377
11.1 General Concept and Components
characteristic is lower work with increased flow rate). Of course, the turbine reacts
to this change in pressure by a lower mass flow rate. So, it not easy to determine
with simple means what the precise outcome is of pressure ratio and mass flow
rate at reduced load, but it is sufficient here to understand (at least believe) that the
mass flow rate increases and the pressure ratio decreases. The detailed study of the
change of the operating point of a gas turbine with changing load is not easy and
becomes even more complex when the gas turbine has many degrees of freedom,
as multiple spools and variable geometry on the compressor. This detailed study is
far above the objectives of the present book and we refer to specialised books [3,
7]. Higher air flow rate at part load reduces the efficiency of the compressor. Lower
fuel flow rate and higher air flow rate at part load reduces the fuel-air ratio in the
combustion chamber. The fuel-air ratio is always low in a gas turbine, in the sense
that full mixing of fuel with air leads to a mixture that is much too lean for stable
combustion. This makes the operation of the combustion chamber delicate (see discussion in a further section). Variation of the fuel-air ratio with load makes the
realisation of good combustion, which means keeping low the noxious combustion
products (CO and NOx), more difficult. So, it is better for compressor efficiency
and for combustion chamber performance that the mass flow rate is reduced at part
load and that the fuel-air ratio stays approximately constant. This then means approximately constant turbine inlet temperature, but reduced pressure ratio [7]. The
mass flow rate may be reduced by variable stator geometry in the front stages of
the compressor. As Fig. 11.3 shows, the mass flow is reduced by turning the stator
vanes more towards the tangential direction. By the same action, the stage work
may also be reduced (lower flow turnings in rotor and stator). This leads then to
a reduced pressure ratio. By doing this in a number of stages, the mass flow rate
through the compressor is reduced, but also the density rise is reduced. This makes
that in further stages the discrepancy between the through-flow velocity in design
conditions and at reduced load may become sufficiently small so that variable stator
geometry becomes unnecessary. In principle, the part load efficiency benefits from
more stages with adjustable stator vanes, but at the other hand, leakage in the stator parts increases which diminishes the efficiency, also in design conditions. So, a
practical optimum is with not many variable geometry stages.
11.1.5 Turbine Part
Figure 11.5 sketches the blade and vane profiles and the velocity triangles at the hub
of an axial turbine. A turbine stage consists of a stator (with vanes) followed by a
rotor (with blades). As with steam turbines, we may use the term nozzle for the channels formed by the stator vanes. Figure 11.6 is a view on the rotor of SGT6-5000F.
The back part is the turbine with four stages. Vanes and blades in a heavy duty
gas turbine are similar to those in the first stages of an LP part of a steam turbine.
But, typically, the through-flow velocity and the work coefficient are larger (see
11.1 General Concept and Components
characteristic is lower work with increased flow rate). Of course, the turbine reacts
to this change in pressure by a lower mass flow rate. So, it not easy to determine
with simple means what the precise outcome is of pressure ratio and mass flow
rate at reduced load, but it is sufficient here to understand (at least believe) that the
mass flow rate increases and the pressure ratio decreases. The detailed study of the
change of the operating point of a gas turbine with changing load is not easy and
becomes even more complex when the gas turbine has many degrees of freedom,
as multiple spools and variable geometry on the compressor. This detailed study is
far above the objectives of the present book and we refer to specialised books [3,
7]. Higher air flow rate at part load reduces the efficiency of the compressor. Lower
fuel flow rate and higher air flow rate at part load reduces the fuel-air ratio in the
combustion chamber. The fuel-air ratio is always low in a gas turbine, in the sense
that full mixing of fuel with air leads to a mixture that is much too lean for stable
combustion. This makes the operation of the combustion chamber delicate (see discussion in a further section). Variation of the fuel-air ratio with load makes the
realisation of good combustion, which means keeping low the noxious combustion
products (CO and NOx), more difficult. So, it is better for compressor efficiency
and for combustion chamber performance that the mass flow rate is reduced at part
load and that the fuel-air ratio stays approximately constant. This then means approximately constant turbine inlet temperature, but reduced pressure ratio [7]. The
mass flow rate may be reduced by variable stator geometry in the front stages of
the compressor. As Fig. 11.3 shows, the mass flow is reduced by turning the stator
vanes more towards the tangential direction. By the same action, the stage work
may also be reduced (lower flow turnings in rotor and stator). This leads then to
a reduced pressure ratio. By doing this in a number of stages, the mass flow rate
through the compressor is reduced, but also the density rise is reduced. This makes
that in further stages the discrepancy between the through-flow velocity in design
conditions and at reduced load may become sufficiently small so that variable stator
geometry becomes unnecessary. In principle, the part load efficiency benefits from
more stages with adjustable stator vanes, but at the other hand, leakage in the stator parts increases which diminishes the efficiency, also in design conditions. So, a
practical optimum is with not many variable geometry stages.
11.1.5 Turbine Part
Figure 11.5 sketches the blade and vane profiles and the velocity triangles at the hub
of an axial turbine. A turbine stage consists of a stator (with vanes) followed by a
rotor (with blades). As with steam turbines, we may use the term nozzle for the channels formed by the stator vanes. Figure 11.6 is a view on the rotor of SGT6-5000F.
The back part is the turbine with four stages. Vanes and blades in a heavy duty
gas turbine are similar to those in the first stages of an LP part of a steam turbine.
But, typically, the through-flow velocity and the work coefficient are larger (see