Elevated OH production from NPHFD and its effect on ignition

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Abstract

This work explores ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous OH planar laser-induced fluorescence (PLIF) and schlieren imaging were utilized at a frequency of 50 kHz to examine the time dependent radical generation during and after the plasma discharge. The results indicate that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of the pulse repetition frequency (PRF). In addition, it is demonstrated that the intensity of the OH-PLIF signal during and shortly after the discharge is elevated for PRF≥10 kHz, indicating that both the volume and concentration of OH are built up at high PRF. This accumulation of OH radicals in the inter-electrode region is directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. It is shown that in cases in which the PRF is below a certain threshold (<10 kHz), OH accumulation does not occur, and it is believed that this condition, along with lower discharge temperatures, is directly responsible for reduced ignition probability.

Introduction

Ignition in reactive flows is a competition between heat and radical generation and loss mechanisms across a range of temporal (ns-ms) and spatial (µm-cm) scales [1], [2]–3]. The interplay between these scales via the governing phenomena of energy deposition, thermalization, advection/convection, and chemical heat release is critical to ignition success, where a mismatch can impede the development of self-sustained flame propagation. For example, some scenarios/environments—diluted mixtures within internal combustion engines or high altitude relight within gas turbine engines—lead to insufficient chemical heat release to overcome the inherent convective loss mechanisms. The primary means of overcoming these challenging environments is to deposit a larger quantity of energy to overdrive the initial ignition kernel development. Unfortunately, this approach has diminishing benefits since it only affects the earlier time scales (energy deposition duration is on the order of 10–100 µs) and does little to affect the many loss processes involved prior to self-sustained flame propagation.

An alternative means to aid ignition in flowing environments is to increase the duration of the energy deposition to traverse longer time scales in order to interact with and manipulate the competing phenomena. This is typically performed with inductor-based systems and allows for energy deposition on the order of milliseconds in order to drive ignition kernel development. While the long duration spark discharge interacts with the flow to enlarge the kernel, the arc tends to elongate, quench, and reinitiate, producing uncontrolled, variable energy deposition [4].

Recent studies using nanosecond-pulsed high-frequency discharges (NPHFD) in reactive flows have allowed for control of the energy deposition across a range of time scales from ns to ms [5,6]. A burst of short-duration pulses (approximately 10 ns full-width half-maximum per pulse) at high pulse repetition frequency (PRF) (up to 300 kHz) allows for fixed energy-per-pulse but variation of inter-pulse times, producing average power levels during a burst of 1–1000 Watts (using for example 3-mJ energy deposition per pulse). The previous studies indicated that ignition using this method can produce multiple regimes of “inter-pulse coupling,” resulting in different degrees of ignitability [5]. The different degrees of ignitability are caused by the interaction amongst critical time scales, particularly the timescale of thermal expansion and early flame propagation, advection of the ignition kernel by the bulk flow, and heat losses to the electrodes and surrounding gas [7,8]. For lower PRF, the interaction between discharges (or incipient ignition kernels) occurs downstream between discrete ignition kernels after leaving the inter-electrode gap due to convective motion. This is termed the “decoupled” regime. At the highest PRFs (up to 300 kHz), a synergy between discharges occurs in the inter-electrode gap and produces significantly elevated temperatures above the single-pulse values [7] that can persist in the kernel to drive increased ignition probability and more rapid kernel growth rates [8]. This is termed the “fully coupled” regime. In the transition between these two distinct regimes (at intermediate PRFs), the ignition probability is reduced as compared to both of the other regimes. This is termed the “partially coupled” regime.

For the different ignition regimes, while the factors determining heat generation are fairly well understood, radical production and loss are not well understood, and measurements of the species concentrations remain limited. In the work by Stancu et al. [9], two-photon absorption laser-induced fluorescence (TALIF) measurements showed significant production of atomic oxygen for a single nanosecond pulse, which was verified by modeling from Popov [10]. The results showed that a large portion of the deposited energy dissociated the O2 via quenching with excited N2 and highlighted one of the main initiators for chemical heat release [11]. In the work by Stepanyan et al. [12], planar laser-induced fluorescence (PLIF) measurements showed significant OH production from a single nanosecond pulse and then OH redistribution because of the discharge-induced flowfield. While these works highlighted the significant benefits of repetitive pulsing for radical production in a (near) quiescent environment, the inclusion of a moderate flow (of order 10 m/s) adds a new dimension in which pooling and sustainment of elevated radical concentrations over a large volume driven by the flow may be possible. This pooling process has been predicted numerically (i.e. [13,14]) but not yet demonstrated experimentally, as both high repetition rate and spatially resolved diagnostics for low concentration radical species are required for such an effort.

The current study aims to fill this gap by exploring the role that inter-pulse coupling plays in radical generation across the different ignition regimes. Simultaneous time-resolved 50-kHz schlieren imaging and PLIF measurements of OH generation in the discharge and developing ignition kernel were performed across a range of PRFs in a methane-air flow. The impact of PRF on the quantity and distribution of OH and the connection to ignitability is presented and discussed.

Section snippets

Flow tunnel

All experiments were performed in a tunnel with a square cross section of side-dimension 38.1 mm. Flow is introduced into a plenum and passed through multiple meshes to ensure uniform flow is achieved. The reactants of methane (99% purity) and dry air were metered via mass flow controllers with measured uncertainties of ±2%. The pressure and temperature in the tunnel were maintained at 100 kPa and 295 K, respectively. A pin-to-pin electrode geometry (1.6mm diameter lanthanated tungsten with

Results and discussion

Sample overlay OH-PLIF and schlieren images of the ignition events resulting from a sequence of 10 pulses at PRF=2, 5, 10, 25, 50, 100, and 200 kHz are provided in Fig. 2, along with the inter-pulse time (IPTtriple bond1/PRF). In addition, magnified OH-PLIF images are provided in Figure S1 for added clarity up to 165 µs after the discharge. The image sequences in Fig. 2 are of individual ignition events extracted from the 20 repeated experiments at each condition. While out of plane motion of the kernel

Conclusions

Simultaneous time-resolved (50-kHz interrogation frequency) schlieren imaging and OH planar laser-induced fluorescence measurements were conducted and have shown the unique benefits of high PRF in the context of radical production. For methane/air mixtures at ϕ=0.6, bulk flow velocity of 10 m/s, and an electrode gap distance of 2 mm, PRF values >5 kHz resulted in overlapping regions of OH formation, and PRF≥10 kHz produced an accumulation of OH that can remain elevated throughout the burst of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

A portion of the research was performed while the corresponding author held a National Research Council Research Associateship Award at the United States Air Force Research Laboratory.

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