2.3 Spray Characterisation
50
Initially, only dripping of liquid drops occurs and no defined liquid core length
can be defined, as displayed by dashed lines in Figure 2.3 (A). In the Rayleigh break-
up regime, (Figure 2.3 {B}) the jet velocities are slow and a smooth jet
exits from the
nozzle. Rayleigh [121] showed that this results from surface tension induced
hydrodynamic instability and in droplets with a constant size of 1.98 times the jet
diameter. Break-up length increases to a maximum at which point aerodynamic forces
begin to have an effect and break up length starts decreasing.
The first wind-induced break-up regime (Figure 2.3{C})
is reached with
increased jet velocity that enables aerodynamic forces between the jet and ambient
gases in conjunction with surface tension forces to result in equivalent drop sizes to
the jet diameter. The jet break-up length is shorter than in the Rayleigh regime. In both
the Rayleigh and the first wind-induced break-up regimes drop formation is thought
to be caused by capillary pinching at the end of the jet.
In the second wind-induced break-up regime (Figure 2.3{D}), with yet higher
jet velocities, a divergence of spray jet after the intact section occurs. This is due to
flow of liquid within the nozzle reaching critical Reynolds
number and becoming
turbulent. Larger jet velocity and increased aerodynamic forces promote unstable
growth of short wavelength waves that result in smaller than jet diameter droplets.
However, break-up length becomes increasingly difficult to evaluate.
Figure 2.3(E) represents the final break up regime - the atomisation regime.
This is reached with further increase in jet velocity and formation of a conical spray
due to increased aerodynamic forces and droplet sizes are much smaller than the
nozzle diameter. Although some researchers have found liquid length to approach zero
in
this regime, others have noted an increase in the break-up length. This has been
explained by an increased dependence on flow conditions within the nozzle [122]. Due
to high injection pressures, it is expected that spray break-up in current project occurs
in the atomisation regime.
Yule and Filipovic [123] note that liquid core break-up length in DI engines
under the atomisation regime can be as much as the distance from the injector nozzle
to the piston surface meaning liquid spray core can accumulate on the piston surface.
This wetting of the piston surface can contribute towards decreased fuel economy and
increased unburned hydrocarbon emissions. Moreover, due to slow vaporisation of un-
atomised fuel, burning of liquid spray core can occur which results in high levels of
2.3 Spray Characterisation
51
soot formation [124]. As the spray progresses, more air is entrained within the fuel jet
and it further spreads out.
The spray density around the exit of the injector nozzle can be very dense and
results in optical methods such as phase Doppler technique
or Particle Imaging
Velocimetry being unsuited for analysis [125]. This means photographic techniques
are often used to measure spray cone angle or spray penetration length instead.
The angle between the outer edges of the spray tip and the injector exit is called
the
spray cone angle.
Large cone angles are preferred because of air entrainment
needed for spray atomisation and better fuel distribution within the combustion
chamber. At lower pressures where the fuel carries less momentum and the effect of
aerodynamic drag forces is larger the cone angle is larger. Cone angle is dependent
upon fuel injection pressure, ambient pressure and fuel viscosity [126, 127]. Roisman
et al. [125] note that ambient pressure seems to have much larger effect on the cone
angle than the injection pressure.
Patel et al. [128] note that the elliptical shape of
the spray often results in
different cone angles being measured with varying downstream distance from the
injector nozzle. As a result, comparing the maximum width of the spray was deemed
more relevant. Similar technique of measuring spray plume width but at 30 mm
downstream from the injector nozzle was employed by Zeng et al. [129].
A further parameter used for characterisation of sprays is the
spray penetration
length
. This determines the maximum distance of any part of the spray from the
injector nozzle. The spray penetration length depends on the fuel injection pressure
and ambient back pressure. The higher the back pressure
or lower the injection
pressure the shorter the penetration length. The fuel speed reaching this point reduces
rapidly with increased distance due to aerodynamic drag forces applied to the fuel
droplets.
In general short distances are sought after in order to avoid impingement of
fuel on piston crowns and the subsequent increase in unburned hydrocarbon and soot
emissions. Park et al. [130] carried out tests on a second generation spray guided direct
injection system on a single cylinder research engine and found at pressures above 200
bar fuel penetration length to be enough to reach the piston crown and advised for use
of particulate filter in order to meet emissions regulations.
2.3 Spray Characterisation
52
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