Effect of protrusions and leading edge ribs on the local heat transfer characteristics in a two-pass cooling channel under rotation

Optimised internal cooling schemes for turbine blades aim at enhancing the heat transfer between the hot channel walls and the relatively cold fluid while preventing overcooling. Often, a combination of different turbulator types adapted to the specific turbine blade application are required. Prior research has demonstrated that a promising design featuring high heat transfer in exchange for a reasonable pressure loss penalty is achieved through the use of ribs and protrusions. As regards to rotor blades, rotation represents a key factor with a pronounced impact on the secondary flow field, and thus on the heat transfer characteristics. In the present work, local heat transfer measurements using the transient thermochromic liquid crystal (TLC) technique are conducted under engine-similar test conditions in the rotating rig of the Institute of Aerospace Thermodynamics (ITLR) at the University of Stuttgart. A two-pass cooling channel channel with two different turbulator designs is investigated: a ribbed geometry (baseline) and a rib-protrusion geometry with leading edge ribs (enhanced). Additionally, numerical simulations performed with ANSYS CFX support the experimental approach and complement it with flow field visualisations. It is shown that the rib-protrusion compound is able to improve the heat transfer under both rotating and non-rotating conditions. Nevertheless, a strong reduction in heat transfer caused by rotation occurs with the rib-protrusion geometry, especially considering the radially inward flow of the second passage. The importance of rotation on the secondary flow field and thereby on the internal heat transfer is demonstrated, contributing to a better understanding of cooling air usage under engine-realistic test conditions.