5. Conclusions
To determine the critical breakage conditions for double glazing in fires, a total of nine glass panes were heated by a pool fire, with the burner-glazing distance changing from 750 mm to 450 mm. Important parameters of double glazing, in terms of breakage time, critical incident heat flux, temperature on four surfaces and cracking behavior, were obtained. Theoretical models are developed to reveal the heat transfer mechanism and predict the thermal breakage behavior of double glazing unit. Numerical study, using finite element method, is employed to predict surface temperatures, making a comparison with experimental result. It is established that the breakage condition of double glazing is significantly different from single ones. The specific conclusions are as follows: (1) In double glazing the pane at ambient side can survive three or four times longer than the pane exposed to fire. Thus, the fire resistance of double glazing is much better than that of ordinary single glazing. (2) For Pane 1, the critical temperature difference and heat flux are around 60 C and 6 kW/m2 ; while due to no frame covered area and radiation filter, the temperature needed for Pane 2 breakage is higher, and its critical heat flux is more than 25 kW/m2 . (3) With the burner-glazing distance decreasing, the breakage time of Pane 1 decreases. Using the prediction model based on incident heat flux, the breakage time of Pane 1 can be well predicted. (4) Through comparison, it is found that radiation transmitted through Pane 1 may be significant to Pane 2 heating in the beginning of fire and thus may not be ignored when predicting the thermal performance of Pane 2, but it needs to be further verified. (5) The breakage of Pane 2 means the failure of whole glazing unit, but it may be affected by multiple factors, such as the thickness of air gap, breakage behavior of Pane 1. Much more work is needed to enhance the understanding of double glazing thermal performance in fire.
