Page 349 - Advanced thermodynamics for engineers

338 CHAPTER 15 COMBUSTION AND FLAMES

mixtures at the same turbulence intensity. Figure 15.10 shows that if the Karlovitz number is increased,
by increasing the turbulence level in the combustion chamber, then the product KLe will also increase:
this makes it more likely for the flame to be extinguished. Hence, there is a limit to how much the lower
flame speed caused by a weak mixture can be compensated by increasing the turbulence level: at some
point misfire will occur.
Flammability limits were introduced in Table 15.1, where they were listed with the explosion
limits. The flammability limit of a mixture is defined as the mixture strength beyond which, lean or
rich, it is not possible to sustain a flame. The flammability limit in practice is related to the situation in
which the flame is found. If the flame is moving in a confined space it will be extinguished more easily
because of the increase in the interaction of the molecules with the walls: this is known as quenching.It
is also possible for the flame to be extinguished if the level of turbulence is too high, when the flame is
stretched until it breaks. The lean flammability limit is approximately 50% of stoichiometric, while the
rich limit is around three times stoichiometric fuel–air ratio.
It can be seen from Table 15.1 that the spread of flammability for hydrogen is much higher than
the other fuels. This makes it an attractive fuel for homogeneous charge engines because it might be
possible to control the load over a wide range of operation by qualitative governing rather than
throttling. This would enable the hydrogen-powered engine to achieve brake thermal efficiencies
similar to those of the diesel engine. The restricted range of flammability limits for hydrocarbon
fuels limits the amount of power reduction that can be achieved by lean-burn running; it also restricts
the ability of operating the engine lean to control NO x . Honda quote one of their engines operating as
lean as 24:1 air–fuel ratio, which enables both the engine power to be reduced and the emissions of
NO x to be controlled. Such lean-burn operation is achieved through careful design of the intake
system and the combustion chamber. In practice, in a car engine the lean limit is set by the drive-
ability of the vehicle and the tendency to misfire. A small percentage of misfires from the engine will
make the unburned hydrocarbons (uHC) emissions unacceptable – these misfires might not be
perceptible to the average driver.
The rich and lean flammability limits come closer together as the quantity of inert gas added to a
mixture is increased. Fortunately, the rich limit of flammability is more affected than the lean one,
and basically the lean limit, which is usually the important one for engine operation, is not much
changed.

15.4.5 TURBULENT FLAME SPEED
It was shown previously that the laminar flame speed is too low to enable engines to operate efficiently,
particularly if they are required to work over a broad speed range. The laminar flame speed must be
enhanced in some way, and turbulence in the flow can do this. A popular, relatively simple, model
describing how turbulence increases the flame speed is the wrinkled laminar flame model, which is
shown on a simplified diagram in Fig. 15.11.
The effect of turbulence on the flame, as introduced more comprehensively in Fig. 15.9,is
threefold:

• The turbulent flow distorts the flame so that the surface area is increased;
• The turbulence may increase the transport of heat and active species and
• The turbulence may mix the burned and unburned gases more rapidly.