A new burner to characterize laminar diffusion flames generated from wildland fuels

Journal Pre-proof A new burner to characterize laminar diffusion flames generated from wildland fuels G. Severino, A. Cabrera, J. Contreras, P. Pinto, J.J. Cruz, A. Fuentes PII:

S0379-7112(19)30536-3

DOI:

https://doi.org/10.1016/j.firesaf.2020.102947

Reference:

FISJ 102947

To appear in:

Fire Safety Journal

Received Date: 30 September 2019 Revised Date:

7 January 2020

Accepted Date: 13 January 2020

Please cite this article as: G. Severino, A. Cabrera, J. Contreras, P. Pinto, J.J. Cruz, A. Fuentes, A new burner to characterize laminar diffusion flames generated from wildland fuels, Fire Safety Journal (2020), doi: https://doi.org/10.1016/j.firesaf.2020.102947. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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A new burner to characterize laminar diffusion flames generated from wildland fuels Severino, G.a , Cabrera, A.a , Contreras, J.a,b , Pinto, P.a , Cruz, J.J.a , Fuentes, A.a

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a

Departamento de Industrias, Universidad T´ecnica Federico Santa Mar´ıa, Av. Espa˜ na 1680, Valpara´ıso, Chile b Escuela de Ingenier´ıa de Transporte, Pontificia Universidad Cat´ olica de Valpara´ıso, Avenida Brasil 2147, Valpara´ıso, Chile

Abstract

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A new burner is presented, designed to generate laminar and axisymmetric

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diffusion flames from wildland fuels. This burner consists of a porous sam-

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ple holder where wildland fuel elements are deposited. The sample holder is

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mounted on a cylindrical structure which allows an oxidizer coflow injection

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around the fuel samples. When the sample is ignited, a well-controlled flame

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is generated because the coflow surrounds the flame and prevents the forma-

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tion of instabilities. This was proved with flame stability measurements in

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several scenarios where sample holder diameter, size distribution of the fuel

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and porosity of the sample were varied. Ignition of the fuel was achieved with

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ethanol. Two fuels were tested: Pinus radiata pine needles and Eucalyptus

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globulus leaves. Both fuels were cut and sieved into three size distributions.

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The CH* spontaneous and visible emission from the flame were detected in

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order to analyze the flame structure after the ethanol is consumed. The vis-

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ible flame height decreased linearly with time after the ignition period, and

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as the oxidizer flow rate increased. Finally, deconvoluted soot pyrometry

Preprint submitted to Fire Safety Journal

January 17, 2020

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measurements at two wavelengths were performed in order to demonstrate

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the ability of the burner to generate stable and axisymmetric flames apt for

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non-intrusive diagnostics.

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Keywords: wildland fuel, forest fuel, diffusion flame, flame stability, soot

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temperature

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1. Introduction

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Wildfires cause human, ecological and economic losses in urban areas,

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agricultural and forestry industries and protected areas. For example, a

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wildfire in the south-central zone of Chile caused eleven deaths, destroyed

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more than a thousand dwellings and burned over 580,000 hectares in 2017

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[1]. Metrics from around the world indicate that fire seasons and the affected

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areas have increased in the past decades, both in length and size respectively

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[2]. The propagation of a wildfire is a complex process that involves several

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scales ranging from micrometers to several kilometers [3, 4]. This means that

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studying wildfires in real scale is a difficult and expensive task. A satisfactory

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understanding of this phenomenon involves solving the flame dynamics equa-

41

tions coupled with the thermo-chemical processes associated to the thermal

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degradation of solid fuels and oxidation of gaseous fuels [5]. This approach

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is difficult to implement mainly because of the turbulent structure of flames

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and lack of a complete comprehension of the kinetics of soot formation and

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oxidation. In view of these problems, semi-empirical methods are used by

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the combustion community to model medium and large scales flames from

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liquid fuels (pool fires) in terms of their burning rate, diameter, flame height,

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emissivity, etc. [6]. In the case of wildfires, this approach faces additional

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complexity, namely the thermal degradation (pyrolysis) of the solid wild-

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land fuel. Since at the mesoscopic scale the elements of vegetation and air

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form a porous medium [4], quantification of geometric and thermo-physical

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properties of the forest fuel, such as permeability [7] are imperative. Other

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drawbacks are the cost of implementing medium and large scale experiments

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and inherent uncertainty of their results due to buoyancy effects and the dif-

55

ficulty of achieving complete scaling [8]. These problems have been tackled

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by means of numerical flame models on a small scale, where multi-step reac-

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tions are implemented among other algorithms [9]. Usually, these numerical

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models need to be validated with small scale flames produced from surrogate

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fuels or from samples of wildland fuels processed in a standardized fashion,

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in order to obtain repeatable results [10].

