图1 冲击电流发生器外观图
纸质出版日期:2021-05-25,
网络出版日期:2020-11-10,
收稿日期:2019-11-06,
录用日期:2020-02-03
扫 描 看 全 文
引用本文
阅读全文PDF
雷击是森林火灾的重要点火源。文章研究了雷击的位置与地表可燃物的关系,采用冲击电压发生器测量了三种典型针叶床的击穿电压值。结果表明:新疆云杉针燃料床和西伯利亚松针燃料床的击穿电压值差别不大,马尾松针燃料床击穿电压最低。另外,研究了不同含灰率对泥土电阻率的影响,结果表明:含灰率越大,电阻率越低,经过人工烧除后的迹地更容易遭受雷击。采用冲击电流发生器来模拟雷击过程,并结合Taylor的假设探究了森林雷击火的形成过程。最后,比较了典型植被结构中树木和草的雷击临界着火特性,发现草比木片更容易被雷电引燃。
Lightning strike is an important ignition source of forest fires. As the global warming, forest lightning fire is becoming more and more serious. In this paper, the relationship between the position of lightning strike and ground combustible matter was studied, and the breakdown voltages of three typical conifer beds were measured by the impulse voltage generator. The results show that there is little difference in the breakdown voltage between Xinjiang spruce needle bed and Siberian pine needle bed. The breakdown voltage of Pinus massoniana Lamb. needle bed is the smallest. In addition, the effect of ash content on the resistance of soil was studied. The results show that the higher the ash content is, the lower the resistance is. The site after prescribed burning is more vulnerable to lightning strike. The impulse current generator was used to simulate the lightning strike process, and it is found that the combustible matter can be ignited and produce mars splashing under the action of impulse current. Combined with the hypothesis of Taylor, the formation process of forest lightning fire was discussed. Finally, the lightning critical ignition characteristics of typical vegetation structure (tree and grass) were compared, and it is found that the grass is more likely to be ignited by lightning than the wood chip.
雷击是引起森林火灾的重要起因,它作为天然火源,是一种难以控制的自然现象,并且形成机理复杂。另外,雷击火通常发生在一些偏远的深山密林,与人为火相比,雷击火的预防与扑救难度要大,容易形成大面积的森林火灾,危及生态环境和人类安全。我国的雷击火在大兴安岭和新疆阿尔泰山最为严重,约占地区森林火灾总数的38%和18%[
雷击火与地理因素有关,雷击的位置对环境有选择性。经验表明,沼泽地、草塘等是雷击的高发区[
雷击火形成机理复杂,以往很多研究采用火灾发生后的统计结果来发掘雷击火的规律。彭欢采用Logistic模型研究大兴安岭的雷击火,并对模型参数进行了修正[
Fuquay对蒙大拿地区发生的雷击火进行了观测研究。在对引起雷击火的云地闪特征数据进行分析后,发现:这些云地闪放电过程中均伴随长时间连续电流,这些长时间连续电流放电时间超过40 ms。此后,Fuquay等将LCC作为能够引发雷击火云地闪的重要判定依据。这一理论得到大部分学者的认同,并应用于美国国家火险等级系统的雷击火险预测模块。此后,长时间持续电流成为雷击火研究领域的重要研究对象[
本文将研究植物含灰率对泥土电阻率的影响,通过冲击电压发生器测量不同地表可燃物的击穿电压,进而探讨人工烧除迹地后的雷击选择性。另外,通过冲击电流发生器来模拟闪电点燃植物的过程,发现冲击电流的点火规律。
新疆额尔齐斯河和内蒙古呼伦贝尔草原是中国雷击火高发区。实验所用的泥土采自新疆额尔齐斯河畔的一块天然泥土地。经考察,该泥土并没有烧过的灰烬痕迹,实验前去除泥土中的根须等杂质。所用针叶分别是新疆云杉针,西伯利亚松针和马尾松针,采样时从地表干枯针叶中选取。草样和木料采自呼伦贝尔草原,分别是糙隐子草和樟子松木,采样时从草原上采割新鲜的活糙隐子草,从活树干上截取一部分樟子松树枝。材料经过105°C烘箱烘干后使用。
