A fluid static drop is a drop formed under the influence of the gravity force only. The surface tension force tends to maintain the integration of the fluid bulk, and keep it attached to the tube surface. A drop with a specific mass is formed once the gravity force exceeds the surface tension force, and this happens with no external pressure. The effect of the surrounding air in further drop breakup is neglected here. Although this type of fluid disintegration action seems to be basic and impractical for atomisation applications, the analysis of static drop formation may give an important indication of the influence of fluid physical properties, such as density and surface tension, on the process (or ability) of atomisation using different nozzle diameters.
The figure below shows the static drop diameter against nozzle diameter for four different liquids, water, diesel, kerosene-like Jet-A fuel, and gasoline, indicating the points where the particle diameter equals to the nozzle diameter . This graph could partly justify the diversion in results obtained in particle diameter when applying two different fluids to the same fuel injector. This was clear in our experimental work when the evaluation of an injector was attempted using deionised water then jet fuel.
Once a liquid drop is released into a flowing air, it becomes prone to air pressure upon its surface. The balance between the air pressure and the tension pressure is responsible for the droplet solidity. The air pressure is generated by increasing the relative velocity between the spray and the surrounding air. When the pressure forces around the droplet exceeds the opposing surface tension pressure, the fluid droplet tries to balance the forces again by increasing the surface exposed to the air pressure, which eventually results in fragmentation of the drop into smaller drops within a specific time. This time is called “the droplet breakup time” which increases as the droplet size decreases. Fluid drops keep splitting up until they reach the size where the breakup time is infinite. Here, the effect of the fluid viscosity appears as a force which resists the deformation of the drop by extending the breakup time.
– Lefebvre A H, “Atomization and Sprays”, Chapter 2, Hemisphere Publishing, USA 1989