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How to Make Sure Wildfire Shelters Save Firefighters’ Lives

2022-07-26
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Wildfires burn hot, fast and unpredictably. Although wildland firefighters receive extensive training to keep themselves safe, they sometimes become cut off by flames that can reach temperatures of 1,600 to more than 2,000 degrees Fahrenheit. To protect themselves in these extremely dire situations, each carries a portable fire shelter (essentially a small, specially formulated foil tent) that can be deployed to shield them from flames and hot gasses. But this technology has serious limits, and researchers are now exploring new materials and designs—and putting prototypes through a gauntlet of fiery tests.

The National Wildfire Coordinating Group (NWCG), a federal government organization that sets standards for wildland fire equipment, reports that fire shelters have been deployed more than 200 times between 2006 and 2020. But this last line of defense does not always work. For example, 19 of the 20 members of the Granite Mountain firefighting crew tragically perished despite using their shelters in Arizona’s 2013 Yarnell Hill Fire. The need for better shelters will only become more crucial as fire seasons continue to grow more severe: last year alone, firefighters battled almost 60,000 wildfires that turned seven million acres of U.S. forest into blackened ash.

“The fire seasons are lengthening, getting more severe, and wildland firefighters are seeing fire behavior that we haven’t seen before,” says Camille Stevens-Rumann, an assistant professor of forest and rangeland stewardship at Colorado State University, who was not involved in the recent fire shelter tests. “In Colorado, we had a fire that burned 6,000 acres an hour. Those conditions lead to more risk for firefighters.” Such situations are particularly dangerous when flames spread quickly in a short period of time, forcing firefighters to retreat to their emergency shelters. “We’re also seeing an increase in extreme fire events, where you see many acres burning over a short period of time,” Stevens-Rumann adds. “With factors like wind, lack of moisture and plentiful fuels, you see fires exploding quickly in a single day.”

The current M2002 fire shelter, the only model approved for use by government-agency firefighters, folds down to a 4.3-pound packet about the size of a loaf of bread. It is typically stored in a plastic sleeve and carried in a special compartment on wildland firefighters’ backpacks. The packet can be unfolded into a half-tube that is just large enough for one person to lie down inside. Its fire resistance comes from a two-layer construction, with an air gap in between for added insulation. The outer layer consists of woven silica that is laminated, or bonded, to aluminum foil. The inner layer is fiberglass laminated to a separate layer of aluminum foil.

A wildland fire emergency shelter folds down to a packet about size of a loaf of bread. Credit: USDA Forest Service photo by Ian Grob

In 2019 a five-year NWCG review recommended retaining the existing fire shelter design. But the organization is always looking for improvements. Now researchers at North Carolina State University (N.C. State) have followed NWCG fire protection guidelines to evaluate the M2002, along with four prototypes developed by university researchers. Their work was detailed in a report published this past spring.

“We’re just trying to improve on [the current design],” says the study’s lead author Joseph Roise, a professor of forestry and environmental resources at N.C. State. “The domelike shape is really as good as you can get. You want to be close to the ground because heat rises—the closer you are to the ground, the less heat you have on your body.” Because the existing shape is hard to beat, the new emergency fire shelter prototypes focus on other ways to improve heat resistance. From the outside, they look a lot like the M2002. But some add an additional layer of advanced heat-resistant material. Others experiment with the placement of the seams, which can be a weak point.

To test the fire shelters’ thermal protective performance in a controlled laboratory environment, the N.C. State researchers used a specially built fire chamber called the PyroDome Turbulent Flame Fire Shelter Test System. Within the chamber, propane burners blasted full-size fire shelters with a direct flame for one minute. Sensitive instruments measured the time needed for the temperature at their floor to hit 302 degrees F, the established maximum temperature level for survivability in a shelter. A video camera inside each test shelter documented how the inside walls and seams changed with exposure to the flames.

During such tests, fire shelters must withstand two types of heat. First, there is radiant heat—think of it as the warmth experienced when standing by a campfire. The outer layer of aluminum reflects approximately 95 percent of this heat, according to Roise. He notes that aluminum is very durable, and when combined with a silica base, which slows the rate of heat transfer to lower the temperature inside the shelter, the materials work together well at reflecting radiant energy.

A more serious challenge is convective heat, which is experienced when a fire moves through a shelter deployment site and the flames or hot gasses directly touch the outside of the shelter. The outer layer can absorb this convective heat, raising its temperature. As that temperature nears 500 degrees F, the adhesives that bind the layers together can break down. If the aluminum foil exterior is separated from the cloth of the shelter’s outer layer, it can be torn away by turbulent winds, destroying much of the shelter’s reflective protection. Any torn spots in the material can also allow convective heat to breach the shelter and rapidly raise internal temperatures.

