Atomization is defined as the disintegration of a liquid into small drop or
droplets. The resultant suspension of fine droplets in a surrounding gas is
termed spray. Atomization of a liquid into droplets can be achieved by various
means: aerodynamically, mechanically and ultrasonically (Liu,
2000). The breakup of a liquid into droplets, for example, may be achieved
by the impingement with a gas in a two fluid atomization and by centrifugal
forces in rotary atomization. The processes of atomization could also be classified
according to the energy used to produce instability in a liquid. Sprays can
be defined as a dispersion of droplets produced by using a nozzle with sufficient
momentum to penetrate the surrounding medium which is gaseous (Nasr
et al., 2002). Shape, pattern and some measure of droplet sizes are
indication of spray characteristics. Atomization can be divided into two types.
The first is primary atomization, which is near the nozzle and secondary atomization,
which is the break-up of drops further downstream.
Practical atomization processes of liquid will generally include two fluid
atomization, pressure atomization and rotary atomization. Atomization of normal
liquids has been long studied in spray combustion (Djavareshkian
and Ghasemi, 2009) and spray drying (Lolodi, 2011),
with combustion being the widest spread application (Liu,
2000). In industrial production processes, sprays are widely used in food
processing, pharmaceutical processes (Davidov-Pardo et
al., 2008), agriculture (Alam and Khan, 2002)
and paper manufacturing (Nasr et al., 2002). For
processes involving vaporization, cooling or cleaning of gases, sprays used
in fire suppression, air humidification and gas cleaning and conditioning. Processes
in agriculture, surface cleaning and treatment, spray painting, coating and
printing require sprays that have high momentum impact to achieve the desired
results. Different industrial applications require different types of sprays
in order to deliver their functions (Nolan, 2000). Full
cone nozzles will result in complete spray coverage in a round, oval or square
shaped area. In generating sprays, the liquid is usually swirled within the
nozzle and mixed with non-spinning liquid that has bypassed an internal vane
and then discharges through an orifice, forming a conical pattern. Full cone
nozzles are most extensively used in industry (Bartell et
The size of droplets is one of the properties normally used for the correlation
of the combustion behaviour. Droplet sizes are generally different for different
fluid properties and even for the same liquid droplets may differ in size due
to influence of other conditions. Furthermore, sprays may be of monodisperse
or polydisperse distributions and thus, there is a need for averaging in order
to determine a the mean size that corresponds to necessary droplet properties
(Lefebvre, 1989). Sauter Mean Diameter (SMD) or D32
is commonly used, which is given by:
where, ni is the number of droplets within a range of is centered
on diameter Di and k is the number of ranges. Factors that
affect the droplet sizes include nozzle type, spraying pressure, flow rate and
spray angle (Lefebvre, 1989). An increase in the liquid
flow rate or pressure through the nozzle will decrease the droplet size, while
a decrease in flow rate will decrease the pressure drop and increase the drop
size. As for spray angle, generally, an increase in spray angle will reduce
the drop size and vice versa.
In studies of sprays involving liquid fuels such as diesel, the experiments can be hazardous and costly due to issues such as the need for repeated measurements, the need to properly discharge the fuel after the end of each experiment and the potential health hazards. It is suggested that replacing the diesel fuel spray with simple liquids such as water in non-combusting experiments may reduce these problems. Thus, in this study, the similarity between the characteristic droplet size of water and diesel sprays is studied using Phase Doppler Anemometer (PDA) system.
MATERIALS AND METHODS
A schematic of the spray rig is shown in Fig. 1. The system comprised of a tank, which was used to supply the liquids to the nozzle. A stainless steel centrifugal pump, which maximum capacity was rated at 4 bars, was used to supply pressure to the nozzle. A pressure gauge was mounted at the discharge side of the pump to indicate the liquid pressure. A back pressure valve was used to return the flow back to the tank. An F-75s full-cone spray nozzle manufactured by Akoka was used to produce the spray. The nozzle can be operated at pressures of up to 5 bars. The air inlet diameter was 6.35 mm and the nozzle outlet diameter was 1.3 mm.
Phase Doppler Anemometry (PDA) was used to measure droplets sizes of both liquids.
The system consists of a transmitter, a receiver, a signal processor and a computer.
Laser was split by utilization of a beam splitter and frequency shift module.
With a lens mounted on the transmitter, the two lasers would intersect at a
point referred as probe volume or measurement volume. When a drop passes through
the probe volume, the scattered light forms an interference fringe pattern.
The phase shift between the scattered light and incident light enabled quantification
of the droplet diameter through the use of data processing in the computer using
the BSA Flow Software. The PDA system measures droplet sizes individually instead
of globally such as that featured by systems like Malvern Particle Sizer or
Digital Image Analysis Technique (Lad et al., 2011)
and thus it is also known as a point measurement system. Detailed description
of this setting was reported by elsewhere (Sulaiman and
|| Schematic of the spray rig and liquid flow loop
|| Measurement points relative to nozzle tip
The experiments were conducted at the different distances from the nozzle tip,
as shown in Fig. 2, for different spray pressures and liquid.
The readings were taken only on the middle plane of the sprays when they were
fully developed. The horizontal and vertical intervals between the measurement
points were 0.5 cm. Assuming that the sprays were almost symmetrical, measurements
were conducted for only half portion of the sprays.
