14. HEGABOX

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General introduction

HEGABOX is the HGSYSTEM model that describes the initial phase in the spreading of an initially stagnant, dense cloud. For an instantaneous release like this, gravity driven spreading dominates during the first phase of the dispersion process.

HEGABOX is a fully time-dependent model.

HEGABOX either uses the full hydrogen fluoride (HF) chemistry and thermodynamics or the standard HGSYSTEM multi-compound, two-phase aerosol thermodynamics model. See the Technical Reference Manual for details on the two thermodynamical descriptions.

HEGABOX can only simulate the initial cloud behaviour where the influence of ambient turbulence is still small. For longer times, a transition to a far-field model must be made. As HEGABOX models transient, dense gas dispersion, a link to HEGADAS-T, the time-dependent heavy gas dispersion model, will be made. The transition to HEGADAS-T will be made when the Richardson number reaches a critical value (normally 10). Note that when linking to HEGADAS-T, both a link file (.HTL) and an observer data file (.HBO) are needed. HEGABOX will create these files automatically.

Range of applications and limitations

HEGABOX can only be used for the very specific release conditions it was developed for. The initial cloud must be (almost) stagnant and dense. Possible scenarios are: dense gas releases with low momentum at low wind speeds and sudden releases of large quantities of dense gas.

HEGABOX has been validated with the Thorney Island data. See the HEGABOX chapter in the Technical Reference Manual.

Problems with significant liquid pools on the ground cannot be dealt with as liquid dropout from the cloud to the ground is not being modelled. If the initial cloud composition as found by HEGABOX results in a cloud with a very high liquid fraction then increasing the user-specified initial entrainment (see input parameter discussion below) might be helpful. For evaporation of liquid pools the LPOOL model of HGSYSTEM should be used.
HEGABOX INPUT PARAMETERS

A description of all the input parameters that can occur in a HEGABOX input file will be given.

The HEGABOX input file has the DOS filename 'casename.HBI' where 'casename' is the user-supplied name of the problem.

In the following, actual keywords are given in capitals and in bold. The descriptions of less important parameters or parameters that need not normally be set by the user, are given in a smaller font.

All parameters, except TITLE, occur in blocks preceded by a specific block keyword. For HEGABOX these block keywords are: CONTROL, SPILL, GASDATA, AMBIENT, DISP and MMESOPT.

The TITLE keyword does not occur in a parameter block.

TITLE The title of the current problem to be run with HEGABOX.
  At most 50 alphanumeric characters.
  Optional, no default.


The optional CONTROL block contains several run control parameters.

TLAST Maximum time for which HEGABOX calculates cloud dispersion (s).
  0.01 <= TLAST <= 9000.
  Optional, default is 500 s.
  If during the calculation the time variable reaches the value TLAST, the run will be stopped. Usually TLAST is set sufficiently large so that the run ends because the Richardson number stop criterion has been met.
DTMAX Maximum time step in output table and numerical integration (s).
  0.01 <= DTMAX <= 100.
  Optional, default is 2 s.
  Only to be changed if the output table produced by HEGABOX in the report file ('casename.HBR'), is too long or too short, depending on the time scale of the problem.
PRTCODE Output printing control code.
  PRTCODE = 0, 1, or 2.
  Optional, default is 1.
  Normally not to be changed. PRTCODE = 0 suppresses printout and with PRTCODE = 2 debug printout is generated.
RIMIN Final (termination) Richardson number (-).
  0.01 <= RIMIN <= 50.
  Optional, default is 10.
  When during the HEGABOX run the Richardson number falls below RIMIN, the run is ended, creating a link file for HEGADAS-T. This is the standard way to end the HEGABOX run. The default value of 10 should normally not be changed by the user.

The SPILL block contains data to describe the spill of pollutant. Wet pollutant = dry pollutant plus water contained in the pollutant (set by WATERPOL).

SPILLTOT Spill size (excluding water pickup) (kg of wet pollutant).
  1 <= SPILLTOT <= 106.
  Mandatory.
RSTART Initial cloud radius (m).
  10-2 <= RSTART <= 103.
  Mandatory.
  Note that in HEGABOX the cloud is always assumed to be a cylinder.
  The user might want to use the HGSYSTEM module LPOOL to find an estimated value for RSTART.

