AFGL-TR-S -23 5

A PROPELLENT AND CHEMICAL

SPILL AND DISPERSION MODEL

Phani K. Raj and John A. Morris

TECHNOLOGY & MANAGEMENT SYSTEMS, INC.

nBurlington, MA 01803-5128

o and DTIC0 Bruce A. Kunkel CT4188

Air Force Geophysics Laboratory S cHanscom AFB, MA 01701< H

1.0 ABSTRACT

Many of the chemicals, propellants and oxidizers handled andused by the defense agencies and NASA produce heavier-than-airvapors if released into the atmosphere. Some of the chemicals,such as nitrogen tetroxide, may react with ambient moistureresulting in the formation of nitric acid vapors and liquidaerosols. Other chemicals, such as liquefied gases, whenreleased flash vaporize and form vapor clouds containing airborneliquid aerosols. All of these vapor clouds have mean densitieshigher than that of ambient air and consequently disperse in theatmosphere in a manner completely different from the dispersionand dilution of a neutral density cloud.

A dense gas dispersion model has been developed (called "AirForce Pisper%.ion Assessment Model" - ADAM) which takes intoconsideration a variety of release types (instantaneous;continuous; pi'ff and jet releases; liquid, gas or two-phase;diked tanks, uidiked tanks; fixed facilities, transports, etc.).The model incluies the effects of liquid aerosols on the cloud orplume behavior, reaction kinetics of the chemical-water vapormixing and the entrainment of air. Atmospheric stability isrepresented on a continuous scale from the extremely unstable toextremely stable. The dispersion model is a hybrid box-volumesource Gaussian mcdel.

The ADAM dispersion model is applicable to most chemicals ofinterest. The mootl, at present, includes the thermodynamicmodules for nitrogen tetroxide, ammonia, chlorine, phosgene,hydrogen sulphide an sulphur dioxide. The dispersion of anyother reactive chemical can easily be evaluated by adding onlythe therzmodynamic modu..e relevant to the chemical.

The results from the mcdel have been compared to test data fromfield experiments conducted over the past 40 years involving therelease of several chemicils of interest to the defense services.

D TMUTION STATEMAENT A

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~AM ~PL~-OiMNU tu;,'.iATON 6o OrEiCE SYMBOL 7a NAMtE OF MONITORING ORGANIZATION

Air Force Geophysics Laboratoryj LYA

Ut, AL)DdESS Grj State. ifd .';?C'x;V) 7b ADDi\ESS (City, State, arid ZIP Code)

Hariscom AFB

M )assachusetts 01731-5000

L1.O,.,j~UI. 80 Or-rICE SYN'MdOL rI.C t.LTit',, TRuMENT i0L,%TiFCATiON NUMBERA T 10.1% (if jpphicjbie)

I S AJ 2U (6ty. State, Iiu( Zt'P:UU0 )j SOur'CE Oi FuNDiPNG NUIVJERS

PR1OCI-1AM PROjECT TASK WORK UNITELEM,,ENT NO -NO NO ACCESSION NO

e 61 1 310., G7 BB

A Prpellent . Chemical Spill Dispersion Mode

'.AL A;('hani K. Raj*; John A. M1orris*; Bruce A. unkel

nrK' , %1 Ct- COV E RE D 14 DA'TE OF REPORT Year, Month, Day) 15 PAGE COUNTK EIRINT rlSGM T_ TO 1988 September 221

u ' rL~~\1~,Y &T.T~ *Technology & M nagement Systems Inc, Burlington, MA 01803Ruprintezd from JANNAF SiAfety & Envir nrnental Pr otiction Subcommittee Meeting, Naval Post-ArIJdae ScIoo %1-2 (.V 1988, Mo &ev CA

1/'1 AIlI CLUL, 18WJECT TERMS (Continue on reverse if necessary and idenhify by block number)

"dCisOUP Heavy gas dispersion) Chemical hazard.Hazard Assessment) rAtmospheric dispersion

P) Av, '- I IkcolitmIuv on)I ive it iwess~ity .*nd itentity by block number)

The defense services transport, store and use many kinds ofa chemicals including fuels, oxidizers, propellants and weapons

related chemicals. Many of these chemicals are volatile and mayfoirn dense vapor clouds if they are released into the atmosphere.Depending on the physical properties of the chemical, storageconditions, release conditions and weather conditions differenttypes of vapor clouds may be formed (heavy clouds, aerosol

a bearing clouds, instantaneous puffs, continuous plumes, etc.). Inaddition, some of the chemicals may react with ambient moisture.It has been shown in the literature that the behavior of heavyvapor clouds is considerably different from that of neutraldensity vapor clouds.

