Optimisation of Thermal and Geometric Parameters of Cylindrical Fins during Natural Convection

Abstract

The efficient use of energy is a crucial goal related to the world economy, and efforts are aimed at reducing energy demand for the same quality and quantity of products and services. The drive to improve energy efficiency in the context of technological development, as well as the intensification of industrial and other processes, imposes a constant demand for more efficient heat exchange devices and systems. As devices in which the heat transfer process is crucial, heat exchangers play an essential role in many engineering and other applications. There are several generic techniques for improving the performance of thermal devices, which ultimately manifest through an increase in the heat transfer coefficient as the primary efficiency indicator. These methods include initiating or intensifying the turbulent flow of working fluids, increasing the heat exchange surface by applying specific shapes and additional elements, and using working fluids with improved thermophysical characteristics. This work presents a model for optimising a heat-exchanging surface with cylindrical fins based on the assumption that the material has a constant volume. In doing so, the maximum heat flux at the optimal diameter of the cylindrical fins made of different materials is compared. This research was conducted through a combination of three methods, namely, analytical, numerical (based on the computational fluid dynamic (CFD) technique), and experimental support, for the verification of the obtained results. The optimisation model is based on the analytical and numerical simulation of the heat flux through the fins to obtain the relevant thermophysical parameters of the investigated fin profiles. The optimisation of cylindrical fin profiles is performed for three different fin materials (steel, aluminium, and copper) based on the constant heat transfer coefficient and for different fin materials based on variable heat flux. The heat flux change along the fin was evaluated using analytical and numerical methods. The analysis showed that, due to the trade-off between the convective heat transfer surface area for each of the fin materials, an optimal fin diameter could be identified. Furthermore, the temperature along the cylindrical fin was evaluated through analytical, numerical, and experimental methods. The numerical simulation of the fin model was performed using CFD simulations, and the practical experimental research was carried out on a physical installation using a TESTO 875-2 thermal camera to obtain a clearer picture of the temperature profile. The results include heat flux through cylindrical fins as a function of the fin diameter, the material used, and the convective heat transfer coefficient. Moreover, a comparison of analytical and numerical solutions for the heat flux of cylindrical fins is presented. The temperature profile as a function of the fin element height x, obtained using three methods, has been used as a possible comparison between the methods. The optimisation of the fins has implications on the heat transfer efficiency, as well as on the material used to build the thermal devices and other heat-exchanging equipment.