Critical review of heat transfer characteristics of nanofluids

Abstract

Researches in heat transfer have been carried out over the previous several decades, leading to the development of the currently used heat transfer enhancement techniques. The use of additives is a technique applied to enhance the heat transfer performance of base fluids. Recently, as an innovative material, nanometer-sized particles have been used in suspension in conventional heat transfer fluids. The fluids with these solid-particle suspended in them are called ‘nanofluids’. The suspended metallic or nonmetallic nanoparticles change the transport properties and heat transfer characteristics of the base fluid. The aim of this review is to summarize recent developments in research on the heat transfer characteristics of nanofluids for the purpose of suggesting some possible reasons why the suspended nanoparticles can enhance the heat transfer of conventional fluids and to provide a guide line or perspective for future research.

Introduction

Conventional fluids, such as water, engine oil and ethylene glycol are normally used as heat transfer fluids. Although various techniques are applied to enhance the heat transfer, the low heat transfer performance of these conventional fluids obstructs the performance enhancement and the compactness of heat exchangers. The use of solid particles as an additive suspended into the base fluid is a technique for the heat transfer enhancement. Improving of the thermal conductivity is the key idea to improve the heat transfer characteristics of conventional fluids. Since a solid metal has a larger thermal conductivity than a base fluid, suspending metallic solid fine particles into the base fluid is expected to improve the thermal conductivity of that fluid. The enhancement of thermal conductivity of conventional fluids by the suspension of solid particles, such as millimeter- or micrometer-sized particles, has been well known for more than 100 years [1]. However, they have not been of interest for practical applications due to problems such as sedimentation, erosion, fouling and increased pressure drop of the flow channel. The recent advance in materials technology has made it possible to produce nanometer-sizes particles that can overcome these problems. Innovative heat transfer fluids-suspended by nanometer-sized solid particles are called ‘nanofluids’. This suspended nanoparticles can change the transport and thermal properties of the base fluid. The aim of this paper is to present a review of the open literature describing recent developments in the enhancement of heat transfer by using nanofluids. In Section 2, the preparation of nanofluids is described. In the following sections, an overview of recent experimental works on thermal properties, the heat transfer performance, and the proposed theoretical investigations of nanofluids is given. In Section 7, some concluding remarks and future expectations are discussed.