20~,,rM'raA.',~LT Or ABUSTRACT 1 AbiSTRACT SECURITY CLASSIFICATION

D~N.A ui .,,i ~TIF b EAM ,ASvI rrrT DJ Ic iCSE RS UnclassifiedI 22a NA110 Ui HLft'O .1',BLt INDIVIDUAL :'2U If LrinONF (include Ar'a Coide) 122c OFFICE SYMBOLhKUCF A . KUNKEL AFGL/LYA

DD FORM 147 3, t,,IA 83 APR- e j~tori maf Lie ustd untlt eAt-awtd SFCUJRITY CLASSIFICATION OF THIS PAGEAll cther edltons are oosoete Unclassified

a88 10 4'01 2

We have developed a model that takes into consideration thedetermination of vapor source strengths from over 18 differentrelease scenarios. The model includes puff releases, jet releasesand plume releases, with or without aerosols. The dispersion ofthe heavy vapor cloud or plume resulting from the source is

Ar rodeled. The thernodynanics and kinetics of the reaction of thecherical .ith the a.bient roiture has been modeled for severalccron che.icals (W204, Ammonia, Phosgene, Chlorine, etc.) ofinterest to tre defense services. These thermodynamic models havebeen incorporated into the dispersion models. Heavy gasdispersion of the chemical vapors, evaporation of the aerosols,

and other related phenomena have been modeled. The model alsosmoothly transits to a passive, Gaussian dispersion phase whenthe density difference between the dispersing cloud and theatmosoheric air is small.

9J The results from the model developed have been compared to testdata from field experiments conducted over the past 40 yearsinvolving the release of several chemicals of interest to thedefense services. These chemicals include, phosgene, chlorine,anhydrous ammonia, nitrogen tetroxide and freon. These test havebeen conducted over different weather conditions, terrains(including over the sea) and release conditions (instantaneousreleases, explosive releases, and continuous releases ofpressurized liquids). The quantities released range from smallvolumes to very large vapor volumes (2000 m3 ). We have comparedour model results with the data from these field tests. Theresults compared include: (i) the variation of peak concentrationat ground level as a functio n of distance, (ii) the spread areaof the vapor cloud at different down wind locations, (iii) theacceleration and down wind velocity of a puff of vapor, (iv) thejet length, depth and horizontal cross wind extent in the case ofcontinuous releases and (v) the far field concentrationcorridors. The agreement between the field test data and themodel results is extremely good.

The entire source and the dispersion models, including theeffects of reaction, rans on an IBM-AT type personal computerwith execution times on the order of 2 minutes. The toxiccorridors and other results can be displayed graphically to scaleon the monitor or superimposed on a digitized map of the area of

a interest.

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These chemicals include phosgene, chlorine, anhydrous ammonia,* nitrogen tetroxide and freon. These tests have been conductedover different weather conditions, terrains (including over thesea) and release conditions (instantaneous releases, explosivereleases, and continuous releases of pressurized liquids). Thequantities released range from small volumes to very large vaporvolumes (2000 m3). The following data from the field tests havebeen compared with model predictions: (i) the variation of peakconcentration at ground level as a function of distance, (ii) thespread area of the vapor cloud at different downwind locations,(iii) the acceleration and downwind velocity of a puff of vapor,(iv) the jet length, depth and horizontal cross wind extent inthe case of continuous releases, and (v) the far fieldconcentration corridors. The agreement between the field testdata and the model results is extremely good.

The ADAM model runs on an IBM-AT type personal computer withexecution times on the order of 2 minutes. The toxic corridorsand other results can be displayed graphically to scale on themonitor or superimposed on a digitized map of the area ofinterest.