含植物灰的泥土制备方法:烘干的泥土通过研磨后,用50目的滤筛过滤;把烘干的马尾松针放入燃烧池燃烧后,获得的灰烬研磨并通过50目的滤筛过滤。最后把泥土,水和灰烬混合,通过改变加入的灰烬重量来制备不同含灰率的土样。
采用冲击电压发生器测量地表可燃物的击穿电压,冲击电压发生器依据Marx回路原理设计。Marx回路原理简单说就是利用电容器并联充电,然后串联放电而产生高压,通过调节电阻、电容以及放电间隙等参数来调整输出电压的波形。冲击电压发生器广泛地应用于雷电过电压耐受性能试验[
泥土电阻率采用试样盒测量,试样盒用亚克力板制成,尺寸为10 cm×5 cm×1 cm,盒两端接铜电极,最后电极处接上电阻表来测量泥土的电阻。泥土电阻率通过电阻率公式ρ=RS/L计算。其中,ρ为电阻率,Ω·m;R为电阻,Ω;S为横截面积,m2;L为长度,m。
雷击着火模拟实验在冲击电流发生器上进行,如
图1 冲击电流发生器外观图
Fig.1 Impulse current generator
图2 雷击点火装置示意图
Fig.2 Schematic diagram of lightning ignition device
图3 典型的电压电流曲线
Fig.3 The typical voltage and current curves
测量不同地表可燃物的击穿电压步骤是:逐步升高冲击电压的峰值,直到电压能击穿地表可燃物为止,如果该电压值能重复击穿植被4次,则记录该值作为击穿电压值。重复上述步骤4次,得到4个击穿电压值,并对4个值求平均和标准偏差。
测量土样电阻率的步骤:把泥土样本装入试样盒后,接上电阻表测量、重复测量,求取平均值和标准偏差。
测量可燃物的雷击着火临界电流的步骤是:保持冲击电流波形为8/20 µs不变,逐步升高冲击电流的峰值,直到可燃物刚好能着火为止,记录输出的电流、电压波形,此时的电流即为着火临界电流。重复测量,求取平均值。
地表可燃物是由可燃物和空气组成的燃料床,是典型的两相体。两相体空间的电场分布情况非常复杂,非气相物质颗粒的存在使得两相体中的局部电场发生畸变,畸变程度与非气相物质颗粒自身参数和分布情况相关[
实验测量了森林地表三种常见松针燃料床的击穿电压值,结果如
图4 三种松针燃料床的击穿电压值
Fig.4 Breakdown voltage of three kinds of pine needle bed
泥土的含灰率与电阻率关系如
ρ=34+8×0.84φ, | (1) |
其中ρ为电阻率,Ω·m;φ为含灰率,%。并求得R-Square = 0.97。
图5 含灰率与电阻率的关系
Fig.5 Relationship between ash content and resistivity
影响土壤电阻率的因素很多,主要有三类:第一类是与土壤结构有关的因素,包含孔隙率、含水量和土壤结构;第二类是表征土壤颗粒特征的因素,包含土壤颗粒形状与方位、土壤颗粒粒度分布、离子交换能力与润湿性等;第三类是与土壤溶液有关的因素,它随着土壤外界环境条件的变化而改变,主要有孔隙水电阻率、孔隙水中离子组成与外界温度等。这三类因素对土壤电阻率的影响并不是独立的,而是相互影响、相互作用的[
可燃物是火灾燃烧的物质基础,枯死的杂草松针极易引起火灾。计划烧除方式可人为干预减少可燃物载量,改变可燃物立体结构,切断可燃物的连续分布,达到预防、减少高强度火灾的目的[
采用冲击电流发生器可以模拟得到雷击的可燃物着火过程。放电后可燃物的着火现象如
图6 冲击电流作用下的可燃物着火现象
Fig.6 Ignition phenomenon of combustible matter under the impulse current
自然界雷电放电时,由于树木突出地表,更容易成为雷击的目标。雷电在树干上形成放电通道,同时产生热效应和机械破坏效应。机械破坏效应是由于焦耳热使树木内部水分受热急剧气化,封闭于树体内的气体剧烈膨胀,致使内外压差逐步变大,进而在被击物体内部出现了强大的机械力,使树木遭受破坏[
呼伦贝尔草原雷击火重点区域的主要植被类型是樟子松+糙隐子草。我们通过冲击电流发生器,对比了樟子松木片和糙隐子草燃料床的雷击临界着火特性,如
项目 | 糙隐子草燃料床 | 樟子松木片 |
---|---|---|
Ip / kA2) | 21.44 | 95.48 |
∫τ0I2dt /(A2·s)3) | 7 208 | 142 547 |
∫τ0UIdt /J 4) | 133 | 6 276 |
1)冲击电流波形8/20 µs;糙隐子草燃料床的堆积密度为0.159 2 g/cm3,含水率0%,厚度1 mm;樟子松木片含水率0%,厚度1 mm;2)Ip冲击电流波形的电流峰值;3)∫τ0I2dt电流波形的平方对时间积分;4)∫τ0UIdt冲击电流输入的总能量。
在草床和木片厚度、含水率相同情况下,取草床典型的堆积密度和木片进行比较,可以发现糙隐子草燃料床的Ip、∫τ0I2dt、∫τ0UIdt比樟子松木片少,临界着火所需能量更少,更容易着火,这主要是由于两种可燃物的着火温度和比热容不同。
本文首先研究了雷击的位置选择性,通过测量三种典型的地表可燃物的击穿电压,得到它们的击穿电压差异,其中马尾松针燃料床的击穿电压最小,绝缘性能最差;通过测量不同含灰率的泥土电阻率,发现随着含灰率增加,电阻率减少,人工烧除后的迹地更容易遭受雷击。
其次,本文通过冲击电流发生器产生了模拟闪电,冲击电流不仅可以加热可燃物至着火,同时也能产生巨大的机械破坏效应,使着火的可燃物飞溅甚至熄灭,这影响了初始火苗转为森林火灾的可能性。观察得到的雷击着火现象与文献和自然界较吻合,进一步验证了Taylor的假设。另外,我们比较了典型植被结构中的树木和草的临界着火特性,发现该结构中草比木片更容易被雷电引燃。
