Internal flow and air core dynamics in simplex and spill-return pressure-swirl atomizers

Abstract

It is well known that the spray characteristics of pressure-swirl atomizers are strongly linked to the internal flow and that an unstable air core may cause instabilities in the spray. In this paper, a 10:1 scale transparent Plexiglas (PMMA) model of a pressure-swirl atomizer as used in a small gas turbine is introduced. The internal flow was examined using high-speed imaging, laser-Doppler anemometry and computational fluid dynamics tools. The experimental and numerical results were analysed and compared in terms of the air core morphology and its temporal stability. Two different liquids were used, Kerosene-type Jet A-1 represented a commonly used fuel while p-Cymene (4-Isopropyltoluene) matched the refractive index of the Plexiglas atomizer body. The internal flow characteristics were set using dimensionless numbers i.e. the Reynolds number and Froude number. The flow test conditions were limited to inlet Reynolds numbers from 750 to 1750. Two atomizers were examined to represent a Simplex and Spill-return (SR) geometries. In a comparative manner, the SR atomizer features a central passage in the rear wall of the swirl chamber. The main advantage of this concept is that the fuel is always supplied to the swirl chamber at a high pressure therefore providing good atomization over a wide range of the injection flow rate. However, the presence of the spill orifice strongly affects the internal flow even if the spill-line is closed. The air core in the Simplex atomizer was found fully developed and stable. The SR atomizer behaved differently, the air core did not form at all, and the spray was therefore unstable. KeywordsInternal flow dynamics, Pressure-swirl, transparent nozzle, CFD Introduction Pressure-swirl (PS) atomizers are used in many applications where a large surface area of droplets is needed or a surface must by coated by a liquid e.g. combustion, fire suspension or air conditioning. PS atomizers are easy to manufacture, reliable and provide good atomization quality. They convert the pressure energy of the pumped liquid into kinetic and surface energy of the resulting droplets. The pumped liquid is injected via tangential ports into a swirl chamber, where it gains a swirl motion, under which it leaves the exit orifice as a conical liquid sheet. The centrifugal motion of the swirling liquid creates a low-pressure zone in the centre of the swirl chamber and generate an air core along the centreline. The flow inside the atomizer is rather complex; it is two-phase with secondary flow effects. There is a strong link between internal flow conditions and the resulting spray characteristics, however, not all aspects of the internal flow are well understood. Before the advent of computational fluid dynamics, a number of authors attempted to describe the internal flow by relatively simple analytical approaches. One of the first was presented by Taylor [1] which focused on an inviscid analysis using Bernoulli's equation and the principle of maximal flow. Taylor derived an equation for the discharge coeffi...

Injector and Combustion Chamber Design for a Nitrous Oxide and Ethanol Rocket Engine

“…Different spray patterns were introduced, and the spray cone angle is given an equation for computing it, which is also a function of geometrical properties of the injector. (Maly, Jedelsky, et al, 2018) and (Maly, Slama, et al, 2019) discuss the numerical simulation of a pressure-swirl atomizer by taking advantage of the axisymmetry of the problem. The Volume of Fluid (VOF) method from ANSYS Fluent commercial software is employed to good agreement with experimental data, using a laminar model.…”