Exploitation of Acoustic Effects in Film Cooling

There have been numerous studies of the behavior of shaped film cooling holes for turbine applications. It is known that the introduction of coolant is an unsteady process, and a handful of studies have described and characterized the unsteadiness. To the best of our knowledge, there are no studies in which unsteady acoustic effects have been actively exploited such that they have led to novel designs with improved cooling performance. This paper discusses the fundamental mechanism of pressure wave propagation through cooling holes and describes systems in which holes which have been acoustically shaped have led to a direct improvement in film cooling hole performance. The mechanism relies on sequential pressure wave reflection within an acoustically shaped hole and is therefore applicable in regions where the external surface is subject to large pressure wave fluctuations at high frequency. The principle is developed analytically, and then demonstrated with a number of computational fluid dynamics (CFD) simulations. We demonstrate that a desired temporal mass flow rate profile can be achieved by appropriate acoustic shaping of the cooling hole. The purpose of this paper is to describe the fundamental design considerations relevant to acoustic shaping. The discussion is developed with reference to a film cooling system for the over-tip region of an unshrouded rotor. The performance benefit of the system in terms of modulation of unsteady mass flux and ingestion characteristics is quantified. It is believed that this is the first time this significant effect has been exploited in film cooling design.