A Controllable Twin- Fluid Internally Mixed Swirl Atomizer

Abstract

The atomizer serves as an energy conversion mechanism to convert a volume of the liquid into a multiplicity of small droplets and then ejects these droplets so as to produce a high ratio of surface to mass in the liquid phase and thereby achieve high rates of mixing and evaporation. For the current needs, a compact configuration arrangement is needed to meet the design requirements of small volume and high maintenance availability. As compared with the traditional design, the new atomizer designs should possess better energy conversion efficiency (from pressure to kinetic energy). The technique of combining two or more principles has well served in the field of atomization in the past. The drawbacks of one type of atomizer can be overcome by combining it with other types of atomizer, in what is generally referred to as a hybrid atomizer. The atomizer discussed in this paper combines the principles of twin-fluid internally mixed atomizers and conventional pressure swirl atomizers to produce a spray the characteristics of which can be controlled over a wide range of operating conditions. Atomization of liquids is widely used in several applications, e.g. spray combustion, spray-painting, spray drying, crop spraying and many other applications. In some atomizer applications the control of droplet and/or particle size is very critical. In some applications, extremely small droplets are preferred (less than a micron), while in others, droplet diameters on the scale of several hundred microns are required. Spray combustion is used in domestic heating burners, industrial heating furnaces, gas turbines, diesel engines and rockets. For the different applications, a wide range of spray devices have been developed and they are generally designated as atomizers and nozzles (1-3). Spray may be produced in various ways. The process of atomization is a process where a liquid jet or sheet is broken up by the kinetic energy of the liquid itself or by exposure to high- velocity air or gas. Some atomizers accomplish this by discharging liquid at high velocity into a relatively slow-moving stream of air or gas. Examples of this type of atomizers include various forms of pressure atomizers and also rotary atomizers that eject the liquid at high velocity from the periphery of rotating disc. An alternative approach is to expose a relatively slow-moving liquid to a high- velocity air stream. This method is generally known as twin- fluid, air-assist or air-blast atomization. In the context of the present invention, it is worthwhile to mention about the conventional swirl atomizer and an internally mixed atomizer reported in the prior art. In swirl atomizer liquid is made to flow through a swirler before it exits from the atomizer, providing a tangential motion to the liquid. This swirling liquid forms a hollow cone outside the atomizer and this cone breaks up into droplets once the kinetic energy of the molecules of the cone overcomes the surface energy of the liquid. In an internally mixed atomizer, a small amount of gas is introduced into the liquid inside the injector forming a two-phase mixture of liquid and gas. This two-phase mixture produces liquid droplets owing to two-phase flow dynamics. However, due to the absence of any tangential velocity component in the flow (because it is a one dimensional flow), this atomizer always forms a solid cone spray.