Enhancement of fluid thermal conductivity by the addition of single and hybrid nano-additives

The inherent high thermal conductivity of many nanomaterials has a great potential for enhancing fluidic heat transfer applications. Conductive nanomaterials such as carbon nanotubes (CNTs), copper nanoparticles (CuNPs) and gold nanoparticles (AuNPs), as well as their hybrids such as CNT–CuNP or CNT–AuNP were used in this study to enhance the thermal conductivity of fluids. Mono-type nanoparticle suspensions showed the greatest enhancement in thermal conductivity, among which the enhancement with CuNPs was the highest. Hybrid suspensions did not show the same degree of improvement. The experimentally measured thermal conductivities of several nanofluids were consistently greater than the theoretical predictions obtained from existing models. Mechanisms for the thermal conductivity enhancement are discussed. The stability of nanofluids was estimated by UV–vis–NIR spectrophotometer and it was observed that the stability was influenced by characteristics of nanoparticles.