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A small layer of wildland fuel in a basket has been deemed to be a practi-

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cal configuration to produce a laminar, axisymmetric, time-varying and non-

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premixed flame. Physical models of gas phase combustion were improved

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by Tihay and co-workers by conducting experiments in such configuration

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[11, 12]. Mu˜ noz-Feucht et al. [13] used a basket filled with particles of pine

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needles to study the structure and soot production of laminar flames. El

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Houssami et al. [14] tested a numerical model to study the processes control-

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ling the burning behavior of wildland fuels with results from a flame of pine

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needles contained in a basket. In this experiment, thermocouples were used

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to measure vertical profiles of temperature inside the porous bed. Study-

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ing laminar flames is justified because understanding non-premixed laminar

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flames is a natural prerequisite for considering more complex diffusion tur-

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bulent flames [15]. The information obtained can also be applied to analyze

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and interpreting the properties of non-premixed turbulent flames by exploit-

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ing laminar flamelet concepts of turbulent flames [16].

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In spite of the simplicity and low cost of this experimental setup, the

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production of such flames is difficult because small disturbances in the sur-

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rounding ambient provoke instabilities in the flames, thus affecting the re-

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peatability of the experiment. In small scale diffusion flames, hydrodynamic

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shear instability due to convective effects can induce the motion of toroidal

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vortices around the flame front, which alter the oxygen supply and deform

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the flame geometry at frequencies between 10 and 20 Hz [17]. Malalasekera

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et al. [18] provided a review on this subject, in which it was stated that there

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is a critical fuel velocity below which there is no flickering.

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The motivation of this study is to provide a modified approach to tech-

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niques previously reported with the design of a standard burner for wildland

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fuels. This setup is capable to produce a stable diffusion flame from fuels

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in a wide range of scenarios allowing the production and characterization of

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laminar time-varying diffusion flames from wildland fuels using non-intrusive

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techniques.

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2. Experimental methodology

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2.1. Burning configuration

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The burner consisted of a porous sample holder (porosity 50 pores per

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inch) mounted on top of an aluminum cylindrical structure (Figure 1a). The

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sample holder was drilled in order to form a cylindrical space where fuel

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elements were deposited. Three diameters of this space were studied: 2.5,

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3.0 and 3.5 cm. The depth of the space was the same in all three cases (5 4

1. CCD camera 2. Burner

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1

3. Analytical scale 4. Oxidizer flow controller

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5. DAQ 2

6. Photodiode

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7. Pulse and delay generator

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(b) Oxidizer supply and optical ar(a) Burner.

rangement.

Figure 1: Experimental setup.

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mm). The oxidizer consisted of air taken from the atmosphere by means of

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a compressor and stored in a tank at 5 bar(g). A control valve allowed a

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desired flow of air to the burner, which was measured with an Aalborg flow

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meter at a resolution of 0.1 L/min and a maximum flow rate of 30 L/min.

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Four supply points allowed the injection of air to the burner, where alumina

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balls supported by a mesh homogenized the air flow. At 30 L/min, the air

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velocity and the Reynolds number upstream the porous sample holder was

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approximately 70 mm/s and 445, respectively (considering properties of air

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at 25◦ C and 1 atm).

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2.2. Sample preparation and characterization

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Experiments were performed with pine needles of Pinus radiata and leaves

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of Eucalyptus globulus . These species were selected as the wildland fuels to

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be tested in the experiments because of their availability in the Valpara´ıso

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region, Chile (33◦ 03’S, 71◦ 37’W), and the possibility of modeling the surface-

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to-volume ratio more accurately. Pine needles and eucalyptus leaves were

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collected from the bottom layer of a forest floor. In the laboratory, both 5

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wildland fuels were cut and sieved to obtain fuel elements classified into

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three size distributions: 0.35 to 0.53 mm (S1); 0.53 to 0.85 mm (S2) and 0.85

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to 1.00 mm (S3). The fuel elements were then dried in a Memmert HCP

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50 humidity chamber at 333 K and 0% of relative humidity for 12 h [13].

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After this time no further changes in mass were observed, suggesting that no

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moisture remained. However, fuel loading before each test indicated weak

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moisture content in the range 1–3%, coming from self-rehydration of the fuel

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elements. These values were consistent with those reported in [11].

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Fuel density was determined following the procedure proposed by Tihay

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et al. [10]. A volume of 2 ml of ethanol was introduced into a graduated

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tube, then a small quantity of fuel particles was submerged in the ethanol.

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Fuel density was calculated as the mass of the fuel particles divided by the

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displaced volume of ethanol. To estimate the surface-to-volume ratio, a cylin-

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drical shape was assumed for the fuel elements obtained from pine needles

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and the relation reported by Mu˜ noz-Feucht et al. [13] was used. Fuel ele-

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ments from eucalyptus leaves were modeled as discs of a fixed thickness (0.39

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mm). Length and thickness of the pine elements and diameter of the eucalyp-

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tus elements were obtained by image processing. Table 1 summarizes these

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properties for each size distribution. Each property varied no more than 9%

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with respect to the reported value. Samples used in these procedures in order

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to determine fuel density and surface-to-volume ratio were not burned in the

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experiments.

