Crossflow around a Four-Start Spirally Fluted Tube

Previous studies have observed higher convective heat transfer from a four-start spirally fluted (SF) tube than from a round tube in crossflow over the Reynolds-number (ReD) range 3,000 = Re = 9,000. Furthermore, its Reynolds number dependence is greater than the reference round tube in this Reynolds number range. Thus far, the associated fluidic mechanisms that underpin both aspects are largely conjectured. This study, therefore, aims to experimentally investigate these two specific aspects. To this end, a series of flow measurements using particle image velocimetry and hot-wire anemometry have been conducted. Our results reveal that crossflow convection from both round and SF tubes compared at ReD = 3,000 occurs over similar fore and aft areas. Reverse flow with a comparable magnitude and level of turbulence develops downstream of both tubes and thus equivalently cools the aft surfaces. Thus, the similar overall crossflow convection takes place. On the other hand, at ReD = 9,000, the fore area of the SF tube is enlarged due to delayed flow separation while that of the round tube remains unchanged. Furthermore, the reverse flow behind the SF tube becomes strengthened with increased turbulence intensity due to the intensified interaction between the bled flow across a crossflow plane via spirals and the primary crossflow from the four-start SF tube becomes more pronouncedly enhanced than the round tube referenced, as increasing the ReD in the range considered.