HEAT TRANSFER, FLUID FLOW, AND PRESSURE MEASUREMENTS INSIDE A ROTATING TWO-PASS DUCT WITH DETACHED 90° RIBS

摘要

Transient thermochromic liquid crystal thermography. a laser-Doppler velocimeter. and pressure transducers have been used to measure the local heat transfer, velocity, and wall static-pressure distributions, respectively, in a rotating two-pass square duct with 90° ribs detached from the leading and trailing walls. The ribs were square in cross-section and their detached-distance/height ratio was 0.38. The rib-height/duct-height ratio and the pitch/rib-height ratio were 0.136 and 10, respectively. The duct Reynolds number was 1 x 10~4 and rotation number ranged from 0 to 0.2. Results are compared with attached rib cases in terms of regional averaged Nusselt number, transverse mean velocity component, pressure coefficient distributions and variation of friction factor with rotation number. The competition between convection effect of the wall jet and downwash effect of the rib-top separated shear layer on the heat transfer augmentation is addressed in detail. Discussion on local Nusselt number distribution, mean velocity components, and turbulent kinetic energy is included. Simple expressions are obtained to correlate friction factor with rotation number. Rib detachment is found to enhance heat transfer on the leading wall of the first outward pass and on the trailing wall of the second inward pass over as compared to the attached rib case. The trend is reversed on the other two walls. Nevertheless, detached ribs create more uniform heat transfer distributions on the leading and trailing walls than attached ribs.