Single-Shaft Turbojet
Previous remarks have noted that this engine is now used only for special
purposes in modern aircraft because of the conflicting requirements of
engine and propulsive efficiencies. The component arrangement is, however,
a building block for many engines. Studying a simple jet engine, which is
sketched in Fig. 1.14, provides an understanding of the behavior of more
complicated ones. A picture of a production engine, which includes an
afterburner, is compressor and turbine maps of Figs. 1.12 and 1.13. This is where these
components are supposed to operate when the design value of the ETR is
imposed. Performance of the components is assumed and conventional
cycle analysis enables the design point performance to be estimated. An
approximate technique can be used for anticipating the way that the
compressor and turbine operating points change when the ETR is raised
or lowered. Such a procedure is now described. It ignores Reynolds
number effects, but one should recognize that anything that changes the
Reynolds number has the potential of producing other noticeable changes
in performance.
Method of approximate analysis. We shall assume an ideal gas having
the properties expressed in Sec. 1.3. This allows the major trends to be
easily revealed. To establish these trends, it is also convenient to make the
reasonable assumption that the gas flow rate in the turbine just behind the
combustor is about the same as that in the compressor immediately in front
of it. Moreover, if we confine our observations to the cases where the
turbine is choked (see Fig. 1.13a), we can plot straight lines on the
coordinates of a compressor map to show the constant corrected turbine
We may use a constant value (e.g., 0.95) for Pr, l/Pc, o without obscuring
any important trends. The equation then indicates that compressor pressure
ratio varies linearly with the corrected weight flow when the turbine is
choked. The slope of the line is proportional to the square root of ETR.
Lines A-A and B-B of Fig. 1.16 thus represent the requirements of
continuity for two values of ETR. An operating point would be indicated
by the intersection of a line representing the given ETR with the curve for
an appropriate corrected speed.
Previous remarks have noted that this engine is now used only for special
purposes in modern aircraft because of the conflicting requirements of
engine and propulsive efficiencies. The component arrangement is, however,
a building block for many engines. Studying a simple jet engine, which is
sketched in Fig. 1.14, provides an understanding of the behavior of more
complicated ones. A picture of a production engine, which includes an
afterburner, is compressor and turbine maps of Figs. 1.12 and 1.13. This is where these
components are supposed to operate when the design value of the ETR is
imposed. Performance of the components is assumed and conventional
cycle analysis enables the design point performance to be estimated. An
approximate technique can be used for anticipating the way that the
compressor and turbine operating points change when the ETR is raised
or lowered. Such a procedure is now described. It ignores Reynolds
number effects, but one should recognize that anything that changes the
Reynolds number has the potential of producing other noticeable changes
in performance.
Method of approximate analysis. We shall assume an ideal gas having
the properties expressed in Sec. 1.3. This allows the major trends to be
easily revealed. To establish these trends, it is also convenient to make the
reasonable assumption that the gas flow rate in the turbine just behind the
combustor is about the same as that in the compressor immediately in front
of it. Moreover, if we confine our observations to the cases where the
turbine is choked (see Fig. 1.13a), we can plot straight lines on the
coordinates of a compressor map to show the constant corrected turbine
We may use a constant value (e.g., 0.95) for Pr, l/Pc, o without obscuring
any important trends. The equation then indicates that compressor pressure
ratio varies linearly with the corrected weight flow when the turbine is
choked. The slope of the line is proportional to the square root of ETR.
Lines A-A and B-B of Fig. 1.16 thus represent the requirements of
continuity for two values of ETR. An operating point would be indicated
by the intersection of a line representing the given ETR with the curve for
an appropriate corrected speed.
No comments:
Post a Comment