given and the water circulating system is adjusted to 5 lit/min. Once steady state 57

2 is attained, readings from eleven thermocouples, voltmeter and ammeter are recorded. Thermal conductivity of the sample is computed from the recorded steady state temperatures for different power inputs using Fourier s conduction law. For better results, an average value of five measurements is taken Thermal conductivity of nanofluids Thermal conductivity is measured at different of temperatures from 30 o C to 80 o C for the different concentration of nanoparticles i.e. 0.2 %, 0.4 %, 0.6 %, 0.8 % and 1.0 % by volume fraction. The data obtained by the experiment is shown in fig.4.2. Fig.4.2 Thermal conductivity of nanofluids as a function of temperature Equations 4.1 to 4.5 are the generalized polynomial equation for Al 2 O 3 -water nanofluid thermal conductivity with volume fractions 0.2 %, 0.4 %, 0.6 %, 0.8 % and 1.0 % respectively, derived from the experimental data obtained. K 0.2 = *10-8 T *10-6 T T T (4.1) K 0.4 = *10-8 T *10-5 T T T (4.2) K 0.6 = *10-8 T *10-5 T T T (4.3) K 0.8 = *10-7 T *10-5 T T T (4.4) K 1.0 = *10-7 T *10-5 T T T (4.5) 58

3 From figure 4.2, it is evident that thermal conductivity of nanofluids shows great enhancement with increase in nanoparticles volume fraction and with rise in fluid temperature. It is also observed that thermal conductivity of nanofluids gets enhanced by 44% to 55% for the volume fractions of 0.8%, and 1.0% respectively when the temperature is increased to 80C. In nanofluid the main mechanism for thermal conductivity enhancement can be attributed as the stochastic motion of the nanoparticles, this Brownian like motion is dependent on fluid temperature MEASUREMENT OF SPECIFIC HEAT Specific heat is measured using guarded hot plate method. After placing the fluid sample into the equipment, energy required for 1 o C raise in temperature is noted. Specific heat of the sample is obtained by using simple energy balance equation. Specific heat of known fluid i.e. water is found for calibration and is as shown in (A2.2). For better results the procedure is repeated for five times and the average value is recorded Specific heat of nanofluids The experimental data obtained for specific heat of nanofluid at different volume fractions and temperatures is shown in fig Equations 4.6 to 4.10 are the generalized polynomial equations for 0.2 %, 0.4 %, 0.6 %, 0.8 % and 1.0 % volume fractions of Al 2 O 3 -deionized water to represent specific heat. Cp 0.2 = *10-10 T *10-7 T *10-5 T T (4.6) Cp 0.4 = 7.197*10-9 T *10-6 T T T (4.7) Cp 0.6 = *10-9 T *10-6 T *10-5 T T (4.8) Cp 0.8 = *10-9 T *10-7 T *10-6 T T (4.9) Cp 1.0 = *10-8 T *10-6 T T T (4.10) The figure 4.3 shows that specific heat of nanofluid is in decreasing trend as the nanoparticle volume fraction increases. The variation of specific heat for different concentrations of nanofluids is very small, but there is much variation in specific heat with change in temperature. For a nanofluid of large nanoparticle volume fraction, this result indicates that low heat energy is needed to obtain the same temperature increment. 59

4 Fig.4.3 Specific heat as a function of temperature for water-al 2 O 3 nanofluids 4.3. MEASUREMENT OF VISCOSITY The viscosity of nanofluid is estimated experimentally using Redwood viscometer-i. The viscosities of the nanofluid at different volume fractions and at different temperatures are measured. The Redwood viscometer-i is as shown in fig.4.4. Before introducing the nanofluid, the equipment is tested with water whose viscosities are known. Tests are conducted to measure the viscosity of water at different temperatures for calibration purpose. A maximum of 5% error is obtained (shown in A3). The kinematic viscosity of the nanofluid is calculated using the relationship. B v = At 10 t -4, m 2 /s (4.11) where ν is the kinematic viscosity of the nanofluid A= & B=1.791 are Redwood viscometer constants t is the time taken in sec to collect 50 c.c. of nanofluid 60

5 Fig.4.4 Redwood viscometer-i (Make: AIMIL, India) The effect of nanoparticle volume fraction and the fluid temperature on the viscosity of nanofluid is analyzed. Viscometer containing the fluid sample is placed in the water bath and steady state condition is maintained at the required temperature. The time taken for 50cc collection of fluid under steady state condition is noted. The process is repeated for the nanofluid samples at different temperatures. Regression equation is developed based on the experimental data at different concentrations and temperatures Viscosity of nanofluids Fig.4.5 shows the experimental values for viscosities of fluids which are obtained. Viscosity is found gradually decreasing with increase in temperature but it increases with increase in nanoparticle volume fraction as shown in fig Behavior of viscosity of nanofluids with temperature is in same trend for all concentrations of nanoparticles added to water. 61

6 Fig.4.5 Viscosity as a function of te mperature for water-al 2 O 3 nanofluids Equations 4.12 to 4.16 represent the generalized polynomial equation of viscosity for 0.2 % to 1.0 % volume fractions when Al 2 O 3 nanoparticles dispersed in deionized water which are derived from the data that is obtained by the experiments conducted µ 0.2 = *10-8 T *10-5 T T T (4.12) µ 0.4 = *10-8 T *10-5 T T T (4.13) µ 0.6 = *10-8 T *10-6 T T T (4.14) µ 0.8 = *10-8 T *10-6 T T T (4.15) µ 1.0 = 2.537*10-8 T *10-6 T T T (4.16) 4.4. MEASUREMENT OF DENSITY For the measurement of density of nanofluids, specific gravity bottle method is used (fig 4.6). First, the empty bottle is weighed and then filled with the deionized water. The bottle filled with water at different temperatures ranging from 30C to 80C is weighed for calibration (shown in A4). Then, the filled bottle with nanofluid samples is weighed again at different temperatures. The specific gravity is estimated by taking the ratio of the net weight of the nanofluid and deionized water. 62

7 Fig.4.6 Specific gravity bottle Density of nanofluids The measured densities of nanofluids are presented in fig 4.7. It is found that as nanoparticle volume fraction increases from 0.2% to 1%, density also increases. Fig.4.7 Density as a function of temperature for water-al 2 O 3 nanofluids According to fig. 4.7, it can be understood that temperature has much influence on the density of nanofluids. Density of nanofluids is higher than the base fluids i.e. deionized 63

8 water and with increase in temperature, density of nanofluids goes on decreasing. But at high range of temperature i.e. above 60C, density of nanofluids appeared for all volume fractions to be closer compared with at lower temperatures. Equations 4.17 to 4.21 represent the generalized polynomial equation of viscosity for 0.2 % to 1.0 % volume fractions when Al 2 O 3 nanoparticles dispersed in deionized water which are derived from the data that is obtained by the experiments conducted. ρ 0.2= *10-6 T T T T (4.17) ρ 0.4= *10-5 T T T T (4.18) ρ 0.6= *10-6 T T T T (4.19) ρ 0.8= *10-5 T T T T (4.20) ρ 1.0= *10-6 T T T T+1092 (4.21) 4.5. SUMMARY Nanofluids are prepared with optimum process parameters. Basic thermo physical properties like thermal conductivity, viscosity, specific heat and density of nanofluids are measured. Empirical models for nanofluid properties are developed based on experimental data, these correlations are used for characterization of nanofluids. However, the above correlations are valid for the specified nanofluids only. 64