通过冲击电流发生器可以进一步探究雷击火的着火机理,并可以进行定量比较不同含水量和堆积密度对可燃物着火的影响。本文将为以后开展雷击火实验提供一种方法借鉴。在将来的工作中,需要把可燃物雷击临界着火能量与自然界的雷击着火实地数据进行对比,以更好地验证人工模拟雷击设备的正确性。
舒立福,王明玉,田晓瑞,等. 我国大兴安岭呼中林区雷击火发生火环境研究[J]. 林业科学,2003,39(6):94-99. [百度学术]
SHU L F, WANG M Y, TIAN X R, et al. The fire environment mechanism of lightning fire formed for Daxing'an mountains[J]. Scientia Silvae Sinicae,2003,39(6): 94-99. [百度学术]
PRICE C, RIND D. The impact of a 2 × CO2 climate on lightning-caused fires[J].Journal of Climate, 1994, 7(10): 1484-1494. [百度学术]
白嘉懿. 四川木里森林火灾确认为雷击火,整个火场已全面控制[N]. 中国新闻网,2019-04-05. [百度学术]
BAI J Y.The forest fire in Muli, Sichuan Province was confirmed as a lightning fire, and the whole fire site has been fully controlled[N].WWW.CHINANEWS.COM, 2019-04-05. [百度学术]
于建龙,刘乃安. 我国大兴安岭地区森林雷击火发生的火险天气等级研究[J]. 火灾科学,2010,19(3):131-137. [百度学术]
YU J L, LIU N A. Lightning-caused wildland fire weather danger rating in Daxing'anling region[J]. Fire Safety Science, 2010, 19(3): 131-137. [百度学术]
ALBRECHT R I, GOODMAN S J, BUECHLER D, et al. Where are the lightning hotspots on Earth?[J]. Bulletin of the American Meteorological Society, 2016, 97(11): 2051-2068. [百度学术]
WOTTON B M, MARTELL D L. A lightning fire occurrence model for Ontario[J]. Canadian Journal of Forest Research, 2005, 35: 1389-1401. [百度学术]
张吉利,毕武,王晓红,等. 雷击火发生的影响因子与预测研究进展[J]. 应用生态学报,2013,24(9):2674-2684. [百度学术]
ZHANG J L, BI W, WANG X H, et al. Lightning-caused fire, its affecting factors and prediction: a review[J]. Chinese Journal of Applied Ecology, 2013, 24(9): 2674-2684. [百度学术]
TAYLOR A R.Ecological aspects of lightning in forests[J].Proc Tall Timbers Fire Ecol Conf,1974,13: 455-482. [百度学术]
李良福,胡怀林. 森林雷电防护研究[M]. 中国:气象出版社,2004. [百度学术]
周世濂. 雷电放电过程中雷击点选择性的机理研究[D]. 广西:广西大学,2004. [百度学术]
ZHOU S L. Mechanism of lightning strike selectivity during the process of discharge[D]. Guangxi: Guangxi University, 2004. [百度学术]
HERMAN R. An introduction to electrical resistivity in geophysics[J]. American Journal of Physiology, 2001, 69(9): 943-952. [百度学术]
张润霞,王益权,解迎革,等. 盐分对土壤电阻率的影响研究[J]. 干旱地区农业研究,2015,33(2):208-213. [百度学术]
ZHANG R X, WANG Y Q, XIE Y G, et al. Influence research of salts on the soil resistivity properties[J]. Agricultural Research in the Arid Areas, 2015, 33(2): 208-213. [百度学术]
王乐凡,余承华,顾强康,等. 不同含盐量饱和盐渍土电阻率试验[J]. 空军工程大学学报(自然科学版),2014,15(4):25-28. [百度学术]
WANG L F, YU C H, GU Q K, et al. A study of resistive experiment on saturated saline soil with different salinity[J]. Journal of Air Force Engineering University (Natural Science Edition), 2014, 15(4): 25-28. [百度学术]