The AMBIENT block specifies the conditions of the ambient atmosphere.

Z0 Reference height for windspeed U0 (m).
  0.1 <= Z0 <= 50.
  Mandatory.
U0 Ambient wind velocity at height Z0 (m/s).
  1.0 <= U0 <= 20.
  Mandatory.
ZAIRTEMP Reference height for temperature AIRTEMP (m).
  0 <= ZAIRTEMP <= 50.
  Mandatory.
AIRTEMP Ambient air temperature at height ZAIRTEMP (°C).
  -50 <= AIRTEMP <= 50.
  Mandatory.
RHPERC Relative humidity of ambient air (%).
  0.0 <= RHPERC <= 100.
  Optional, default is 0.
TGROUND Temperature of the earth's surface (°C).
  -50 <= TGROUND <= 50.
  Mandatory.

The DISP block contains the atmospheric dispersion parameters. Note that the ambient pressure is always taken to be 1.0 atm (760 mm Hg).

ZR Land surface roughness (m).
  10-5 <= ZR <= 1.
  Mandatory.
PQSTAB Pasquill/Gifford stability class (single character).
  PQSTAB = A, B, C, D, E or F.
  Mandatory.
MONIN Monin-Obukhov length (m).
  -500 <= MONIN <= 1020 .
  Optional, default calculated by HEGABOX based on ZR and PQSTAB.
  The user can override the correlation-based value that HEGABOX normally will use, e.g. if measurements are available.

In the GASDATA block we specify the physical properties of the pollutant. Most of the parameter values can be generated using DATAPROP and for a pollutant consisting of several compounds it is strongly recommended to use DATAPROP prior to HEGABOX to generate these values.

THERMOD Thermodynamical model used (-).
  THERMOD = 1, 2.
  Optional, default is 1 (non-reactive aerosol model).
  THERMOD = 1 implies the use of the standard HGSYSTEM non-reactive, multi-compound two-phase thermodynamic model. In the absence of an aerosol, ideal gas relations will hold.
  For THERMOD = 2 the full HF thermodynamical model will be used.
TGAS Initial wet pollutant temperature (°C).
  -273 <= TGAS <= 100.
  Mandatory.
  TGAS is the wet pollutant temperature before any dilution with ambient wet air (see INICONC below) and before water pickup (see WPICKUP below).
  Wet pollutant = dry pollutant plus water contained in the pollutant.
WATERPOL Mole fraction water (liquid plus vapour) in wet pollutant (-).
  0 <= WATERPOL <= 0.2.
  Optional, default is 0.0.
  WATERPOL refers to the released pollutant only, it should not take
  the water pickup into account (see WPICKUP below).
WPICKUP Mole fraction water picked up from earth's surface during release (-).
  0 <= WPICKUP <= 0.2.
  Optional, default is 0.0.
  WPICKUP is extra water added to the (wet pollutant) mixture during release, the original pollutant may already contain water (see WATERPOL above).
  The picked up water has temperature TGROUND.
INICONC Dilution by initial entrainment (mole wet pollutant plus water pickup per mole total mixture).
  10-5 <= INICONC <= 1.0.
  Optional, default is 1.0.
  The mixture after water pickup can be diluted by an initial entrainment with wet ambient air. The air composition and temperature are as specified in the AMBIENT block.
  Useful for very `violent' releases where at the start of the HEGABOX simulation significant entrainment already has occurred.
MMGAS Molecular mass of the dry pollutant (kg/kmole).
  2 <= MMGAS <= 200.
  Mandatory.
  Will be generated by DATAPROP.
CPGAS Specific heat at constant pressure of the dry pollutant (J/(mole K)).
  5 <= CPGAS <= 200.
  Mandatory.
  Will be generated by DATAPROP.
DIFFDT2 Thermal diffusivity of the dry pollutant at a representative
   