RESULTS AND DISCUSSION
The experiments on the sprays were conducted to determine the characteristics of water and diesel spray by using the PDA systems. The primary data provided information on the data counts and the individual droplets diameters. The SMD values were calculated by using Eq. 1.
Shown in the Fig. 3a is a typical distribution of water droplet
diameters at a vertical distance of 0.5 cm below the nozzle tip at a pressure
of 50 kPa. The data are in the range of between 200 μm and 1000 μm.
The SMD at this set of data is 736 μm with a standard deviation of 218
μm. Clearly the spray droplets are polydisperse since due to the wide range
of diameters as verified by the large standard deviation.
||Typical frequency of droplet diameters for (a) Water and (b)
Diesel recorded using the PDA system
|| SMD of water and diesel sprays at 50 kPa
The same trend in Fig. 3a is also displayed by the typical
diesel sprays shown in Fig. 3b which was obtained at the same
pressure and same location relative to the nozzle tip. Also a polydisperse spray,
the SMD for the diesel spray is 715 μm with a standard deviation of 224
The comparison of Sauter Mean Diameter (SMD) between water and diesel at 50 kPa at different vertical distances from the nozzle tip is shown in Fig. 4. Water is shown to have slightly larger SMDs than that of diesel although the difference becomes smaller at farther vertical distance from the nozzle tip. The largest SMD value is at 0.5 cm distance from nozzle tip which is around 740 μm and the smallest value is around 670 μm. The SMD of water decreases slightly with the distance from the nozzle tip. For diesel, the SMDs are almost constant around 700 μm at each distance. Both liquids display nearly the same pattern i.e., decrease in SMD with the vertical distance.
Figure 5a shows the comparison of Sauter Mean Diameter (SMD)
between water and diesel sprays at 100 kPa at different vertical distances from
the nozzle tip. It displays the same pattern as in Fig. 4,
in which water has the larger SMD than that of diesel sprays at any vertical
distance from nozzle tip except at 2.5 cm. The largest SMD value (740 μm)
is measured at 0.5 cm below the nozzle tip and the smallest value is around
650 μm. The SMD of water decreases with the distance from the nozzle tip.
There is a sudden drop by about 70 μm at the point of 2.5 cm below the
nozzle tip. For diesel, the SMD are almost constant around 700 μm at each
distance. The trends of SMD variation with distance for both liquids in Fig.
5a are shown to be of similar patterns to that displayed in Fig.
4 i.e., SMD decreases with the vertical distance.
Shown in Fig. 5b is the comparison of Sauter Mean Diameter (SMD) between water and diesel sprays at 150 kPa at different vertical distances from the nozzle tip. The SMD for water is shown to be larger than that of diesel at any location. The largest SMD value (750 μm) is at 0.5 cm below the nozzle tip and the smallest value being around 710 μm. The SMD of water decreases with the vertical distance from the nozzle tip. There is a sudden drop by about 60 μm at the distance of 2.5 cm below the nozzle tip. For diesel, the SMD are displayed to be almost constant at about 700 μm. The SMDs for both liquids show similar patterns as those in Fig. 4 and 5.
Measurements were also conducted to determine the Sauter Mean Diameter (SMD)
at different radial distances for both water and diesel sprays. The distance
between points to point was 0.5 cm. In general, it was found that the trends
of SMD in radial direction were of mixed values. Table 1 and
2 display the SMD values at different radial distances for
water and diesel sprays.
|| Droplet sizes for water and diesel sprays at (a) 100 kPa
and (b) 150 kPa
|| Sauter mean diameter (SMD) of water droplets at different
spray pressure and radial distance from nozzle
|| Sauter mean diameter (SMD) of diesel droplets at different
spray pressure and radial distance from nozzle
It was assumed that the sprays were symmetrical and thus measurements were
performed only on one side of the spray as shown earlier in Fig.
2. There is a large difference in SMD values between the middle point measurement
and radial distance of both liquids. The SMD at middle region of the spray are
generally larger in size than that at locations away from the spray axis. For
the SMD of radial distance, the values are approximately constant with distance.
The SMD of water are larger than that of diesel at any point. There is a sudden
decrease in SMD at 0.5 cm away from the axis but the SMD values start to increase
back until the border of the spray. As the pressure increases, the SMD is shown
A study on the similarity between water and diesel sprays was attempted by comparing the temporal distributions of Sauter Mean Diameter (SMD). The pressures were varied at 50, 100 and 150 kPa. From this work, the following conclusions can be drawn:
||The sprays of water and diesel are polydisperse as the droplets
has a wide range of diameters
||Both water and diesel sprays have the largest droplet sizes close to the
nozzle tip (0.5 cm below) at any pressure. At 2.5 cm away from the nozzle,
the droplet sizes for both liquids were found to be the smallest
||Generally water has the larger droplet sizes that of diesel at each pressure
||As the pressure increases, the SMD for both liquids at the center increases
and as the vertical distance from nozzle tip increases, the SMD decreases
||As the pressure increases, the SMD for both liquids decreases in radial
distance away from the axis
There is only a small difference between the values of SMD for water and diesel sprays. On average, their trends of SMD distributions are almost the same. Therefore, it can be concluded that the SMD of water and diesel are identically same and it is probable that water sprays can be used to represent diesel sprays in non-combusting experiments pertaining to droplet diameters.