2.0 DESCRIPTION OF HEAVY VAPOR CLOUD DILUTION

When a vapor cloud* which is heavier than air due to itsvapor density or the combined density of vapor and any aerosolsis released** into the atmosphere, it undergoes dilution indifferent stages. Depending on the nature of release (explosive,passive, jet, etc.), there may be an initial rapid entrainment ofair into the cloud. The cloud next goes through a gravitationalslumping stage due to its excess density. During this secondstage, the lateral dimensions of the cloud increase due togravity driven flows and the cloud is accelerated downwind due tomomentum transfer from the wind. The entrainment of air isprimarily controlled by the density stratification in the cloudand by the lateral spread rate. When the cloud density is withia few percent of the density of air, a third stage of dispersi TC

Soccurs in which the dilution of the cloud depends on both t catmospheric turbulence characteristics as well as the clouC Pc '

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density. In the fo-rth and final stage, the cloud dilution isdue principally to atmospheric turbulence.

Air is entrained into the cloud primarily at the edges and* at the top. Therefore, the concentration of the chemical

decreases first at the edges and the top. The chemicalC

The term "vapor cloud" used in this paper is assumed to meanboth a puff of vapor and a plume.

** We have modeled a variety of chemical release modes andsources (pressurized liquid release, instantaneous &continuous, cryogenic liquid release, gas release, etc.). -The details of these sources and models to determine source Codalstrength have been published in a recent report (Raj and I/orMorris, 1987).

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concentration at the core remains high until sufficient air hasdiffused into the central regions. If there is chemicalreaction, the concentrations of the products of reaction are highat the edges and the top. In effect, in a real cloud theconcentrations within the cloud are non-uniform and may showdistinct bi-modal distributions in the lateral direction forreaction products. However, as the dilution continues, it isanticipated that the reaction product distributions will show amore uniform or modified Gaussian type distributions.

Our approach in modeling the above phenomena is based on theassumption that the chemical reaction, if any, and the dispersionprocess can be decoupled. The interaction between the twoprocesses occurs through the overall density of the cloud. Thatis, the air entrainment rate is determined by the overall densityof the cloud and the meteorological conditions. The overalldensity of the cloud and the mean concentration of the species inthe cloud at any instant of time are determined solely by: (i)the mass, phase, and thermodynamic conditions of the chemical atrelease, (ii) the mass of air mixed, its temperature and

S humidity, and (iii) the total net heat input into the cloud fromexternal sources. Our second assumption is that reaction betweenair and the chemical ceases when the concentration of the primarychemical is very low. In fact, we assume that no reaction occursafter the transition from the heavy gas dominated dispersion tothe atmospheric dominated dispersion. The third assumption inour model is that the initial stages of dispersion can bedescribed by a modified "box" model and the atmosphericdispersion stage is described using a modified Gaussian, volumesource based model. The details of these are described in theappropriate sections below.

3.0 CHEMICAL EFFECTS

Many different types of behavior are possible if a chemicalvapor is released into the atmosphere and it contains liquidaerosols (which may react with water vapor or air). The firsttype is the one in which the cloud formed is heavier than air andcontains liquid aerosols. The chemical may not react with theair or water vapor and liquid aerosols may not dissolve in liquidwater (ex., chlorine). In this case, the entrainment of air intothe cloud lowers the vapor concentration and the chemical vaporpressure. The aerosols evaporate at the expense of the sensibleheat of the cloud and, in the absence of any external heating,

a depress the cloud temperature. The density of the cloudincreases due to the lowering of temperature.

A second type of chemicals normally behave as above, buthave a completely different behavior when liquid water drops arepresent in the cloud (due to rain, fog, snow or water droplets

* from a fireman's hose). A classic example of this is phosgene.If no liquid water is present in the cloud, phosgene vapor andaerosols are not reactive with water vapor in the atmosphere. Ifliquid water drops are present due to condensation or other

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reasons, however, phosgene liquid aerosols readily dissolve inwater forming hydrochloric acid drops and carbon dioxide vapor.These types of behaviors obviously have profound influences ondispersion predictions.

