Glass Breaking In Fires

Abstract

Glass breaking in compartment fires is an important practical problem since a window acts as a wall before breaking and as a vent after breaking. If sufficient excess pyrolyzates have accumulated in the hot layer, this sudden geometric change can lead to backdraft and flashover. As Emmons explained at the First Symposium, windows break in fires due to thermal stress from the differential heating of the central portion and the shaded edge. The focus of this paper is on quantifying the connection between the compartment fire and the glass temperature to predict the window breaking time, tb. Techniques are presented for accurately calculating the history of the central glass temperature profile, T (x .r), for any fire exposure. Two-dimensional temperature histories, T (x ,y ,t), where x is depth and y is toward the center, and mean stress histories, a zz (y ,t), are also calculated. It is determined here that breaking occurs when the mean glass temperature difference is~T = (ablE~)g, where ablE is the maximum glass tensile strain,~is the thermal coefficient of linear expansion and g is a geometry factorpf order one. Calculations suggest that the edge remains at its initial temperature, T; , so that~T = T (tb) -Ti, when the shading is large, sIL~2, and the heating is fast, atb Is2::;1, where L is the glass thickness, s is the shaded edge width and «is the glass thermal diffusivity,

Experimental investigation of flame impingement on vertical and inclined glazing facades

“…In a compartment fire were oxygen may be scarce, the ability of a fire to generate cracks in external glazing systems due to increased thermal stresses can result in an increased level of oxygen availability which in turn has a major effect on the heat release rate of the fire. This scenario was studied previously by Pagni [1]. Research by Shields [2] also predicted that an island must be created in the glazing panel prior to the vent scenario.…”

“…It is appearant that the height of the flames are important and must be assesed. The flame height is dependant on the dimensions of the fuel source and the heat release rate of the fire and can be calculated as shown in Equation (1).…”

“…A large amount of experimental and numerical research has been conducted in relation to temperature and stress distribution in glass. Pagni [1] predicted that a temperature difference of 58 • C across a glazing panel would lead to the initiation of cracks. Keski-Rahkonen [3] predicted that a temperature difference of approximately 80 • C, across the glazing was required for the initiation of cracking.…”

Implementing a Glass Fracture Module in the BRANZFIRE Zone Model

“…The program is based on the heat transfer solution from Joshi and Pagni [5] and so models the glass as a distributed mass with through-thickness radiation absorption and nonlinear radiative boundary conditions. As well as specifying the dimensions and the thermophysical properties of the glass as constants, the user is required to enter the flame radiation flux history for windows close to the fire source, the hot layer temperature development, the heat transfer coefficient for the unexposed side (constant), the time-varying heat transfer coefficient for the fire-exposed side and the emissivity of the gas layer.…”

“…A numerical solution method for solving the through-thickness temperature distribution, based on the standard explicit finite difference method is presented. The resulting temperature distribution can be averaged and compared against the glass fracture criterion from Pagni and Joshi [5]. Sincaglia and Barnett [12] pay particular attention to radiant energy absorption, transmission and emission within the glass and its functional dependence on wavelength.…”