彭欢,史明昌,孙瑜,等. 基于Logistic的大兴安岭雷击火预测模型[J]. 东北林业大学学报,2014,42(7):166-169. [百度学术]
PENG H, SHI M C, SUN Y, et al. Lightning fire forecasting model of Daxing’an Mountain Based on Logistic Model[J]. Journal of Northeast Forestry University, 2014, 42(7): 166-169. [百度学术]
孙瑜,史明昌,彭欢,等. 基于MAXENT模型的黑龙江大兴安岭森林雷击火火险预测[J]. 应用生态学报,2014,25(4):1100-1106. [百度学术]
SUN Y, SHI M C, PENG H, et al. Forest lighting fire forecasting for Daxing’anling Mountains based on MAXENT model[J]. Chinese Journal of Applied Ecology, 2014, 25(4): 1100-1106. [百度学术]
郭福涛,胡海清,马志海. 应用空间点模式方法研究大兴安岭雷击火空间分布格局[J]. 生态学报,2009,29(12):6741-6747. [百度学术]
GUO F T, HU H Q, MA Z H. Spatial point process for spatial distribution pattern of lightning-caused forest fires in DaXing'an Mountains[J]. Acta Ecologica Sinica, 2009, 29(12): 6741-6747. [百度学术]
高永刚,顾红,张广英. 大兴安岭森林雷击火综合指标研究[J]. 中国农学通报,2010,26(6):87-92. [百度学术]
GAO Y G, GU H, ZHANG G C. Integrated Index Study on Forest Lightning Fire for Daxinganling Mountains[J]. Chinese Agricultural Science Bulletin, 2010, 26(6): 87-92. [百度学术]
HARTFORD R A.Smoldering combustion limits in peat as influenced by moisture, mineral content, and organic bulk density[C]//Proceedings of the Conference on Fire and Forest Meteorology, 1993: 282-286. [百度学术]
NIETO H, AGUADO I, GARCIA M, et al.Lightning-caused fires in central Spain: Development of a probability model of occurrence for two Spanish regions[J]. Agricultural and Forest Meteorology,2012,162-163: 35-43. [百度学术]
PODUR J, MARTELL D L, CSILLAG F. Spatial patterns of lightning-caused forest fires in Ontario, 1976-1998[J]. Ecological Modelling, 2003, 164(1):1-20. [百度学术]
LATHAM D J, SCHLIETER J A. Ignition probabilities of wildland fuels based on simulated lightning discharges[R]. US: Intermountain Research Station, 1989. [百度学术]
朱易,刘乃安,邓志华,等. 雷击引燃森林可燃物概率的实验研究[J]. 火灾科学,2012,21(2):71-77. [百度学术]
ZHU Y, LIU N A, DENG Z H, et al.Experimental study on the probability of lightning induced ignition of forest fuels[J].Fire Safety Science,2012,21(2): 71-77. [百度学术]
DARVENIZA M, ZHOU Y. Lightning-initiated fires: Energy absorbed by fibrous materials from impulse current arcs[J]. Journal of Geophysical Research, 1994, 99(D5): 10663-10670. [百度学术]
FUQUAY D M, TAYLOR A R, HAWE R G, et al. Lightning discharges that caused forest fires[J]. Journal of Geophysical Research, 1972, 77(12): 2156-2158. [百度学术]
Society of Automotive Engineers.SAEARP 5412.Aircraft lightning environment and related test waveforms[S].USA:Ae-2 Lightning Committee,2005. [百度学术]
郭云力.碳纤维增强树脂基复合材料雷击损伤的研究[D]. 山东:山东大学,2014. [百度学术]
