Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 N A N O R E V I E W Open Access Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment Gopalan Ramesh and Narayan Kotekar Prabhu* Abstract The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment. Introduction Quench hardening is a commonly used heat treatment process in manufacturing industry to increase the ser- vice reliability of components where the material is heated to the solutionizing temperature, held for a parti- cular period of time and then quenched into the quenching medium. Quenching during heat treatment involves simultaneous occurrence of different physical events such as heat transfer, phase transformation and stress/strain evolution, and heat transfer is the driving physical event as it triggers other processes [1]. The two phase (boiling) heat transfer is the predominant mode of heat transfer during quenching. When the hot metal submerged into the liquid pool, heat transfer is con- trolled by different cooling stages known as vapour blanket stage/film boiling stage, nucleate boiling stage and convective or liquid cooling stage [1-3] (Figure 1). Quenching from high temperature is enough to produce a stable vapour film around the surface of component. * Correspondence: prabhukn_2002@yahoo.co.in Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Srinivasnagar, Mangalore, India During this vapour blanket stage, heat transfer is very slow because the vapour film acts as an insulator and occurs by radiation through the vapour phase. Nucleate boiling starts when the surface temperature of the com- ponent drops slowly where the vapour film starts to col- lapse and allowing liquid to come into contact with the surface of component. The stage is characterized by vio- lent bubble boiling as heat is rapidly removed from the part surface and maximum cooling rate is obtained. This continues till the surface temperature drops below the boiling temperature of the liquid. Quenching is a non-stationary process where the occurrence of these local boiling phenomena is a function of time and posi- tion along the surface of the component. This behaviour leads to the occurrence of a wetting front, which is the locus of the boundary between the vapour film and the occurrence of bubbles [4]. The final stage of the quenching, i.e. convection cooling occurs when the metal surface is reduced below the boiling point of quenchant. During this stage, boiling stops and heat transfer occurs directly by direct contact between the surface and liquid and the rate of heat removal is low. © 2011 Ramesh and Prabhu; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.