Design of Aspirated Compressor Blades Using Three-Dimensional Inverse Method

A three-dimensional viscous inverse method is extended to allow blading design with full interaction between the prescribed pressure-loading distribution and a specified transpiration scheme. Transpiration on blade surfaces and endwalls is implemented as inflow/outflow boundary conditions, and the basic modifications to the method are outlined. This paper focuses on a discussion concerning an application of the method to the design and analysis of a supersonic rotor with aspiration. Results show that an optimum combination of pressure-loading tailoring with surface aspiration can lead to a minimization of the amount of sucked flow required for a net performance improvement at design and off-design operations. INTRODUCTION Turbo-compression technology has been advanced continuously by higher work capacity per stage as a result of increases in rotor speed, aerodynamic loading, and throughflow Mach numbers. With the advent of sophisticated diagnostic tools involving CFD and measurement techniques, more suitable blade shapes having relatively low losses at higher diffusion and Mach number levels have been deployed. While incremental performance advancements can be made through geometric optimization and improved design methods, severe aerodynamic limitations such as increased losses and decreased operability are often encountered when attempting to push significantly beyond current loading levels. Thus, techniques for achieving low losses with wide operability at increased aerodynamic loading levels have received renewed interest [1,2]. As shown by Loughery et al. [3], surface transpiration, properly focused, can be effective at mitigating some deleterious effects associated with increased aerodynamic loading of compressor blades. Surface transpiration is effected either through suction (i.e., aspiration) or blowing of a relatively small amount of flow along the blade or endwall surfaces. Various tactics are possible including controlling profile aerodynamics with or without shocks, managing secondary flows, and tailoring profile and endwall aerodynamic interactions. To effectively execute these schemes in an optimal sense, not only requires a good understanding of the underlying mechanisms but also availability of effective design tools. In this paper, a CFD tool that can be used to design compressor blades with surface flow transpiration is described. The proposed method is an extension of a three-dimensional inverse method reported by Dang et al. [4] and Medd [5], whereby the blade pressure loading distribution is prescribed and the derived quantity is the three-dimensional blade camber surface. Transpiration boundary conditions are incorporated within this framework, thereby allowing full interaction between the prescribed pressure loading distribution and the transpiration scheme. Following a brief exposition of the method, this paper focuses on a discussion concerning the aerodynamic design and performance aspects of a highly-loaded supersonic rotor with aspiration. The intent is not to develop a complete aspirated rotor design that can be manufactured and experimentally tested, but rather to showcase the utility of the inverse method. In general, aspiration is used as an add-on to improve operability of highly-loaded blades and tends to suffer from large sucked flow rate requirements and lack of a unified approach to aspirated transonic blading design. Herein, the blade design objective is an optimum combination of pressureloading tailoring with surface aspiration resulting in a minimal amount of sucked flow for a net aerodynamic performance improvement at design and off-design operations.