The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 229
Fig. 6.22 The combustion chamber diverts a small proportion of the mass flow so that combustion can take
place at low airspeed. The burning gases are then recombined with the main flow.
kerosine will only burn efficiently at an air/fuel ratio of about 15:1, when temperatures
of as high as 2000 ◦ C are reached. In practice a range of fuel ratios between 45:1 and
130:1 will be required, and lower temperatures must be arranged at the power turbine
to avoid blade damage.
Figure 6.22 shows how the combustion chamber is designed to overcome these problems.
Only a small part of the compressor delivery enters the area of the burner, and this
restricted flow is slowed down so that combustion can take place. The remainder of the
airflow passes around the outside of the burner thereby cooling it. Within the burner,
fuel leaves the nozzle in a conical spray and begins to burn with a roughly optimal
mixture. The burning gases are joined by the annular airflow from the compressor and
the result is that the flame turns inwards and produces a toroidal vortex. Burning fuel
can recirculate in the vortex to ensure complete combustion. The high temperatures
reached during this process are insulated from the walls of the flame tube by further
air from the compressor. As combustion is completed, the hot gases are diluted by
further compressor air. As a result the temperature of gases reaching the power turbine
is controlled, and a much weaker overall mixture is achieved.
The burner nozzle does a similar job to the carburettor in a piston engine. In order
to achieve atomization, the burner nozzle contains a small chamber into which fuel is
admitted under pressure by a series of tangential ports. The chamber diameter reduces
to the orifice. As the rotating fuel moves to the orifice, the radius of rotation must reduce
and conservation of momentum suggests that the rate of rotation must increase. As it
emerges from the orifice the fuel is spinning rapidly and is thrown into a cone-shaped
spray. The burner has a similar problem to the carburettor when handling the reduced
fuel flow needed to sustain idle power. Passing a small fuel flow through a burner
intended for full power results in slow rotation, poor atomization and combustion.
Figure 6.23 shows that the solution is to build a slow running jet into the nozzle.
A nozzle of this kind is called a duplex burner and is controlled by a spring-loaded
valve that prevents fuel flowing from the main nozzle until sufficient pressure has been
applied to make it work properly.
Several small annular combustion chambers may be arranged around the engine, each
fed from the compressor. The combustion chamber outputs are then merged together
to drive the power turbine. As an alternative, one large annular combustion chamber
may be used. This has several advantages. Flow from the compressor is annular, as is
the flow into the power turbine so an annular combustion chamber gives better pressure
and flow distribution. The surface area of an annular combustion chamber is also less
than that of a multi-chamber system and this reduces losses.