A refined cylinder fire model for thermal radiation calculation from localized fire to exposed horizontal surface

Abstract

Fire is the most frequent hazard to buildings. The current design methods for predicting the temperatures of structures in fire are originally developed for the standard fire that assumes a uniform gas temperature field and are invalid for natural fires with high non-uniform gas temperature fields. Localized fire is a typical natural fire with high non-uniform gas temperature field, which is often considered in fire safety design. There are currently no validated simple general models for calculating the heat transfer from localized fire to exposed structures. To address the need of design method, this paper develops a refined cylinder fire model for calculating the incident radiant heat fluxes on horizontal structural members inside and outside the flame region of a localized fire. The model uses a cylinder, centered at the fire plume axis, to represent the localized fire. The height of the cylinder is not fixed, different to any other fire models. The temperatures in the cylinder are highly non-uniform in three dimensions and are determined based on the classic fire plume theory. For radiation calculation, the cylinder is evenly divided into several elements along the radius and height. Each element is assumed to be a homogenous, isotropic, and absorbing-emitting (participating) graybody. By careful consideration of the spatial gas temperature distribution, the radiation properties (emissivity and transmissivity) of the elementary graybody, and the configuration factor between the elementary graybody and the horizontal target surface, the model can get relatively accurate and conservative incident radiant heat flux. The relative deviations against both the experiment data and numerical results are within ± 30 %. The model introduces a new concept of calculating the heat fluxes from fire to structures, is a relatively simple general model (compared with the sophisticated computational fluid dynamics models), and is more precise and accurate than other simple ones. The model is recommended for use in performance-based fire safety design.