The measurements have been made for heat transfer coefficient to an air pulsated flow downstream of an axisymmetric abrupt expansion in a circular pipe with constant wall heat flux. The upstream flow was unheated and fully developed. Runs were made with small diameter (d) to large diameter (D) ratios of 0.32, 0.49, and 0.61 and Reynolds number range of 7760 to 40084 (based on test section diameter) and frequency range of 1 to 13 Hz. Results represent the effect of sudden pipe expansion ratio on the heat transfer characteristics. The influence of pulsation frequency in addition to sudden pipe expansion on heat transfer is also presented in this work. The results showed that the mean Nusselt number of sudden pipe expansion increases as the d/D ratio decreases. With d/D = 0.32, the enhancement of heat transfer in absence of pulsation due to the sudden expansion was about 101 % depending on Reynolds number value while it was a bout 50% with d/D = 0.61. For pulsated flow, the heat transfer results showed that the heat transfer was strongly affected by Reynolds number while it was slightly affected by the pulsation frequency values for any d/D ratios. With low Reynolds number, Re = 7760, the mean Nusselt number increased up to 138 % at f = 10 Hz and d/D = 0.61. ANSYS FLOTRAN CFD computer code (2000) was applied to predict the flow pattern velocity and heat transfer coefficient to support the experimental results. The results show the flow separation, recirculation, reattachment, and wake regions. The computational results of heat transfer coefficient was investigated where good agreement was found. The experimental correlations of the relative mean Nusselt number of the pulsated flow through sudden pipe expansion are developed in terms of Reynolds number and dimensionless frequency |
