Session: 32-08 Fan & Propulsor Design

Paper Number: 101639

101639 - Historical Developments in Fan Technologies for Aeroengines 

The fan is a key component of a turbofan engine. A major part of the engine thrust is derived from the fan and its contribution to the overall engine specific fuel burn is a close second only to the efficiency of the low-pressure turbine. This paper reports on the evolution of fan aerodynamics starting with the development of turbofan engines similar to the CF6-80, with bypass ratios between 4-6, whose performance is used as a yard stick against which following aerodynamic technology is evaluated. From the CF6-80 onward, improving propulsive efficiency involved larger diameter fans with high aspect ratios that required the use of a part span support (shroud or snubber) to prevent aeromechanical instability. The part span shroud was generally situated in the supersonic flow region and resulted in performance penalty and reduction in airflow. Some unique methods used by designers to reduce these losses associated with the shroud are discussed. A numerical assessment of the differences in aerodynamic performance between part span shrouded and un-shrouded fan blades at take-off and at cruise are presented. The need to go to higher bypass ratios and more efficient fans led to the introduction of wide chord fan blades that eliminated the need for part span shrouds for aeromechanical stability. The use of wide chords in fan blades was facilitated by the development of composite materials and hollow fan blade technology, and provided significant improvement in fan efficiency. However, the wide chord fan blade technology was not adequate for the engine performance requirements necessitated by the fuel crisis in the early seventies. This crisis resulted in the engine designers to develop radical new engine architectures that began the introduction of a decade long un-ducted fan (UDF) and propfan era. Some of the UDF and propfan developments are presented in the paper. The price of oil dropped considerably in the late eighties and engine companies decided to shelve these radical technologies at that time. However, the insatiable thirst to improve the engine performance continued with attention being directed to reduce shock losses in fan blades. Aerodynamic sweep started to creep into the designer’s vocabulary and aft sweep was the most prevalent driver with the intent to achieve performance gains by reducing bow and passage shock strengths. Well-designed aft swept rotors seemed to have very good efficiency, but in most cases fell significantly short on stability requirements. Historically, pre-occupation with trying to reduce passage shock strength tended to omit from consideration the aerodynamic sweep benefits derivable from radial flow migration and suction surface boundary layer centrifugation. Results from a first of its kind controlled scientific single stage transonic fan test with three rotors (radial, aft-swept and forward swept) are presented that solved the dilemma of swept rotors and resulted in the modern compound swept fan blade used in all new turbofan engines for most compression components. The performance improvement with the swept rotors is estimated by OEMs to be approximately two percent. With continuous requirements for higher thrust, bypass ratio demand has also increased. Almost all engine companies have responded to these challenges by reducing the number of fan blades. The use of advanced 3-D aerodynamics of blade using CFD tools has played a vital role to address and solve these issues. Higher bypass ratios and lower fan pressure ratios favors propulsive efficiency. To meet future performance and stability requirements of lower fan pressure ratios, new fan architectures are discussed.

Presenting Author: Chetankumar Mistry Indian Institute of Technology Kharagpur

Presenting Author Biography: Dr. Chetan S. Mistry is an Assistant Professor in the Department of Aerospace Engineering, IIT Kharagpur. He has 22 years of experience in teaching and research. He has done his graduation in Mechanical Engineering from REC, Surat (Presently NIT Surat). He received his Master of Engineering in Turbomachinery from NIT, Surat; Ph.D. from IIT Bombay. His Ph.D. thesis on “Experimental Investigation on the Performance of a Contra Rotating Fan Stage under Clean and Distorted Inflow Conditions” was awarded with “Award for Excellence in Thesis Work”, IIT Bombay in 2014. He is also a recipient of “ASME IGTI Young Engineer Travel Award” in 2013. He is editor for Proceedings of the National Aerospace Propulsion Conference and Proceedings of Recent Advances in Computational and Experimental Mechanics, Volumes I and II. He had reviewed papers for many reputed national and international conferences and journals, books for different publishers, sponsored research proposals and Ph. D. theses. His areas of research are: Design and performance augmentation strategies for turbomachines, experimental and computational study of turbomachines including contra rotating axial flow turbomachines, electric propulsion as well as fluid mechanics and heat transfer and experimental aerodynamics. He is a fellow of the Institution of Engineers and member of AIAA, ASME and ISTE.

Authors:

Ernesto Benini University of Padova
Chetankumar Mistry Indian Institute of Technology Kharagpur
Aspi R. Wadia GE Aviation