Investigating potential of metal mesh to contain wildfires

Investigating potential of metal mesh to contain wildfires

This work concerns an exploration of the ability of various metal screens to contain both fire radiant heat flux and firebrand attacks in a wildfire. Wildfires are a growing concern in many parts of the world today. Fighting wildfires is a highly complex and dangerous venture due to their unpredictable nature and fast propagation velocity caused by firebrand attack and radiant heat flux. The unchanged trends of losses indicate the inefficiency of current firefighting techniques and motivate the development of new techniques that are capable of effectively containing wildfires and therefore avoiding losses.

Metal meshes are currently being used as flame arrestors and their practice in fire engineering sector has precedent. Metal screens are used with limited applications to protect structures in wildfire-prone areas. The Australian Standard for construction in bushfire-prone areas mandates covering all house openings by metal meshes to minimise the penetration of firebrands into the structures and contain the effect of fire radiation. Likewise, American standards such as the California residential code mandate screens to cover house ventilation openings in order to contain firebrands and flame impingements. Similar recommendations are evident in standards published by the International Code Council (ICC) and the National Fire Protection Association (NFPA) for structures in wildfire-prone areas. All indications imply that metal screens have a potential to contain wildfires propagation mechanisms. Therefore, it is expected that metal screens can be effective as barriers in containing the wildfires. This study aims to investigate the potential of metal screen to be used as wildfire barriers.

In this study, the performance of various metal screens to contain both fire radiant heat flux and firebrand attacks was explored in two steps. In the first step, an experimental setup using a light source and UV-visible spectrometer was designed and manufactured to determine the direct radiant flux through square woven wire screens with porosities ranging from 41% to 66%. The tunnel-vision effect of screens was measured and an empirical correlation between screen porosity and the angle of tunnel vision was developed. The results indicate that screens are able to block radiation more than the value suggested by their porosity, and more importantly, screens are more effective for larger radiant sources. Three empirical formulas were
presented to calculate the direct radiant heat flux of different size fires through screens.

In the second stage, an Ember Shower Simulator (ESS) was developed to study the performance of various metal screens against firebrand attacks. The ESS is capable of generating firebrands from various types of vegetation and performing experiments with low porosity screens at relatively high wind speeds. In a series of experiments, the effects of screen opening size, opening shape, wire diameter, screen manufacturing type and orientation of screens with respect to the firebrand flow on their performance against a Eucalyptus leaf firebrand shower were investigated. Further in this study, the combined effect of screen with different opening sizes and a buffer zone behind the screen was examined.

Two important mechanisms of firebrands passing through the screens were identified. Some firebrands shatter into smaller firebrands called secondary firebrands and then pass the screen opening. Some others that are less vulnerable keep burning behind the screen to reduce their size and pass through the screen opening. The screen opening size and wire diameter were found to have a great effect on the shattering intensity and the size of firebrands leaving the screen. Flat screens had a lower shattering intensity with respect to woven screens. An inclined screen increased the retention of firebrands behind screens in comparison with a vertical screen. This study could confirm that buffer zones are effective in improving the protection of objects shielded by a screen against firebrands. The experiments showed a relation between screen opening size and the size of the buffer zone as more firebrands quenched within the buffer zone for smaller opening size screens. It was found that the combination of buffer zone and screen remarkably reduced the number and size of firebrands on the fuel bed. As a result, the number of firebrand-induced ignitions significantly decreases in comparison with the condition that the screen is absent.

A discussion on the effectiveness of screens to contain the two propagation mechanisms is provided. Based on the discussion, it was found that metal screens are effective in containing the fire radiant heat flux and to considerably reduce the safety distance from a fire. The screens are efficient in substantially mitigating firebrand showers when a relatively short buffer zone is established. The results imply that screens may be capable of eliminating the risks of both firebrand attacks and radiant heat flux if an appropriate buffer zone is established.