Heat transfer measurements in a two-pass square duct via a transient liquid crystal image method

Abstract

In this research air flow transient heat transfer in a two-pass channel via thermochromic liquid crystals is studied. An 180° turn and rib roughened surface are part of the channel. This geometric combination simulates turbine airfoil cooling passages. The test section consists of two 24-inch channel passes separated by an 180° turn. Heated air flows into the lower channel, around the turn, and exits the test section thru the upper channel. The inside opaque surface (ribs included) of the 180° turn region is air gun spray coated with liquid crystals. An RGB (Red, Green, and Blue) camera faces the inside opaque test section surface. Plexiglas ribs are double side taped at designated intervals and configurations to the inside surface of the test section that faces the camera. This camera is focused in the region of interest and relays the liquid crystal color change data during the transient heat test to a color frame grabber board; the board is hardware of an IBM PC. The data is reduced to obtain heat transfer coefficients. Heat transfer measurement distributions at 3 Reynolds numbers (10,000, 25,000, and 50,000) were studied. There were two geometric surface rib patterns attached to the channel. First was a 90° continuous rib and the other was a 60° continuous rib. The ribs are designed at P/E=10, and E/D=0.125. The channels are designed at W/D=1 (square channels). In addition, a heat transfer measurement distribution for a smooth surface was also studied. Data was taken at 3 Reynolds numbers for the rib studies and the smooth surface study. In these tests the results showed that the Nusselt number ratios in the second pass are higher than in the first pass with enhanced heat transfer via ribs and centripetal forces. Also, Nusselt number ratios decrease in the second pass after the turn with increasing Reynolds number. Secondary flow separation and reattachment were also observed after the turn and within the ribs. Finally, the awesome power of this liquid crystal transient test technique is in the ability to accurately obtain detailed convection heat transfer data/coefficients; the detailed heat transfer coefficient before and after the turn is very useful in the cooling design of advanced turbine blades. And it is to this end that this research has been well directed.