9.4.2 Efficiency of Small Gas Turbine in Missile Hydraulic
System
Small gas turbines of aircraft use gas of solid retardant propellant as working fluid.
Generally speaking, they are pure impulse turbines. Because this kind of gas turbine
uses high enthalpy working fluid, its weight and size are small, its specific power is
high, and its starting speed is fast. Because of its simple structure, there are no parts
for reciprocating motion. It is not easy to get stuck at high temperature and has high
reliability. It is widely used in the hydraulic system of medium and short-range
tactical air defense missiles at home and abroad. Therefore, it is necessary to discuss
and optimize many factors affecting turbine performance, so that gas turbine will be
more viable in the competition with other primary energy sources of various
hydraulic systems.
Here, only the efficiency of this gas turbine is discussed preliminarily.
Generally speaking, efficiency is a very important index for a prime motor. The
gas turbines discussed here are used in extremely compact spaces and in situations
where weight restrictions are quite stringent. Therefore, the pursuit of efficiency
must take into account the size and weight, or even the pursuit of efficiency is
mainly to reduce the size and weight, followed by the economy. In fact, for this
particular occasion, the working time of the whole power plant is not long, the total
power is very small, and the economic loss caused by low efficiency is small. This
is quite different from the general power plant. Moreover, in order to improve
efficiency, the corresponding economic cost must be paid. It is necessary to find a
way to improve efficiency significantly at a low cost.
In this kind of small gas turbine, there are several kinds of losses when working
fluid works: 1 nozzle loss; 2 blade loss; 3 excess speed kinetic energy loss; 4
leakage loss; 5 wheel disk friction blast loss; 6 part intake loss; 7 kinds of
mechanical losses (gear, bearing, coupling, etc.).
From the calculation and analysis of the following examples, it can be seen that
various losses account for the proportion of total turbine input power. The known
parameters are as follows:
Gas initial pressure p 0 ¼ 4:98 MPa, gas initial temperature T 0 ¼ 1500 K, gas
constant R ¼ 437:6 J= kg K
ð
Þ, gas adiabatic index K ¼ 1:27, nozzle throat diameter
d kp ¼ 2:5 mm, nozzle outlet angle a r ¼ 22
, turbine calculated diameter
D CD ¼ 290 mm, design speed n ¼ 6000 r=min.
Table 9.12 shows the value and percentage of wheel circumferential work,
effective work and losses per kilogram of gas turbine at design speed 6000 r=min.
From the comparison of various losses, it can be seen that the largest part of the loss
is the residual kinetic energy loss, accounting for 26.80% of the total input energy.
Obviously, this loss must be reduced first.
From the velocity triangle, it can be seen that the simpler and more effective way
to reduce the turbine outlet speed is to increase the speed of revolution, and with the
increase of the speed of revolution, a minimum outlet speed can be obtained, and
140
9 High-Temperature and High-Speed Gas Turbine Pump …
System
Small gas turbines of aircraft use gas of solid retardant propellant as working fluid.
Generally speaking, they are pure impulse turbines. Because this kind of gas turbine
uses high enthalpy working fluid, its weight and size are small, its specific power is
high, and its starting speed is fast. Because of its simple structure, there are no parts
for reciprocating motion. It is not easy to get stuck at high temperature and has high
reliability. It is widely used in the hydraulic system of medium and short-range
tactical air defense missiles at home and abroad. Therefore, it is necessary to discuss
and optimize many factors affecting turbine performance, so that gas turbine will be
more viable in the competition with other primary energy sources of various
hydraulic systems.
Here, only the efficiency of this gas turbine is discussed preliminarily.
Generally speaking, efficiency is a very important index for a prime motor. The
gas turbines discussed here are used in extremely compact spaces and in situations
where weight restrictions are quite stringent. Therefore, the pursuit of efficiency
must take into account the size and weight, or even the pursuit of efficiency is
mainly to reduce the size and weight, followed by the economy. In fact, for this
particular occasion, the working time of the whole power plant is not long, the total
power is very small, and the economic loss caused by low efficiency is small. This
is quite different from the general power plant. Moreover, in order to improve
efficiency, the corresponding economic cost must be paid. It is necessary to find a
way to improve efficiency significantly at a low cost.
In this kind of small gas turbine, there are several kinds of losses when working
fluid works: 1 nozzle loss; 2 blade loss; 3 excess speed kinetic energy loss; 4
leakage loss; 5 wheel disk friction blast loss; 6 part intake loss; 7 kinds of
mechanical losses (gear, bearing, coupling, etc.).
From the calculation and analysis of the following examples, it can be seen that
various losses account for the proportion of total turbine input power. The known
parameters are as follows:
Gas initial pressure p 0 ¼ 4:98 MPa, gas initial temperature T 0 ¼ 1500 K, gas
constant R ¼ 437:6 J= kg K
ð
Þ, gas adiabatic index K ¼ 1:27, nozzle throat diameter
d kp ¼ 2:5 mm, nozzle outlet angle a r ¼ 22
, turbine calculated diameter
D CD ¼ 290 mm, design speed n ¼ 6000 r=min.
Table 9.12 shows the value and percentage of wheel circumferential work,
effective work and losses per kilogram of gas turbine at design speed 6000 r=min.
From the comparison of various losses, it can be seen that the largest part of the loss
is the residual kinetic energy loss, accounting for 26.80% of the total input energy.
Obviously, this loss must be reduced first.
From the velocity triangle, it can be seen that the simpler and more effective way
to reduce the turbine outlet speed is to increase the speed of revolution, and with the
increase of the speed of revolution, a minimum outlet speed can be obtained, and
140
9 High-Temperature and High-Speed Gas Turbine Pump …