Steady and transient pyrolysis of thick clear PMMA slabs Yannick Pizzo a,b , Christine Lallemand c , Ahmed Kacem a,b,d , Ahmed Kaiss a,b , Jonathan Gerardin e , Zoubir Acem e , Pascal Boulet e , Bernard Porterie a,b,⇑ a Aix Marseille Université, CNRS, IUSTI UMR 7343, 13453 Marseille, France b ETiC (IRSN/AMU/CNRS), 13453 Marseille, France c DGA Tn, BP 40195, 83050 Toulon Cedex, France d Institut de Radioprotection et de Sûreté Nucléaire (IRSN), BP 3, 13115 St Paul-Lez-Durance Cedex, France e LEMTA, Université de Lorraine, CNRS, 2 Avenue de la Forêt de Haye, TSA 60604-54518 Vandœuvre lès Nancy Cedex, France article info Article history: Received 28 April 2014 Received in revised form 1 July 2014 Accepted 2 July 2014 Available online 8 September 2014 Keywords: PMMA Pyrolysis Spectral radiation Model Experiments abstract Thermal degradation of solid fuels is an issue of major importance in the prediction of fire ignition and growth. The pyrolysis rate is generally deduced from the energy balance at the surface of the solid mate- rial, while all or part of the in-depth loss into the solid material or the net radiation at its surface is dis- regarded. The aim of the present study is to improve the accuracy and predictive capability of solid fuel pyrolysis models. Both the steady and transient burning of thick clear polymethyl methacrylate (PMMA) slabs are investigated. First, experiments are conducted to evaluate quantitatively the energy flux com- ponents at the surfaces of steady-burning vertically oriented slabs. The total heat flux from the flame is measured, showing a decrease from 30.9 to 23.4 kW/m 2 as the sample height increases from 2.5 to 20 cm. A specific procedure for estimating the surface reradiation of burning slabs is conducted, changing from the vertical to the ceiling configuration, where flame self-extinction occurs. At midheight of the slab, the surface reradiation heat flux is as high as 11.5 kW/m 2 , which corresponds to the emissive power of an equivalent blackbody at a temperature of 671 K, in accordance with current spectroscopic measurements. Local steady-burning mass loss rates are then deduced from the energy balance at the fuel surface and compared with direct measurements, showing good agreement. Second, the transient burning of ther- mally thick slabs of clear PMMA is studied experimentally and numerically. Pure pyrolysis, for incident heat fluxes of 14 and 18 kW/m 2 , and flaming experiments are conducted. A one-dimensional numerical model is proposed to solve the problem of conjugated heat transfer into the semitransparent material, assuming that the exposed surface is a perfect emitter and a perfect absorber (soot-covered). The Schus- ter–Schwarzschild approximation is employed to calculate in-depth radiation, using data for the absorp- tion coefficient of clear PMMA with a very fine spectral resolution. Model results exhibit the same trend as that revealed in experiments for the rise in temperature in the sample and the regression rates. Results show that the surface temperature tends asymptotically to a constant value, which increases with the incident heat flux, and that steady burning occurs when the surface temperature saturates. Similar steady-burning regression rates are obtained for the 18 kW/m 2 and flaming configurations. It is found that for the flaming case, the increase in the steady-burning regression rate due to higher incident heat flux and surface temperature is nearly compensated for by the decrease of regression rate caused by higher losses (outward reradiation and heat of gasification) Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction For a semitransparent polymer, here clear polymethyl methac- rylate (PMMA), the local mass loss rate (MLR) due to pyrolysis, _ m 00 , can be related to heat fluxes through an energy balance equation at the surface of the burning material as (Fig. 1a). q conv fl |ffl{zffl} convection from the flame þ q rad fl |{z} radiation from the flame þ _ m 00 h ng |fflffl{zfflffl} heat carried to the surface by non volatile PMMA ¼ q rr |{z} outward re-radiation þ q cond id |ffl{zffl} conduction into the solid þ q rad id |{z} net radiation into the solid þ _ m 00 h g |ffl{zffl} heat carried by the PMMA vapors away from the surface þ q refl fl |{z} reflection of flame radiation : ð1Þ The left-hand terms of Eq. (1) represent the heat fluxes from the flame and the heat carried to the surface by the polymer in its nonvolatile state. The right-hand terms represent the outward http://dx.doi.org/10.1016/j.combustflame.2014.07.004 0010-2180/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. ⇑ Corresponding author at: IUSTI, 5 rue Enrico Fermi, 13453 Marseille cedex 13, France. E-mail address: bernard.porterie@univ-amu.fr (B. Porterie). Combustion and Flame 162 (2015) 226–236 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame