A Joint Experimental and LES Characterization of the Liquid Fuel Spray in a Swirl Injector

Abstract

The quality of liquid fuel spray injection determines to a large extent the steady state performance and dynamics of gas turbine and aero-engine combustors. The present investigation is focused on the detailed characterization of the liquid fuel spray in a single sector test rig targeted at aero-engine applications. The liquid fuel (heptane) is injected in a hollow cone spray pattern by a simplex atomizer and the injector comprises a radial swirler. Two features of the droplet distribution that are less commonly found in the technical literature are identified. First, the distributions of mean droplet diameters exhibit non-axisymmetric patterns, a lack of symmetry that is investigated for three types of swirlers differing by their swirl number and/or head loss. Second, it is found that the size-conditioned velocity distributions feature a single wide peak for small droplets and become bimodal for the largest droplets, with a first peak at low velocities, and a second one at higher velocities. The spray behavior analysis is complemented by making use of Large Eddy Simulations with Lagrangian Particle Tracking. Droplet injection is achieved with a model in which the initial size and velocity distributions are specified from experimental data in the atomizer near field. The initial spray interacts with the lateral injector surface and requires a droplet-wall interaction model accounting for the existence of a liquid film. Simulations do not retrieve the lack of rotational symmetry that is found experimentally indicating that this is not linked to the nature of the swirling flow. This is also consistent with further experiments with a different atomizer confirming that this is due to imperfections in the initial atomizer geometry. Another result is that certain swirler designs appear to be more robust to these atomizer imperfections. Simulations accounting for the liquid film yield a bimodal distribution for the droplets’ axial velocity distribution which would not be obtained without this model indicating that it is important to represent the droplet-wall interaction, a feature that is not commonly found in the literature.

Do flame describing functions suitably represent combustion dynamics under self-sustained oscillations?

“…The air flow rate measured by a Bronkhorst EL-FLOW mass flow controller with a relative accuracy of 0.6% is injected at the bottom of the plenum. The stream of air enters the chamber through an injection unit described in detail in a recent publication [61]. This unit comprises an air distributor leading to a tangential swirler with six channels.…”

“…A swirler, designated as 716, is used in the present investigation. The swirl number determined experimentally by integrating the velocity profiles at the outlet of the injector at a height of 2.5 mm above the backplane is S=0.70 (refer to [61] for swirler characteristics). The burner is operated at a global equivalence ratio of φ = 0.95 which corresponds to an airflow rate of 2.3 g s −1 and a fuel flow rate of 520 g h −1 .…”

Swirler effects on combustion instabilities analyzed with measured FDFs, injector impedances and damping rates

“…Here R inj = 4 mm is the radius of the injector outlet, d sc is the diameter of the swirler channels and 2R 0,sc is the distance separating the axis of two opposing channels. Adapted from [22,23]. plenum fed by air at atmospheric conditions by a Bronkhorst EL-FLOW mass flow controller with a relative accuracy of 0.6%.…”

“…The plenum is connected to the combustor through a spray-swirl injector. The injector contains an air distributor leading to a tangential swirler with six channels [22]. The passage of air through the swirler channels induces a clockwise rotation of the incoming flow.…”

“…The swirl numbers are determined experimentally by integrating the velocity profiles at the injector outlet at a distance of 2.5 mm above that section. A detailed characterization of swirlers, velocity profiles, and droplet sprays may be found in [22,23]. Liquid heptane fuel is supplied through a central tube in the plenum, and its mass flow rate is set by a Bronkhorst CORI-flow controller with a relative accuracy of 0.2%.…”