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Bulk density was determined as the ratio between the mass of a sample

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and the volume occupied by that sample in the cylindrical space of the sample

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holder. Porosity was determined as one minus the ratio between bulk density

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Table 1: Fuel density and surface-to-volume ratio (SVR) of the fuel elements for each size distribution. Species

Size

Fuel density

Length

Thickness

Disc diameter

SVR

distribution

(g/cm )

(mm)

(mm)

(mm)

(m−1 )

Pinus

S1

0.739

0.32

0.0147

-

279017

radiata

S2

0.704

0.48

0.0198

-

206886

S3

0.699

2.16

0.4348

-

10145

Eucalyptus

S1

0.841

-

-

0.11

41547

globulus

S2

0.831

-

-

0.37

15984

S3

0.819

-

-

0.75

10478

3

139

and fuel density. Table 2 shows these parameters for the selected experiments.

140

A Kern PFB scale with a resolution of 0.001 g was used to measure mass of

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the samples.

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2.3. Experimental procedure

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Ignition of each sample was achieved by burning 0.3 ml of ethanol poured

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on the sample surface as reported by Mu˜ noz-Feucht et al. [13]. When the

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ethanol was consumed, the combustion of the wildland fuel sample produced

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a laminar, axisymmetric, time-varying and non-premixed flame. Flame ex-

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tinction occurred at approximately 90 seconds after ethanol was completely

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burned. This is because a char layer generated in the surface of the sample

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reduced heat transfer to the rest of the fuel, reducing the production rate of

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gases that sustained the flame.

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Mass loss as a function of time was determined with a Shimadzu UW620H

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analytical scale (0.001 g of resolution) in order to determine the moment in

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which the ethanol was completely consumed. Temporal resolution of mass 7

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loss measurements was 0.9 s.

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The frequency spectrum of the signal from a photodiode aimed at the

156

flame was analyzed to assess flame stability in several scenarios. A char-

157

158

acteristic value of the frequency spectrum determined in each scenario was P P estimated as F = i (fi Ai )/ i (Ai ) where fi is a given frequency and Ai is

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the normalized amplitude of the spectrum at that frequency. This method

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was proposed by Huang et al. [19]. A Thorlabs SM05PD1A photodiode lo-

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cated at 72 mm from the burner axis was used for these purposes (Figure 1b).

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Its signal was sampled at 50 Hz with an Iotech Personal DAQ 55.

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Images of the visible flame were obtained with an Imperx B1410 CCD

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camera located at 630 mm from the burner axis (Figure 1b) and processed

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with the method described by Mu˜ noz-Feucht et al. [13]. The CCD camera

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was equipped with a Tamron 18 mm (f = 2) lens, and the exposure time was 5

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ms. Spontaneous emission of the CH* from the flame was obtained by placing

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a 430 nm filter in front of the camera and processing the corresponding images

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with the aforementioned method.

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Soot temperature was estimated with the two-color pyrometry technique

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proposed by Legros et al. [20], where the ratio of two deconvoluted radial dis-

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tributions of flame emission at different wavelengths determined temperature

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with the formula given by Jenkins and Hanson [21]. Since flame intensity

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varied with time, a filter wheel allowed alternately placing band-pass filters of

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660 and 800 nm (10 nm FHWM in both filters) in front of the CCD camera.

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Since the switch between the two filters took 0.8 seconds, it was assumed

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that both images were taken at the same time in order to determine the soot

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temperature field as a function of time.

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3. Results and discussion

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3.1. Flame stability

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Experiments with Pinus radiata were performed to determine optimal

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burner configuration. It is important to note that size distributions smaller

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than 0.35 mm and larger than 1.00 mm were discarded. The tinier fuel el-

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ements penetrated into the porous holder affecting the ignition procedure

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and the wildland fuel burning stage. In contrast, the surface of the wild-

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land fuel samples were also affected by the larger elements, which eventually

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perturbed the optical diagnostics performed during the burning rate stage.

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Table 2 shows the selected cases to illustrate how flame stability was im-

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pacted when one parameter of the configuration (sample holder diameter,

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size distribution or porosity) was varied while others were kept constant. It

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can be seen in Figure 2(a) that a conical shaped laminar flame was produced

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in all cases, with more intense soot emission in a conic zone near the flame

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tip.

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If the sample holder diameter was increased from 2.5 to 3.0 cm while keep-

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ing all other parameters constant (Figure 2(a) and Figure 2(b)), a slightly

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higher flame was produced because of an increased amount of solid fuel avail-

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able for pyrolysis. However, when the sample holder diameter increased from

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3.0 to 3.5 cm the flame geometry did not change appreciably (Figure 2(b)

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and Figure 2(c)). In experiments performed with sample holder diameters

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larger than 3.5 cm (not shown in this report) the flames were no longer ax-

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isymmetric: they presented two or more flame tips, uncontrolled geometry

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and unstable behavior.

