FRICTION AND HEAT TRANSFER IN A ROTATING RIB-ROUGHENED SQUARE U-DUCT UNDER HIGH ROTATION NUMBERS

Experimental investigations have been conducted on a rotating two-pass square channel, in which staggered ribs (attack angle of 45 degree) are roughened on both leading and trailing surfaces. The hydraulic diameter of the channel is 24 mm, and the pitch-to-height ratio and diameter-to-height ratios of the ribs are both 10:1. Reynolds number and rotational speed range from 20000 to 40000 and zero to 1000 rpm respectively. Since the absolute pressure in this channel is increased above 5 atm, the maximum rotation number reaches to 1.025. Regional averaged heat transfer coefficients are measured by classical copper plate technique. Pressure drops are measured by newly designed rotating pressure measurements module. Data are compared to that obtained in rotating smooth U-duct. It shows that the ribbed U-duct achieves enhanced regional heat transfer performances than the smooth case under stationary and rotating conditions at almost all locations except the turn region which has no ribs placed in. In the first passage of the ribbed case, the trends of stream-wise heat transfer distribution on both leading and trailing surfaces are altered compared to the counterparts in smooth case at rotation number range of 0-1.025. Besides, different from the smooth case in which the critical rotation number on heat transfer in the first leading passage decreases as X/D increases, the trend of critical rotation number in the ribbed case is not clear. Moreover, various phenomena reveal that the inserting ribs can offset the effect of rotation on heat transfer. The trends of friction factor and thermal performance as a function of rotation number in ribbed case are totally different to smooth case and they both achieve optimized value at Ro=0.6.