Local heat/mass transfer distributions in rotating two-pass square channels

摘要

The detailed local heat transfer distributions in rotating two-pass square channels are investigated using the naphthalene mass transfer technique and the heat/mass transfer analogy. Reynolds number is varied from 5,500 to 14,500, and rotation number up to 0.24, for air flows through a smooth channel and rib-roughened channels with ribs on the leading and trailing walls. With the rib-roughened channels, the effects of rib blockage and rib angle (90$spcirc$ and 60$spcirc$) are studied.;Since the test channel walls and the air flow are both at room temperature during the mass transfer experiments, there is very small density variation in the flow field. Attention is therefore focused on the sole effect of the rotational Coriolis force under the zero buoyancy condition, and its coupling with the effects of the turn-induced secondary flows and the rib-induced local flow fields. The normalized local Sherwood number distributions on the leading and trailing walls are presented as well as the streamwise distributions of the spanwise averaged Sherwood number ratios.;Results show significant spatial variations of the local mass transfer around the turn and between the ribs on the rib-roughened surfaces. In the first straight pass, the mass transfer on the leading wall decreases with increasing rotation number, and, in the smooth channel case, it decreases monotonically along the streamwise direction. Published heat transfer results showed the strongly coupled effects of Coriolis forces and buoyancy forces caused an increase of the downstream heat transfer at high rotation numbers. Comparison of the present results with published heat transfer results reveals that buoyancy effects are far less significant in a ribbed channel than in a smooth channel.;The complex flow field in the turn region is characterized by the interactions between the turn-induced and the rotation-induced vortices, which produce the highly complicated mass transfer distributions around the turn and in the upstream half of the second pass, in the case of the smooth channel. However, the pattern of the spanwise averaged mass transfer distribution remains almost invariant with change of the rotation number from 0.09 to 0.24. This implies a strong dependency of the flow field on the local geometry of a sharp turn.;Ribs on the trailing wall alter the whole flow field in the first pass at high rotation numbers. Larger ribs on the trailing wall make the streamwise velocity profile more symmetric, weakening the rotational effects, and enhance the opposite leading wall mass transfer significantly.;Skewed ribs (60$spcirc$) enhance more mass transfer than normal ribs (90$spcirc$) in the first pass. In the first pass of a channel roughened with skewed ribs, the coupling effects of the rotational vortices and the rib-induced secondary flows cause very high mass transfer that increases along the streamwise direction.;Reynolds number effects are evident in the turn with a substantial increase of the Sherwood number ratios at lower Reynolds numbers. In the first pass of a rotating smooth channel, the differences between the Sherwood number ratios on the leading wall and those on the trailing wall become greater as the Reynolds number decreases.