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Size distribution of the samples affected the mean surface-to-volume ratio 9

Table 2: Selected experimental conditions to study the influence of sample holder diameter, size distribution and sample porosity. Oxidizer coflow was 10 L/min in all cases. Parameter

Condition

Case

Value (a)

(b)

(c)

Varied

2.5

3.0

3.5

Size distribution

Constant

S1

S1

S1

Bulk density (g/cm3 )

Constant

0.279

0.279

0.279

Varied

0.686

0.987

1.344

Constant

0.622

0.622

0.622

(d)

(e)

(f)

Constant

3.0

3.0

3.0

Size distribution

Varied

S1

S2

S3

Bulk density (g/cm3 )

Varied

0.273

0.260

0.259

Mass of the sample (g)

Varied

0.966

0.921

0.914

Constant

0.630

0.630

0.630

(g)

(h)

(i)

Sample holder diameter (cm)

Mass of the sample (g) Porosity Case Sample holder diameter (cm)

Porosity Case Sample holder diameter (cm)

Constant

3.0

3.0

3.0

Size distribution

Constant

S2

S2

S2

Bulk density (g/cm3 )

Varied

0.255

0.283

0.311

Mass of the sample (g)

Varied

0.900

1.000

1.100

Porosity

Varied

0.638

0.598

0.558

10

(a)

(b)

25 20 15 10 5 -10

0

10

25 20 15 10 5 -10

(d)

HAB (mm)

(c)

25 20 15 10 5 0

10

-10

(e)

20

20

20

15

15

15

10

10

10

5

5

5

-10

0

10

-10

(g)

0

10

-10

(h) 20

20

15

15

15

10

10

10

5

5

5

0

10

-10

0

10

0

10

(i)

20

-10

0

(f)

10

-10

0

10

r (mm)

Figure 2: Images of the visible flame at 75 s from ignition for each case described in Table 2.

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of the particles, a property that controls net energy influx to the particles and

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their pyrolysis rate. In general, Figure 2(f) demonstrates that finer particles

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burned more efficiently than coarser particles, which produced a larger flame

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height and the conic zone near the flame tip previously mentioned. However,

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in this case the reaction zone cannot be clearly discerned because its intensity

209

was more uniformly distributed in space.

210

Sample porosity did not significantly affect the spatial distribution of soot

211

emission, as can be deduced from Figure 2(g) to (i). However, it is interesting

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to note that decreasing porosity was equivalent to less air contained inside the

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sample, which tends to impede a thorough mixing of the pyrolysis gases and

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(a) F =3.4253 Hz

Normalized amplitude [-]

100

0

10

20

(d) F =3.0564 Hz

100

0

10

100

0

10

0

20

10

0

10

100

0

10

0

20

10

20

(f) F =2.4351 Hz

100

20

(h) F =2.4953 Hz

(c) F =3.5667 Hz

100

20

(e) F =2.5777 Hz

100

20

(g) F =2.4923 Hz

(b) F =2.6484 Hz

100

0

10

20

(i) F =2.6417 Hz

100

0

10

20

Frequency [Hz]

Figure 3: Frequency spectra of the photodiode signal (ordinate axis is in log scale) for each case described in Table 2.

214

air. Thus, a less efficient burning of the sample is generated, and consequently

215

a less intense emission from soot particles.

216

The frequency spectra of the photodiode signals obtained for the cases

217

described in Table 2 are shown in Figure 3. These plots are normalized with

218

respect to the amplitude at 0 Hz. In all cases, the normalized amplitude was

219

less than one for all frequencies higher than 0 Hz. Therefore flickering at

220

some dominant frequency was absent in these cases. Figure 3 also shows the

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characteristic frequency estimated with the formula proposed by Huang et

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al. [19]. This frequency was small when compared to the frequencies usually

223

reported for diffusion flames from gaseous fuels (10 to 20 Hz) [17]. The ability

224

of the burner to provide stable flames was thus proved for the selected cases.

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225

3.2. Mass loss

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Mu˜ noz-Feucht et al. [13] measured the time evolution of the mass of the

227

sample and determined that ethanol was completely burned when the original

228

mass of fuel sample was reached: for samples of 3.5 cm of diameter and 1.5

229

g this duration was 50 seconds [13]. In the present study, for experimental

230

conditions corresponding to case (a) in Table 2, the mass of the sample was

231

measured every 0.9 s, the normalized mass loss was thus determined as η =

232

(minitial −m(t))/(minitial −mf inal ) and the results are shown in Figure 4. The

233

criteria used by Mu˜ noz-Feucht et al. [13] to distinguish between ethanol and

234

wildland fuel burning indicates that ethanol was completely burned at 36 and

235

45 s for pine and eucalyptus, respectively. The flame duration was 120 and

236

138 s for pine and eucalyptus, respectively. It can be seen in Figure 4 that

237

at a given time, Pinus radiata samples have lost more mass than Eucalyptus

238

globulus samples. Consequently, pine requires less time than eucalyptus to

239

reach a given normalized mass loss. Therefore, the flame extinction (η = 1)

240

for pine occurs earlier than for eucalyptus fuel elements.

