Juha Nikmo

The homogeneous equilibrium approximation in models of aerosol cloud dispersion

Abstract The dispersion of heavier-than-air aerosol clouds is of great relevance ti the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion but, in most cases, two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact almost all of those models which do treat two-phase phenomena rely on the “homogeneous equilibrium” assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release. Here we test the homogeneous equilibrium model against a more sophisticated aerosol approach, in the context of two-phase ammonia clouds released in dry and moist air. It is shown that the simpler model does indeed provide a good description for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequate. The homogeneous equilibrium model seems to be adequate for evaluating the dispersion of heavier-than-air clouds containing droplets smaller than 100 μm.

A model for mass and heat transfer in an aerosol cloud

Abstract In mathematical models for heavy gas clouds dispersing in the atmosphere, two-phase dynamics is usually modelled using assumptions of homogeneous equilibrium. This implies that the mixture of liquid droplets and gas is homogeneous and in thermodynamic equilibrium at all times. This paper discusses a more rigorous approach for modelling the thermodynamical aspects of heavy gas dispersion. A model is presented for the evaporation and condensational growth of a monodisperse binary droplet population, including also the influence of air entrainment. The model evaluates the thermodynamical evolution of a five-component aerosol mixture, consisting of two-component droplets and a three-component gas. The model has been applied for evaluating two-phase ammonia clouds released in both dry and moist air. The numerical results show the influence on contaminant concentration of the droplet size, the atmospheric moisture and the non-ideality of a liquid solution.

A hybrid plume model for local-scale atmospheric dispersion

Abstract This paper presents a local-scale dispersion model, based on atmospheric boundary layer scaling theory. In the vicinity of the source, Gaussian equations are used in both the horizontal and vertical directions. After a specified transition distance, gradient transfer theory is applied in the vertical direction, while the horizontal dispersion is still assumed to be Gaussian. The dispersion parameters and eddy diffusivity are modelled in a form, which facilitates the use of a meteorological pre-processor. We present a novel model of the vertical eddy diffusivity (Kz), which is a continuous function of height in various atmospheric scaling regions. The model also includes a treatment of the dry deposition of gases and particulate matter. The accuracy of the numerical model was analysed by comparing the model predictions with two analytical solutions; the numerical deviations from these solutions were less than 2% for the computational regime. The model has been tested against the Kincaid experimental field data. The agreement of the predictions and the data is good on the average, although the internal variation of the predictions versus data scatter plot is substantial.

Modelling heavy gas cloud transport in sloping terrain

Abstract A model is presented for the transport of a denser-than-air gas cloud on a slope. We discuss the structure of the model and derive detailed model equations for the special case in which the wind is directly uphill or downhill. The model was designed as a hazard analysis tool, and its com- puter implementation can be used as a subprogram in heavy gas dispersion models. We have compared model predictions with results of the Thorney Island phase I field experiments. Although these trials were conducted on flat terrain, the comparison is useful for understanding the cloud transport processes. We have also analysed numerical calculations of heavy gas cloud dispersion on a slope.

Modelling aerosol processes related to the atmospheric dispersion of sarin

We have developed mathematical models for evaluating the atmospheric dispersion of selected chemical warfare agents (CWA), including the evaporation and settling of contaminant liquid droplets. The models and numerical results presented may be utilised for designing protection and control measures against the conceivable use of CWAs. The model AERCLOUD (AERosol CLOUD) was extended to treat two nerve agents, sarin and VX, and the mustard agent. This model evaluates the thermodynamical evolution of a five-component aerosol mixture, consisting of two-component droplets together with the surrounding three-component gas. We have performed numerical computations with this model on the evaporation and settling of airborne sarin droplets in characteristic dispersal and atmospheric conditions. In particular, we have evaluated the maximum radii (r(M)) of a totally evaporating droplet, in terms of the ambient temperature and contaminant vapour concentration. The radii r(M) range from approximately 15-80 microm for sarin droplets for the selected ambient conditions and initial heights. We have also evaluated deposition fractions in terms of the initial droplet size.

Dispersion from Strongly Buoyant Sources

This paper reviews the project “Dispersion from strongly buoyant sources — BUOYANT” (1994–1997), which has addressed the atmospheric dispersion of pollutants emitted from typical fires in warehouses and chemical stores (Ramsdale et al., 1997). Such fires may represent a major hazard to people and the environment, and the fire plumes may contain a variety of harmful or toxic chemical compounds.

Utilization of Meandering in a Receptor-Oriented Solution of the Line Source Dispersion Equation

Lateral dilution in line source dispersion models is usually treated as the sum of short and long period variations of the wind direction and dilution by wind speed perpendicular to the road. We present an alternative solution, where the influence of meandering is estimated assuming normally distributed long term fluctuations of wind direction. The average of receptor-oriented wind direction, which is based on probability distribution, differs substantially from the meteorological mean value in case of wind directions nearly parallel to the road.

The homogeneous equilibrium approximation in heavy gas dispersion models

The dispersion of heavier-than-air aerosol clouds is of great relevance to the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion but, in most cases, two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact almost all of those models which do treat two-phase phenomena rely on the homogeneous equilibrium assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release. Here the authors test the homogeneous equilibrium model against a more sophisticated aerosol approach, in the context of two-phase ammonia clouds released in dry and moist air. It is shown that the simpler model does indeed provide a good description for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequate. The homogeneous equilibrium model seems to be adequate for evaluating the dispersion of heavier-than-air clouds containing droplets smaller than 100 micrometers. In this paper the dispersion model DRIFT is compared with the aerosol model AERCLOUD.

Comparison of Models for Aerosol Vaporisation in the Dispersion of Heavy Clouds

The dispersion of heavier-than-air aerosol clouds is of great relevance to the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion, but two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact essentially all of those models which do treat two-phase phenomena rely on the “homogeneous equilibrium” assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release.