The third type of chemical behavior is exemplified by therelease of a high vapor pressure liquid into the atmosphere (ex.,Manhydrous ammonia). In this case, the gold vapor-aerosol cloudformed after release condenses moisture from the atmosphere. Theanhydrous aerosols dissolve in the condensed water formingaqueous liquid aerosols. These liquid droplets can persist for along time if the partial vapor pressure of the chemical over adilute aqueous mixture is very low.

The fourth type of behavior involves spontaneousdissociation and/or reaction of a chemical with the moisture inthe atmosphere, producing new chemical species. These newspecies may condense, depending on their partial pressure, andform liquid chemical aerosols. These formations have profound

* Ieffects on the vapor cloud temperature, density and on the cloudsize. Hazards may be posed by the secondary product chemicals aswell as by the primary chemical. The aerosols formed may be verystable and persist for a considerable distance. In addition, theaerosols may extend the region over which the cloud behaves as aheavy gas. Example chemicals for this type of behavior arenitrogen tetroxide (N204 ) and hydrogen fluoride (HF). N204 formsnitrogen dioxide by dissociation and forms nitric acid aerosolsdue to reaction with the moisture in the atmosphere. The nitricacid further dissolves in the water and forms aqueous nitric acidaerosols which may persist until considerable dilution of thecloud has occurred.

The overall model calculation procedure is illustrated inthe flow diagram shown in Figure 1. The user inputs a variety ofdata through easy-to-use menus. (Selected data from a defaultfile can be overridden by the user.) There are four subsystemmodules containing (i) source models, (ii) models to represent

* atmospheric parameters, (iii) thermodynamic models, and (iv)dispersion models. In addition, a chemical property database(library) is also a part of the system. The output isgraphically displayed as concentration or dose isopleths. Inaddition, the output results are stored in numeric files.

4.Q PTSPERSION MODEL

The principal feature of our dispersion model is that itsimulates the density dominated dispersion phase as well as thedispersion in the passive (near neutral density) regime without

* having to resort to equivalent point sources, invoking transitioncriteria, etc. The dispersion of both heavy plumes as well aspuffs are modeled. The initial conditions of the vapor puff (orplume) are calculated using the source models and thethermodynamic models. The starting condition for a puff is a

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cylindrical box and for a plume is a given flow rate through arectangular "window" of specified dimensions. In both cases, thethermodynamic condition of the vapor air mixture is fullyspecified.

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ImFigure 1: Dispersion Model Subnodules and Calculation

Flow Chart

The details of the dispersion model are illustratedschematically in Figure 2 and Figure 3 for the case of a puff

[] dispersion (a similar approach is used for plumes also). Thecylindrical cloud at position A (at X = 0) is subject to a windspeed U10. Air is entrained at the top and on the sides. Alsothe cylindrical cloud expands radially due to gravitationalcollapse. The cloud is also accelerated by the prevailing wind.In our model, it is assumed that the vapor concentration

* distribution is radially symmetric.

Referring to Figure 3, the initial concentration and otherintensive properties (temperature, aerosol fractions, etc.) areuniform ("Top Hat") within the initial cylindrical box (at X =0). As the cloud moves downwind and entrains air, the top hat

* concentration profile is smoothed at the edges by side (and top)entrainment with the central core still having a uniform, butlower, concentration. If all of the core is diluted, theconcentration distribution in the puff is essentially Gaussian.

The above physical process is simulated in our model by the* following process. We recognize that the dilution and puff

expansion is due to two simultaneous processes. The first one isdue to gravitational expansion and air entrainment associatedwith it. The second is due to the effect of ambient turbulence.We treat these effects as being linearly superposed. First, we

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model the gravitationally driven radial expansion, downwindmotion and air entrainment using the classic "box" approach (fordetailed mathematical representations, we refer the reader toseveral literature citations such as, Havens, 1982; Jagger, 1983;Raj, 1986; Raj, 1985; Weber, 1983). The air entrained into thebox is assumed to react with the chemical and result in adownwind cylindrical box at location X with uniform concentrationCX and other properties which are also top hat distributed. Thisis shown in Figure 3.

