Heat transfer measurements in a leading edge cooling geometry under rotating conditions

In this study, a cold bridge type leading edge cooling system has been experimentally investigated, with the aim to determine rotational effects on heat transfer. A radial channel feeds seven circular holes, which in turn generate seven impingement jets in the leading edge inner cavity. The spent coolant flow is extracted by means of five rows of extraction holes, replicating showerhead and film cooling systems. Different mass flow rates are set through the various extraction rows, thus simulating the effect of blade external pressure distribution. Jet Reynolds number ranging from 20,000 to 50,000 have been investigated in static and rotating conditions (corresponding to a jet Rotation number of 0.008). The effect of different jet feeding conditions, representative of hub, midspan and tip section of the blade, has also been analyzed. Measurements have been carried out exploiting a transient technique with thermochromic liquid crystals. In order to respect the correct sign of buoyancy forces, the test is performed by uniformly heating up the test article with a small amount of mass flow rate, and then by recording the surface response to a sudden decrease in air temperature (down to ambient value) and increase in mass flow rate (up to the nominal value). The obtained results show that jet Reynolds number is the main driving parameter of heat transfer, while jet feeding and extraction conditions effect is limited to the heat transfer pattern shape. In rotating conditions, the interaction between radial residual momentum and Coriolis force on the jet causes a reduction in jet lateral spreading. Moreover, Coriolis forces on the feeding channel appear to disturb the jet generation especially at the blade tip. The combination of these effects causes rotation to have a detrimental effect on heat transfer.