Preliminary Design and Performance of Counter Rotating Turbines for Open Rotors: Part I — 1-D Methodology

Abstract

Direct Drive Open Rotors (DDORs) have the potential to significantly reduce fuel consumption and emissions relative to conventional turbofans. However, this engine architecture presents many design and operational challenges both at engine and aircraft level. At preliminary design stages, a broad design space exploration is required to identify potential optimum design regions and to understand the main trade offs of this novel engine architecture. These assessments may also aid the development process when compromises need to be performed as a consequence of design, operational or regulatory constraints. Design space exploration assessments are done with 0-D or 1-D models for computational purposes. These simplified 0-D and 1-D models have to capture the impact of the independent variation of the main design and control variables of the engine. Historically, it appears that for preliminary design studies of DDORS, Counter Rotating Turbines (CRTs) have been modeled as conventional turbines and therefore it was not possible to assess the impact of the variation of the number of stages (Nb) and rotational speed of the propellers. Additionally, no preliminary design methodology for CRTs was found in the public domain. Part I of this two-part publication proposes a 1-D preliminary design methodology for DDOR CRTs. It allows an independent definition of the Nb, rotational speeds of both parts of the CRT, inlet flow conditions, inlet and outlet annulus geometry as well as power extraction. It includes criteria and procedures to calculate: power extraction in each stage, gas path geometry, blade metal angles, flow conditions at each turbine plane and overall CRT efficiency. The feasible torque ratios of a CRT are discussed in this paper. A form factor for the CRT velocity triangles is defined (similar to stage reaction on conventional turbines) and its impact on performance and blade design is discussed. A method for calculating the off-design performance of a CRT is also described in Part I. In Part II, a 0-D design point (DP) efficiency calculation for CRTs is proposed as well as a case study of a DDOR for a 160 PAX aircraft. In the case study, three main aspects are investigated: A) the design and performance of a 20 stage CRT for the DDOR application; B) the impact of the control of the propellers on cruise specific fuel consumption, C) the impact of the design rotational speeds and Nb of the CRT on its DP efficiency, engine fuel consumption and engine weight.

Non-Dimensional Parameters for Comparing Conventional and Counter-Rotating Turbomachines

“…CRTs have also been proposed for use in aero-engines designed for civil passenger aircraft where they would power counter-rotating turbofans [8], propfans [9] or open rotor devices [10,11]. Coupling the counter-rotating propulsors with a CRT is attractive as it reduces the need for a gearbox as well as reducing the size and weight of the engine.…”

“…Various studies have addressed these issues, either implicitly or explicitly, in a number of ways and there is currently no agreed approach. This has led to varying design methodologies [5,8,10,14] which result in different conclusions about the use of counter-rotation.…”

“…The thickness distribution is defined by a power law based on the location of maximum thickness, blade stagger angle and a profile shape power which, in this case, is set to 3. The blades have a prescribed leading edge thickness of 10% of the maximum thickness, and zero trailing 9) and (10), corrected for deviation, typically around 1 • , depending on the loading. An example blade can be seen in Fig.…”

“…Another application where high power density is important is the turbopump used in rocket engines. CRTs can be used to drive the liquid oxygen fuel pumps, and here, counter-rotation is claimed to reduce the size and mass of the turbine as well as being more stable at off-design conditions [6,7].CRTs have also been proposed for use in aero-engines designed for civil passenger aircraft where they would power counter-rotating turbofans [8], propfans [9] or open rotor devices [10,11]. Coupling the counter-rotating propulsors with a CRT is attractive as it reduces the need for a gearbox as well as reducing the size and weight of the engine.…”

Analyses of the Design of Multirows Vaneless Staggered Counter-Rotating Turbine

“…Counter rotating turbine is a turbine in which two adjacent rotors rotate in opposite directions [6,7]. Wintucky and Stewart [8] were the first to investigate counter rotating turbine technology, mainly through theoretical analysis of the relationship between efficiency and the turbine speed ratio and output power ratio.…”