  temperature T, divided by T2 (m2/s/K2).
  3×10-11 <= DIFFDT2 <= 3×10-9
  Mandatory.
  Division by T2 is used to try to scale out the temperature dependency of the thermal diffusivity.
  Used in calculation of natural convection heat flux.
  Will be generated by DATAPROP.
VISCDT2 Kinematic viscosity of the dry pollutant at a representative temperature T, divided by T2 (m2/s/K2).
  2×10-11 <= VISCDT2 <= 3×10-9.
  Mandatory.
  Division by T2 is used to try to scale out the temperature dependency of the kinematic viscosity.
  Used in calculation of natural convection heat flux.
  Will be generated by DATAPROP
  The value of HEATGR (communicated to HEGADAS-T) is calculated as: HEATGR = (DIFFDT2×VISCDT2)2/3×CPGAS×1000/VISCDT2. The factor 1000 arises because we want the specific heat per kmole. HEATGR is not a HEGABOX keyword, but it is a HEGADAS keyword.
SPECIES Pollutant compound properties. Using this keyword at least once implies the use of the full two-phase (aerosol) model for a mixture consisting of at least one compound (excluding water).
  If the SPECIES keyword is not specified, ideal gas thermodynamics is used with gas properties given by CPGAS and MMGAS. Condensation or freezing of water is still taken into account.
  The SPECIES keyword plus parameters must be specified for every compound in the mixture, except water. The sum of the molar fractions (see parameter #2 below) must equal 1.0 - WATERPOL.
  The use of DATAPROP to generate the input parameters when the SPECIES keyword is being used, is strongly recommended.
  Currently a maximum of 8 species can be specified (excluding water). Please note that DATAPROP allows for more species to be specified. DATAPROP also splits dry air (if specified) up into nitrogen and oxygen, thus generating two compounds instead of one. Thus the HEGABOX link file generated by DATAPROP could contain the SPECIES keyword more than 8 times. The user should combine or remove compounds if this occurs.
  Please note that, in contrast with SPILL and AEROPLUME, HEGABOX and HEGADAS do have some restrictions: either a single two-compound aerosol or a number of separate one-compound aerosols are allowed (not regarding water and dry air), other combinations are not supported.
  Following the SPECIES keyword of a certain compound a block of 12 parameters (#1 to #12) must be specified:
#1 compound name (maximum of 12 characters).
#2 mole fraction in pollutant mixture (-).
0<= #2<= 1.
#3 aerosol class (-).
-1<= #3 <= 50.
#4 specific heat of vapour (J/(mole K)).
5<= #4 <=300.
#5 specific heat of liquid (J/(mole K)).
0<= #5 <=10 3 .
#6 heat of vaporisation (J/mole)
0<= #6 <=10 5
#7 critical temperature (K).
0<= #7 <=10 4
#8 critical pressure (atm).
0<= #8 <=10 3
#9 vapour pressure function coefficient B1.
-10 8 <= #9 <=10 8 .
#10 vapour pressure function coefficient B2.
-10 8 <= #10 <= 10 8 .
#11 vapour pressure function coefficient B3.
-10 8 <= #11<= 10 8 .
#12 vapour pressure function coefficient B4.
-10 8 <= #12 <= 10 8 .
Note: the saturated vapour pressure of the compound is described by the Wagner function:

P v (T) = P c x exp { [ B 1 x Q + B 2 x Q 1.5 + B 3 x Q 3 + B 4 x Q 6 ] / T r }
where T is the vapour temperature, P c the critical pressure, T c the critical temperature, T r = T/T c and Q = 1 - T r .

The MMESOPT block contains the 'switches' which indicate the use of the extra options developed by Earth Technology and sponsored by Martin Marietta Energy Systems. An indicator value of 1 means that the corresponding option is used, a value of 0 means that the option is inactive.

IMETP Indicator for use of meteorological pre-processor (-).
  IMETP = 0 or 1.
  Optional, default is 0.
  For IMETP = 1, an additional file 'casename.MET' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
IDEP Indicator for use of wet and dry deposition model (-).
  IDEP = 0 or 1.
  Optional, default is 0.
  For IDEP = 1, an additional file 'casename.DEP' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
ICANY Indicator for calculation of canyon effects (-).
  ICANY = 0 or 1.
  Optional, default is 0.
  For ICANY = 1, an additional file 'casename.CNY' must be supplied. See Chapter 18 for a description of the data which must be given in this file.
IFLUC Indicator for calculation of centre-line concentration fluctuations (-).
  IFLUC = 0 or 1.
  Optional, default is 0.
  For IFLUC = 1, an additional file 'casename.PTL' must be supplied. See Chapter 18 for a description of the data which must be given in this file.

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