A NEW EXPERIMENTAL APPROACH FOR HEAT TRANSFER COEFFICIENT AND ADIABATIC WALL TEMPERATURE MEASUREMENTS ON A NOZZLE GUIDE VANE WITH INLET TEMPERATURE DISTORTIONS

Modern gas turbines lean combustors allow to limit NOx pollutant emissions by controlling the flame temperature, while maintaining high turbine inlet temperatures. On the other hand, their adoption presents other challenges, especially concerning the combustor-turbine interaction. Turbine inlet conditions are generally characterized by severe temperature distortions and swirl degree, which, in turn, is responsible for very high turbulence intensities. Several past studies have focused on the description of the effects of these phenomena on the behavior of the high pressure stages of the turbine, both considering them as separated aspects, and, in very recent years, accounting for their combined impact. Nevertheless, very limited experimental results are available when it comes to evaluate the heat transfer coefficient (HTC) on the nozzle guide vane external surface, since relevant temperature distortions present a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, annular, three-sector test rig, made by a combustor simulator and a NGV cascade. Making use of three real hardware burners of a Baker Hughes heavy-duty gas turbine, operated in similitude conditions, it can reproduce a representative swirling flow, with temperature distortions at the combustor-turbine interface plane. This test apparatus was exploited to develop an experimental approach to retrieve reliable heat transfer coefficient and adiabatic wall temperature distributions simultaneously, in order to overcome the known limitations imposed by temperature gradients on state-of-the-art methods for HTC calculation from transient tests. A non-cooled mockup of a NGV doublet, manufactured using low thermal diffusivity plastic material, was used for the tests, carried out using IR thermography with a transient approach. In the authors' knowledge, this presents the first experimental attempt of measuring a nozzle guide vane heat transfer coefficient in the presence of relevant temperature distortions and swirl.