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A Possible Analog of theLewis Number for Gases ina Porous Medium J. C. J ONES *

Department of Engineering, University of Aberdeen

(Received June 28, 2007)

The interesting paper on gypsum board [1] in a recent issue of this journal concludes with a statement that in a room fire where walls areconstructed of gypsum board, diffusion of hot gases and vapors throughthe gypsum board will play a part in heat transfer supplementary toconduction through the board. It is also stated that further work on thisis planned and the present author offers the following point of possibleinterest.

In a doctoral thesis he supervised many years ago [2] (most of theresults from which were reported in JFS: [3,4]) both thermal diffusivity(usual symbol , units m 2 s 1 ) and diffusion coefficient for air (usualsymbol D, units m 2 s 1 ) were measured for packed beds of forest litter.The point was made in [2] that the quotient:

Dis dimensionless and for gases is called the Lewis number, afterBernard Lewis. Lewis numbers for gases are of the order of unity.It was asserted in [2] that the counterpart of the Lewis number for a gas-phase system had been determined for a system whereby heatand mass are being transferred through a porous solid, and valuesfor various packing densities of the solid were presented and found tobe quite distant from unity.

*E-mail: j.c.jones@abdn.ac.uk

JOURNAL OF FIRE SCIENCES, VOL. 26 – May 2008 213

0734-9041/08/03 0213–2 $10.00/0 DOI: 10.1177/0734904107082243 SAGE Publications 2008

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Were this concept to be extended to the work in [1], the thermaldiffusivity could be determined using crushed gypsum board and thesimple experimental method involving heating and cooling described bythe present author (e.g., [5]) and applied by him to materials as diverseas coals, cereal granules, and upholstery materials. The diffusioncoefficient D would be more challenging to determine experimentally.However, a reasonable estimate could be obtained by calculation fromknowledge of the particle density of the gypsum and the bulk density of the gypsum board using the method originally attributed to Penman,described in full by Bowes [6].

The present author hopes that the authors of [1] will take these ideas onboard as they follow very naturally from a point raised in [2] which wasnever followed up, yet merits attention; that is, the extent to which ananalog of the Lewis number for a porous medium is helpful inunderstanding the interplay of mass and heat transfer in such a medium.

REFERENCES

1. Wakili, K.G., Hugi, E., Wullschleger, L. and Frank, T. (2007). Gypsum Boardin Fire – Modelling and Experimental Validation, Journal of Fire Sciences ,25 : 267–282.

2. Ramahti, H. (1995). Ignition and Combustion Properties of Australian ForestMaterials, PhD thesis, University of New South Wales, Sydney.

3. Jones, J.C., Bridges, R.G., Rahmati, H., Fowler, D. and Vorasurayakarnt, J.(1990). The Self-heating and Thermal Ignition Propensity of Forest FloorLitter, Journal of Fire Sciences , 8: 207–223.

4. Jones, J.C., Wake, G.C. and Rahmati, H. (1991). The Unpiloted Ignition of Eucalyptus Leaves Treated as a Parallel Reaction System, Journal of FireSciences , 9: 311–329.

5. Jones, J.C. (1998). Measured Thermal Diffusivities of Upholstery Materials, Journal of Fire Sciences , 16 : 327–334.

6. Bowes, P.C. (1984). Self-heating: Evaluating and Controlling the Hazards ,Elsevier, Amsterdam.

BIOGRAPHY

Clifford Jones

Clifford Jones is Reader in Engineering at the University of Aberdeen.He has over 25 years experience in the area and about four publicationsincluding seven books. He holds the blue ribbon degree of Doctor of Science from the University of Leeds.

214 J. C. J ONES

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