EXPERIMENAL STUDIES ON TURBULENT FLOW IN RIBBED RECTANGULAR CONVERGENT DUCTS WITH DIFFERENT RIB SIZES

Experimental investigations on Nusselt number and fiction loss behaviors of airflow through a rectangular convergent duct fitted with square ribs of different sizes are presented here. The test section has two sloped surfaces and two parallel vertical surfaces. The ribs are placed only on the sloped surfaces of the test section. A specified heat-flux is applied to the ribbed sides of the test section and the ribs are arranged in the form of a staggered array. Measurements were carried out for a convergent rectangular channel with inlet aspect ratio of 1.25 and an exit aspect ratio of 1.35. The heights of the rib turbulators (e) were 6 and 9 mm. This yields a rib height (e) to mean hydraulic diameter of the duct (D-m) ratio of 0.0697 and 0.1046 respectively with a fixed rib pitch (p) to test section width (w) ratio of 0.6. Reynolds number based on the mean hydraulic diameter (D-m)of the channel were kept in a range of 20,000 to 60,000. The results from the ribbed ducts are compared with that of the smooth convergent duct, to estimate the heat transfer enhancements from the ribbed ducts. The comparison shows that among the tested smooth, 6 mm and 9 mm ribbed ducts, the heat transfer and pressure drop increases with the increased rib height. The local variation of thermo hydraulic performance ratio of 6mm ribbed ducts at different Reynolds numbers showed that the ratio is greater than 3 for all Reynolds numbers with CD-6 duct, which indicates that the ribbed ducts with 6 mm ribs are enhancing the heat transfer by 3 times more than the corresponding smooth ducts. The corresponding value for CD-9 ducts is close to 1.2 only. The peak values of THPR are found to be close to 9 for CD-6 duct at X/D-m = 2.8 for Re= 20,000, whereas for the CD-9 duct it is close to 6 at X/D-m = 2.0 for the same Reynolds number. The values of THPR decreases with increasing Reynolds number, which indicates that the pressure drop penalty increases with increase in Reynolds number.