CHAPTER THREE
3. DEVELOPMENT OF HYPOTHESES AND RESEARCH MODEL 1 Hypotheses
3.1.1 Formulating hypotheses
The droplet size and velocity distributions for Jet-A1, diesel, PME and RME on one side of the centreline are shown in Fig. 5.26. The droplet concentration and volume flux profiles, normalized with their respective peak magnitudes of their profiles at y = 10 mm, are superimposed on the droplet size-velocity distributions. The general trend for all fuels shows the distribution of smaller droplet SMD at the centreline region of the spray. The droplet velocity at the spray centreline is relatively lower. The droplet velocity peaks located at a radial distance from the centreline before decreasing towards the spray boundary. The lower droplet velocity at the centreline is due to the direct competition between the strong centre reverse flow induced by the centre toroidal recirculation zone and the droplet axial velocity from the atomizer outlet. The adverse pressure gradient between the spray core region and the main swirling flow is induced by the aerodynamic blockage of the bluff body and the radial pressure gradient induced by the swirl, thus forcing the flow to recirculate towards the inner core from
Reacting spray results downstream. The stronger reverse flow at the centreline reduces the droplet velocity while causing the spray to spread radially. For profiles at y = 10 and 15 mm downstream, the droplet SMD shows the gradual increase towards the spray boundary.
The larger droplets at the spray boundary region attain a low droplet velocity at a low concentration of droplets. The droplets at y = 20 mm downstream however, show a rather evenly distributed droplet size that still attain a significant value of velocity.
The scarce droplets at the spray centreline and boundary are completely vaporised.
The superimposed droplet concentration and volume flux profiles shows the spatial distribution of droplets within the spray flame. At y = 10 mm downstream, the droplet concentration peak moves radially outwards. The droplet volume flux profiles show a distribution at different spatial locations due to the weighting effect of droplet sizes. However, the droplet concentration volume flux profiles coincide at the same spatial locations for y = 15 and 20 mm. The overall spray structures and the droplet distribution trends of the fuels considered are almost similar, despite the difference in magnitude. PME and RME show very similar spray structures within the flame. This is expected due to the similar physical characteristics between the two fuels.
0 8 16 24
10 15 20 25
=8 μm
=30 ms-1
(a) Jet-A1 flame
Axial position (mm)
Radial position (mm)
(x50) (x2)
(x20)
0 8 16 24
10 15 20 25
=8 μm
=30 ms-1
(b) Diesel flame
Axial position (mm)
Radial position (mm)
(x10) (x20) (x2)
Reacting spray results
0 8 16 24
10 15 20 25
=8 μm
=30 ms-1
(c) PME flame
Axial position (mm)
Radial position (mm)
(x2)
(x1) (x20)
(x8)
0 8 16 24
10 15 20 25
=8 μm
=30 ms-1
(d) RME flame
Axial position (mm)
Radial position (mm)
(x2) (x20)
(x1) (x8)
Figure 5.26: Radial distribution of mean ( ) droplet velocity, (o) SMD, (—) concentrations (#/cm3) and (---) volume flux (cm3/cm2/s) for (a) Jet-A1, (b) diesel, (c) PME and (d) RME at the axial locations of 10, 15 and 20 mm downstream of the atomizer outlet under the same power output condition of 6 kW.
The droplet distribution and trajectories within the swirling spray flame are investigated for the spatial positions indicated in Fig. 5.27. The investigated swirling flame here is established using PME with the fuel mass flow rate of 0.16 g/s at the global equivalence ratio of φ = 0.47. The probability density function (PDF) of the droplet size and velocity distributions is shown in Fig. 5.28. At the centreline of the axial profile y = 10 mm, the droplet velocity shows a normal distribution that spans between -35 and 110 m/s. The droplet density is high at this particular location as shown in Fig. 5.25 and most droplet sizes are distributed within the mean diameter size D10 of 3 μm. The droplet velocity-size correlation shows the wide range of velocities attained by these droplets at this position. The negative velocity exhibited by the droplets indicates the influence of the reverse flow on the spray induced by the central recirculation zone.
