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Method for Computing Heat Transfer Between Connected Compartments in a Zone Fire Model.


pdf icon Method for Computing Heat Transfer Between Connected Compartments in a Zone Fire Model. (1945 K)
Forney, G. P.; Moss, W. F.

NISTIR 6190; 32 p. July 1998.
Order number: PB98-148216

Keywords:

heat transfer; compartments; zone models; fire models; radiation modeling; radiative heat transfer

Abstract:

This note describes a method for calculating conductive heat transfer between connected compartments in a zone fire model. The conduction problem is solved by dividing a compartment into six partitions corresponding to the floor and ceiling and the front, right, back and left walls. The wall partitions are further divided into several equally sized horizontal slabs or strips in order to approximate the vertical wall temperature profile. Radiative and convective flux boundary conditions are computed for the front and back surface of each partition/strip. Some of the difficulties encountered when computing radiation exchange for large numbers of wall surfaces are alleviated by taking advantage of symmetry and the numerical characteristics of the linear system of equations that are solved. All strips have the same height. Strips on opposite walls have the same width. Strips do not change size with time. Using this information, the number of necessary configuration factor and transmissivity factors are reduced substantially as is the computing time for these factors. For wall surfaces that are nearly black, i.e. emissivities close to one, the system of equations to be solved (the net radiation equations) are strongly diagonally dominant. Therefore iterative algorithms (analogous to ray tracing) rather than direct methods such as Gaussian elimination can be used for determining the net radiative fluxes.