Thermal Characteristics and Convection in Nanofluids

The fundamentals and the introduction to the thermal convection are discussed in the chapter. The heat transfer is the movement of energy by the temperature difference. The heat transfer is occurred by various modes between two bodies. Different modes of heat transfer are briefly discussed. Thermal convection or the convective heat transfers and the thermophysical properties associated with it have been discussed. The application of the convective heat transfer is listed and discussed. The demand of highly efficient heat transfer device or equipment is increasing day by day. Different applications of the heat transfer field are required to develop new technologies or methods to cope up with the demand. Different active and passive methods to enhance the heat transfer among the two bodies are briefly discussed.

The need of advanced heat transfer fluid leads to develop nanofluids. Nanofluids are the conventional heat transfer fluids containing solid nanoparticles. The chapter deals with basic introduction of nanofluids, development history of nanofluids and different classifications of nanofluids. Different conventional fluids have been used as the working fluid to transfer the heat in various processes. As a working fluid, water is used extensively due to its immense availability, but not considered as an efficient heat carrier due to low thermal conductivity. The alternates of water, like engine oil, ethylene glycol, etc., are also applied to the various applications, but higher viscosity and toxic nature have restricted the employability of these substitutes in the heat transfer processes. Thus, water has remained the only accessible option as working fluids. However, during the last few decades, it is observed by the researchers that these conventional working fluids have low thermophysical properties which confine the convection heat transfer rate. Hence, by improving the thermophysical properties of the working fluids, the heat transfer can be increased. Nanofluids could be a new dawn to the highly efficient heat flow technologies

The uniform dispersion of nanoparticles in the base fluid is applied to many heat transfer and fluid flow processes to investigate the effect of the suspension on the efficiency of the system. The first and foremost condition for practical applicability of nanofluids is the uniform dispersion of nanofluids. So, the stability should be taken care during the preparation of nanofluids. The uniform dispersion of nanoparticles is applied to many heat transfer processes to investigate the effect of the colloidal suspension on the effectiveness of the system. The procedures applied in the nanofluids preparation of nanofluid are the most influencing factors in the heat transfer characteristics of nanofluids. The prevention of overall agglomeration and clustering of nanoparticles and to achieve a stable colloidal suspension depend upon the method of preparation. The procedures to prepare the nanofluids can be divided into the one-step method and two-step methods depending on the procedure used.

Numerous studies have been conducted on the characterization of nanofluids, including the measurement of thermophysical properties of nanofluids. The chapter includes the discussion on the various thermophysical properties of the nanofluids. The thermophysical properties mainly comprise thermal conductivity, viscosity, heat capacity and density. Thermal conductivity and viscosity play a vital role to enhance the convection heat transfer in nanofluids. The effects of various parameters on the thermal conductivity, viscosity, density and specific heat of the nanofluids are epitomized in a systematic manner. A colloidal suspension of nanoparticle into the base fluid can adjust the thermophysical properties to the desired requirement. Many models have been reported in the literature to analytically estimate the thermophysical properties (especially thermal conductivity) of colloidal suspension, but still there is some contradiction between the theories. Some of the important experimental studies on the thermal conductivity and other properties of nanofluids are summarized.

The nanofluids are the promising smart fluids having advanced thermophysical properties. The earlier chapters have talked about various fascinating facts about the nanofluids. One of the important aspects of the nanofluids is the stability of the nanoparticles. The uniform and long-term suspension is the key and critical feature required in the nanofluids for the industrial applications of these advanced fluids. The chapter summarizes various stability evaluation methods and techniques to enhance the stability of nanofluids. There are number of methods to investigate the stability of the nanofluids. The various types of surfactants and additives are used by the researchers to enhance the stability of the nanofluids. The key factor of using additives is that it is influencing the thermophysical properties and chemical properties of such fluids. The mechanics behind the stability of nanofluids are also discussed in the article.

After discussing the various thermophysical properties of the nanofluids and the effects of various parameters on these thermophysical properties, we are now entering in the field of forced convective heat transfer. The researchers across the worlds have found out that nanofluids (the colloidal suspension of nano-sized particles into the conventional base fluid) reveals some exciting results for the convective heat transfer. The chapter summarizes and analyzes the various experimental and numerical results reporting the convective heat transfer using nanofluids. The enhancement in the heat transfer by using nanofluids is not a common conclusive argument have reported by the researchers. Some researchers have reported the deterioration, and some shows anomalous enhancement in the heat transfer. The conclusion on the mechanism behind the change in the convective heat transfer is still not clear using the nanofluids. On the other hand, pressure drop enhancement is reported by the researchers while using the nanofluids, and it is increased with the concentration. At high particle concentration, the increment is noted around 5% compared to water. The dimensionless number such as Nusselt number and Reynolds number are used to study the convective heat transfer. The fundamentals of the heat transfer and nanofluids have been discussed already in the previous chapters. This chapter discusses the convective heat transfer and pressure drop characteristics in different flow regimes of the internal forced convection.

The natural convection is ubiquitous in nature, and technology and abundant of experimental and numerical studies have been conducted to investigate the fascinating characteristics of the phenomena. The natural convection has the wide range of application in almost every field of technologies such as solar energy, electronics cooling and nuclear energy. The enhancement of heat transfer by the natural convection is a challenging task, and nanofluids are considered to have a potential for the enhancement in heat transfer. The main aim of the chapter is to comprehensively discuss the mechanism about natural convection in nanofluids and explore the various characteristics related to nanofluid natural convective heat transfer. As discussed in previous chapters, the thermal conductivity of the nanofluid plays an important role in the heat transfer. The article has discussed the dependence of natural convection in nanofluids on the thermophysical properties. The various studies have systematically summarized to discuss the effects of various parameters on the heat transfer and to optimize these parameters according to the application. The experimental case studies with alumina/water nanofluid and magnetic nanofluid are considered to discuss the various effects on the heat transfer. The effect of concentrations, shape and size of the nanoparticles is discussed in the case studies.

Nanofluids are the smart fluids with the potential to replace the conventional heat transfer fluid to improve the system efficiency in a sustainable way. The chapter is focused on the current and future applications of the nanofluids. The enhanced thermophysical properties of the nanofluids are the key factors for the applications. Nanofluids are applicable to a number of engineering applications like cooling of car radiator, solar panel, electronics panel, nuclear reactor, space application, etc. All the investigations regarding the application of the nanofluids are systematically summarized and comprehensively discussed in the chapter.