H. Data acquisition system
3.5 Comparison of the convective heat transfer
characteristics of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid
nanofluids
The comparison of the convective heat transfer coefficients of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number at the weight concentration of 0.025wt% is shown in Fig. 3.15. The convective heat transfer coefficients of MWCNT, Fe3O4 and MWCNT/Fe3O4 hybrid nanofluids at a weight concentration of 0.025wt% increased from 1722.2 to 1914.3 W/m2K as Reynolds numbers increased from 1000 to 1600. Those were increased from 3.9% to 6% in the convective heat transfer coefficient compared to that of the base fluid. It reveals that remarkable improvement in the convective heat transfer coefficient of MWCNT/Fe3O4 hybrid nanofluid was shown at a weight concentration of 0.025wt% to compare with MWCNT and Fe3O4 nanofluids.
Fig. 3.16 shows the comparison of the convective heat transfer coefficient results of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.05wt% according to Reynolds number. The convective heat transfer coefficients of MWCNT, Fe3O4 and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.05wt% increased from 1730.6 to 1958.5 W/m2K when the Reynolds number increased from 1000 to 1600. Those were increased from 4.4% to 8.5% in the convective heat transfer coefficient compared to the base fluid. The improvement of convective heat transfer of MWCNT/Fe3O4 hybrid nanofluid was 6.9%, 8.1%, 7%, and 8.5%, respectively, when Reynolds numbers was
1000, 1200, 1400 and 1600 at the weight concentration of 0.05wt%.
The comparison of convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.1wt% according to Reynolds number is shown in Fig. 3.17. The convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 at the weight concentration of 0.1wt% increased from 1792.7 to 2030.5 W/m2K as Reynolds number increased from 1000 to 1600. Those were increased from 8.2% to 12.4% in the convective heat transfer coefficient compared to the base fluid. The maximum increase in the convective heat transfer coefficient was 2030.5 W/m2K at the 0.1wt% Fe3O4
nanofluid, which was 12.4% higher than the base fluid when Reynolds number was 1600.
Fig. 3.18 shows the convective heat transfer coefficients at the weight concentration of 0.2wt% of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number. The convective heat transfer coefficients of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.2wt% increased from 1798.2 to 1988.4 W/m2K when the Reynolds number increased from 1000 to 1600. Those were increased from 8.5% to 10.1% in the convective heat transfer coefficient compared to the base fluid. It is observed that the improvement of the convective heat transfer coefficient of 0.2wt%
MWCNT/Fe3O4 hybrid nanofluid was 10.1% higher than that of the base fluid. The improvement of the convective heat transfer coefficient of MWCNT/Fe3O4 hybrid nanofluid is restricted by the thermo-physical properties and the random movement of the nanoparticles with the Reynolds number at a high weight concentration of 0.2wt%. Based on these results, the improvement in convective heat transfer is mainly depended on their thermal conductivity, viscosity, thermal
boundary layer, flow condition, shape, and size of the nanoparticle. In all studies of the convective heat transfer, the convective heat transfer coefficient increases with the concentration of the nanoparticles as well as Reynolds number. In this experiment, the convective heat transfer coefficient was dependent on the concentration, and their viscosity. It can be assumed that extra thermal resistance on the inner surface of the test section was caused by the agglomerated Fe3O4
nanoparticles, and they coated the inner surface of the test section at high Fe3O4
nanofluid concentrations. Besides, the viscosity of the Fe3O4 nanofluid increased with the increase of nanofluid concentration. This phenomenon is due to the shear rate in the cross-section of the tube, which reduces the thermal boundary layer thickness.
