Studies on Thermal Characteristics of Nanofluids in Heat Pipe

Abstract

High heat fluxes due to densification of electronic components in integrated circuits pose a challenge in their heat dissipation rates. Use of forced convective or micro conductive fin attached to the integrated circuits limit their performance at high heat flux loads. Phase change heat transfer devices play an excellent role in dissipating the heat from the highly concentrated heat sources, thus providing superior cooling capabilities.

Over the past decade, comprehensive research work has been carried out on the operating characteristics of heat pipe using conventional working fluids. The heat pipes using nano-suspensions of metallic and oxide particles in the base fluid termed as “nanofluids” have attracted the attention of researchers worldwide because of enhanced thermophysical properties of the nanofluids.

In the present work, the potential of using Al2O3 / Deionised (DI) water based nanofluids as working fluids in heat pipe has been investigated at various orientations. The nanofluids prepared by two step method without the use of any surfactant are stabilized by ultrasonication. Structural modifications in the manufacturing of heat pipe have been made to delineate any vapor velocity fluctuation effects. A heating chamber has been constructed around the evaporator section of the heat pipe for uniform heat flux and the condenser section has been cooled by natural convection.

The thermal conductivity enhancement of Al2O3 / DI water nanofluids measured by transient hot wire (THW) method demonstrates a nonlinear relationship with increase in volume fraction of dispersed nanoparticles and attains a maximum enhancement of 15 % for 1 vol. % of Al2O3 loading in distilled water at 70°C.

The thermal performance of heat pipe has been investigated at increasing concentrations of Al2O3 / DI water nanofluids (0.005 %, 0.05 %, 0.5 % and 1 %) and Watt loads of heat inputs (12 W, 32 W & 72 W) to cover the operating temperature range of the electronic equipments ( < 70oC). Achieving favorable results, the average wall temperature along the length of the heat pipe reduces with the use of nanofluids as compared to conventional fluids i.e. DI water. Interestingly, the wall temperature decreases further with the increase in the Al2O3 volume concentration from 0.005 to 1 vol. % in the base fluid. The effective thermal conductivity of heat pipe increases with the increase in the nanoparticle concentration. An increase of 28 %, 22.6 % and 23.21 % has been achieved in the effective thermal conductivity of heat pipe tested at 12 W, 32 W and 72 W heat input respectively when loaded with 1 vol. % of Al2O3 nanoparticles as compared to DI water.

Secondly, the heat pipe tested at various favorable tilt angles in the range 0o to 90o demonstrates an optimum thermal performance at 30o favorable tilt angle using DI water as working fluid. Interestingly, the use of nanofluids as working fluid has shifted the optimum favorable tilt angle from 30o to 45o irrespective of the selected concentration level (0.005 & 0.05 vol. %) of nanofluids and given heat inputs.

Thirdly, the operational limitations of modified heat pipe are calculated based on the thermophysical properties of Al2O3 / DI water nanofluids. The changed properties of nanofluids show an enhancement in the boiling, entrainment, viscous and sonic limit except the capillary limit of heat pipe. The surfactant free nanofluids provide higher margin of safety to the boiling limit of the heat pipe, simultaneously eliminating possibility of vapor lock. The capillary limit decreases with the use of nanofluids due to increase in the vapor and liquid flow path resistance rather than capillary pressure in the evaporator section of heat pipe.

Lastly, the nanofluids are prone to sedimentation and clogging with respect to time. The study will be incomplete without conducting the temporal study on the thermal performance of heat pipe. The heat pipes loaded with different volume concentrations (0.005, 0.05, 0.5, 1 vol. %) of Al2O3 / DI water nanofluids is investigated after 0, 3, 6 and 9 months from the date of manufacturing. The heat pipes are kept non-operational in between the specified durations. The consistency in the heat pipe performance is compared at different Watt loads (12 W, 32 W & 72 W) of heat input. The results indicate temporal deterioration in the thermal performance of heat pipe at low volume concentration in the selected range of heat inputs. The consistency in the operating characteristics of heat pipe has been observed at high vol. % concentration (1 vol. %) and high heat input of 72 W. The optical microscope images of the used mesh in the heat pipe reveal and authenticate our observations.