Experimental investigation on the effects of shaped ribs in circular ducts

Electrochemical machining (ECM) is a conventional manufacture technology in drilling radial cooling channels within gas turbine blades. Cooling air flows inside these internal channels and passes through holes located at the blade leading and trailing edge. Generally ECM is used to manufacture cylindrical holes; nevertheless a specific kind of ECM, named Shaped Tube Electrolytic Machining (STEM) technique, allows to produce holes with variable diameter, that can be considered as tubes of circular cross section having a repeated-rib roughness. Application of this kind of ribbed channels can virtually improve heat transfer performance than cylindrical holes, without increasing manufacture costs. This paper presents an experimental study concerning the heat transfer and friction factor measurements of four ducts with shaped ribs for fully developed turbulent internal flows. The four circular ribbed ducts show two different pitch-to-rib height ratios (Px/e=10-13.3) and two different rib height-to-hydraulic diameter ratios (e/D=0.0714-0.0952). Moreover the four models are classified by the rib shape, and named Sharp and Smooth. Transient tests were performed varying the Reynolds number at the inlet section from 10000 to 40000. Heat transfer coefficients maps were evaluated using narrow band Thermo-chromic Liquid Crystals (TLC), while friction coefficients were calculated measuring pressure drop along the ducts with piezoresistive pressure sensors in cold tests. Results report heat transfer coefficient profiles and pressure losses along the stream direction for all tested geometries as function of the Reynolds number. Heat transfer and friction coefficients are also compared to values for a smooth tube. Moreover, for each geometry, the streamwise averaged Nusselt numbers have been fitted in order to highlight Reynolds number dependence. A comparison with some published correlations for rectangular ribbed channels showed that using a correction factor it is possible to obtain the heat transfer coefficient for circular ribbed ducts. Experimental results highlight that, keeping constant e/D, the Smooth geometries provide same global heat transfer performance with lower pressure losses than the Sharp ones.