Influence of particle size on the effective thermal conductivity of nanofluids: A critical review

•Review of particle size-dependent thermal conductivity of nanofluids is presented.•The relevant experimental and analytical reports are discussed.•Factors responsible for inconsistencies in the experimental results are emphasized.•Summarized the particle size-dependent thermal conductivity models of nanofluids.•Presented nanofluid applications for improving the efficiency of the energy systems. Nanofluid is an innovative class of nanotechnology-based thermal fluids and has been proven to improve the energy conversion process efficiency significantly. Thermal conductivity of the nanofluids, the fundamental thermophysical property determining their performance, is a subject of extensive controversies over the years and thereby incites the fundamental doubts in the commercial application of these innovative thermal fluids. A possible justification of these inconsistencies is the lack of comprehensive data over a wide range of sensitive parameters characterizing the effective thermal conductivity of the nanofluids including particle morphology (size and shape) and concentration, fluid temperature, particle and hosting fluid properties, measurement and stability techniques. Particle size, the most discernible feature differentiating nanofluids from micrometre-sized suspensions, contributes not only in ensuring dispersion stability but predominantly influences their heat transport characteristics. Therefore the study is aimed at presenting a critical review of all the experimental, theoretical and numerical investigations on the particle-size-dependent effective thermal conductivity of the nanofluids to comprehend the influence of nanoparticle size variation on the thermal performance of the nanofluids in diverse nanofluid combinations and operational conditions. The study also incorporates a systematic comparison of the experimental results to explicate anomalies in reported results and the mutual impact of imperative parameters on the particle-size-dependent thermal conductivity of the nanofluids.