Structure of igniting ethanol and n-heptane spray flames with and without swirl

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Abstract

This paper explores the ignition and subsequent evolution of spray flames in a bluff-body configuration with and without swirl. Ethanol and n-heptane are used to compare the effects of volatility. Ignition is performed by a laser spark. High speed imaging of OH*-chemiluminescence and OH-PLIF collected at 5 kHz are used to investigate the behaviour of the flames during the first stages of ignition and the stable flame structure following ignition. Swirl induces a wider and shorter flame, precession, and multiple reaction zones, while the non-swirling flames have a simpler structure. The reaction fronts seem thinner with ethanol than with heptane. The dataset can be used for model validation.

Highlights

► Spark ignition of swirling flames by laser shows timescales of successful and failed sparks. ► Ethanol and n-heptane flames show different reaction fronts. ► Fast-response OH-PLIF system reveals flame evolution from ignition to complete flame establishment.

Introduction

Spark ignition of recirculating spray flames is a topic of technological importance which contains significant fundamental challenges as it involves complicated combustion phenomena [1]. In the present paper, we extend the investigation of Marchione et al. [2] by examining flames of various fuels, flows with and without swirl, a laser-induced breakdown for sparking rather than disturbing electrodes, and we focus on flows with higher bulk velocities. Also, fast imaging diagnostics are deployed to probe the flame structure [3]. There are various phases associated with the successful ignition of a spray burner: (i) kernel initiation; (ii) flame growth; and (iii) overall flame stabilisation. Each of these phases involves a certain stochasticity, which leads to a wide range of different behaviours for a particular spark event [1]. Space limitations do not allow a thorough review of spray ignition here and so the reader is directed to Refs. [1], [2] for a discussion of previous work on the topic. This study aims to measure the timescales of these individual processes, quantities that can be used for validating numerical simulations of ignition, while fast OH-PLIF is also used to examine the flame structure and the development of the kernel in the first stages of growth.

Section snippets

Burner

The burner was previously used by Marchione et al. [2] and slightly modified here, with a replica installed at the University of Sydney. It consists of an outer duct for air injection, a central inner duct for fuel injection, and a combustion chamber (Fig. 1). Air is injected through two opposite slots located at the top of a 35 cm long circular duct of 37 mm inner diameter. The air flow rate is set through rotameters, calibrated by a mass flow controller (Bronkhorst, IN-Flow, [0–600] L/min). The

Ignition probability

In the non-swirling burner, it is not possible to ignite a flame when sparking very close to the bluff-body (z < 4 mm, i.e. z < 0.16 dBB). At a given distance z from the bluff-body, the probability of ignition was found to change from 0 on the air-side to 1 in the jet over a very short distance (typically the spacing is less than the mesh grid, i.e. less than 5 mm). Thus only two conditions with ignition probabilities of respectively zero and 100% were chosen at each spark location for the

Conclusions

An experimental study of laser ignition on lab-scale non-swirling and swirling spray flames has been conducted. Ethanol and n-heptane were used as fuel and the behaviour of the flames was compared. Fast OH*-chemiluminescence movies allowed a classification of successful and failed events. Time-scales of initiation, full flame ignition and extinction were also extracted from the movies. Concerning the non-swirling flames, the ignition behaviour is relatively simple: the small kernel generated by

Acknowledgments

We are grateful for the hospitality of the University of Sydney that made this collaboration possible. The work at Cambridge University has been funded by the European Commission through project TECC-AE (ACP7-GA-2008-211843).

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