Experimental and numerical study of ¯ammability limits of gaseous mixtures in porous media L. di Mare a, * , T.A. Mihalik b , G. Continillo c,d , J.H.S. Lee b a Dipartimento di Ingegneria Chimica, Universit  a di Napoli Federico II, Naples, Italy b Department of Mechanical Engineering, McGill University, Montreal, Canada c Facolt  a di Ingegneria, Universit  a del Sannio, Benevento, Italy d Istituto di Ricerche sulla Combustione CNR, Naples, Italy Received 20 June 1999; received in revised form 15 November 1999; accepted 1 December 1999 Abstract The present work deals with an experimental and numerical study of ¯ammability limits for hydrocarbon±air mixtures in porous media and their dependence on the physical and geometrical properties of the solid phase. Experimental data have been obtained in a standard ¯ammability apparatus modi®ed through the insertion of a packed bed of spheres. Numerical data have been obtained through integration of the equations of a one-dimensional model with single step kinetics. The model accounts for heat transfer to the solid phase in the porous medium. The kinetic parameters of the model are tuned in order to match ¯ammability limits for freely propagating de¯agrations; then, the model is applied to cases with porous medium. Numerical results are compared with experi- mental results and at least qualitative agreement is found. Particularly, both sets of data show that ¯ammability limits are more sensitive to the geometric properties of the porous medium than to its physical properties. An explanation is given through the analysis of the heat transfer process. The results show that, in modelling ¯ammability limits within porous media, heat losses to the solid phase should be taken into account, along with ¯uid-dynamic eects, to correctly understand extinction mechanisms and predict ¯ammability data. Ó 2000 Elsevier Science Inc. All rights reserved. Keywords: Flammability; Porous combustion 1. Introduction Flammability limits do not exist for adiabatic ¯ames, at least in the sense of thermodynamical property of a given gas mixture, as demonstrated rather recently by Smooke and Giovangigli [1]. The earliest models for extinction phenomena [2] explained ¯ammability limits in terms of heat losses, due either to radiation or con- vection, towards the surroundings. More recent works have pointed out the role played by other eects in ¯ammability limit mechanisms, such as ¯ame stretch [3] or chemical kinetics eects [4]. The eects of duct ge- ometry on ¯ammability limits has also been investigated [3]. However, although combustion within porous media is an important issue in both safety and technology of industrial processes, only little attention has been de- voted to the experimental and theoretical study of ¯ammability limits in porous media [5±7]. The large speci®c surfaces available within porous media and the complicated ¯uid dynamics may alter the mechanisms responsible for the extinction of freely propagating premixed ¯ames. In the experimental part of this work data were col- lected on lean and rich ¯ammability limits of propane± air and methane±air de¯agrations passing through po- rous media of dierent physical and geometrical char- acteristics. Then, a numerical model was constructed, based on a set of one-dimensional transient balance equations for mass, momentum and energy for the two phases. The data obtained with the numerical solution of the model are compared to the experimental data and the ¯ame structure is analysed to understand possible mechanisms of ¯ame extinction. 2. Experiment The experiments are carried out in a 1.8 m long transparent Plexiglas tube of 5 cm internal diameter. Experimental Thermal and Fluid Science 21 (2000) 117±123 www.elsevier.nl/locate/etfs * Corresponding author. 0894-1777/00/$ - see front matter Ó 2000 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 9 9 ) 0 0 0 6 1 - 8