Finite element modeling

Non-reacting spray results

Non-reacting spray results and efficient combustion. The swirl also allows the increase of lateral spreading of the spray and hence the larger spray cone angle. The presence of swirl in the current atomizer also explains the difference in droplet radial distribution compared to other result obtained from atomizer without swirl [5].

At 30 mm downstream from the nozzle tip, the droplet SMD profiles of the fuels considered are rather similar. Jet-A1 fuel shows a slightly noticeable lower SMD value compared to the other heavier fuels. The discrepancy of the droplet SMD become more pronounced at further downstream axial positions. At y = 50 mm from nozzle tip, RME shows distinctively higher SMD values especially near the spray boundary whereas Jet-A1 remains consistently low. At the radial position x = 20 mm of axial location y = 50 mm, the droplet SMD of Jet-A1 droplets is 20 % lower compared to PME and diesel, and 43 % lower compared to RME droplets. The low Jet-A1 SMD indicates the higher tendency of Jet-A1 to breakup and atomize compared to other heavier fuels. This is because Jet-A1 fuel has lower viscosity and surface tension values compared to other heavier fuels. Besides, vaporisation of Jet-A1 droplets occurs more easily due to the higher surface area and the increased convective mass transfer. Diesel and PME exhibit relatively similar SMD values at y = 30 and 50 mm downstream but the difference becomes more pronounced at y = 80 mm. PME shows higher SMD values than diesel due to the higher viscosity and surface tension. The radial distribution of the droplet SMD profile shows a slight increase of ~ 2 μm at the spray centreline region for all fuels. The slight increase of droplet size at the spray centreline region could be due to the entrainment of droplets from the spray boundary as a result of the radial pressure difference induced by the swirling atomizing air flow. Another possibility is the effect of coalescence as a result of high collision rates between droplets.

Non-reacting spray results 5.1.2.2 Droplet mean velocity and RMS velocity distribution

0 6 12 18 24

0 5 10 15 20

Mean axial velocity (m/s)

Radial position (mm) (a) y=30 mm Diesel

Jet A-1 PME RME

0 6 12 18 24

0 5 10 15 20

Mean axial velocity (m/s)

Radial position (mm) (b) y=50 mm Diesel

Jet A-1 PME RME

0 8 16 24 32

0 5 10 15 20

Mean axial velocity (m/s)

Radial position (mm)

(c) y=80 mm Diesel Jet A-1 PME RME

Figure 5.11: The radial distribution of droplet mean velocity profiles at downstream axial positions of (a) 30 (b) 50 and (c) 80 mm from the atomizer outlet under the condition of same fuel mass flow rate of 0.14 g/s and ALR = 2.

The radial distributions of the droplet mean axial velocity at the downstream axial positions of y = 30, 50 and 80 mm from atomizer outlet are shown in Fig. 5.11.

Overall, the plain-jet airblast atomizer shows the characteristic droplet velocity distribution with the highest value at the spray centreline (x = 0 mm) region. The droplet velocity decreases with the increasing radial position towards the spray boundary. Droplets attain the highest momentum near the nozzle outlet but slowly decay as the droplets travel downstream and results in a lower velocity. Comparison of the velocity profiles shows that the four fuels considered exhibit indistinguishable profiles, indicating the independent influence on the fuel physical properties. Instead, the droplet velocity is mainly governed by the momentum of the atomizing air. At ALR = 2, the atomizing air velocity is ~ 100 m/s whilst the liquid jet velocity is ~ 0.8 m/s. The relatively higher velocity of the gas phase shows the dominating influence of air over the liquid. The kinetic energy from the air is used to shear the liquid jet into droplets. At position y = 30 mm downstream of the atomizer outlet, centreline droplet velocity peaks at ~ 17 m/s. The profile shows a narrow curve as the spray is still in a developing phase. Further downstream the spray, the droplets decelerate as the spray spreads wider. The peak velocities at the downstream axial position of y = 50 mm and 80 mm are ~ 13 m/s and ~ 10 m/s respectively.

Non-reacting spray results

0 6 12 18 24

0 2 4 6

RMS velocity (m/s)

Radial position (mm)

(a) y=30 mm

Diesel Jet A-1 PME RME

0 6 12 18 24 30

0 2 4 6

RMS velocity (m/s)

Radial position (mm)

(b) y=50 mm ^Diesel Jet A-1 PME RME

0 6 12 18 24 30

0 1 2 3 4

RMS velocity (m/s)

Radial position (mm)

(c) y=80 mm ^Diesel Jet A-1 PME RME

0 6 12 18 24

0 1 2 3 4

RMS velocity/Velocity

Radial position (mm)

(d) y=30 mm ^Diesel Jet A-1 PME RME

0 6 12 18 24 30

0 2 4 6

RMS velocity/Velocity

Radial position (mm)

(e) y=50 mm

Diesel Jet A-1 PME RME

0 6 12 18 24 30

0 1 2 3 4

RMS velocity/Velocity

Radial position (mm)

(f) ^Diesel_{Jet A-1}^{y=80 mm}

PME RME

Figure 5.12: The radial distribution of rms velocity profiles (a,b,c) and ratio of rms to velocity (d,e,f) at downstream axial positions of y = 30, 50 and 80 mm from the atomizer outlet under the condition of same fuel mass flow rate of 0.14 g/s and ALR = 2.

The corresponding radial distributions of droplet rms velocity at different downstream axial locations are shown in Fig. 5.12a-c. In general, the droplet velocity rms profiles are almost similar for all the fuels tested. The profiles show a peak at a distance from the centreline where the velocity gradient is highest. For the downstream axial location y = 30 mm, the rms velocity is found to increase near the spray boundary as the droplet velocity increase between the radii x = 13 mm and 18 mm.

The profiles at y = 50 and 80 mm show a decreasing trend after the peak near the centreline. However, this does not mean that the droplet velocity fluctuation is low. By dividing the velocity rms with the respective spatial velocity as presented in Fig. 5.12d-f, the ratio shows high fluctuation near the spray boundary due to the presence of a wide range of droplets with different size. Large droplets tend to lag in the flow due to the drag force imposed compared to small droplets. The common trend shown in all

Non-reacting spray results increases towards the spray periphery. The plots show higher degree of scatter when the ratio of velocity rms/velocity is more than 1, where the variability of droplet velocity indicates the presence of the unstable shear layer as the inner jet entrains outer stagnant air.