An Experimental and Computational Parametric Investigation of Flow Conditions in Intersecting Circular Passages

In some industrial applications such as turbine airfoil film cooling, coolant passage configurations must maintain sufficient pressure margin to prevent hot gas ingestion whilst using holes of large enough diameter to avoid foreign object blockage. This frequently means that some regions of a film-cooled surface are provided with excess air, or that exhausting film jets are over-blown, adversely amplifying local mixing or coolant separation and therefore enhancing heat flux. A cooling system featuring intersecting passages would allow a high pressure margin to be obtained using discrete, localized loss mechanisms where flows intersect. The degree of loss could be tailored to the local internal and external flow conditions by altering the intersection extent (i.e. the degree of intersecting passage offset), thereby optimizing the use of coolant. Furthermore, localized in-passage convective heat transfer enhancements caused by thin boundary layers and impinging flows in the vicinity of the intersections would improve total heat flux (Watts per square meter) despite surface area lost to intersection voids. As the heat transfer and loss enhancements do not rely on intricately manufactured flow features, the cooling performance is likely to be robust in industrial applications, extending component life. An experimental and computational investigation of the flow through two intersecting cylindrical pipes has been carried out at turbine engine-representative conditions to test these hypotheses. While previous workers have characterized loss and heat transfer in co-planar intersecting holes, this first-of-a-kind study parametrically investigates both fully and partially-intersecting passages, accounting for passage offsets due to typical manufacturing tolerances or purpose-built localized loss enhancements. The loss coefficient across the intersection has been experimentally determined for a range of intersection angles and degrees of intersection in a large scale model running at near atmospheric conditions. The results are used to develop an empirical correlation for the loss coefficient for the isolated intersecting circular channel. A commercial computational fluid dynamics (CFD) code, FLUENT^©, has been used to model heat transfer locally within selected intersecting geometries, and thus to examine the average heat transfer coefficient compared to that predicted by the well-known Dittus-Boelter correlation and other investigators. Insight gained from the CFD predictions enables a first-order estimate of the impact of adding intersections to the convective cooling performance of these advanced cooling configurations. Results show that even an imperfectly machined (i.e. partial) intersection can provide a significant improvement to heat transfer as well as enhanced loss.