Fig. 3.15 Comparison of the convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at a weight concentration of 0.025wt%
1000 1200 1400 1600
1200 1400 1600 1800 2000 2200 2400
H T C , h (W /m
2 K )
Reynolds number, Re
0.025wt% MWCNT nanofluid 0.025wt% Fe3O4 nanofluid
0.025wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
Fig. 3.16 Comparison of the convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at a weight concentration of 0.05wt%
1000 1200 1400 1600
1200 1400 1600 1800 2000 2200 2400
0.05wt% MWCNT nanofluid 0.05wt% Fe3O4 nanofluid
0.05wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
H T C , h (W /m
2 K )
Reynolds number, Re
Fig. 3.17 Comparison of the convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at a weight concentration of 0.1wt%
1000 1200 1400 1600
1200 1400 1600 1800 2000 2200 2400
0.1wt% MWCNT nanofluid 0.1wt% Fe3O4 nanofluid
0.1wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
H T C , h (W /m
2 K )
Reynolds number, Re
Fig. 3.18 Comparison of the convective heat transfer coefficient of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at a weight concentration of 0.2wt%
1000 1200 1400 1600
1200 1400 1600 1800 2000 2200
2400 0.2wt% MWCNT nanofluid
0.2wt% Fe3O4 nanofluid
0.2wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
H T C , h (W /m
2 K )
Reynolds number, Re
The comparison of pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to the Reynolds number at the weight concentration of 0.025wt% is shown in Fig. 3.19. The pressure drop for all nanofluid increased with an increasing of Reynolds number. The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid was ranged from 2.96 to 3.1 kPa when Reynolds number increased from 1000 to 1600. Those were increased from 36.7% to 43% in pressure drop compared to the base fluid. It reveals that the maximum increment of pressure drop of MWCNT/Fe3O4 hybrid nanofluid was 3.1 kPa which was 43%
increased pressure drop at the weight concentration of 0.025wt%.
The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.05wt% is shown in Fig. 3.20. The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid was ranged from 3.09 to 3.16 kPa when Reynolds number increased from 1000 to 1600. Those were increased from 42.6% to 46% in pressure drop compared to the base fluid. The maximum increment of pressure drop of MWCNT/Fe3O4 hybrid nanofluid was 3.16 kPa which was 46% increased pressure drop at the weight concentration of 0.05wt%. Besides, the increment of pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids was 3.09, 3.11, and 3.16 kPa, respectively. Those were 42.6%, 43.3%, and 46% increased one when compared to the base fluid.
Fig. 3.21 shows the pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.1wt% according to Reynolds numbers.
The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid was ranged from 3.12 to 3.29 kPa when Reynolds number increased from 1000 to 1600. Those were 44.1% to 51.9% increased pressure drop compared to the base fluid. The maximum increment of pressure drop of MWCNT/Fe3O4 hybrid nanofluid
was 3.29 kPa which was 51.9% increased pressure drop at the weight concentration of 0.1wt%. In addition, pressure drop enhancement of MWCNT and Fe3O4, nanofluids were 44.1% and 47%, respectively, at the weight concentration of 0.1wt% compared to that of the base fluid.
The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number at the weight concentration of 0.2wt% is presented in Fig. 3.22. The pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids ranged from 3.24 to 3.52 kPa when Reynolds number increased from 1000 to 1600. Those were 49.6-62.4% increased pressure drop compared to the base fluid. It indicates that the maximum increment of pressure drop was 62.4%
in pressure drop of MWCNT/Fe3O4 hybrid nanofluid at the weight concentration of 0.2wt%. The increment of pressure drop of MWCNT and Fe3O4 nanofluid was 49.6% and 55.2%, respectively, compared to that of the base fluid. From this analysis, the pressure drop of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids are higher than that of the base fluid under laminar flow condition, which can increase the pumping power in the system.
