The Effect Of The Water Film On The Breakage Behavior Of Glass Exposed To Fire

The glazing assemblies are used more and more widely in modern buildings for aesthetic elegance and economic reasons, and the type of glass is also more diverse. These buildings make our living environment more comfortable and bright. However, the glazing system is the weakest part of high-rise buildings when subjected to a fire hazard. The breakage of the glass could significantly affect the compartment fire dynamics because the glass pane acts as a wall before breaking but as a vent after breaking. Once the new vent format, it will exacerbate the spread of fire, affecting evacuation, and then resulting in greater loss of life and property. In order to prolong the fire-resistant time of glazing system, window sprinklers are installed in some high-rise buildings.Domestic and foreign scholars have launched some research work in protecting ordinary glass, toughened glass, fire-resistant glass and so on with sprinkler system, and they drew some useful conclusions. After surveying lots of experimental and theoretical literature, this paper studied the influence of water film on the breakage and fallout behavior of heated glass experimentally.An experimental setup including water supply system, water film system, fire source simulation system, measurement and control system and recycling water film system was designed according to experimental purposes and programs.Tempered glass and two types of typical non-tempered glass (float glass and Low-E glass) were studied in the present work. To simulate that the water film discharged at different stage of the fire, the glass samples were heated to different temperatures and then the water film was initiated manually, the breakage behavior of glass was studied. The breakage and fallout behavior of glass was compared under the condition of with and without the water film.The results show that the water film can protect the glass very well as long as it can be initiated when the glass temperature is relatively low (namely the early stage of the fire). However, the water film will accelerate the breakage process of the tempered glass rather than protect it once the glass temperature is relatively high (the late stage of the fire. Under the installation condition of this paper, the maximum temperature to withstand water film for6mm tempered glass exposed to fire is250℃. That is to say, the water film will lead to the breakage of the glass when the glass temperature outnumbered this critical value.In the cases without the water film, the tempered glass could endure up to400℃without cracking. The thermal shock caused by the sudden cooling can account for this phenomenon. It is also found that crazing of tempered glass generally occurs in the rapid cooling process, rather than the rapid heating process. The water film will not only accelerate the breakage process of high temperature glass, but also increase the fallout ratio of the glass, because it has an impact effect when in contact with the glass.Similar phenomena was found in Low-E glass. Under the condition of this work, without water film effect the average fracture temperature for6mm Low-E glass is93.6℃. And the water film wouldn’t protect the Low-E glass any more once its temperature outnumbered70℃, on the contrary it will advance the crack and fallout process of Low-E glass. Under the continuous cooling of the exposed glass surface by the cold water film, the thermal stress caused by the temperature difference lead to the breakage of the glass. Under the installation conditions of this work, without water film effect the average crack temperature for6mm float glass is102.4℃. The water film wouldn’t accelerate the crack process of float glass when it is discharged below100℃.The difference between Low-E glass and clear float glass is that the Low-E glass is coated with a thin film including low emissivity material and metal oxides. Its heat transfer coefficient U is reduced significantly due to the existence of this layer of film. The characteristics of low penetration rate of thermal radiation result in greater temperature difference between the exposed and the unexposed glass surface, and that is the reason why Low-E glass is easier to crack than float glass with the same thickness.The heat absorbed by the water film, the average velocity and thickness of the water film and the utilization of the water film were also calculated in the paper, which revealed the protective effect more accurately.