How does the Design of Ultrasonic Nozzles Affect Flow Velocity?
Unlike traditional pneumatic nozzles, ultrasonic nozzles do not rely on the force of air to decompose liquid flow for atomization. On the contrary, atomization is generated by a completely different process. In the case of traditional pneumatic nozzles, the flow rate or liquid velocity is mainly driven by the pressure (psi) used in the system, that is, the higher the air pressure, the higher the flow rate. On the other hand, ultrasonic nozzles do not necessarily rely on air pressure to control flow. The obvious advantage of not requiring air to generate atomization is that the resulting plume has almost no kinetic energy. Ultrasonic nozzles can be considered as airless systems under certain conditions. Air shaping is commonly used to shape atomized plumes to give them strength and direction. In this case, the air is used as an auxiliary, and the real airless system is limited to applications using plumes, such as chemical vapor deposition, spray pyrolysis, spray drying, vacuum applications, etc. Typically, it involves applications that involve chambers and generate plumes.
So if the flow rate is not a function of air, what drives the liquid velocity in the case of an ultrasonic nozzle? The answer is the pumping mechanism and the orifice or hole that distributes the liquid onto the surface of the ultrasonic nozzle for atomization.
Hole:
When liquid is pumped through an ultrasonic nozzle, it is crucial that the flow rate does not exceed the critical speed designed by the nozzle. The critical velocity is when the liquid leaving cannot correctly wet the tip of the nozzle. In an extreme example, if the pump causes the nozzle to "urinate" in layman's terms, the liquid velocity far exceeds the critical velocity. Even if the nozzle is powered off, this situation will still exist. More commonly, it occurs when the critical speed is only moderately exceeded. The generated atomization is incomplete, where some liquids wet and atomize appropriately, while other materials do not atomize and produce large droplets. For this reason,
The other extreme of critical velocity is when the liquid velocity or flow rate is too low, causing the liquid to not wet concentric on the tip correctly. Imagine water flowing out of a garden hose under high pressure and a small stream flowing out of a large sewer. A garden hose, the water flowing out will contact all parts of the nozzle opening, while the drainage pipe of the sewer will only allow the water to contact the lowest point opening. To achieve proper atomization, a minimum flow rate is required to allow the liquid to wet 360 degrees appropriately at the outlet point.
Ultrasonic nozzles can have various aperture sizes for a given frequency and tip design. Understanding the application requirements is important, therefore it is necessary to know the flow rate range. FUNSONIC can help you determine the appropriate frequency nozzle, nozzle design, and orifice.
Pump:
Ultrasonic nozzles can be used with various liquid delivery systems, such as infusion pumps, gear pumps, peristaltic pumps, pressure tanks, and gravity fed reservoirs. No matter what system is used, as long as the liquid is transported in a stable flow within the working range of the nozzle, any of these systems can work. However, pulsation should be avoided as it can cause the liquid to fall outside the operating range, even temporarily. This is particularly evident in low flow applications such as bracket coatings. It is not uncommon to expect the nozzle to be pushed below its design range, which can result in poor atomization and/or pulsation.
