COMBINED EFFECTS OF RIB ANGLE-OF-ATTACK AND PITCH-TO-HEIGHT RATIO ON MASS TRANSFER FROM A SURFACE WITH TRANSVERSE RIBS

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Abstract
An experimental study is performed to investigate the effects of rib angle-of-attack ($ ) and pitch-to-height ratio (pit) on the mass transfer from a rib-roughened surface. For a given Reynolds number, Re - 4 x 104 distributions of local mass transfer coefficient with p/ e = 5 and 10 and θ = 60° and 90° are obtained in great detail. By integrating the measured local data, the surface with p/ e = 5 and 6 90° is found to have the highest pitch-averaged mass transfer coefficient among the four cases studied. This is in sharp contrast to the conventional notion that a rib surface with p/ e - 10 always yields a higher heat transfer enhancement than with p/ e - 5. A change of 9 from 90° to 60° results in opposite effects on the average mass transfer for pie = 5 and 10. The $ variation also strongly affects the local mass transfer characteristics, as expected. Very high mass transfer coefficients exist immediately ahead of a rib corner. This is speculated to be caused by vortex helical motion and periodic boundary layer burst in the region.

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