Fig. 3.19 Comparison of pressure drop of MWCNT, Fe3O4 and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.025wt%
1000 1200 1400 1600
1.0 1.5 2.0 2.5 3.0 3.5 4.0
P re ss u re d ro p ( k P a)
Reynolds number, Re
0.025wt% MWCNT nanofluid 0.025wt% Fe3O4 nanofluid
0.025wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
Fig. 3.20 Comparison of pressure drop of MWCNT, Fe3O4 and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.05wt%
1000 1200 1400 1600
1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.05wt% MWCNT nanofluid 0.05wt% Fe3O4 nanofluid
0.05wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
P re ss u re d ro p (k P a)
Reynolds number, Re
Fig. 3.21 Comparison of pressure drop of MWCNT, Fe3O4 and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.1wt%
1000 1200 1400 1600
1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.1wt% MWCNT nanofluid 0.1wt% Fe3O4 nanofluid
0.1wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
P re ss u re d ro p (k P a)
Reynolds number, Re
Fig. 3.22 Comparison of pressure drop of MWCNT, Fe3O4 and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.2wt%
1000 1200 1400 1600
1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.2wt% MWCNT nanofluid 0.2wt% Fe3O4 nanofluid
0.2wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
P re ss u re d ro p ( k P a)
Reynolds number, Re
The comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number at the weight concentration of 0.025wt%
is shown in Fig. 3.23. The maximum friction factor of MWCNT/Fe3O4 hybrid nanofluid was 0.0331, which was 26.9% increased friction factor at the weight concentration of 0.025wt% as compared with the base fluid. The results observed that the friction factor of MWCNT and Fe3O4 nanofluids were 0.0329 and 0.0298 which were 26.2% and 14.5% improved ones, respectively, compared to the base fluid. From this, the Reynolds numbers increased friction factors for all nanofluids.
Fig. 3.24 shows the comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid. The friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids was ranged from 0.0310 to 0.0339 when Reynolds number increased from 1000 to 1600. Those were 19.1%-30.2% increase in friction factor compared to the base fluid. The friction factor of MWCNT/Fe3O4 hybrid nanofluid was 30.2% at the weight concentration of 0.05wt%. Besides, the enhancements of MWCNT and Fe3O4 nanofluids were 27.7% and 19.1%, respectively, at the weight concentration of 0.05wt% compared to that of the base fluid.
The comparison of the friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4
hybrid nanofluids according to Reynolds number at the weight concentration of 0.1wt% is shown in Fig. 3.25. The friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid was ranged from 0.0320 to 0.0355 when Reynolds number increased from 1000 to 1600. Those were 22.8%-36.3% increase in friction factor compared to that of the base fluid. The enhancements of the friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluid was 28.4%, 22.8%, and 36.3%, respectively, compared to that of base fluid.
Comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number at the weight concentration of 0.2wt%, is presented in Fig. 3.26. The friction factor of MWCNT/Fe3O4 hybrid nanofluid was 0.0407 which was 56% increased at the weight concentration of 0.2wt% compared to that of the base fluid. The friction factor of MWCNT and Fe3O4 nanofluid at a concentration of 0.2wt% was 0.0342 and 0.0355, which were 31.3% and 36%
increase in friction factor, respectively, compared to that of base fluid. The fiction factor of MWCNT nanofluid showed little enhancement compared to that of the Fe3O4 nanofluid. Besides, the maximum enhancement of friction factor was shown at the 0.2wt% MWCNT/Fe3O4 hybrid nanofluid. It can be inferred that the movement of the nanoparticles through the test section due to thermophoresis and Brownian diffusion significantly increases the viscosity of nanofluid, which is flattened velocity. This flattened velocity decreases the temperature difference of the wall and bulk in constant heat flux condition.