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Figure 2: Different Types of Releases Modeled

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Raj, P.K., "Summary of Heavy Gas Spills Modeling Research",AProceedings of the Heavy Gas (LNG/LPG) Workshop. Toronto,

Ontario, p. 51, January 1985.

Raj, P.K. (1986), "Dispersion of Hazardous Reactive ChemicalVapors in the Atmosphere", Proc of 1986 Haz Mat Spills Conf, St.Louis, MO, pp. 305.

Raj, P.K., and J.A. Morris (1987), "Source Characterization andHeavy Gas Dispersion Models for Reactive Chemicals", Report #AFGL-TR-88-0003(I) submitted to the USAF by Technology &Management Systems, Inc., Burlington, MA 01803, December 1987.

Raj, P.K., J.A. Morris and R.C. Reid (1987), "A Hybrid Box-Gaussian Dispersion Model Including Considerations of ChemicalReactions and Liquid Aerosol Effects", paper presented at theInternational Conference on Vapor Cloud Modeling, Cambridge, MA,November 1987.

Slade, D.H., ed., MeteoroloQy and Atomic Energy 1968, U.S. AtomicEnergy Commission, 1968.

Webber, D.M., "The Physics of Heavy Gas Cloud Dispersal", UKAEAReport, SRD-R243, Sheffield, England, 1983.

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Figure 10: Comparison of Model Pre-

dictions for Downwind CenterlineConcentration with Test Data fromThorney Island 13 Freon Release

In all of the model runs, only the conditions of thereleases and the chemicals were changed. The model internalparameters (constants in entrainment, friction and otherequations) were kept the same in all comparisons. It is seenthat in all cases the model predicts the concentration dataexceedingly correctly. Also, the predictions are uniformlyaccurate both in the heavy gas regime () and in the passivedispersion regime (Figure 7) and in the transition regions. Themodel predicts reasonably accurately the observed cloud height,the area of spread as a function of time and the times of arrivalof puffs at given distances.

The entire model described in this paper runs on an IBM-ATtype computer and calculation times range from 30 seconds to 1.5minutes. The results are plotted on a graphics screen.

This model was developed under a U.S. Air Force project andis intended to be used for determining toxic corridors arisingfrom the release of chemicals which form heavy gas clouds orplumes. This model will be an adjunct to the AFTOX modelcurrently being used by the U.S. Air Force. The model is usefulfor determining the toxic corridors arising from the release ofchemicals from both: (i) transportation, launch pad area andstorage tank accidents, and 2) chemical weapons.

REFERENCES

Havens, J.A. (Editor), (1982), "Dispersion of Dense Vapors-MIT-GRI LNG Safety Research Workshop, Vol II", Published by the GasResearch Institute, Chicago, IL.

Kunkel, B.A., Development of an Atmospheric Diffusion Model forToxic Chemical Releases, AFGL-TR-85-0338, Hanscom AFB, 1985,ADA169135.

Jagger, S.F., "Development of CRUNCH: A Dispersion Model forContinuous Releases of a Denser-Than-Air Vapour Into theAtmosphere, UKAEA, SRD R229, 1983.

topographic conditions. The comparisons have included downwindground level concentration variation as a function of distance,height and width of visible clouds and plumes, cloudtemperatures, and cloud arrival times. Details of thecomparisons are given in our report (Raj & Morris, 1987). Only asample of these comparisons are presented here.

Figure 6 shows predicted and measured concentrations for theexplosive release of phosgene. The cloud contained a significantquantity of liquid aerosols. Figure 7 shows similar results fora continuous release of chlorine on the ocean. The results for acontinuous release of anhydrous ammonia are shown in Figure 8.Figure 9 shows the results for nitrogen tetroxide and showsresults from one of the tests of the Thorney Island Seriesinvolving the instantaneous release of freon.