241

Figure 5 compares the evolution of mass loss with Pinus radiata to data

242

reported by Mu˜ noz-Feucht et al. [13] with the same species and Tihay et al.

243

[22] with Pinus pinaster, Pinus halepensis and Pinus laricio. The present

244

experiment was performed employing 1.5 g of fuel elements with a S2 size

245

distribution burned in the 3.5 cm sample holder, which are the same condi-

246

tions reported in Ref. [22]. An oxidizer coflow of 10 L/min was supplied in

247

the burner in order to improve flame stability. The Figure 5 shows the entire

248

duration of a current experiment carried out in this study. In this case, the

249

ethanol burning stage lasts approximately 50 s. During the wildland fuel

13

Figure 4: Normalized mass loss as a function of time. Sample holder diameter is 2.5 cm, size distribution of fuel elements is S1, sample porosity is 0.622 and oxidizer coflow is 30 L/min. Vertical dashed lines indicate the time when ethanol burned completely and the time when flame extinction occurs.

250

burning stage (t > 50 s) it is observed that the mass loss rate for the present

251

study is larger. This effect can be attributed to the oxidizer coflow, which

252

promotes mixing of combustible gases with oxygen, thus increasing the rate

253

at which pyrolysis takes place.

254

3.3. Flame structure

255

Flame height is an important characteristic of coflow laminar diffusion

256

flames since flame height measurements can be used to test models of flame

257

structure [23] and to estimate residence time of particles [24]. In the present

258

study, the flame was unsteady because its height decreased with time. How-

259

ever, three stages could be qualitatively distinguished. The first stage cor-

260

responded to the interval where the flame height had an erratic behavior

14

2.2 Pinus radiata (This work) Pinus radiata (Muñoz-Feucht et al.) Pinus pinaster (Tihay et al.) Pinus halepensis (Tihay et al.) Pinus laricio (Tihay et al.)

2

m (g)

1.8 1.6 1.4 1.2 1 0

50

100

150

200

t (s) Figure 5: Mass loss of Pinus radiata fuel elements of size distribution S2 as a function of time. Experimental conditions were oxidizer coflow 10 L/min, sample holder diameter of 3.5 cm and sample mass of 1.5 g. Previous data for Pinus radiata was taken from Ref. [13], while data for Pinus pinaster, Pinus halepensis and Pinus laricio is from Ref. [22].

261

because of the ethanol poured over the wildland fuel sample. This flame was

262

blue and of low intensity. The second stage occurred when the combustion of

263

the wildland fuel began and was sustained over time. The main characteris-

264

tic of this stage was that the evolution of flame height had a decreasing and

265

stable behavior (Figure 6). The flame acquired a yellow color in this stage,

266

was of low intensity between 30 and 60 s, and high intensity near the flame

267

tip between 60 and 90 s. The third stage was characterized by an abrupt

268

decrease of flame height until the flame extinguished at approximately 120 s.

269

The reaction zone was studied by means of spontaneous emission of

270

CH* radicals. CH* radicals, which are produced spatially close to the first

271

sharp temperature rise in the reaction zone of a flame, create an outline

272

of blue/violet photons typically referred to as the ‘flame’. This distinction 15

273

is useful in determining flame parameters that depend on its identification

274

(e.g. base location, stretch, and flame speed). Because emission is passive

275

and non-intrusive, researchers have explored the potential of measuring com-

276

bustion parameters (e.g. equivalence ratio [4, 5] and heat release [6]) using

277

CH* chemiluminescence. Thus, CH* radical spontaneous emission could be

278

useful to explore the reaction zone. The latter is reported as a trace of

279

the reaction zone and then, a marker where maximum temperature is lo-

280

cated [25]. Figure 6 shows flame images taken with a 430 nm filter (10 nm

281

FWHM). The utility of chemiluminescent emission as a diagnostic tool is

282

compromised in flames containing additional background radiation, particu-

283

larly from soot. As these flames present low soot concentration, as reported

284

by Mu˜ noz-Feucht et al. [13], they work well for CH* chemiluminescence.

