by David C. Wisler
(Aerodynamics Design Topics) and
Leigh Koops. Al McDaniel. Jerry Juenger, Jay Cornell (Mechanical Design Topics)
INTRODUCTION
The purpose of fans and compressors in turbojet engines is to impart energy to the airflow. This is accomplished by a rotor consisting of spinning wheels (or disks) filled with airfoil projections (blades) to increase the kinetic energy of the airflow and therefore bring about a total
pressure rise. Directly following the rotor airfoils is a stage of stator airfoils (vanes). In this process both the rotor and stator turn the airflow, slowing the velocity, and yielding a rise in the static pressure of the airflow.
Multiple stages or rows of rotor/stator stages are stacked in axial flow compressors to achieve exit to inlet total pressure ratios on the order of 8 to 12. Fans by compari-son have fewer stages and are generally used to move large volumes of air with pressure ratios of 1.5 to 3.0.
Other components in fans and compressors include shafts connected to the turbine to turn the rotor and cas-ings which act as pressure vessels to contain the air and support the stator vanes. In some engines many of the stator vanes are mechanically actuated to change their angle of pitch providing a better match of airfoil inci-dence angle at various engine operation points. These el-ements of fans and compressors are shown in Figure 3.1.
The design of air compression turbomachinery has ad-vanced greatly since the early part of the 20th century when GE started to produce aircraft engine supercharg-ers based on Dr. Sanford Moss's designs. These turbosu-perchargers utilized a centrifugal compressor and an axiai flow turbine, such as the one shown in Figure 3.2.
Centrifugal compressors have design advantages of be-ing compact and capable of high pressure ratios. Durbe-ing Worid War II, GE's experience in steam turbines and tur-bos uperchargers led to being selected to produce Ameri-ca's first jet engine, the I-A, which also utilized a centrifugal compressor. It became apparent that with the demand for higher thrust engines flying at higher
speeds that centrifugal compressors were not the proper choice due to the large frontal area and resulting drag.
So. GE developed an axial flow compressor for the world's first turboprop engine, the TG100, designed in the mid-l940s. An example of an axial compressor can be seen in the J85 turbojet (Figure 3.3a) and a centrifu-gal compressor can be found incorporated in the CT7 en-gine (Figure 3.3b). Centrifugal compressors can now achieve a pressure ratio of 8:1 in a single stage, making them highly appropriate for applications such as helicop-ters.
Axial flow compressors advanced in the 1950s with the incorporation of variable stator vanes on the J79 engine.
This allowed the turbojet engine to perform efficiently both at Mach 0.9 cruise speeds and at Mach 2.0 dash to combat speeds. Commercial derivatives of the J79. the CJ805 series of engines, were developed in the late
1950s. The aft fan version of the CJ805. shown in Fig-ure 3.4, provided increased thrust and somewhat better SFC.
In the 1960s new transport engine designs at GE placed the fan at the front of the engine where some of the fan exit air could supercharge the compressor. The rest of the fan discharge air would "bypass" the compressor to provide additional thrust due to the movement of large volumes of air. The ratio of bypass airflow to core air-flow in GE's first production high bypass turbofan en-gine, the TF-39, is 6.5:1. Further additions of stages to the fan rotor, such as the CF6-50 and CF6-80, where most or all of the exiting air would enter the core, are called "boosters."
In the 1980s advances were made in the area of compos-ites. NASA funded research in propfans and a 1970's technology base of fan blade mechanisms with variable pitch came together to produce the latest advance in fan technology, the unducted fan or UDF. A cutaway dia-gram of the UDF is shown in Figure 3.5. The UDF re-sembles a large turboprop engine. One difference that can be noted is that the fan blades are swept back so that they resemble scimitars. This blade design allows the propulsion efficiency of the UDF fan to approach that of turboprops but maintains the peak performance at higher flight airspeeds. While turbofan engines are able to fly efficiently al still faster flight airspeeds, the vast im-provement in SFC provided by UDF engines makes them an efficient propulsion choice for air transport de-signs for the early 21st century.
The first part of this chapter provides an overview of the aerodynamics of advanced compressor and fan systems.
Topics include basic principles and definitions, an histor-ical perspective of the trends in compressor and fan
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Figure 3.3a Turbojet Engine with an Axial Flow Compressor
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design, design and analysis methods including some cur-rent thinking in fan/prop-fan configurations, and lastly a presentation of variable geometry and the effects of tip clearance, shrouds, leakage, and clearance control on performance. The second part of the chapter deals with the mechanical design aspects of fans and compressors.
It must be remembered that behind each of the topics dis-cussed lies a wealth of proprietary technology used in the design of compressor and fan systems that cannot, for obvious reasons, be discussed.