| | Author | Types | Applications | Ratio | Research method | Main conclusions |
| | Jiang [35] | Cu | Horizontal serpentine buried tube sandbox bench | 1 and 3 wt% | Indoor tests and numerical simulations | (1) The stable maintenance time of 1 wt% Cu–water nanofluid with a mass ratio of 22 : 1 between nanocopper particles and sodium nitrite is the longest, but only 10 hr, which cannot meet the experimental requirements. The stability of suspension can be improved by increasing the concentration of dispersant in a certain range. The higher the concentration of Cu–water nanofluid, the shorter the stable suspension time
(2) Cuo–water nanofluid improves the heat transfer performance of buried pipe, and the heat transfer ratio of horizontal snake buried pipe is close to 35% compared with water. Although the increase in the viscosity of the nanofluid leads to the increase of 16.4% in the running power of the pump, the use of Cuo–water nanofluid improves the heat transfer performance of horizontal snake buried pipe by close to 16% under the test condition |
| | Diglio et al. [36] | Ag, Cu, Al, Al2O3, CuO, graphite, and SiO2 | Borehole heat exchanger | 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 vol% | Numerical simulations | (1) Cu-based nanofluid is characterized by highest Rb reduction, that ranges from about 3.5%–3.8% varying ϕ from 0.1% to 1%, respectively; while, CuO-based nanofluid is the worst solution, with ΔRb that varies from about 0.020%–0.20%
(2) Ag-based nanofluid is characterized by highest convective heat transfer coefficient, following by Cu-based nanofluid
(3) Cu- and CuO-based nanofluids are characterized by lowest volumetric heat capacity reduction, while graphite by highest one
(4) Ag-based nanofluid is characterized by highest pressure drop, and it is followed by Cu-based nanofluid |
| | Li et al. [37] | Al2O3, CuO, and SiO2 | U-shaped deep buried pipe | 0.1, 0.3, 0.5, and 1–vol% | Indoor tests and numerical simulations | (1) CuO/water has the best effect on enhancing heat transfer in buried tubes, and the growth rate of the hourly average heat transfer per linear meter for 240 hr of buried tube heat transfer is calculated to be 0.624% at 1 vol%
(2) The enhanced heat transfer effect of the same type of nanofluid increases with the increase of the volume concentration of nanoparticles, and the higher the volume concentration the more obvious the increase
(3) The difference in the enhanced heat transfer effect of buried tubes is very small when the volume concentration of nanoparticles is the same for different types of nanofluids |
| | Hassan and Harmand [38] | Cu, CuO, and Al2O3 | Rotating heat pipes | 0.04% | Numerical simulations | (1) Rotating heat pipes with Cu–water nanofluid have maximum heat transfer compared with CuO–water and Al2O3–water nanofluids. The maximum heat transfer by rotating heat pipe at ΔT = 20°C and ω = 3,000 rpm increases by about 56% due to using Cu–water nanofluid with Cu nanoparticles of volume fraction 0.04 and radius 5 nm
(2) The results show that for the same input conditions of the heat pipe and same volume fraction and diameter of solid nanoparticles, Cu–water nanofluid has the largest heat transfer compared with CuO–water and Al2O3–water nanofluids. Increasing taper angle of the condenser decreases the heat transfer of the rotating heat pipe for pure water and nanofluids |
| | Niu et al. [39] | SiO2 | Miniature heat pipe | 0%, 0.2%, 0.6%, and 1.0% | Indoor tests | (1) The utilization of SiO2–water nanofluids as working fluids enhances the performance of the miniature heat pipe
(2) As compared with the heat pipe using DI water, the decreasing of the thermal resistance in heat pipe using nanofluids is about 43.10%–74.46% by air cooling and 51.43%–72.22% by water cooling |
| | Bhullar et al. [40] | Al2O3 | Straight heat pipe | 0.005, 0.05, 0.5, and 1.0 vol% | Indoor tests | (1) The experimental results show an optimum reduction of 22% in the thermal resistance value using 1 vol% of Al2O3/DI nanofluids as compared to DI water at low heat input of 12 W. The stabilized operation of the heat pipe is observed at high heat input of 73 W and at low concentration of 0.005 vol% Al2O3/DI water nanofluids |
| | Barua et al. [41] | Al2O3 and MgO | The straight pipe and coiled pipe designs | 0, 0.5, 1, 1.5, and 2 vol% | Indoor tests | (1) The nanofluids can significantly improve the heat and exergy gains. At the Reynolds number of 10, for the nanoparticles volume fraction of 2%, the heat gain of the nanofluid with MgO is about 6% greater than the nanofluid with Al2O3, and it is about 70% greater than the base fluid (water). The nanofluid with MgO nanoparticles shows better heat and exergy gains compared to the nanofluid with Al2O3 nanoparticles |
| | Kumaresan et al. [42] | CuO | Sintered wick heat pipe | 0.5, 1.0, and 1.5 wt% | Indoor tests | (1) The increase in heat transport capacity of the heat pipe is about 31.2% for 1 wt% of CuO nanofluid
(2) Thermal resistance of heat pipe is also reduced when the DI water is replaced by CuO nanofluid and the changes observed for 0.5, 1.0 and 1.5 wt% are 38.3%, 66.1% and 54.1%, respectively
(3) The thermal conductivity of heat pipe increases with the use of nanoparticles and its concentration. The enhancement registered for 0.5, 1.0 and 1.5 wt% are 46.6%, 63.5% and 55.9%, respectively. Thermal efficiency of heat pipe is improved by 24.9% for 1.0 wt% CuO nanofluid compared with the DI water at the optimum tilt angle of 45 |
| | Ghozatloo et al. [27] | Graphene | Shell and tube heat exchanger | 0.05, 0.075 and 0.1 wt% | Indoor tests | (1) The effect of graphene on thermal conductivity of nanofluids is much more than heat transfer coefficient of nanofluids and this effect increases with increasing the concentration of graphene. The thermal conductivity of graphene nanofluids at 25 °C increased by 15.0%, 29.2 and 12.6 at 0.05, 0.075, and 0.1 wt%, respectively
(2) With increasing graphene concentration and fluid temperature, the heat transfer coefficient of graphene nanofluids will be enhanced. By increasing the temperature from 25 to 38 °C, 13.1% increase in the convective heat transfer of 0.1 wt% graphene nanofluids was observed. The effect of graphene concentration in water at higher temperatures is more noticeable. By increasing the concentration of graphene from 0.025 to 0.1 wt%, the heat transfer coefficient of graphene nanofluids increased by 15.3% at 25°C, whereas at 38°C, an enhancement of 23.9% on heat transfer coefficient was occurred |
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