可燃气云爆炸强度计算综述

可燃气云爆炸强度计算综述Ξ

毕明树ΞΞ王淑兰　丁信伟

(大连理工大学)

摘　要　综述了可燃气云爆炸强度的计算研究现状,对已提出的两种计算方法进行了讨论。

关键词　气云爆炸　燃烧　设计计算

符　号　说　明

C o———大气声速,m/s;

E c———燃烧能,M J;

p o———大气压力,Pa;

Δp

s

———爆炸超压值,Pa;

Q f———气云的爆炸热,M J/kg;

Q T NT———T NT的爆炸热,M J/kg,Q T NT=4.12～4.69M J/kg;

R o———爆源半径,m;

R———至爆源中心的距离,m;

V———受限气云的体积,m3;

W f———气云的质量,kg;

W T NT———气云的T NT当量,kg;

α———T NT当量效率。

在可燃气体的输送、贮存、加工和使用过程中,如果向大气泄漏而形成的可燃气体与空气混合物被意外地点燃,就会发生爆炸,这种现象称为开敞空间蒸气云爆炸(Unconfined Vapor Cloud Ex2 plosion),简称UVCE。可燃气云爆炸已给人类社会造成了巨大灾难,要采取防治措施,就必须掌握气云爆炸的特性和爆炸强度。然而,由于气云爆炸的危险性和复杂性,研究工作难度较大。欧洲国家在这方面做了很多研究工作,进行了理论分析、实验研究和数值模拟,但至今尚未形成一致的结论。目前,可用的数据基本上是经验型的。文献中报道的可用于气云爆炸设计计算的方法主要有两种:T NT当量法和T NO多能(Multi2Energy)模型法。笔者对这两种方法进行了评述。

1　TNT当量法

由于对T NT炸药的爆炸威力已进行了大量研究工作,目前已能有效地预测其爆炸场和对物体的破坏作用。因此研究者[1]设想把气云爆炸的破坏作用转化成T NT爆炸的破坏作用,从而把气云的量转化成为的T NT量,称为T NT当量。因爆炸威力与爆炸反应热直接相关,故转换表达式为:

W T NT=αW f Q f/Q T NT(1)

这样,气云的爆炸威力就是质量为W T NT炸药的爆炸威力,其最大超压可由图1查取,换算公式为:

λ=R

3

W T NT

(2)

研究者[2]对23起事故的120余个点进行了统计分析,发现T NT当量效率分布在0.02%～15.9%之间,当量效率低于10%的情况占97%,当量效率近似于4%的情况占60%。显然,α值过于分散,实际应用中,偏差过大。这是因为气云爆炸与T NT爆炸有本质的区别:其一,T NT爆炸时的爆源体积可忽略,而气云的体积较大,不能忽略,且随着爆炸的进行,爆源体积在增大;其二, T NT爆炸时能量是瞬间释放的,而气云爆炸过程中能量的释放速率有限;其三,T NT爆炸过程形成的冲击波强度大,但衰减速度快,而气云爆炸多属爆燃过程,正压作用时间较短,负压作用时间较

Ξ

ΞΞ毕明树,男,1962年12月生,副教授,化工过程与装备教研室副主任。辽宁省大连市,116012。

国家教委博士点基金资助课题。

长。因此,T NT 当量法只适用于很强的气云爆炸,且用以描述远场时偏差小,用于近场时偏差较大。

图1　T NT 爆炸压力2距离曲线

2　TNO 多能模型

试验表明,完全开敞空间内气云爆炸产生的压力是很小的,而当气云内有障碍物时,即使气云体积较小,也能产生较高的压力,造成严重的生命与财产损失。可见,爆炸场中的障碍物是使爆炸压力升高的重要因素。依此,Van den Berg 等[3～6]提出了多能模型。基本观点是,既然约束条件是增强气云爆炸威力的关键因素,则只有受约束的那部分气云才对爆炸强度有作用,而不受约束的那部分气云对爆炸强度几乎没有贡献。T NO 多能方法以半球形气云为模型,假设中心点火,火焰以恒定速度传播,从而以数值方法计算不同燃烧速度下的气云爆炸强度,获得了一组爆炸强度曲线。图2是最大超压曲线,换算公式为:

Δ p s =

Δp s

p o

(3) R =

R

(E c /p o )1/3

(4) E c =3.5V (5) R o =

R o (E c /p o )1/3

(6)

图中有10条曲线,每条曲线代表不同的爆源强度,也就是火焰速度不同。曲线10代表爆轰的情况,曲线1～10爆源强度依次增大。曲线6～7代表爆源强度居中的情况,对一般的气云爆炸,处

理远场问题较合适。

这样,如果受限气云的体积V 已知,就可由式(5)计算出对爆炸有贡献的燃烧能量,由式(4)算得所关心点的 R ,然后从图2查得Δ p s ,再由式(3)得到超压值

。

图2　最大超压2距离曲线

多能模型在理论上比较合理,已受到广泛地

关注,但在具体应用中仍存在以下缺点:

a.该模型认为,处于完全开敞空间的气云是混合不均匀的混合物,不会维持爆轰的传播,因而可以忽略其对爆炸强度的贡献。这对大多数反应不太激烈的气云(如氨、甲烷、氯乙烯等)的爆炸是正确的,但如果气云形成的时间较长,尤其是反应激烈的气云,在完全开敞的空间内形成了均匀混合物,则它对爆轰的作用就很大。这种现象在工业实际中已出现过。

b.如何确定受限区域的尺寸是个难以解决的问题。

c.如果将整个气云分成几个爆炸源,它们的总强度如何叠加尚无定论。

d.爆炸强度图中的10个级别如何选取?3　结束语

以上介绍了两种相对比较实用的开敞空间气云爆炸强度的计算方法,但还远未完善。要获得普遍应用,还有很多工作要做。除上述两种方法外,还有许多其他方法,但其应用性更差,这里不做介绍。

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图5　实验值与拟合值的比较

从图中可以看出,实验值与拟合值之间存在一定的差异。这主要是由于测量手段不精确、流体的流型比较复杂、影响参数过多造成的。平均相对误差为15%。

5　结论

5.1　

要的因素。平均气含率和循环液速均随表观气速的增大而增大,但增加的程度因结构的不同而异。　

5.2　反应器结构参数的变化也有一定的影响,但是远远没有气量的影响程度大。底部间隙增加时,平均气含率及循环液速均有所增加;循环液速基本上随液位(顶部间隙)的增加而增加,但平均气含率随液位变化的情况较复杂;随筒节间隙的增加,平均气含率也有所增加,循环液速变化不明显;当筒节数减少时,平均气含率和循环液速均增加。

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随着计算机的迅速发展,实验手段的不断更新,数值模拟与大型实验相结合将是进行这方面研究的主要方法,必将提出更好的、实用的气云爆炸灾害防治措施。

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