Hydrothermal characteristics of turbulent flow in a tube with solid and perforated conical rings

Metallic inserts are well-recognized in modern research as efficient passive thermal performance enhancement methods. However, only scarce studies examined conical rings mainly due to the high corresponding pressure drop. This study aims to achieve a balanced hydrothermal performance using solid and perforated conical rings by focusing -for the first time- on the number and shape of holes in both convergent and divergent rings. The results show that larger Nusselt numbers (up to 360.2 at Reynolds number of 26,000) can be achieved using solid divergent rings due to thermal boundary layer disruption and near-wall flow acceleration, but this configuration also shows the highest friction factors (5.04 at Reynolds number of 6000). Perforating the rings reduces both Nusselt number and friction factor for more balanced performances, especially for higher numbers of holes. Circular and triangular holes result in the highest and lowest values of both parameters, respectively. A maximum thermal enhancement factor of 1.1 is obtained when using solid divergent rings at Reynolds number of 6000. However, a thermal enhancement factor of 1.06 can be achieved using circularly perforated rings with significantly lower pumping power requirements. When referencing solid rings instead of the plain tube, thermal enhancement factors can reach 1.2.