Generalized correlations for friction factor and Nusselt number in corrugated tubes

The geometric characteristics of corrugations are recognized as critical factors affecting both flow resistance and heat transfer performance in corrugated tubes. However, significant discrepancies persist among experimental measurements and existing correlations for the friction factor ( f ) and Nusselt number ( Nu ). In this study, we develop new empirical correlations for f and Nu based on a recently published dataset of periodic inward corrugated (PIC) tubes, obtained through Large Eddy Simulations (LES). A nonlinear optimal subset regression method was employed to derive the correlations. Additionally, the critical Reynolds number marking the transition from laminar to turbulent flow was identified and correlated with geometric parameters, enabling a rapid prediction of flow transition point in PIC tubes. The accuracy of the developed correlations was assessed by comparing them with numerical simulation results, which shows less than 10 % deviation. To further evaluate their generality, the correlations were applied to predict f and Nu values reported in previous studies. The results demonstrate that the proposed correlations exhibit strong universality and are capable of accurately predicting the flow and heat transfer performance of PIC tubes across a wide range of geometric configurations and Reynolds numbers.