摘要
随着现代工业和社会环境中储存、运输或使用着大量的危险液化气体介质,这些介质通常不是有毒就易燃易爆,被加压以液态形式存在。密封这些液化气体介质的容器一旦因为意外事故发生破坏就会引发泄漏事故,其中瞬时泄漏由于危害最大且往往会伴随蒸气爆炸最危险。尽管目前学术界的相关研究已取得了可观的成果,但对事故后果演化机理仍不清晰,因此研究液化气体瞬时爆炸性蒸发产生的云团演化过程的机理和特征规律,可以为科学应对此类事故提供理论依据;成功预测云团的变化过程,对事故继发灾难的防范具有决定性的价值。
本文针对液化气体爆炸性瞬时泄漏产生的蒸气云演化的机理进行了研究。以过热理论为基础,把崩裂形成的液滴群作为切入点,将液体汽化潜热、饱和蒸气压等物性参数考虑到液滴表面蒸发计算中,完善了描述液滴蒸发的传热传质计算,按照时间发展建立了描述液化气体瞬时泄漏产生的蒸气云演化理论模型。采用计算流体力学(CFD)方法,依据液化气体瞬时泄漏产生的蒸气云演化理论模型,建立了瞬时泄漏产生的云团演化数值模拟模型。对Pettitt在1990年进行的小尺度玻璃容器瞬时破坏后的液体瞬时泄漏的小尺度试验进行了数值模拟,并与试验数据进行了对比分析,验证了本文所建立的理论模型和数值模拟模型的科学性及优越性。该模型可以预测云团演化过程中从微观到宏观的物理变化,并优越于之前学者所建立的模型。
以氟利昂为研究介质的瞬时泄漏产生蒸气云演化过程的数值模拟计算结果表明,氟利昂的瞬时泄漏并不激烈,云团膨胀速度较为平稳。进一步对云团宏观尺寸、云团内介质浓度和温度分布变化以及微观上液滴群的停留轨迹、液滴浓度分布以及蒸发速度等参数的研究结果显示,云团演化的最初大约0.4 s以内可以被定义为初始膨胀阶段,该阶段有云团快速膨胀同时温度下降、介质和温度分布区域不均匀化等特征。
本文还以液氯和液氨作为典型毒性危险液化气体介质、液化石油气作为典型的易燃易爆危险介质,在相同条件下进行了瞬时泄漏爆炸数值模拟计算,并从宏观云团和微观液滴两个方面进行了结果分析。结果表明这三种介质云团在演化过程中云团温度的变化趋势和介质浓度的分布变化规律与氟利昂云团类似,但由于这三种介质室温条件下过热度均远大于氟利昂,所以云团的直径增长在最初0.4 s内的初始膨胀都比氟利昂云团明显。而且通过对比发现影响温度下降幅度的重要参数是汽化潜热,在该阶段里,液滴大量气化造成云团温度下降是重要特征。
本文还根据国家相关部门对危险介质的伤害极限规定,对液氯和液氨这两种以毒性为主的危险介质发生瞬时泄漏后,预测了在不同标准浓度下的介质云团影响范围直径。并通过分析可以得到,与液氨相比液氯瞬时泄漏产生的云团毒性大,持续时间长,影响范围广,对环境的伤害更严重;对于易燃易爆的液化石油气,根据可燃极限范围,预测了液化石油气云团的爆燃后果云团中可燃区域的大致演化过程以及云团最大可燃直径的变化范围。
本文的研究成果不仅在理论上发展了液化气体瞬时产生蒸气云的演化机理,数值模拟计算结果也丰富了蒸气云的演化中关于介质输运、热量传递和相变过程的变化规律,为国外同领域的科研提供新的参考数据,并为我国特种设备爆炸事故后果的预测与预案的制订提供理论和应用依据。
It is usual in industry or common life that certain dangerous materials are stored as liquid form in pressurized containers. These materials are often toxic, flammable, or both. Herein, any accidental release can results in potentially disastrous consequences. Although not the most common one, instantaneous flashing release is the most severe mode, which often has explosive characteristics. Although some relevant studies for this issue have been done, some mechanisms of the accidents evolution are not clear and there still are lots of problems. The study for the evolution of two-phase cloud by release can provides scientific basis for the optimal design of the safety system, and it is critical to predict these release by reliable tool.
The study on the mechanism of the two-phase cloud evolution from an instantaneous release was presented in this paper. On the basis of super-heat theory, more properties including the phase change latent and saturated vapor pressure were considered in the calculation of the droplets evaporation, with which the model of the heat and mass transfer between droplets and gas-phase. The model for the prediction of cloud expansion was built by the Computational Fluid Dynamics (CFD) approach, and this model was used for the simulation of the small scale instantaneous release by Pettitt in 1990. To validate the optimal performance and scientific properties of model, the averaged velocity of droplets from the calculations of Freon cloud was used to directly compare with experimental data. The model can provides the variation of the microscopic and macroscopic physical change in cloud expansion, and the performance of this model is better than some relevant research before.
The simulated results of the Freon release indicate that the expansion of Freon cloud is not violent, and the some important calculated results including cloud size, the distribution of species concentration and temperature, droplets residence and evaporation rate indicated that the first about 0.4 s can be identified as the cloud formation stage, and some phenomena including rapid increasing of cloud size, sharp decreasing of temperature, the distribution of species concentration and temperature tend to inhomogeneous is the stage characteristics of this initial expansion process.
The dispersion process of different dangerous materials including liquefied chlorine, liquefied ammonia and liquefied petroleum gas were simulated by successful model in same condition to analysis the characteristics of the expansion process. By the comprehensive analysis of calculated results about droplets and cloud, the simulated results indicate that the law of the cloud evolution is similar with Freon cloud, although the superheat of these three materials is much bigger than Freon’s. Because of the violent phase change, the expansion and the decreasing of temperature is more significant than Freon cloud. It is found that the phase change latent is an important factor for the initial cooling.
The further analysis of the release consequence was made for three dangerous materials by the reference of damage criterion of each material. According to criterion published by governmental institute, the range of toxic influence of chlorine and ammonia cloud from instantaneous release was predicted. The simulated results show that the chlorine cloud is more dangerous than ammonia’s, and the evolution of flammable zone and size of LPG cloud was also presented in this paper.
The work presented in this paper develops the evolution mechanism of two-phase cloud from an instantaneous release of liquefied gases. This paper not only rich the mechanism of cloud evolution, but also provides a reliable tool and theoretical basis for the prediction of consequences of vapor explosion accidents and relevant safety system.
引文
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