Thermal conductivity, viscosity and heat transfer process in nanofluids: A critical review

Heat transfer efficiency has always been at the center of attractions for many researchers and industries and demand for higher efficiency methods and materials are increased in the last decades. One of the effective methods to enhance the thermal performance of instruments is using nanofluids as a solid-liquid suspension.  In the present paper, the thermal conductivity, viscosity, and heat transfer process of the nanofluids are discussed. The effect of volume fraction, temperature, particle size, shape, base fluid type, and other factors on thermal conductivity, viscosity, and convective heat transfer coefficient of nanofluids are presented. Also, the preparation methods of nanofluids are exhibited and the stability and the effective factors on the stability of nanofluids are brought in the manuscript. Besides, a summarized number of experimental and mathematical studies about the thermal conductivity, viscosity, and stability of nanofluids are listed, compared, and analyzed. The works about the Nusselt number in fluids and nanofluids are presented in detail to determine the future challenges of nanofluids.