Because real wildfires create such unpredictable conditions, it is necessary to test fire shelters in the field, as well as in the laboratory. The N.C. State researchers also conducted eight field tests in four locations around North America. The test sites offered different fuel types, such as chaparral (where the ground is covered with shrubs or small trees), grassland and boreal forest, as well as multiple kinds of topography, from flat to hilly. The tests exposed the prototype shelters to different flame configurations, temperatures and weather conditions. This variety made it challenging to compare the M2002 model with prototypes, however. High winds, inconsistent amounts of fuel, and varying fire behavior produced different conditions for each shelter tested.

Emergency fire shelters after field-testing. Credit: John Williams

“There are a lot of variables in a wildfire environment. It’s very unpredictable. You could have a shelter next to another shelter, and when fire runs through the area, you’ve got surprisingly different results between the two,” says David Maclay-Schulte, an equipment specialist at the U.S. Forest Service’s National Technology and Development Program, who did not work on the new N.C. State report. “It’s very challenging to get repeatable and reliable data that way.”

Despite such challenges, the M2002 and all four prototypes passed the fire protection tests: they preserved a survivable air temperature inside the shelters during the field test operations, even in situations where flames breached the outer layer. The tests also proved the effectiveness of prototypes that included advanced thermal-insulation layers. And adding an insulating layer actually prevented some of the convective energy from burning through a shelter’s outer layers of aluminum foil, according to Roise: all of the prototypes outperformed the M2002 in the lab tests, and the one that performed best included a layer of a heat-resistant material called Kapton, developed by chemical company DuPont.

But this prototype was also much heavier and bulkier than the current model. That’s a problem because a shelter’s weight (as well as its durability and cost) are important considerations. Wildland firefighters already must carry 45-pound packs of equipment, sometimes in sweltering heat, so a shelter that adds too much to that burden will not make the cut.

Another critical criterion is toxicity, which the N.C. State study did not test for. Researchers have studied certain materials that offer increased protection from radiant and convective heating. When exposed to high heat, however, these substances release toxic fumes that would endanger firefighters using the shelters. “Firefighters can survive the fire but suffer negative consequences because the thermally decomposed material becomes poisonous,” Maclay-Schulte says.

Although the prototypes proved promising, they failed to dethrone the M2002 as the model wildland firefighters carry into the field. But the quest for better fire shelters still continues. “We’re always testing different materials and materials composites to see if they react differently and perform better,” Maclay-Schulte says. “There’s always something going on with fire shelter development.”

