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Journal of Loss Prevention in the Process Industries 25 (2012) 989e992

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Journal of Loss Prevention in the Process Industries

journal homepage: www.elsevier .com/locate/ j lp

Numerical study on the initial expansion of two-phase cloudfrom an instantaneous release

Wei Tan a,*, Xiao Liu a, Liyan Liu a, Yujin Liu a, Zejun Wang b

a School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, Chinab Tianjin Special Equipment Inspection Institute, Tianjin 300192, China

a r t i c l e i n f o

Article history:Received 10 August 2011Received in revised form24 March 2012Accepted 23 May 2012

Keywords:Two-phase cloudDangerous liquefied gasInstantaneous releaseInitial expansionCFD

* Corresponding author. Tel.: þ86 22 27408728.E-mail address: tjuwtan@yahoo.com.cn (W. Tan).

0950-4230/$ e see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.jlp.2012.05.014

a b s t r a c t

In industries some dangerous liquefied gases may accidentally release and it may form a flammable ortoxic mixture after mixing with air. One tool that is being developed in industry for two-phase clouddispersion modeling is computational fluid dynamics (CFD). In this paper, the dispersion processes ofdifferent dangerous materials including liquefied chlorine, liquefied ammonia and liquefied petroleumgas were simulated in the same condition to analyze the characteristics of the initial expansion processesby CFD tool. The heat and mass transfer between droplets and the vapor after an instantaneous releaseevent was calculated by using the EulerianeLagrangian method. The results from a number of 3-D CFDbased studies were compared with the available small-scale experimental results. The results show thatthe present model and numerical simulation are reliable.

� 2012 Published by Elsevier Ltd.

1. Introduction

Various dangerous gaseous materials are stored or transportedas liquid in pressurized containers. Once the containers acciden-tally fail, the released dangerous materials can result in disastrousconsequences (CCPS, 1999; Lonsdale, 1975). Typically the instan-taneous release is believed to be the most severe release mode. Asudden depressurization makes a liquid get into a superheatedstate, and the resulting violent boiling can result in physicalexplosion which can break the container into pieces (AbbasiTasneem, & Abbasi, 2007; Birk & Cunningham, 1999; Khan &Abbasi, 1998). With the total loss of a container, the instanta-neous release can result in a rapidly expanding two-phase cloud,which consists of liquid droplets generated from flashing boilingand vapor evaporated from the liquid. The two-phase cloud is themedium that is potentially disastrous (Leslie & Birk, 1991). Topredict the evolution of this cloud, some simulation and laboratorywork have been done (Aamir & Watkins, 2000; Calay & Holdo,2008; Gilham, Mitchell, Woodburn, & Deaves, 1999; Makhviladze& Yakush, 2005). However, due to the limitation of available data,the computational capabilities and modeling conditions, resultsobtained from previous simulations have been unable to reflectexperimental data (Gilham et al., 1999).

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The cloud from an instantaneous release will undergo an initialexpansion stage (the cloud formation), which is presumably themost important, but unfortunately, least understood phase (Abbasi& Abbasi, 2006; Gilham et al., 1999). In this stage, some liquid boilsas a result of sudden reduction of pressure, and rapidly expandingbubbles break into droplets, mixing with the expanding vaporevaporated from the liquid and entrained air, forming the cloud.The aim of this paper is to deepen the understanding of the initialexpansion of a sudden release event and provide a reliable tool forprediction of the release. Our results may enrich the theoreticalbasis for the optimal design of safety systems for handlingdangerous liquefied gases.

2. Model set-up

2.1. Theoretical background

At themoment total loss of container, a superheated liquid comesin contact with ambient conditions. The liquid can get its heatequilibrium by releasing its super-heat through violent boiling andevaporation of droplets, characteristics of an explosion (Abbasi &Abbasi, 2007; Reid, 1976, 1980). With the violent boiling, the pres-surized liquid breaks up into small droplets. The combination offlashing boiling of liquid and hydrodynamic instability results in theformation of droplets. It is still difficult to describe or explain the realphysical process of the cloud formation stage. Therefore, someassumption is necessary to develop an optimal numerical model.

