On the role of inserts in forced convection heat transfer augmentation

The role of inserts in internal forced convection has been widely acknowledged as a passive device in the heat transfer enhancement. The present study is aimed to empirically investigate the heat transfer enhancement in a tube fitted with a square-cut circular ring insert in the transitional and the fully turbulent flow regimes. By performing an in-depth analysis on the experimental data, the role of insert has been quantified by deriving a new non-dimensional group. This new non-dimensional group is proposed to characterize the effect of inserts on the heat transfer enhancement. While the findings show the incorporation of insert in the flow passage enhances the heat transfer rate, the characteristics of the flow in the transitional and the fully turbulent flow regimes induced by the effect of insert are distinct. The new non-dimensional group provides interesting insights into the role played by the insert. The physical significance of the non-dimensional number which provides a measure of the change of enthalpy relative to the change of flow energy in the flow direction can be used to explain the decrease of heat transfer augmentation in the turbulent flow regime relative to the transitional flow regime. Based on the analysis of the non-dimensional group, it can be deduced that the contribution of the axial pressure drop in the heat transfer augmentation is marginal albeit not negligible compared to the temperature rise in the characterization of the heat transfer augmentation with the incorporation of insert. The evaluation of heat transfer augmentation efficiency based on the rate of change of internal energy shows that the performance efficiency of an insert would be identical in different flow regimes, contradictory to the widely held axiom that the effect induced by the insert on the heat transfer augmentation diminishes in the turbulent flow regime.