Parametric study on the thermal performance enhancement of a thermosyphon heat pipe using covalent functionalized graphene nanofluids

•Highly dispersible graphene nanofluids prepared using covalent functionalization.•The heat transfer and thermal conductance potency promoted by by graphene nanosheets.•A trade-off between filling ratio, tilt angle and performance were identified.•The apparent enhancements of effective thermal conductivity were 105 at φ = 5%.•The reduction in thermal resistance was 26.4% for prepared nanofluid at φ = 5%. Heat transfer characteristics of copper sintered heat pipe explored using a modified graphene nanoplatelets (GNP)-containing nanofluid with great dispersion stability as a novel working fluid. Firstly, a water dispersible GNP with specific desire was synthesized by the reaction of GNP sheets with the diazonium salt (DS) of sodium 4-aminoazobenzene-4-sulfonate. An X-ray photoelectron spectroscopy (XPS) test shown successful covalent functionalization of GNP using DS which provided special water dispersibility characteristics. The results indicate that the thermal conductivity enhancement was up to 17% by adding modified GNP sheets in the base fluid. It also, exhibited a maximum sedimentation of 16% after 840 hrs. Further research works were carried on thermal performance of heat pipe by varying nanofluid concentrations, filling ratio, input heating powers and inclination angles of heat pipes. The results proof that the maximum enhancements of the effective thermal conductivity and reduction in thermal resistance for purposed nanofluid atφ = 5% were 105% and 26.4%, respectively. Moreover, these good features of the GNP/DS nanofluid make it a very promising working fluid to enhance the thermal performance and efficiency of the current heat pipe systems.