参考译文
如何确保野火庇护所拯救消防员的生命
野火燃烧迅速,迅速,难以预测。尽管野外消防员接受了大量的安全培训,但有时他们会被高达1600至2000多华氏度的火焰切断消防通道。为了在这些极端可怕的情况下保护自己,每个人都携带一个便携式的消防掩体(本质上是一个特殊配方的小箔帐篷),可以用来保护他们免受火焰和热气体的伤害。但这项技术有严重的局限性,研究人员现在正在探索新材料和新设计,并对原型进行了激烈的火焰测试。美国国家野火协调小组(NWCG)是一个为野外消防设备制定标准的联邦政府组织,该组织报告说,2006年至2020年期间,消防庇护所已经被部署了200多次。但这最后一道防线并不总是有效。例如,2013年亚利桑那州的亚内尔山火灾中,花岗岩山消防队员的20名成员中有19人不幸遇难,尽管他们使用了自己的避难所。随着火灾季节继续变得更加严重,对更好的避难所的需求只会变得更加重要:仅去年一年,消防人员就与近6万起野火搏斗,使美国700万英亩的森林变成了黑色的灰烬。科罗拉多州立大学森林和牧场管理助理教授卡米尔·史蒂文斯-鲁曼(Camille Stevens-Rumann)说:“火灾季节正在延长,变得越来越严重,野外消防人员看到了我们以前从未见过的火灾行为。”她没有参与最近的消防场所测试。“在科罗拉多州,我们经历了一场每小时烧毁6000英亩土地的大火。这些情况给消防员带来了更多的风险。”当火焰在短时间内迅速蔓延,迫使消防员撤退到紧急避难所时,这种情况尤其危险。史蒂文斯-鲁曼补充说:“我们还看到极端火灾事件的增加,你可以看到许多英亩的土地在短时间内被烧毁。”“由于风、缺乏水分和充足的燃料等因素,你会看到火灾在一天内迅速爆发。”目前的M2002消防站是唯一被政府部门消防人员批准使用的型号,它可以折叠成一个4.3磅重的包,大约和一块面包一样大。它通常被储存在一个塑料套筒里,放在野外消防队员的背包里的一个特殊隔间里。这个包裹可以展开成一个只够一个人躺在里面的半管。它的防火性能来自于两层结构,中间有一个空气间隙,以增加绝缘。外层由编织二氧化硅层压或粘合到铝箔上。内层是玻璃纤维层压到一个单独的铝箔层。2019年,NWCG进行了为期五年的审查,建议保留现有的消防掩体设计。但公司总是在寻求改进。现在,北卡罗来纳州立大学(North Carolina State University, N.C. State)的研究人员已经按照NWCG消防指南评估了M2002,以及由大学研究人员开发的四个原型机。今年春天发表的一份报告详细介绍了他们的工作。“我们只是试图改进(当前的设计),”该研究的主要作者约瑟夫·罗伊斯说,他是北卡罗来纳州立大学林业和环境资源教授。“穹顶状的形状真的是你能得到的最好的。你想要靠近地面,因为热量会上升——你离地面越近,你身上的热量就越少。”因为现有的形状很难被击败,新的应急消防避难所原型专注于其他方法来提高耐热性。从外观上看,它们很像M2002。但有些人会额外添加一层高级耐热材料。另一些人则尝试铺设接缝,这可能是一个薄弱环节。 为了在受控的实验室环境中测试防火掩体的热保护性能,北卡罗来纳州立大学的研究人员使用了一个专门建造的名为pyroome湍流火焰防火掩体测试系统的消防室。在燃烧室内,丙烷燃烧器用直接火焰炸开全尺寸的防火掩体,持续一分钟。灵敏的仪器测量了他们的地面温度达到302华氏度所需的时间,302华氏度是避难所生存所需的最高温度水平。每个测试掩体内的摄像机记录了暴露在火焰中的内墙和接缝的变化。在这种测试中,防火设施必须能承受两种类型的高温。首先,有辐射热——可以把它想象成站在篝火旁的温暖。罗伊斯说,外层的铝反射了大约95%的热量。他指出,铝非常耐用,当与硅基材料结合时,可以减缓热传递速度,从而降低庇护所内的温度,这种材料在反射辐射能方面表现良好。更严重的挑战是对流热,当火灾通过避难所部署地点,火焰或热气体直接接触避难所外部时,就会感受到对流热。外层可以吸收对流热量,从而提高温度。当温度接近500华氏度时,粘合层的粘合剂就会分解。如果铝箔外层与防护罩外层的布分开,它就会被湍流的风吹走,破坏防护罩的大部分反光保护。材料上的任何撕裂点也会让对流热量突破遮蔽物,迅速提高内部温度。因为真正的野火会造成如此不可预测的条件,所以有必要在现场和实验室中测试防火庇护所。北卡罗来纳州立大学的研究人员还在北美的四个地方进行了八次实地测试。试验场提供了不同类型的燃料,如灌木丛(地面被灌木或小树覆盖)、草地和北方森林,以及从平地到丘陵的多种地形。测试将原型掩体暴露在不同的火焰配置、温度和天气条件下。然而,这种多样性使它具有挑战性的比较M2002模型与原型。大风、不一致的燃料量和不同的火灾行为对每个测试避难所产生了不同的条件。“野火环境中有很多变量。这是非常难以预测的。美国林业局国家技术与发展项目的设备专家大卫·麦克雷-舒尔特说:“你可以把避难所放在另一个避难所旁边,当大火穿过该地区时,你会得到令人惊讶的不同结果。”他没有参与北卡罗来纳州的新报告。“通过这种方式获得可重复和可靠的数据非常具有挑战性。”尽管有这样的挑战,M2002和所有四个原型都通过了防火测试:它们在现场测试操作中保持了一个可生存的空气温度,即使在火焰冲破外层的情况下。测试还证明了包括先进隔热层的原型的有效性。根据罗伊斯的说法,增加一层绝缘层实际上可以防止一些对流能量通过防护罩外层的铝箔燃烧:所有的原型机在实验室测试中都超过了M2002,表现最好的包括一层名为Kapton的耐热材料,由化学公司杜邦开发。 但这个原型机也比现在的模型重得多,体积也大得多。这是一个问题,因为庇护所的重量(以及它的耐用性和成本)是重要的考虑因素。Wildland的消防队员已经必须携带45磅重的设备包,有时还要在闷热的天气里,所以一个给这种负担增加太多负担的庇护所是不符合要求的。另一个关键标准是毒性,北卡罗来纳州的研究没有测试这一点。研究人员已经研究了某些材料,这些材料可以提供更多的保护,免受辐射和对流加热。然而,当暴露在高温下时,这些物质会释放出有毒气体,危及使用避难所的消防员。麦克雷-舒尔特说:“消防员可以在火灾中幸存下来,但会遭受负面后果,因为热分解的物质会有毒。”尽管这些原型车被证明很有前途,但它们并没有取代M2002,成为野外消防员带进野外的模型车。但对更好的防火设施的追求仍在继续。麦克雷-舒尔特说:“我们一直在测试不同的材料和复合材料,看看它们的反应是否不同,性能是否更好。”“消防站的开发总是有问题。”
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