Comparative Analysis in

Convective Heat Transfer Enhancement by Nanofluids

by Prof. Oronzio Manca

Dipartimento di Ingegneria Industriale e dell'Informazione

Seconda Universita degli Studi di Napoli

Heat transfer enhancement determines the need for new and innovative coolants with improved performance. The novel concept of "nanofluids" has been proposed as a route to surpassing the performance of heat transfer fluids currently available. Several investigations have revealed that the thermal conductivity of the fluid containing nanoparticles could be increased by more than 20% for the case of very low nanoparticle concentrations.

All literature is focused on the theoretical, experimental and numerical study of thermophysical properties and convection of nanofluids, but the modern design concept for a thermal system, pursues not only the enhancement of heat transfer performance, but also requests the minimal power requirements. However, what constitutes enhancement in convective heat transfer in nanofluids is subject to interpretation. According to the established correlations for turbulent and laminar convective heat transfer, the heat transfer coefficient depends on the thermophysical properties and flow parameters. The question of anomalous enhancement is important, because a significant deviation among the data would signal the presence of some nanoparticle-specific heat transfer mechanisms that make nanofluids behave in a fundamentally different way from homogenous fluids.

In any case, enhancement of the heat transfer performance, usually, must be achieved at the expense of power input and this is also the case of nanofluids. In fact, in the study of nanofluid convection, there is the recurrent question of where is the position of the trade-off between the increase in heat transfer and pressure loss. When working with nanofluids, it is of great importance to determine the optimal concentration to use and most convenient particles dimension to utilize. It therefore seems clear that further research is required in order to fully understand the behavior of nanofluids as well as to quantify the extent of their potential use in engineering applications. The optimal trade-off between heat transfer and power input requirement becomes a major issue in the design of a thermal system.

Different performance criteria are usually employed in the evaluation in the comparison between thermal and mechanical performances of nanofluids by a first and a second thermodynamics law analysis. Generally, to characterize the energetic or thermal performance of a fluid flowing in a specific device can be employed the performance evaluation criterion (PEC) based on an energetic global approach. PEC allows to compare heat transfer rate to pumping power. Another criterion can be given by the ratio between the Nusselt number (Nu) and the Euler number (Eu) which represents a measure of pressure loss due to pumping effort to achieve that heat transfer. However, a modern approach for the optimization of a thermal system is based on the second law of thermodynamics. Entropy generation analysis offers a rigorous physical framework to solve the above mentioned problems. Particularly, the entropy generation is used as the parameter for evaluating the efficiency of the system. The system with minimum entropy generation is considered as the optimal design. Study on entropy generation and minimization is essential since viscosity of nanofluids increases with nanoparticles volume fraction.

In the presentation will be compared results carried out with the different approaches with reference to laminar and turbulent convective heat transfer in ducts and impinging jets. < back >