Thermal Cracking Prediction and Thermal Protection by Down-flowing Water Film for Glazing Elements Exposed to Compartment Fires


Student thesis: Doctoral Thesis

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Awarding Institution
Award date28 Aug 2019


In modern buildings, the openings between adjoined compartments or between the outside and the compartment are usually installed with glazing elements. However, glass panes are easy to crack and fall out when subjected to heat loads because of low tensile strength. This is a risk factor of building fire safety in relation to spreading of smoke and fire, since the breakage of glass panes creates openings for fresh air to enter and also creates passages for rapid fire spread to adjacent rooms, accelerating fire development. This study focuses on two aspects of glazing elements regarding building fire safety: (ⅰ) the thermal cracking predictions of glazing elements exposed to compartment fires and (ⅱ) the thermal protection performance by down-flowing water film along glazing elements in compartment fire scenarios.

The prediction of loss of integrity of glazing elements exposed to compartment fires is important in building fire safety. Accurate prediction of glazing thermal breakage is critical for fire-spread and smoke-spread analysis in fire safety evaluation of a building, since glazing elements behave as walls before breakage and as openings after breakage. In cracking prediction, the criterion based on critical temperature difference has been widely adopted in fire safety evaluation of glazing elements in buildings. However, this method is inappropriate for thick glazing and large-height glazing since it omits the thermal bending stress for thick glazing and it omits the flexing stress for large-height glazing. Thus, appropriate and reliable expressions which can be employed in cracking prediction for thick glazing and large-height glazing are needed in order to improve the accuracy of cracking prediction of glazing elements in fire engineering applications.

The transient temperature fields and the corresponding stress fields in glazing of different thicknesses with one side exposed to heat radiation have been investigated. Glass samples of different thicknesses were tested under the same radiation exposure. Besides, a transient two-dimensional heat transfer model was presented to obtain the transient temperature fields in the glass samples. In addition, the temperature fields were loaded to ANSYS to obtain the thermal stress fields. Results show that in the initial heating period, the maximum stress was mainly caused by the temperature gradient component across thickness and the stress value was larger for thicker glazing. As time evolved, the factor determining the maximum stress was gradually dominated by the temperature gradient component along the planar direction. Based on these results, more accurate method on cracking prediction for thick glazing were formulated. It is suggested that for thick glazing under strong heat exposure, thermal bending stress caused by temperature gradient across thickness should be calculated first to evaluate the cracking time. If the thermal bending stress cannot reach the breaking value, the criterion of critical temperature difference should be used.

Thermal stress calculations including the effect of flexing stress were performed based on typical temperature distributions of large-height glazing exposed to compartment fires, then expressions of the maximum stress were derived. Besides, medium-scale experiments with glass panes of 600 mm x 600 mm exposed to heat radiation were carried out to verify the applicability and accuracy of the obtained expressions. Results show that the proposed expression of maximum stress agrees well with the experimental cracking behavior. Flexing stress had significant contribution to the total stress field, with cracks starting from both the bottom and upper edge of the glazing when exposed to typical distribution of heat radiation from compartment fires. Even though the experiments were of medium-scale, the results still confirm the contribution of flexing stress. For large-height glazing, the flexing stress will have a larger contribution to the total stress field and cannot be ignored.

Water is widely used in building thermal protections due to its convenience, low cost, high specific heat capacity, and high specific latent heat of vaporization. Down-flowing water film has great potential for wide applications in thermal protection of glazing elements in fire scenarios. However, a lack of theoretical studies on the heat attenuation performance limits the development of practical guidelines, such as the requirements for the flow rate, the water discharge pressure, and the activation principles. In the present study, the heat transfer process in the system of glazing incorporated with water film exposed to radiant fluxes were investigated, and preliminary expressions regarding the heat attenuation efficiency of the water film incorporating the key influencing factors have been developed. These include the thickness and mean velocity of the water film, the incident heat flux, and the surface temperature of the heated glazing at the instant of water film activation. Based on the expressions developed, the effects of these factors on the heat attenuation efficiency are analyzed. The results show that increasing the thickness of water film is more effective than increasing the velocity in obtaining higher thermal protection efficiency under the same flow rate in fire scenarios. In addition, the appropriate thickness range and velocity range of effective water film design are proposed, which can allow the common glazing elements to serve as fire barriers. Moreover, it is more efficient and safer to activate the water film before the glazing temperature reaches 220 ℃. These findings will be useful in developing practical guidelines for effective design of such water film systems.

    Research areas

  • glass glazing, fire safety, thermal cracking, water film, thermal protection