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Dispersion in the Open Atmosphere

Many different accident situations are conceivable, which can give rise to the inadvertent emission of a flammable substance and which have great influence on the evolution of a vapor cloud. It can be released as a liquid or a gas or a two-phase mixture. The component, from which the substance is released, may be a tank, a pump, a valve, pipe work or other equipment. The orifice, through which it is leaking, can vary over different shapes and sizes. The leaking fluid can flow into different geometries. And finally it is the thermodynamic conditions of the fluid, which determine its release behaviour. Four major categories for the release of liquid or gaseous hydrogen can be identified:

  1. small-scale, moderate hydrogen release from permeation or boil off;
  2. vaporization of a liquid hydrogen pool on a solid or liquid surface;
  3. two-phase jet release of hydrogen after opening a system under pressure;
  4. rapid escape of hydrogen to all sides after the catastrophic failure of a pressure.


The generation of a gas cloud in the atmosphere is principally caused by forces resulting from the internal energy of the gas and/or from energy inside the system, from which the gas has escaped, or from a relative excess energy in the environment. Those opposed are dissipative forces, among which atmospheric turbulence is the most important one.

In case there is no early ignition, the vapour cloud shape is further determined by density differences, atmospheric conditions, and topography. Several phases of a gas cloud formation can be distinguished: In the early phase, the gas cloud is still unmixed and usually heavier than the ambient air. Its spreading is influenced by gravitational force resulting in a near-ground, flat cloud. The following phase is characterized by a gradual entrainment of air from outside into the gas cloud enlarging its volume, thus lowering gas concentration, and changing its temperature. In the final phase, due to atmospheric dispersion, density differences between cloud and ambient air will be leveled out, where concentrations eventually fall below flammability limits. Thus density of the gas mixture vapor cloud varies with time.

The turbulence structure of the atmosphere is composed of large-scale turbulence described by the large-scale wind field, and of isotropic turbulence, which is a rapid variable superimposed to the medium wind field. The latter is generated due to the fact that "roughness elements" withdraw from the medium wind field kinetic energy, which is transferred to turbulence energy. It is this energy and of particular importance the small eddies, which finally determine the spreading of the gas cloud; the larger eddies are responsible for its meandering. Further factors influencing the turbulence structure within a gas cloud, apart from the atmospheric turbulence of the wind and temperature field inside the turbulent boundary layer (5 mm < z < 1500 m), are:

  1. velocity gradient (sheer force between wind field and gas cloud);
  2. current created by buoyancy forces;
  3. heat transfer from ground into cold gas (thermally induced turbulence);
  4. rapid expansion from vaporization of cryogens.

Fluctuations in the concentration as a consequence of the atmospheric turbulence are typically in the order of a factor of 10 above the statistical average.

The spreading of a gas cloud in the atmosphere is strongly influenced by the wind conditions which change with height. Vertical wind profiles can be determined as a function of the so-called stability categories depending on the temperature conditions. As an example, Pasquill suggested the categories A, B, C for unstable, D for neutral, E, F for stable conditions. The spreading mechanism of a gas in the atmosphere is mainly by mixing with the ambient air. The boundary layer between gas and air governs momentum and mass exchange, which is much stronger than molecular diffusion. Horizontal dispersion perpendicular to wind direction is about the same for all stability categories; it is different for vertical dispersion. Under stable conditions, vertical exchange is small leading to a long-stretched downwind gas cloud. In contrast, a temperature decrease with height, which is stronger than the adiabatic gradient (-0.98 K/100 m), results in an effective turbulent diffusion and rapid exchange. This is particularly true for a hydrogen gas cloud, which behaves in a neutral atmosphere as if it were in an unstable condition. Worst-case scenario would be the existence of a large hydrogen gas cloud generated with minimal internal turbulence, on a cold, humid day with high wind velocity and strong atmospheric stability.

The jet release of a liquefied cryogen under pressure is connected with the formation of aerosols. The two (or three)-phase mixture developed exhibits an inhomogeneous concentration distribution. There will be a rapid vaporization, which may create locally high H2 concentrations. It was observed that the larger the liquid fraction of the two-phase jet, the larger was the evolving flammable vapor cloud (KneeboneA:1974). Another effect observed for vertical jet-like gas release under certain conditions is a bifurcation of the plume into two differently rotating vortices. After a short acceleration phase, a double vortex is developing which eventually splits up. This effect may reduce the height of the gas cloud and lead to a stronger horizontal spreading (ZhangX:1993).

With respect to just vaporized LH2, the lifetime as a heavier-than-air cloud (1.3 kg/m3) is relatively short. It needs only a temperature increase of the hydrogen gas from 20 K to 22 K to reach the same density of the ambient air (1.18 kg/m3). This short time span of negative buoyancy is slightly prolonged by the admixed heavier air, before the buoyancy becomes positive and enhances with further temperature increase. Unlike pool vaporization leading to only weak vapor cloud formation, instantaneous release of LH2 or high release rates usually result in intensive turbulences with violent cloud formation and mixing with the ambient air. If LH2 is released onto water, rapid phase transitions occur, which are connected with very high vaporization rates. The exiting vaporized gas also carries water droplets into the atmosphere increasing the density of the vapor cloud and thus influencing its spreading characteristics.