W. Tan et al. / Journal of Loss Prevention in the Process Industries 25 (2012) 989e992990

The simulation described in this paper was performed with theassumption that the container was suddenly totally failed and theformation of cloud started at themoment when the liquid had beenshattered into droplets at the location of the lost container (Fig. 1).

Fig. 2. Schematic of computation domain and boundary conditions.

2.2. Computational model

The instantaneous release experiment used for the simulation ismade by Pettitt in 1990 (Pettitt, 1990), which represents the suddenrelease of liquefied Freon-11 at 280 kPa to the atmospheric condi-tion. A spherical flasks positioned in the center of a laboratory wassmashed mechanically, so the formation process of cloud could beassumed as an instantaneous and approximately symmetrical two-phase flow. Some detailed measurements for certain aspectsincluding the variation of droplets size and velocity with cloudexpansion process were performed. The microcosmic informationwas obtained by laser-based detectors at 0.22, 0.32, 0.40m from theflask.

Commercially available computational fluid dynamics (CFD)software FLUENT 6.3.26 was used for simulation. TheEulerianeLagrangian approach combinedwith the discrete randomwalkmodel and standard evaporationmodel were used to calculatethe transfers of the mass, momentum and heat among the droplets,vapor and gas in the two-phase cloud from the instantaneousrelease (Fluent, 2006). Physical forces including gravity force,thermophoretic force and Saffman lift force were considered in thesimulation. The air and vapor were treated as the continuous phaseand the droplets were treated as the discrete phase.

As shown in Fig. 2, the “lost flask container”was centered in theflow domain, from which the droplets began to jet into the airenvironment perpendicular to the flask surface, and the compu-tational domain was defined as a square with side lengths of 6 m.

2.3. Boundary condition

The boundary condition and initial computational inlet aboutdroplets were also defined according to the comparable experi-ment by Pettitt. A mass flow inlet boundary was set at the locationof the flask and a pressure outlet boundary for the outer domainwas set as shown in Fig. 2. The velocity of the continuous phasewasestimated as zero in mass flow inlet because of the no-air-flowenvironment. The initial kinetic energy and dissipation rate was5.83m2 s�2 and 18.64m2 s�3 respectively at the inlet boundary. Thecontinuous phase was treated as an ideal gas, and the liquid phaseas an incompressible liquid. Turbulence was modeled with the

Fig. 1. Schematic show of the assumption for the initial computational condition.

standard ke3 model. The initial velocity and size distribution ofdroplets resulting from shattered liquid at the location of the inletboundary were defined according to results of the Pettittexperiment.

3. Results and discussions

3.1. Model validation

The variation of droplets velocity and size were measured bylaser-based detectors at 0.22, 0.32 and 0.40 m from the source ofthe release. To validate the model, an instantaneous release ofFreon-11 was performed and the microscopic information in cloudwas recorded by three dummy planes in simulations. The averagedvelocity of droplets from calculations was directly compared withexperimental data. The decreasing trend of the calculated curveswas similar to the experimental measurements (Fig. 3). However,the simulation results were over predicted (Fig. 3). This over-prediction may be caused by either the inaccuracy of the experi-mental measurements or the definition of the initial condition inthe simulation, or both. Overall, the simulated results fit theexperimental measurements better than previous reports (Deaves,Gilham, Mitchell, Woodburn, & Shepherd, 2001; Gilham et al.,

Fig. 3. Direct comparison of the averaged velocities of droplets.

Fig. 4. The evolution of 3-D shape of LPG cloud.

W. Tan et al. / Journal of Loss Prevention in the Process Industries 25 (2012) 989e992 991

1999). Therefore, the assumptions adopted here appear to befeasible, and the simulation presented in this paper is more reliable.