285

The CH* spontaneous emission images are used to determine the stoi-

286

chiometric flame zone which corresponds to the maximum intensity and is

287

located near the maximum temperature zone [26]. Representative images of

288

CH* spontaneous emission are shown in Figure 6 for flames produced with

289

pine and eucalyptus fuel elements used in the three sample holders studied

290

(2.5, 3.0 and 3.5 cm). These graphs present the arbitrary intensity emission

291

of CH* radicals at different times after the flame is ignited and show the evo-

292

lution of the flame structure with time in terms of the spontaneous emission

293

at 430 nm. A decrease of the reaction zone with time was observed in Fig-

294

ure 6, where the maximum height (around 20 mm) occurred at the beginning

295

of the stage when only the solid fuel burned. On one hand, for pine flames

296

higher intensity was found near the base of the flame, but not on the tip of

297

the flame as can be seen at 45 s. At 75 s, a new region appeared on the tip of

16

20

20

20

20

10

10

10

10

0

10

-10

0

10

-10

0

10

-10

30

30

30

30

20

20

20

20

10

10

10

10

-10

0

10

-10

0

10

-10

0

10

-10

30

30

30

30

20

20

20

20

10

10

10

10

-10

0

10

r (mm)

-10

0

r (mm)

10

-10

0

10

r (mm)

t=45s

HAB (mm)

t=90s 30

0

0

-10

0

30

20

20

20

20

10

10

10

10

0

10

-10

0

10

-10

0

10

30

30

30

30

20

20

20

20

10

10

10

10

-10

0

10

-10

0

10

-10

0

10

30

30

30

30

20

20

20

20

10

10

10

10

-10

0

10

r (mm)

(a) Pinus radiata.

t=90s

30

10

10

t=75s

30

-10

r (mm)

t=60s

30

10

HAB (mm)

HAB (mm)

t=75s 30

-10

HAB (mm)

t=60s 30

HAB (mm)

HAB (mm)

t=45s 30

-10

0

r (mm)

10

-10

0

10

-10

0

10

-10

0

10

0

10

-10

r (mm)

r (mm)

(b) Eucalyptus globulus.

Figure 6: Evolution of the flame structure over time for the sample holder diameters studied: 2.5 cm (upper row), 3.0 cm (middle row) and 3.5 cm (lower row). Size distribution of wildland fuel elements is S2, sample porosity is 0.630 and oxidizer coflow is 30 L/min in all cases.

17

298

the flame where CH* spontaneous emission was captured. This means that

299

an extension of reaction zone was probably found in this stage of the flame

300

life. This behavior was consistent with that reported by Karnani and Dunn-

301

Rankin [27] with the same 430 nm filter for ethylene diffusion flames. At

302

90 s after ignition, the flame was almost extinguished, but its intensity was

303

still significant. The flame extinguished when the rate of pyrolysis gaseous

304

products was not enough to sustain a flame. On the other hand, eucalyptus

305

flames exhibit the aforementioned region on the flame tip at an earlier stage

306

of the flame life (45 to 60 s). Consequently, the flame intensity for eucalyptus

307

is higher than for pine. Analyzing the sample holder size, it is important to

308

observe that larger diameter modifies the flame geometry, in particular the

309

flame height independently of time. For example, at 45 s the tip of the flame

310

produced with a sample holder of 3.5 cm is located 15 mm higher than the

311

one with a sample holder diameter of 2.5 cm. Similar behavior is observed

312

in Figure 6 for other time instants and for eucalyptus flames.

313

The evolution of the visible flame height as a function of time was de-

314

termined as follows: ten measurements of visible flame height were made for

315

six air flows (5 to 30 L/min) and averaged at each time instant. Figure 7

316

shows the average flame height as a function of time and air coflow in the

317

time range of 30 to 80 s for pine and eucalyptus. It can be observed that the

318

flame height decreased linearly during this interval for all the imposed flows.

319

However, a trend can be seen: as the air flow decreased, the average flame

320

height increased slightly. Visible emission from a diffusion flame occurred

321

mainly due to thermal radiation from soot particles. When soot particles

322

crossed the flame reaction zone, they were mixed with the surrounding air

18

(a) Pinus radiata.

(b) Eucalyptus globulus.

Figure 7: Evolution of the visible flame height between 30 and 80 s for six air flows (5, 10, 15, 20, 25 and 30 L/min).

35 Pinus radiata (this work, S1) Pinus radiata (this work, S2) Pinus radiata (this work, S3) Pinus pinaster (Tihay et al.) Pinus halepensis (Tihay et al.) Pinus laricio (Tihay et al.)

30

hf (mm)

25 20 15 10 5 0 0

25

50

75

100

t* (s) Figure 8: Visible flame height for three size distributions (S1, S2 and S3), coflow 10 L/min, sample holder diameter of 3.5 cm and sample mass of 1.5 g. Data for Pinus pinaster, Pinus halepensis and Pinus laricio was taken from Ref. [22]. Time coordinate is defined as t∗ = t − 50.

19

323

and oxidized, thus reducing their radiant emission. The distance traveled

324

by the soot particles between the flame reaction zone and the point where

325

they completely burn out marked the point of the maximum visible flame

326

height at that instant. Since this distance depends on the mixing time and

327

not reaction time with the oxidizer, the higher the air flow imposed on the

328

burner, the lower the distance traveled by the soot particles until they are

329

fully oxidized. This trend can be observed in Figure 7, and is in agreement

330

with the interpretation: at every time instant between 30 and 80 s, an air

331

flow of 30 L/min generated a visible flame height which was roughly 1 to 2

332

mm lower than the one for 5 L/min.