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Figure 6 Comparison of Model Pre- Figure 7 Comparison of Model Pre-

dictions for Downwind Centerline dictions for Downwind CenterlineConcentration with Test Data from Concentration with Test Data from

[]U.S. Army Phosgene Test Lyme Bay V Chlorine Release

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dictions for Downwind Centerline Concentration with Test Data fromConcentration with Test Data from Eagle 3 Nitrogen Tetroxide ReloaseDeser m Tortoise 4 Ammonia Release

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The modeling can be very simple if the chemical is notreactive. The analysis can become very complex if the chemicalreacts with water or air leading to the formation of new specieswhich may be distributed in vapor and condensed phase (as in thecase of nitrogen tetroxide reaction with water vapor). Detailsof the chemical reaction models for 6 chemicals (including N204 )are indicated in our report (Raj & Morris, 1987). An example ofthe calculation of the final state of the mixture for the mixingof N204 vapor and humid air was described in detail in ourearlier paper (Raj, et al., 1987).

Typical results from this model for N204-Humid Air mixingare indicated in Figure 4 and Figure 5. The variation of N204-Vapor Air mixture temperature as a function of dilution is shownin Figure 4. The initial decrease in temperature is due to thedissociation of N204 to No?. Figure 5 shows the concentration ofwater vapor in an equilibrium mixture of N204 vapor and humid airof different relative humidities at different dilutions. Alsoplotted on the same figure is the saturated water vaporconcentration in air of different humidities at mixturetemperature (dotted line). It is clear that in the region wheresaturated water vapor concentration is lower than that calculatedby gas mixture model (solid lines) condensation of water in theform of fog will occur. This will lead to the dissolution ofnitric acid vapors forming aqueous nitric acid fog (or aerosols).The results clearly show how even when the original chemical doesnot have any aerosols, spontaneous condensation and formation ofa new reaction product species can occur.

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7.0 COMPARISON OF DISPERSION MODEL RESULTS WITH FIELD TEST DATA

We have compared the results from the model with most of theavailable field test data for a variety of chemicals (somereactive), different release types and atmospheric and

To obtain the real concentration distribution at X, we nowassume that the box at X with its CX concentration and dimensionRX and HX is dispersing from X - 0 to X = X under the influenceof atmospheric turbulence only. To describe the latter dilution,we use the volume source Gaussian model (Raj & Morris, 1987) on asource of dimensions RX, HX, and uniform concentration CX locatedat X - 0. This model then gives the distribution ofconcentration in a puff whose center is located at X and whichhas the characteristics of "wings" on the sides and uniform corein the center.

The above procedure can be continued until the center lineground level concentration is equal to the specifiedconcentration. Also at every X, the width of a contour ofspecified concentration can be determined. To conservecomputational time, we use only the volume source Gaussian modelto describe the dispersion in the far field; i.e., when the localRichardson number is less than 1. The standard deviation valuesused in the atmospheric turbulence induced dilutions are the

* Pasquill-Gifford values (Slade, 1968) modified to take intoaccount the effects of aerodynamic roughness and theconcentration averaging time.

Model eauations have been presented in an earlier paper(Raj, et al., 1987). More complete details including assumptionsand equation simplifications are described in our recent report(Raj and Morris). Therefore, model description in terms ofequations is not presented here.

5.0 ATMOSPHERIC STABILITY

We calculate the atmospheric stability using the methodologysuggested by Kunkel (1985). This method is based on Golder'smethod (1972), but classifies the atmospheric stability on acontinuous scale from extremely stable to extremely unstable.The standard deviation values are corrected for roughness andaveraging times. The method utilizes either the measured windangle standard deviations (and time of day) or the solar heatinput, time of day, wind speed data, surface roughness, etc. tocalculate the atmospheric stability.

6.0.Q CHEMICAL REACTION CONSIDERATIONS

The mixing of the chemical bnd ambient air is modeled usingequilibrium thermodynamic approach. A unit mass of the chemicalat an initial specified state is mixed isobarically with "imit

units of humid air from the atmosphere. There is a heat exchangeof Q units of energy within the universe. The final mixturethermodynamic conditions are determined by applying the equation

* of mass and energy conservation together with phase equilibriarelationship and reaction kinetics. The final mixture state isexpressed by its temperature, density, number of species in vaporphase, the number in liquid phase, vapor and liquid phase specieconcentrations.

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