NUMERICAL AND EXPERIMENTAL STUDY OF FLOW AND HEAT TRANSFER IN OUTWARDLY CONVEX CORRUGATED TUBES WITH A TWISTED TAPE INSERT

This study involves numerical and experimental investigations on flow and heat transfer in an outwardly convex corrugated tube with various structural twisted tape inserts (CT) by using computational fluid dynamics (CFD) methods. The numerical mode is validated by comparing it with experimental data for a smooth tube and corrugated tubes in a developed experimental apparatus. The study also includes a comparison of thermodynamic performances of a CT, a smooth tube with a twisted tape insert (ST), and of a smooth tube to determine their differences. The influence of a twist ratio is considered to analyze thermodynamic regulation and reveal the mechanism operating in CT. The numerical results also agree well with the experimental results on the Nusselt number and friction factor of the corrugated and smooth tubes. The maximum deviations are controlled within +/- 6% for Nu(c) and +/- 7.6% for f(c). The Nu in the CT exceeds those in the ST and smooth tube by 120-136% and 171-317%, respectively. The friction factor in the CT exceeds those in the ST and plain tube by 148-153% and 476-514%, respectively. The effect of twist ratio in a CT is considered, and the results indicate that the best overall thermal performance (n = 1.97) is obtained with a high twist ratio (y/w = 5). However, the highest thermal performance (Nu(c)/Nu(s) = 4.78) is obtained with the lowest twist ratio (y/w = 1.25). The contour plots indicate that a potential reason for this can be attributed to the coexistence of the corrugated tubes and tapes that provide a synergetic swirling effect, which efficiently reduces the thermal and velocity boundary layer developments and thus effectively increases the heat transfer performance.