333

The evolution of visible flame height in terms of the size distribution of

334

Pinus radiata fuel elements is shown in Figure 8. In this plot the time coor-

335

dinate is defined as t∗ = t − 50 in order to focus the analysis on the wildland

336

fuel burning stage. The flame height decreases almost linearly with time

337

independently of size distribution. Between 0 and 25 s these curves are inde-

338

pendent of size distribution, but the opposite behavior is observed between

339

25 and 75 s: when the extinction approaches, the smallest fuel elements (S1)

340

produced higher flames than the other size distributions (S2 and S3). This

341

effect could be attributed to SVR, which for S1 is about twenty seven times

342

higher than for S3 (Table 1), thus inducing a higher thermal feedback to the

343

fuel elements. Also, Figure 8 shows results reported by Tihay et al. [22].

344

It is important to note that the sample holder diameter employed was 3.5

345

cm, fueled with 1.5 g and coflowed with 10 L/min of oxidizer, replicating

346

the conditions of the work mentioned above. Comparing with data reported

347

by Tihay and co-workers, visible flame height in the present study is lower

20

348

because of the coflow supplied, independently of the species reported, except

349

for Pinus pinaster. The results obtained show similar trends, thus demon-

350

strating that the burner is able to replicate the flame height evolution and

351

keeping its axisymmetry.

352

3.4. Soot temperature

353

Soot temperature for pine and eucalyptus (Figure 9) was determined in

354

the interval when the flame reached its highest intensity near the tip, between

355

60 and 90 s, in order to obtain a higher spatial resolution.

356

The species of the wildland fuel elements affects the spatial soot temper-

357

ature distribution. On the one hand, the temperature field for eucalyptus

358

is more uniform than for pine. On the other hand, the soot temperature

359

field in general is spatially larger for eucalyptus than for pine, validating the

360

observations made for visible and CH* images. Observing the results for

361

eucalyptus, it is possible to indicate that the larger sample holder diameter

362

(3.0 cm) produces a higher soot temperature, for the two times shown.

363

Figure 10a shows soot temperature profiles for pine at 70 s for several

364

heights above the burner (HAB). The retrieved soot temperature was be-

365

tween 1200 and 2000 K approximately, and its magnitude decreased with the

366

HAB. Radial distribution of soot temperature reached a maximum at the

367

flame centerline for all the HABs studied. These observations also apply for

368

the other time instants when soot temperature was determined.

369

According to Tihay et al. [12], some of the pyrolysis gases released by

370

thermal degradation of Pinus halepensis and Pinus laricio are carbon diox-

371

ide, carbon monoxide, water vapor, methane and C4 hydrocarbons, among

372

others. If it is assumed that degradation of Pinus radiata produced simi21

pine 2.5 cm t = 70 s

22

pine 2.5 cm t = 90 s

22

22

eucalyptus 2.5 cm t = 70 s

22

eucalyptus 2.5 cm t = 90 s

22

eucalyptus 3.0 cm t = 70 s

22

eucalyptus 3.0 cm t = 90 s

T (K) 2400

20

20

20

20

20

20 2200

18

18

18

18

18

18 2000

HAB (mm)

16

16

16

16

16

16 1800

14

14

14

14

14

14

12

12

12

12

12

12

10

10

10

10

10

10

1200

8

8

8

8

8

8

1000

6

6 0

1

r (mm)

2

6 0

1

r (mm)

2

6 0

1

2

r (mm)

6 0

1

r (mm)

2

1600 1400

6 0

1

r (mm)

2

800

0

1

2

r (mm)

Figure 9: Soot temperature field at 70 and 90 s for sample holder diameters 2.5 and 3.5. Size distribution of fuel elements is S2, sample porosity is 0.630 and oxidizer coflow is 30 L/min in all cases.

22

2100

1800 HAB =7.8 mm HAB =7.5 mm HAB =7.2 mm HAB =6.9 mm HAB =6.6 mm

2000

1400

T (K)

T (K)

1900

1600

1800

1200

1700

1000

1600

800

1500 0

0.5

1

1.5

t =62 s t =66 s t =70 s t =76 s t =81 s t =87 s

600

2

0

r (mm)

0.5

1

1.5

2

r (mm)

(a)

(b)

Figure 10: Radial profiles of soot temperature for Pinus radiata with a sample holder diameter of 2.5 cm, size distribution S2 and porosity of 0.63: a) for several HABs at 70 s and, b) for a normalized HAB of 0.88 for several time instants.

373

lar mass fractions of pyrolysis gases, then the gaseous fuels (mainly carbon

374

monoxide and methane) were diluted, thus generating a flame with a low

375

soot volume fraction. The same was demonstrated by Mu˜ noz-Feucht et al.

376

[13]. 10a shows that soot reaches temperatures higher than 1500 K for all

377

HABs shown, which means that soot inception (which for butene occurs at

378

about 1600 K [28]) was at a lower HAB. Since soot temperature decreased

379

with HAB, one interpretation is that soot precursors were being oxidized at

380

a higher rate than they were being produced, thus reducing the soot volume

381

fraction and forming a flame with a closed tip without smoke emanating from

382

the flame.