GUO Y L.The research of lightning damage of carbon fiber reinforced polymer composite[D]. Shandong: Shandong University, 2014. [百度学术]
NECHMI H E, BEROUAL A, GIRODET A, et al. Fluoronitriles/CO2 gas mixture as promising substitute to SF6 insulation in high voltage applications[J]. IEEE Transactions on Dielectrics and Electrical Insulation, 2016, 23(5): 2587-2593. [百度学术]
周游. 雷电冲击电压下纳米粒子改变变压器油纸绝缘特性的机理[D]. 河北:华北电力大学,2015. [百度学术]
ZHOU Y. Influence mechnism of nanoparticles on the insulating properties of nanofluid/pressboard under lightning impulse voltage[D]. Hebei: North China Electric Power University, 2015. [百度学术]
孙伟,陈文针.冲击电流发生器的理论分析[J]. 高压电器,1999(4):40-42. [百度学术]
SUN W, CHEN W Z.Theoretical analysis of impulse current generator[J].High Voltage Apparatus,1999 (4):40-42. [百度学术]
刘志强. 雷电环境下复合材料层合板电-磁-热-结构耦合效应研究[D]. 陕西:西北工业大学,2014. [百度学术]
LIU Z Q. Analysis of electro-magneto-thermo-structural coupling effects on composite laminate in lightning environment[D]. Shanxi: Northwestern Polytechnical University, 2014. [百度学术]
董琪. 碳纤维复合材料雷击损伤实验研究与数值模拟[D]. 山东:山东大学,2015. [百度学术]
DONG Q. Experimental and simulation study of the lightning strike damage of carbon fiber composites[D]. Shandong: Shandong University, 2015. [百度学术]
邓鹤鸣. 雷电冲击电压下两相体放电特性研究[D]. 湖北:华中科技大学,2010. [百度学术]
DENG H M. The research on the characteristics of two-phase mixture discharges under lightning impulse voltage[D]. Hubei: Huazhong University of Science and Technology, 2010. [百度学术]
聂向晖,杜鹤,杜翠薇,等.大港土电阻率的测量及其导电模型[J].北京科技大学学报,2008,30(9):981-985. [百度学术]
NIE X H, DU H, DU C W, et al. Electrical resistivity measurement and conductive model of Dagang soil[J]. Journal of University of Science and Technology Beijing, 2008, 30(9): 981-985. [百度学术]
高仲亮,李岩泉,张明远. 大兴安岭南部草甸计划烧除的防火效果评估[J]. 林业机械与木工设备,2015,43(8):19-21. [百度学术]
GAO Z L, LI Y Q, ZHANG M Y. Evaluation of fire prevention effect of planned burning of meadow in southern Daxing’anling[J]. Forestry Mechinery & Woodworking Equipment, 2015, 43(8): 19-21. [百度学术]
ANDERSON K R. A model to predict lightning-caused fire occurrences[J]. International Journal of Wildland Fire, 2002, 11: 163-172. [百度学术]
杜野. 雷击木的特征研究[J]. 森林防火,2018(1):32-35. [百度学术]
DU Y. Study on the characteristics of lightning strike wood[J]. Forest Fire Prevention, 2018 (1): 32-35. [百度学术]
WIERZCHOWSKI J, HEATHCOTT M, FLANNIGAN M D. Lightning and lightning fire, central cordillera, Canada[J]. International Journal of Wildland Fire, 2002(11): 41-51. [百度学术]
0
浏览量
1
下载量
3
CSCD
相关文章
相关作者
相关机构