Turbofan: Difference between revisions

One of the earliest turbofans was a derivative of the [[General Electric J79]] turbojet, known as the [[General Electric CJ805|CJ805-23]], which featured an integrated aft fan/low-pressure (LP) turbine unit located in the turbojet exhaust jetpipe. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades. This aft-fan configuration was later exploited in the [[General Electric GE-36]] UDF (propfan) demonstrator of the early 80s. One of the problems with the aft fan configuration is hot gas leakage from the LP turbine to the fan.{{citation needed|date=November 2010}}

 

Many turbofans have the basic two-spool configuration where both the fan and LP turbine (i.e., LP spool) are mounted on a second (LP) shaft, running concentrically with the HP spool (i.e., HP compressor driven by HP turbine). The [[Rolls-Royce BR700|BR710]] is typical of this configuration. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal (e.g., [[General Electric CFE738]]), double-centrifugal or even diagonal/centrifugal (e.g., [[Pratt & Whitney Canada PW600]]).

 

Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding an intermediate-pressure (IP) compressor between the fan and HP compressor, to supercharge or boost the latter unit helping to raise the [[overall pressure ratio]] of the engine cycle to the very high levels employed today (i.e., greater than 40:1, typically). All of the large American turbofans (e.g., [[General Electric CF6]], [[GE90]] and [[GEnx]] plus [[Pratt & Whitney JT9D]] and [[Pratt & Whitney PW4000|PW4000]]) feature an IP compressor mounted on the LP shaft and driven, like the fan, by the LP turbine, the mechanical speed of which is dictated by the tip speed and diameter of the fan. The Rolls-Royce BR715 is a non-American example of this. The high bypass ratios (i.e., fan duct flow/core flow) used in modern civil turbofans tends to reduce the relative diameter of the attached IP compressor, causing its mean tip speed to decrease. Consequently, more IPC stages are required to develop the necessary IPC pressure rise.

 

 

[[Ivchenko-Progress|Ivchenko Design Bureau]] chose the same configuration for their [[Lotarev D-36]] engine, followed by [[Progress D-18|Lotarev/Progress D-18T]] and [[Progress D-436]].

 

 

===Geared fan===