6. MODELLING OF ATOMIZATION
6.3. Chaotic disintegration model for the size of the drops in spray
The spray generated by an airblast atomizer is influenced by a number of the operating parameters, like the volumetric flow rates of the airflow and of the liquid flow, parameters of the primary spray, pressure in the chamber, material properties of the fluids, etc. Since the mechanism of atomization is extremely complicated the main problem is not the luck in finding formulas relating the operating parameters with the spray parameters.
Such formulations however do not add much to the understanding of the processes occurring inside the atomizer. There is also no confidence that such relations are universal. In this situation the scaling of the problem, relating even one of the parameters with the drop size of the spray can be valuable.
The details of the mechanism of atomization are not considered in the present model. The size of the drop is estimated from the energy balance as in [83].
86
The primary spray creates a liquid film of the thicknessHfilm on the pre-filmer. The flow in the film is accelerated by the fast airflow. At some instant the film breaks up and creates drops. The initial drop velocity is comparable with the film velocity; therefore, the total kinetic energy cannot be used in the energy balance of the atomization. The main assumption is that the reason for the atomization is the liquid film deformation. The kinetic energy of the film deformation goes to the creation of the surface of the drops.
Consider an element in the liquid film of the typical size a. The volume of this element is proportional to a3. Denote γ. the rate of its deformation. Therefore, the kinetic energy of deformation of the element is approximately
. 2
3( )
~ a a
E ρL γ (6.16) During the breakup this energy goes to the creation of the new surface. The corresponding surface energy is estimated as
~ a2
S σ (6.17) The smallest possible drop corresponds to the case when the entire kinetic energy of deformation transforms to the surface energy of the fragment. Therefore, the typical size of the drops, a, can be estimated equating E and S. The resulting expression is obtained in [83]
in the form
3 / 1
. 2
~
γ ρ
σ
L
a (6.18) It is assumed that the main deformation of the liquid and of the gas takes place in a thin turbulent boundary layer. The shear stress at the interface between the liquid and the air can be estimated from the Blasius law in the turbulent boundary layer in the air
4 / 1
~ 2
A A
A A A
w U U
δ ρ ν
τ (6.19)
x x U
A A A
5 / 1
~
−
δ ν (6.20) Now, the rate of deformation of the fluid element in the film at the edge of the pre-filmer (x=L) is basically the shear stress divided by the film thickness and can be estimated in the form
10 / 1 2 / 1
10 / 1 10 / 9 2 / . 1
1 ~
L H U
H L film
A A A L w
film ρ
ν ρ
ρ
γ = τ (6.21)
87
The typical size of the drops in the spray can be thus estimated using (6.18)-(6.21) in the follwong form
5 / 3 3 / 1 15 / 1
15 / 1 3 / 1 3 / 2
~
A A A
film
U L a H
ρ ν
σ (6.22)
In figure 6.23 the dependence of the SMD is shown as a function of the typical size adetermined in expression (6.22). It is shown that SMD is proportional to a:
SMD = 0.3a
This linear dependence confirms our assumption that at the relatively high airflow velocities the mechanism of the liquid film atomization can be well described by the chaotic disintegration theory.
It should be noted that not each deforming liquid flow leads to chaotic disintegration. The atomization condition is the smallness of the typical size a (determined by (6.18)) in comparison with the characteristic size of the entire flow region (in our case the film thickness H). In some cases the experimental values of the drop diameters are comparable with the film thickness. These results are related to the experiments under high-pressure conditions where the air velocity is relatively low. The atomization in such cases cannot be described by the chaotic disintegration theory. The corresponding values of the SMD are marked in figure 6.23 as open circles. As expected, their values lie far from the theoretical line, clearly indicating a different mode of atomization.
Figure 6.23: SMD as a function of the typical size a, for water and kerosene.
88
The new expression for a is also checked for data from other experiments for different operating conditions and found to be applicable. For example, the data provided by [15] for kerosene film breakup at room temperature conditions is used and the results fit well as shown in figure 6.23.
To compare this result with that obtained in the previous section, the expression of a can be normalized to the film thickness H and rewritten as a function of Weg as per the following equation.
10 / 3 30 / 1 15 / 1 30 / 1
30 / 1 15 / 1
g A A
film Hfilm We
L H
a
ρ ν
= σ (6.23)
The theoretical prediction for the ratio a/H behaves as We−g0.3 which is very close to that obtained from the experimental data previously (see the empirical relation (6.15)). The powers of the other terms beside Weg in equation (6.23) are very small; 1/15 and 1/30. These functions change very slowly. Therefore the combination of these terms are almost constant in these experiments.
It should be stated that this model for predicting the SMD of the droplets in the airblast atomizer predicts rather well the Sauter mean drop diameter at the low a/H values. When this value is relatively high the results diverge from the linear dependence as shown in some of the data in figure 6.23.
We should admit that the atomization of the liquid film in the airblast atomizer is rather complicated, but in this work a novel model is introduced which successfully predicts the average diameter of the drops in the spray generated in the airblast atomizer.
The detailed experimental data, collected in the framework of this study and which are used in this model, are listed table 6.2.
89
Table 6.2: Main integral parameters of primary and aiblast sprays.
Chamber pressure Airblast spray Primary spray
Air velocity SMD U SMD
Bar m/s µm m/s µm
1 57.2 54 10.0 55
1 91.1 33 10.6 66
1 130.1 32 11.3 64
1 57.2 58 26.2 47
1 57.2 52 23.2 49
10 5.2 71 5.1 78
10 9.1 98 5.1 78
10 13.0 73 5.1 78
1 57.2 56 36.3 46
1 91.1 41 36.3 46
1 130.1 47 36.3 46
5 10.4 87 19.5 45
90