Study of wood self-extinguishment with a double sliding cone calorimeter

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Abstract

The aim of the present work was to improve the study of wood self-extinguishment under cone calorimeter thanks to a dedicated experimental setup. A cone calorimeter was modified into a double sliding cone calorimeter in vertical orientation for studying samples extinguishment. With this new setup, the external heat flux can be quickly switched from a high to a low value seeking for the heat flux leading to the sample extinguishment. Samples were first submitted to different heat fluxes (60, 82.5 and 93.5 kW.m2) leading to auto-ignition. After different exposure times (6, 9, 15 and 18 min), the heat flux was suddenly reduced to a lower value (between 38.5 and 60 kW.m2) by sliding from one cone to the other one. All in all, 206 tests were performed varying heat fluxes and exposure time. This large number of test made it possible to carry out a statistic study and to deduce a critical Mass Loss rate and a critical heat flux for self-extinguishment. The critical heat flux for self-extinguishment varied from 44 to 51 kW.m2 depending on the heat flux that was used to ignite the sample. These critical heat fluxes were found weakly dependent on the time during which the sample was exposed to the high flux. Time-to-extinguishment was between 40 s and 100 s. Mass Loss Rate Per Unit Area (MLRPUA) for which self-extinguishment occur was found to be equal to 3.95 g.m2s1 and this value is almost independent on experiment parameters (exposure time and heat flux).

Introduction

The use of wood material in facade construction or in building structures appears to be a supportive solution for sustainability development concerns. It has a good thermomechanical behavior in fires, which gives wooden structures a resistance to fire which is long in duration. Consequently, the study of flaming self-extinguishment of wood is more and more addressed with both fundamental and empirical approaches. A review of available contributions was reported and summarized by Bartlett [1]. For instance, in a recent paper, Emberley et al. [2] performed tests at large scale and found that a wooden structure could self-extinguish during a fire when the maximum incident heat flux is reduced below 45 kW.m2. However, in some particular cases the conditions for self-extinguishment could never be achieved as several authors have observed [[3], [4], [5]] who studied extinguishment phenomenon in a full-scale Cross Laminated Timber (CLT) compartment. These works focused on CLT delamination during the tests. Delamination is a random phenomenon that generates the fall of burning wood and exposing additional fuel. Therefore, the delamination contributes to feed the fire and to expose some virgin wood to the heat flux, avoiding conditions for self-extinguishment to occur. Moreover, the experimental study made by McGregor showed that self-extinguishment did not occur for a large compartment made entirely of wood [3].

Studies were also carried out at smaller scale, using a cone calorimeter or a Fire Propagation Apparatus (FPA), to estimate useful parameters such as heat flux and critical Mass Loss Rate Per Unit Area (MLRPUA) at self-extinction [6]. Emberley et al. [7] measured a critical MLRPUA from 2.65 to 8.28 g.m2.s1 and a critical heat flux between 24.1 and 56.6 kW.m2 according to wood species. That paper concluded that the self-extinguishment is independent on wood densities or on the imposed heat flux. Using a FPA, Bartlett et al. [4] performed experiments by decreasing the imposed heat flux provided by the FPA source until the self-extinguishment was observed. A critical MLRPUA equal to 3.48 g.m2.s1 and a critical heat flux equal to 31 kW.m2 was found in this way (at ambient oxygen concentrations) [4]. These experiments, at small scale, have determined critical values for extinguishment after MLR decrease, at steady state. Finally, Crielaard et al. [8] studied the smouldering extinguishment by shifting sample from one cone with a heat flux of 75 kW.m2 to another between 0 and 10 kW.m2 varying air flow. These two cones were horizontally oriented. The critical heat flux for self-extinguishment of smouldering is below 5 to 6 kW.m2. A 0.5 m.s1 airflow led to quicker extinguishment than with no airflow while a 1 m.s1 airflow led to maintain the combustion at 6 kW.m2. In that work MLR data could not be provided since the sample had to be moved from the first cone to the second one.

The aim of the present work was to study the flaming self-extinguishment of spruce wood exposed to heat fluxes provided by two vertically oriented cone calorimeters. For that purpose, the initial cone calorimeter setup was modified by adding a second cone. This new device is very useful for studying sample extinguishment, by suddenly sliding from one cone to the second one, while continuing to record the mass loss. It allows to provide an accurate characterisation of both the critical heat flux and the MLR for self-extinguishment. The setup is completed by a precision scale to record the mass loss and an infrared camera to measure the surface temperature during the tests [9]. In the following, the experimental setup and the material will be first described. Then, results related to the heat flux and the MLR at self-extinguishment will be presented and discussed.

Section snippets

Samples and double sliding cone calorimeter

The experimental setup used for this study is presented in Fig. 1. It is based onto a usual cone calorimeter in vertical orientation which was modified by fixing two cones on a sliding ball bearing table. This allows to switch fast from one heat flux (coming from one of the cones at a fixed temperature) to another heat flux (coming from the other cone at a different temperature). The sample is put in the sample holder which is laid on a precision scale. The main advantage of this setup is that

Heat flux for wood self-extinguishment

Some tests were performed to determine the different parameters governing wood self-extinguishment. As explained before, the sample was exposed to a first high heat flux inducing auto-ignition. After 15 min, the cones were slid and the sample was exposed to a lower heat flux. Table 1 presents the results of 95 tests carried out with a fixed first exposure time equal to 15 min. This table gathers if self-extinguishment occurred or not, and the proportion of self-extinguishment, for different

Influence of the exposure time on sample self-extinguishment

The measurements performed so far were presented only for one exposure time (15 min) to the high heat flux. However, the exposure time might affect the determined parameters especially when the wood decomposition is not yet in steady state. Tests were performed by submitting the sample to the high heat flux during different exposure times: 6 min (38 tests), 9 min (36 tests) and 18 min (37 tests). These times were chosen to avoid the transient state, since the goal of this work was to determine

Conclusion

A dedicated experimental setup using two heater cones was built up in order to study the flaming self-extinguishment of wood by suddenly switching from a high heat flux (leading to sample auto-ignition) to a lower heat flux in a very short time. An experimental campaign with 206 tests was led to provide an accurate characterisation of the heat flux and the critical MLRPUA for wood self-extinguishment. The critical heat flux for self-extinguishment was estimated between 44 and 51 kW.m2 whatever

Author statement

Lucas Terrei: Methodology,Investigation, Formal analysis, Writing- Original draft. Zoubir Acem: Supervision, Methodology, Formal analysis, Writing- Original draft. Paul Lardet: Project administration, Writing - Review & Editing. Pascal Boulet: Writing - Review & Editing.Gilles Parent: Supervision, Methodology, Formal analysis, Writing- Original draft.

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.

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    This latter simplified the setup of the sample before the exposition to the heat flux. More details about the modified cone calorimeter device are given in the work of Terrei et al. [35]. For the purpose of studying the wood degradation without oxygen in the atmosphere, a stainless-steel chamber was built in order to place a sample inside, in vertical orientation.

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