Fig. 3.23 Comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.025wt%
1000 1200 1400 1600
0.02 0.03 0.04 0.05 0.06 0.07 0.08
F ri ct io n f ac to r, f
Reynolds number, Re
0.025wt% MWCNT nanofluid 0.025wt%Fe3O4 nanofluid
0.025wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
Fig. 3.24 Comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.05wt%
1000 1200 1400 1600
0.02 0.03 0.04 0.05 0.06 0.07 0.08
0.05wt% MWCNT nanofluid 0.05wt%Fe3O4 nanofluid
0.05wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
F ri ct io n f ac to r, f
Reynolds number, Re
Fig. 3.25 Comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.1wt%
1000 1200 1400 1600
0.02 0.03 0.04 0.05 0.06 0.07 0.08
0.1wt% MWCNT nanofluid 0.1wt%Fe3O4 nanofluid
0.1wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
F ri ct io n f ac to r, f
Reynolds number, Re
Fig. 3.26 Comparison of friction factor of MWCNT, Fe3O4, and MWCNT/Fe3O4
hybrid nanofluids at a weight concentration of 0.2wt%
1000 1200 1400 1600
0.02 0.03 0.04 0.05 0.06 0.07
0.08 0.2wt% MWCNT nanofluid
0.2wt%Fe3O4 nanofluid
0.2wt% MWCNT/Fe3O4 hybrid nanofluid EG/Water (20:80)
F ri ct io n f ac to r, f
Reynolds number, Re
In this study, the convective heat transfer characteristics of MWCNT, Fe3O4 and MWCNT/Fe3O4 hybrid nanofluids based on EG/Water=20:80 was investigated experimentally. The weight concentrations of nanofluid was ranging from 0.025wt%
to 0.2wt% and Reynolds numbers was changed by 1000, 1200, 1400 and 1600, respectively. The Fe3O4 nanocomposite synthesized by using the co-precipitation method, which has a magnetic property and has a high potential to improve its thermal properties by adding high thermal conductivity material of MWCNT nanoparticles. Besides, the MWCNT/Fe3O4 hybrid nanofluid was fixed at the mixing ratio of 1:1. The following conclusions were made from this investigation.
[1] The convective heat transfer coefficients at the weight concentration of 0.2wt% of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids according to Reynolds number was investigated experimentally. The convective heat transfer coefficients of MWCNT, Fe3O4, and MWCNT/Fe3O4 hybrid nanofluids at the weight concentration of 0.2wt% increased from 1798.2 to 1988.4 W/m2K when the Reynolds number increased from 1000 to 1600. Those were increased from 8.5% to 10.1% in the convective heat transfer coefficient compared to the basefluid. However, the convective heat transfer coefficient of MWCNT/Fe3O4 hybrid nanofluid at a weight concentration of 0.1wt% was 2030.5 W/m2K, which was 12.4% improvement one compared to the base fluid.
It can be assumed that the convective heat transfer coefficient was dependent on the concentration, thermal conductivity, and viscosity. The particle migration effect was occurred when the viscosity of nanofluid increased with
the increase of nanofluid concentration. This particle migration can be due to the shear rate in the cross section of tube, which increase the thermal boundary layer thickness. In addition, the increasing concentration of nanofluid leads to reduce the dispersion stability of MWCNT/Fe3O4 hybrid nanofluid at weight concentration of 0.2wt%, therefore the convective heat transfer performance decreases.
[2] In general, the MWCNT/Fe3O4 hybrid nanofluid is more effective in increasing the convective heat transfer coefficient compared to single nanofluids of MWCNT and Fe3O4. In comparison, the MWCNT/Fe3O4 hybrid nanofluid was found to have the highest convective heat transfer coefficient and pressure drop. The highest flow velocity was also obtained in the MWCNT/Fe3O4 hybrid nanofluid. As a result, the velocity of MWCNT/Fe3O4 hybrid nanofluid can be increased to increase the intensity of synergistic effect which is the heat convection intensity factor of convective heat transfer enhancement.
[3] All nanofluids had a higher convective heat transfer coefficient compared to the base fluid. The convective heat transfer coefficient increased with an increase in the Reynolds number and nanoparticle concentration. The improvement of convective heat transfer were occurred in the entry length region. This improvement in the entrance region was caused by the low thermal boundary layer thickness because the effect of viscous shear rate gradually spread further away from the wall to the centerline. In this study, the friction factor of MWCNT/Fe3O4 hybrid nanofluid was 0.0407 which was 56% increased friction factor at the weight concentration of 0.2wt% compared
to that of the base fluid. The friction factor of MWCNT and Fe3O4 nanofluid at a concentration of 0.2wt% was 0.0342 and 0.0355, which were 31.3% and 36% increased friction factor, respectively, compared to that of base fluid.
Friction factor was decreased with the increase of Reynolds number. The increasing nanofluid concentration with an increase in nanofluid viscosity, which decreased fluid movement. It can be inferred that the movement of the nanoparticles through the test section increased because thermophoresis and Brownian diffusion significantly increased of nanofluid, which was flattened velocity. This flattened velocity decreased the temperature difference of the wall and bulk in constant heat flux condition.
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