Reacting spray results
Figure 5.27: Locations where the transport of droplets within the spray flame was investigated.
The droplet count at the centreline position of y = 15 mm is significantly low due to the close proximity to the flame reaction zone. The intense heat propagated from the flame vaporises most of the small droplets within the upstream distance of y = 0 and y = 15 mm. The remaining droplets at this position show the size distribution that is biased towards smaller drop diameters as shown in Fig. 5.28e.
Evaporation of the droplets results in the decrease of droplet size and subsequently lowers the droplet momentum and velocity. The droplet size-velocity correlation shows the droplet velocity that spans between 0-80 m/s. At the radial position x = 5 mm from centreline and y = 15 mm, the droplet volume flux is relatively high. This location also corresponds to the peak velocity for the radial profile of y = 15 mm. The PDF of the droplet velocity shown in Fig 5.28g indicates a wide distribution of velocity that ranges between 0-100 m/s, similar to the distribution shown in Fig. 5.28a. The droplet size distribution shows a skewed histogram that is biased to small droplet sizes.
The drop size-velocity scatter plots in Figs. 5.28c, 5.28f and 5.28i show that larger droplets attain higher velocities compared to some of the smaller droplets. This indicates the radial spreading of the spray due to the influence of the centreline reverse flow. The change in direction of the droplet trajectories from the centreline axial position is important as the droplet distribution governs the shape of the flame. The
Centreline
(b) y=15, x=0
(a) y=10, x=0 (c) y=15, x=5
(d) y=20, x=7
In mm
Reacting spray results dense fuel vapours at the location where the droplet volume flux is high diffuses into the flame zone for local reactions. At the downstream location of y = 20 mm and radii of x = 7 mm, the presence of larger droplets is evident. The droplet velocity shows a narrower distribution (0-80 m/s) due to the reduced presence of smaller droplets. The larger droplets exhibit the droplet velocity that ranges between 30 and 50 m/s as shown in Fig. 5.28l.
-500 0 50 100 150
0.006 0.012 0.018 0.024
Axial velocity (ms-1) (a) x=0 mm, y=10 mm, PME
0 3 6 9 12
0 0.1 0.2 0.3 0.4
D10 (μm) (b) x=0 mm, y=10 mm, PME
0 8 16 24 32 40
-40 0 40 80 120
Axial velocity (ms-1)
D10 (μm) (c) x=0 mm, y=10 mm, PME
-400 0 40 80 120
0.01 0.02 0.03 0.04 0.05
Axial velocity (ms-1) (d) x=0 mm, y=15 mm, PME
0 6 12 18 24 30
0 0.05 0.1 0.15 0.2 0.25
D10 (μm) (e) x=0 mm, y=15 mm, PME
0 10 20 30 40 50
-30 0 30 60 90 120
Axial velocity (ms-1)
D10 (μm)
(f) x=0 mm, y=15 mm, PME
0 50 100 150
0 0.005 0.01 0.015 0.02 0.025
Axial velocity (ms-1) (g) x=5 mm, y=15 mm, PME
-5 3 11 19 27
0 0.05 0.1 0.15 0.2 0.25
D10 (μm) (h) x=5 mm, y=15 mm, PME
0 10 20 30 40 50
-30 10 50 90 130
Axial velocity (ms-1)
D10 (μm)
(i) x=5 mm, y=15 mm, PME
0 40 80 120
0 0.01 0.02 0.03 0.04
Axial velocity (ms-1) (j) x=7 mm, y=20 mm, PME
0 6 12 18 24
0 0.05 0.1 0.15 0.2
D10 (μm) (k) x=7 mm, y=20 mm, PME
0 10 20 30 40 50
-30 0 30 60 90 120
Mean velocity (ms-1)
D10 (μm)
(l) x=7 mm, y=20 mm, PME
Figure 5.28: The distribution of the droplet (a,d,g,j) axial velocity (b,e,h,k) SMD and (c,f,i,l) droplet size-velocity correlation of the PME swirl flames at locations a, b, c and
Reacting spray results