383

Soot temperature was determined by computation of the ratio of the de-

384

convoluted radial distribution of flame emission at two wavelengths. This al-

23

385

gorithm was further simplified by assuming an optically-thin flame in view of

386

the low soot volume fraction determined by Mu˜ noz-Feucht et al. [13]. Decon-

387

volution was performed with the onion-peeling method and regularized with

388

the Tikhonov procedure, as suggested by Daun et al. [29]. The regularization

389

parameter required by the Tikhonov procedure was determined by means of

390

the L-curve criteria proposed by Hansen and O’Leary [30], which resulted

391

in a value of 1E-5 for all the HABs and time instants studied. Error in the

392

soot temperature calculation was estimated with the procedure described by

393

Escudero et al. [31]. One drawback of the onion-peeling technique was that

394

uncertainty in temperature increased as the radius tended towards zero (i.e.

395

the flame centerline) as shown by the error bars in Figure 10a. However, in

396

this study the technique used to retrieve soot temperature was non-intrusive

397

and further work on reducing this error is expected to establish this method

398

for small flames. This is because reduction of the error induced by using

399

thermocouples is limited by the bead size and the robustness of the physical

400

models involved in their correction.

401

Figure 10b shows the radial distribution of temperature at a ratio of

402

HAB and flame height of 0.88 for several time instants. Between 62 and

403

76 s, the soot temperature remained almost constant. At 81 s, temperature

404

decreased with radius for r > 0.5 mm. At 87 s, the entire radial profile

405

increased 200 K. An interpretation of this trend is that between 62 and 81

406

s, the char front moved to the sample center, where an increasingly smaller

407

surface of unburned fuel remained to be degraded. This decrease in pyrolysis

408

rate induced a lower soot temperature because of lower soot production.

409

However, when the unburned fuel surface reached critical diameter between

24

410

81 and 87 s, the oxidizer velocity passing through this area increased locally,

411

thus promoting soot production and increasing soot temperature during a

412

small time interval. When the char layer covered the sample completely, the

413

flame was no longer sustained and it was extinguished.

414

4. Conclusions

415

A standardized burner design for production of diffusion flames of wild-

416

land fuel samples was successfully tested. Flame stability was proved in

417

several scenarios with varying sample holder diameter, size distribution of

418

fuel elements and sample porosity. Mass loss measurements indicate that

419

experiments with Eucalyptus globulus last longer than with Pinus radiata.

420

Three stages could be distinguished according to the variation of flame

421

structure over time: a first stage where ethanol was being consumed, a second

422

stage where flame height decreased linearly with time, and a third stage

423

where flame height decreased abruptly and extinction occurred. Oxidizer

424

coflow around the sample helped to avoid formation of flame instabilities.

425

Varying the oxidizer coflow up to 30 L/min did not significantly impact the

426

visible flame structure. This is because flame height increased about 2 mm

427

when the air flow was reduced to 5 L/min. Spontaneous emission of CH*

428

radicals was captured to study the reaction zone of the flame, finding that

429

reaction zone is modified between the first and the last stage of the flame.

430

The application of non-intrusive methods to determine soot properties in the

431

diffusion flame produced by the burner was demonstrated by determination

432

of the soot temperature field of the flame with two-color pyrometry. At

433

a fixed time instant, soot temperature decreased with radius and with the 25

434

HAB. The latter is evidence of a diluted gaseous fuel feeding the flame and

435

a low soot volume fraction, thus generating a flame with a closed tip. Soot

436

temperature decreased with time as unburned fuel surface was increasingly

437

smaller. A slight increase of soot temperature in the last stage of the flame

438

may occur due to an increase in the oxidizer velocity through the sample.

439

Finally, the burner is able to replicate the flame geometry and the mass

440

loss obtained previously in similar conditions, using an equivalent ignition

441

procedure of the wildland fuel elements, also giving the possibility to supply

442

different types of oxidizer conditions. An important feature of the burner

443

is an improved stability reducing the flickering at the flame tip, keeping the

444

axisymmetry of the flame geometry, thus facilitating the utilization of non-

445

intrusive laser-based diagnostics.

446

5. Acknowledgments

447

J. Contreras wishes to thank the CONICYT Chile for financial support

448

through FONDECYT Iniciaci´on 11161045. This work was supported by the

449

Chilean CONICYT Research program under Grant 172095 project and by

450

UTFSM through PIIC.

451

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31

CRediT author statement Gonzalo Severino: Software, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision Alejandro Cabrera: Validation, Writing - Original Draft Jorge Contreras: Conceptualization, Validation, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition Pablo Pinto: Validation, Data Curation Juan-José Cruz: Software, Resource, Data Curation Andrés Fuentes: Methodology, Formal analysis, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration

Declaration of interests ☒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.