The spreading behaviour of a large gas cloud is different from a small one meaning that the effects in a small cloud cannot necessarily be applied to a large one. For small releases, the dynamics of the atmosphere are dominant and mainly covering gravitational effects due to the rapid dilution. For large amounts released, the evolving gas cloud can influence itself the atmospheric wind conditions changing wind and diffusion profiles in the atmosphere. This so-called "vapor blanket" effect could be observed particularly at low wind velocities, where the atmospheric wind field was lifted by the gas cloud and the wind velocity inside the cloud dropped to practically zero.

The near-ground release of cryogenic hydrogen resulting in a stable stratification has, in the initial phase, a damping influence on the isotropic turbulence in the boundary layer to the ambient air, thus leading to a stabilization of the buffer layer (so-called "cold sink effect"). For small wind speeds, additional effects such as further heating of the gas cloud due to energy supply from diffusion, convection, or absorption of solar radiation, as well as radiation from the ground will play a certain role, since they reduce gas density and enhance positive buoyancy.

A still deep-cold hydrogen gas cloud exhibits a reduced heat and mass exchange on the top due to the stable stratification. A stronger mixing will take place from the bottom side after the liftoff of the cloud resulting from buoyancy and heating from the ground. The dilution is slightly delayed because of the somewhat higher heat capacity of hydrogen compared to air. In case of a conversion of para to ortho hydrogen, a heat consuming effect (708.8 kJ/kg), reduces the positive buoyancy. This process, however, is short compared to dispersion.

Another effect determining a cold hydrogen cloud behaviour is the condensation and solidification, respectively, of moisture which is always present in the atmosphere. The phase change is connected with the liberation of heat. Therefore density is decreased and thus buoyancy is enhanced. The higher the moisture in the atmosphere, the sooner is the phase of gravitation-induced spreading of the vapour cloud terminated. The effect of condensation also results in a visible cloud, where at its contour lines, the temperature has just gone below the dew point. For high moisture contents, the flammable part of the cloud is inside the visible cloud. For a low moisture content, flammable portions can also be encountered outside the visible cloud. Visible and flammable boundaries coincide at conditions around 270-300 K ambient temperature and humidities of 50-57%.

According to the "model of adiabatic mixing" of ambient air and hydrogen gas, assuming there is no net heat loss or gain for the mixture, there is a direct correlation between mixture temperature and hydrogen concentration, if air temperature and pressure and relative humidity be known. This means on the other hand that thermocouples could be used as hydrogen detectors. The model was found to be in good agreement with measured concentrations. Taking the conditions of the NASA LH2 spill trials as an example, the cloud boundaries were assessed of having had a hydrogen concentration of around 8-9%.

The topography has also a strong influence on the atmospheric wind field and thus on the spreading of the gas cloud. Obstacles such as buildings or other barriers increase the degree of turbulence such that the atmospheric stability categories and their empirical basis are loosing their meaning locally. This situation requires the application of pure transport equations which may become very complex due to the generation of vortices or channeling effects (PerdikarisGA:1993). A gas cloud intersecting a building will be deflected upwards reducing the near-ground concentration in comparison to unobstructed dispersion. On the other hand, if the source is near the building in upwind direction, a vortex is created with a downwards directed velocity component, which may increase the near-ground concentration. This effect, however, may be more important for heavy gases than for the lighter gases.

Experimental Activities

The first hydrogen release experiments conducted with LH2 date back to the late 1950's (CassutLH:1960), (ZabetakisMG:1961). They included, however, only little information on concentrations and were basically limited to visual recordings. The experimental series with LH2 release conducted by Arthur D. Little were dedicated to the observation of the dispersion behavior showing that still cold hydrogen gas does not rise immediately upwards, but has the tendency to also spread horizontally. The initial column-like cloud shape later transforms into a hemispherical shape. Measurements of the translucence reveal large variations in the concentrations indicating incomplete mixing (see Fig. 1). The continuous release at a rate of 2 l/s over 16 min and of 16 l/s over 1 min and for wind speeds between 1.8-7.6 m/s, the developing visible vapor cloud had an extension of up to 200 m before fading away. Gusty winds had the effect of splitting up the gas cloud.

Fig 1: Shape of H2-air cloud, (ZabetakisMG:1961)

The first and up to now most relevant test series to study hydrogen dispersion behaviour was conducted by NASA in 1980 with the near-ground release of LH2. In five tests, a volume of 5.7 m3 was released within 35-85 s; in two more tests the released volumes were 2.8 m3 in 18 s and 3.2 m3 in 120 s, respectively (WitcofskyRD:1984) and (ChirivellaJE:1986). Eight times the concentration was measured at a total of 27 positions. Temperature measurements were also indicators for H2 concentration. These trials have shown that drifting of the H2 vapour cloud can go up to several hundred m, particularly if the ground is able to sufficiently cool down. The tests also demonstrated that the vaporization rate of LH2, much more than that of other cryogens, is strongly dependent on the type of release.

In 1994, the German BAM has conducted LH2 release experiments with the main goal to demonstrate the safety characteristic of a rapidly decaying hydrogen vapour cloud in the open atmosphere in contrast to the behaviour of vaporizing LPG. Six LH2 spill tests were conducted with amounts released of 0.5-1 m3 (total 260 kg) at rates of around 0.6-0.8 kg/s. The tests were also to show the influence of adjacent buildings on the dispersion behaviour (SchmidtchenU:1994), (DienhartB:1995).


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Hydrogen Dispersion

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