3.2. Expansion process

The theoretical expression for initial expansion velocity of cloudhas been studied by a number of researchers. However, theexperimentally measured velocities were much smaller than thetheoretical values because of the assumption for isotropic expan-sion. It can be deduced from Fig. 3 that the calculated expansionvelocity of the cloud is more consistent with the observed resultsalthough the reliability of cloud size by video is doubtful.

The shape evolution of the cloud of liquefied petroleum gas(LPG) and the size evolution of clouds of the different materialswithin the first second is shown in Figs. 4 and 5, respectively. Thecloud underwent a symmetric initial expansion and then graduallyshowed features of a heavy gas (Fig. 4). The clouds of all the threematerials expanded rapidly in the initial 0.3e0.4 s. The expansiongradually slowed down afterward and almost completely stoppedat about 0.7 s (Fig. 5). These simulated results agree generally wellwith experimental observations.

3.3. Variation of cloud temperature

Driven by internal energy, the two-phase cloud underwenta rapid initial expansion with release of its phase-change latentheat. As shown in Fig. 6, for all the three materials, this violent

Fig. 5. Size evolution of clouds in which the concentration of vapor is more than 1%.

evaporation caused sharp decrease of the cloud temperatures. Thecloud temperatures then increased with a much slower speed thanthat of the initial decrease. It also can be seen from Fig. 6 that thetemperature of the LPG cloud was the lowest and the slope of itscurve was the largest, which could be due to the fact that its latentheat is much lower than chlorine and ammonia (Table 1). There-fore, it can be deduced that the lower the latent heat is the lowerthe temperature and the larger the size of the cloud will be. Also,the cloud formed from instantaneous release of a material witha lower phase-change latent heat will expand more violently andhas more severe consequences.

3.4. Evaporation of droplets

The cloud expansion is mainly caused by the evaporation ofliquid and the air entrainment. What can be seen from Fig. 7 is thatthe volume-averaged evaporation rates generally increased whenmore droplets jet into air environment. The evaporationwas greatlyinfluenced by the temperature of the continuous phase, becausethe phase-change led to both the expansion of the cloud and the

Fig. 6. Variation of the lowest cloud temperature in cloud.

Table 1Phase-change latent heat.

Material Latent heat (J/kg)

NH3 801,320Cl2 279,790C3H6 18,670C4H8 22,400

Fig. 7. Variation of volume-averaged evaporation rate of droplets.

Fig. 8. Variation of volume-averaged evaporation rate of droplets in first 0.6 s.

W. Tan et al. / Journal of Loss Prevention in the Process Industries 25 (2012) 989e992992

decrease of the temperature of the gas around evaporated droplets.Therefore, the evaporation rate of NH3 was the highest and that ofLPGwas the lowest among the threematerials. These are consistentwith the calculated variation of temperature given in Section 3.3.

If the initial temperature decrease of a cloud, such as thatformed from LPG, is extremely sharp, the cooling of the gas phaseby the initial violent evaporation may hinder the subsequentphase-change process. Only the LPG curve showed a decreasewithin the initial 0.3 s (Fig. 8). After the initial cooling stage, the

evaporation speed of LPG did not rise until the time (w0.3 s) whenthe cloud started to warm up. All these results are supported by thevariation of the LPG cloud temperature in this time period (Fig. 6).

4. Conclusions

An instantaneous release experimental case was used fornumerical simulation to study the characteristics of the initialexpansion process of different dangerous materials. First of all, thispaper provides a successful application of a CFD approach for theprediction of these releases. Also, the reliability of the simulatedresults was validated by direct comparison of the mean velocityvariation of droplets. The good agreement between the simulationresults and experimental data indicates that the assumption isacceptable and feasible. Finally, the initial expansion of a cloudwithin the first 0.3e0.4 s could be identified by the comprehensiveanalysis of calculated results including volume-averaged evapora-tion rate, size and temperature of the cloud.

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