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Adrian Brown
Adrian Brown

Atomization And Sprays Lefebvre.pdf

The main benefit of pressure-swirl atomizers is their high energy-efficiency, and many industrial processes use them because of their combination of reliability, ability to achieve droplets of small dimensions, and high performance [20]. In many widely used pressure swirl atomizers, a liquid enters the swirl chamber through a number of tangential holes or slots. Centrifugal force causes the liquid to spread within the chamber as a hollow conical spray, with spray angles ranging from 30 to almost 180 depending on the application. Atomization occurs not only because of the break-up of the liquid sheet but also because of collisions between droplets and the interaction between droplets and air [21,22,23]. The finest atomization occurs at high pressures and at wide spray angles [24]. Atomizer performance has been found to be related to physical and experimental properties such as surface tension, viscosity, mass flow rate, density and injection pressure [25,26,27,28]. The mean diameter of drops is strongly associated with the injection pressure. With an increase in the pressure-drop across the atomizer, the liquid exits from the nozzle with a greater velocity, which then creates more intense disturbances on the liquid surface, increasing the quality of atomization. The effect of varying the injection pressure is usually more visible at low injection pressures than at high injection pressures [29]. Rashad et al. found that increasing the injection pressure from 8 bar to 12 bar decreases the D32 drop diameters from 69 µm to 55 µm [30]. However, if increasing pressure from 2 to 10 bar reduces the drop diameters by 45%, then an increase from 10 to 20 bar reduces the drop diameters by 28% and from 20 to 90 bar the reduction rate is just 42% [31]. It has been found that there exists a critical injection pressure of 15 bar beyond which the spray angle and drop sizes become practically independent of the pressure [32].

Atomization And Sprays Lefebvre.pdf

The influence of operation conditions on the atomization quality of the atomizer assembly. (a) dt = 2.0 mm, dn = 0.6 mm, ALR = 0.69, Pw = 2.5 bar, Pa = 0.35 bar; (b) dt = 2.0 mm, dn = 0.6 mm, ALR = 1.19, Pw = 3 bar, Pa = 1 bar; (c) dt = 2.4 mm, dn = 0.8 mm, ALR = 0.67, Pw = 2.5 bar, Pa = 0.35 bar; (d) dt = 2.4 mm, dn = 0.8 mm, ALR = 0.97, Pw = 2.9 bar, Pa = 1 bar.

Ambient pressure plays a role part in forming the spray angle, and many research focused on this point tries to reveal the relationship between ambient pressure and spray angle. Wang and Lefebvre [7] focused on high ambient pressure, and the result shows that the basic effect of an increase in air pressure is to improve atomization, but this trend is opposed by contraction of the spray angle which reduces the relative velocity between the drops and the surrounding air and also increases the possibility of droplet coalescence. Sovani et al. [8] studied the spray cone angle in high ambient density environment for different injection pressures and ambient pressure. Kim et al. [9] investigated the influence of ambient gas pressure on the spray cone angle of single swirl atomizer and liquid sheet fragmentation experimentally; the result shows that the measured spray angles according to the ambient gad density differed before and after the sheet broke up and put forward a kind of boundary mathematical model of spray cone suggesting that density of gas has a deep influence on the spray cone angle. Chen and Yang [10] studied the effect of ambient pressure on the flow dynamics of a liquid swirl injector by means of a combined theoretical and numerical analysis; the result suggests that the pressure drop across the liquid sheet downstream of the nozzle exit increases, thereby suppressing the liquid expansion in the radial direction and decreasing the spreading angle. Kenny et al. [11] studied the flow inside the swirl atomizer and the spray cone angle with combustor of high pressures by experiments, the cone angle decreases when ambient gas pressure increases, but no specific analysis for the causes of the change was provided. Chen et al. [12] studied the spray characteristics of swirl atomizer by using a high-speed shadowgraph system under high ambient pressure; the results show that spray is suppressed by ambient pressure, a critical pressure value was found at which the spray structure converts from a wide hollow cone to a narrow contracting bell, and the discharge coefficient increases while the spray cone angle decreases. Lee et al. [13] reveal that high ambient pressure decreases the vertical extent of the spray penetration and its area.

It should be noted that a very detailed observation of the process of low ambient pressure atomization is needed for a deeper understanding; however, this is beyond the scope of the current work. Such diameter of a droplet will be investigated in our future works.

Based on the energy-based approach, atomization occurs because of kinetic energy loss. The resulting formulation reveals that the MDD is inversely proportional to the atomization efficiency and liquid Weber number.


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