The Handbook of Hazardous Materials Spills Technology
As an all-new update to the classic Hazmat reference book, this text addresses issues such as International law and regulations, risk assessment, biological effects, spill modeling, and more.
TABLE OF CONTENTS The Handbook of Hazardous Materials Spills Technology
Chapter 18: Spreading of Cold Dense Clouds
M. Nielsen, Ris ? National Laboratory,
Roskilde, Denmark
18.1 INTRODUCTION
Emissions from a pressure-liquefied or refrigerated gas storage often produce a cloud which is both heavier and colder than the ambient air. Horizontal spreading is enhanced by gravity, whereas vertical mixing is reduced by density stratification similar to dispersion of an ordinary dense gas cloud. The objectives of this section are to identify the differences induced by the temperature deficit. First, the cloud density depends on a heat balance, which if the entrained air is moist, must include heat of condensation. Another significant difference is the heat convection from the warm ground to the cold gas mixture that gradually modifies the cloud temperature and thereby the density. Finally, the heat convection is an additional source of turbulence which tends to enhance the mixing. From a risk-analysis point of view, the thermally-induced processes are mitigating factors.
18.2 DENSITY CALCULATIONS
18.2.1 Homogeneous Equilibrium
Numerical models of cold dense gas dispersion predict cloud dynamics, temperature, and phase composition in parallel to its dispersion. The documentation often contains a reference to the homogeneous equilibrium assumption (HEA). This means that the aerosol and gas phases are supposed to have the same temperature and that the contaminant partial pressure in the gas phase is equal to the saturation vapor pressure. HEA is a computationally convenient assumption since it allows calculation of the thermodynamic state by a submodel independent of cloud dynamics. The appropriate thermodynamic relations depend on the contaminant properties, e.g., its water...
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