Pressure loss and heat transfer characterisation of intersecting hole heat exchangers

Abstract

Intersecting cooling holes allow very high density heat exchangers to be implemented in the cooling systems of gas turbine blades. The pressure loss and heat transfer characteristics caused by the highly turbulent flow at intersections are poorly understood. These are characterised experimentally and numerically for a single intersection of two equal diameter holes at 90∘ or 60∘/120∘ angles of intersection, 0%, 10%, 49% and 88% offset. Large variations in pressure loss occur for a given intersection geometry; dependent on the mass flow splits, and the level of offset. Pressure losses range from 4 to more than 30 inlet dynamic heads. This allows the flow distribution in an engine application to be tuned, allowing for manufacturing uncertainties. Heat transfer coefficient (HTC) distributions measured in single intersections are reported. These show clear enhancement associated with stripping the fluid boundary layer and mixing of the bulk flow. High local heat transfer gradients were observed at the intersections and circumferentially averaged Nusselt number enhancement of up to 5 found where the degree of pipe intersection is significant. The HTC distributions allow the required packing of hole intersections while controlling the pressure margin. The experimental data are replicated in CFD studies which show good agreement within experimental uncertainty. Thus more detailed numerical models can be relied on for a full range of intersection angles and offsets. Extending the modelling to include multiple linked intersections confirmed that data measured in single intersections are applicable to a matrix of multiply intersecting holes.