Experimental study on the effect of graphene and Al2O3 nanofluids in a miniature flat heat pipe

•The effect of graphene and Al2O3 nanofluid on the thermal performance of miniature flat heat pipes is investigated.•Improved heat transfer performance of mFHP is found with these nanofluids.•The effect of the tilt angle is less in the case of Al2O3 nanofluid-based mFHP compared to graphene-based mFHP.•The optimum tilt angle of mFHP is based on the combined effect of the type of nanofluid used in FHP, the heat load range, and the method of condenser cooling. The miniature flat heat pipe (mFHP) is a promising cooling technique for the thermal control of portable electronic appliances. The thermal behavior of miniature flat heat pipe (mFHP) with graphene-DI water and Al2O3-DI water-based nanofluid is experimentally investigated. The copper-made mFHP has dimensions of 200 mm in length, 10 mm in breadth, and 3 mm in thickness. The effect of nanofluid, tilt angle (0˚, 30˚, 45˚, 60˚ and 90˚), condenser cooling mode (natural convection and forced convection with fan cooling), and heat load on the effective thermal conductivity of mFHP are analyzed in this present work. The results reveal that the mFHP filled with nanofluid as working fluids shows better thermal performance as compared to those with the base fluid alone. The FHP filled with Al2O3-DI water nanofluid achieves the maximum effective thermal conductivity of 6271 W/mK under a heat load of 14 W and 60˚ orientation with natural convection condenser cooling. A heat pipe's wettability and capillary pressure are both enhanced by using nanofluid as the working fluid. As the optimum orientation of mFHP varies based on the combined effect of the type of nanofluid used in FHP, the heat load range, and the method of condenser cooling; hence it is very difficult to obtain its value analytically. Therefore, the experimental findings help in selecting the design parameters of mFHP for suitable applications.