Improvement of lean blow out performance of spray and premixed swirled flames using nanosecond repetitively pulsed discharges

Abstract

Plasma-Assisted Combustion (PAC) has shown potential in improving the ignition, extinction, and dynamic performance of combustion systems. In this work, nanosecond repetitively pulsed (NRP) spark discharges are applied to extend the lean blow out (LBO) limit of the SICCA-Spray burner. This laboratory-scale atmospheric test rig is equipped with a swirl spray injector representing in an idealized fashion a single sector of a gas turbine. Three fuels and injection conditions are considered: perfectly premixed methane–air, liquid heptane, and liquid dodecane injected as hollow cone sprays. The optimal electrode position that extends the LBO limit is found to be near the external edge of the outer recirculation zone (ORZ). Spectroscopic measurements show that the NRP sparks produce atomic species and heat the gas above the adiabatic flame temperature. High-speed chemiluminescence images of blow out sequences indicate that the flame evolves similarly for all three fuels from “M” or “V” shapes prevailing at $\varphi =0.9$ to a configuration where chemical conversion also takes place in the ORZ at $\varphi =0.63$. A low frequency combustion oscillation arises near the LBO limit ($\varphi =0.57$). Spray flames blow out at this point, while the plasma-assisted ones continue to burn. It is shown that PAC provides a significant improvement of the extinction performance, in particular when operating with liquid fuel spray injection.

Introduction

Premixed and pre-vaporized lean combustion allows for reduced pollutant emissions in gas turbines and aero-engines. In order to control this type of combustion, manufacturers rely to a large extent on advances in swirling injector design to anchor compact flames with a high degree of air dilution. Although the lean stability performance of traditional swirling injectors is already quite good, an increase of the lean blow out (LBO) margin is still desirable, especially for operational and safety reasons [1]. For twenty years, plasma-assisted combustion (PAC) has been considered to improve three key areas of combustion: ignition [2], combustion instabilities [3], [4] and extinction [5], [6]. Recent works in these directions are reviewed in [7], [8], [9], [10].

Non-equilibrium NRP (nanosecond repetitively pulsed) spark discharges last a few nanoseconds, with an overvoltage of a few kilovolts. Applied at a repetition rate in the 10 to 100 kHz range in a combustible mixture, they produce thermal, chemical, and hydrodynamic effects [11], [12], [13] that promote the combustion process [2], [4], [6], [14]. These benefits have been extensively demonstrated at low and atmospheric pressures, and some recent studies [15], [16] have also shown positive effects for the ignition of lean mixtures at pressures up to 16 bar. It was shown [14], [17] that the discharge serves as a localized source of heat (thermal effect) and active species (chemical effect), anchoring the flame even in very lean mixtures. In a recent study, Kong et al. [18] used a continuous AC-powered plasma in a methane–air flame and suggested that the predominant contribution might be from thermal effects, probably due to the higher duration and power of their discharge.

In most practical systems, the LBO limit is dictated by the design of the injector and operating parameters. NRP discharges have successfully been used to extend the LBO limit in laboratory scale test rigs, often with minimal modifications to the combustor. The electrode is generally located on the centerline of the burner, close to the injector outlet, inside a gas recirculation zone with low flow velocities [3], [5], [6], [19]. Even in very lean mixtures, well below the extinction limit, a reaction zone is formed in the vicinity of the electrode. The NRP discharges thus establish a pilot flame that extends the LBO limit of the combustor. It is likely, however, that combustion is incomplete under these conditions [14].

In practical applications, specifically in aero-engines, positioning the electrode on the injector centerline may not be practical. The solid electrode will perturb the flow and symmetry, create additional flame anchoring points and also get degraded by the resulting heat fluxes. Moreover, liquid fuel atomizers are commonly located on the centerline to ensure a homogeneous spray distribution, and the discharge might form between the electrode and the atomizer head. To avoid damaging this sensitive component, in the present study, the electrode is placed close to the lateral wall and the chamber backplane, in the outer recirculation zone (ORZ). Thus, the discharges will occur in an area where the reactive species have sufficient time to mix with the fuel and oxidizer.

In this work, three fuels are considered: premixed methane–air (for a baseline), liquid heptane and liquid dodecane, which is comparatively less volatile [20]. We show that the LBO limit of these swirling flames is extended with NRP discharges.