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This paper presents a study of heat transfer to turbulent air flow in the   abrupt axisymmetric expansion of an annular pipe. The experimental   investigations were performed in the Reynolds number range from 5000 to   30000, the heat flux varied from 1000 to 4000 W/m2, and the expansion ratio   was maintained at D/d=1, 1.25, 1.67 and 2. The sudden expansion was created   by changing the inner diameter of the entrance pipe to an annular passage.   The outer diameter of the inner pipe and the inner diameter of the outer pipe   are 2.5 and 10 cm, respectively, where both of the pipes are subjected to   uniform heat flux. The distribution of the surface temperature of the test   pipe and the local Nusselt number are presented in this investigation. Due to   sudden expansion in the cross section of the annular pipe, a separation flow   was created, which enhanced the heat transfer. The reduction of the surface   temperature on the outer and inner pipes increased with the increase of the   expansion ratio and the Reynolds number, and increased with the decrease of   the heat flux to the annular pipe. The peak of the local Nusselt number was   between 1.64 and 1.7 of the outer and inner pipes for Reynolds numbers varied   from 5000 to 30000, and the increase of the local Nusselt number represented   the augmentation of the heat transfer rate in the sudden expansion of the   annular pipe. This research also showed a maximum heat transfer enhancement   of 63-78% for the outer and inner pipes at an expansion ratio of D/d=2 at a   Re=30000 and a heat flux of 4000W/m2

Augmented of turbulent heat transfer in an annular pipe with abrupt expansion