Hazardous Gas Dispersion Research Articles

Study on Theory of Hazardous Gas Puff Dispersion

In order to provide some possible scientific guidance for the investigation to puff dispersion of hazardous gas, some theoretical methods were used to deduce the critical parameters for description of puff dispersion behavior. In addition, some important issues such as how to distinguish dense gas dispersion from non-dense gas dispersion and the whole process to investigate a typical puff dispersion problem are also discussed. Finally some useful algorithms for standard deviation of concentration distribution over time, standard along-wind dispersion coefficient, et al. are deduced. These results can be potential scheme for puff field tests, wind tunnel experiments and especially for numerical simulations.

LES of turbulent wind and gas dispersion in a city

We have conducted LES (large eddy simulation) for turbulent wind flows and dispersion of hazardous gas emitted from a point source on the ground in a large city, Tokyo. Also, for numerical validation, wind tunnel tests using a Dynamic Particle Image Velocimetry (DPIV) and tracer gas measurement system are performed to estimate turbulence statistics of wind and gas concentration in between the compacted tall buildings. We elucidate the predictive accuracy of LES based on the wind tunnel test data. Finally, we discuss the meaning and the potential of LES prediction for dispersion problems in a complex urban area, taking into consideration the limitation of the predicted results by Reynolds Averaged Navier–Stokes (RANS) modeling.

Modeling Hazardous Gas Dispersion in Neutrally and Stably Stratified Atmospheric Boundary Layer Flows

The objective of the present work is to propose a methodology to simulate hazardous gas dispersion in neutrally and stably stratified atmospheric boundary layer (ABL) flows. The profiles of velocity, temperature and turbulence quantities are set based on the stratification effect. Special emphasis is placed on the generation of a horizontally homogeneous turbulent boundary layer (HHTBL). FLUENT software is validated for simulating ABL flows by comparing the results obtained with available numerical data from the literature. The fully developed profiles of velocity, turbulent kinetic energy, turbulent dissipation rate and temperature are implemented as user defined functions in the commercial code FLUENT. The hazardous gas release is modeled using the convection diffusion equation. Four test cases from the classical Prairie Grass experiment are considered for validating the present approach. The predicted concentration of the SO2 gas at various locations of the test field (50, 100, 200, 400 and 800 m) are ...

Local characteristics of cross-unit contamination around high-rise building due to wind effect: Mean concentration and infection risk assessment

In this present work, the characteristics of hazardous gas dispersion and possible cross-unit contamination around a complex-shaped high-rise residential building due to wind effect are thoroughly studied using physical modeling method. Experiments were performed in a boundary layer wind tunnel for a 1:30 scale model that represented a 10-story residential building in prototype. Tracer gas, simulating exhausted room air, was continuously released from different floor levels, and its concentrations on the adjacent envelope surfaces were measured using fast flame ionization detectors. The mean concentration fields were reported and analyzed under different configurations during the experiment to consider the effects on pollutant dispersion behavior due to changes in source position and approaching wind condition, with the main emphasis on the differences between open-window and closed-window conditions. In particular, the measured concentration fields were further examined from a practical point of view, with respect to hazard assessment. Understanding these hazardous plume dispersion features is useful for employing effective intervention strategies in modern residential building environment in case of hazardous substance release. The study on this physical process is not only helpful to reduce the hazardous effect of routine release of harmful pollutant near the building, but also useful for the purpose of prevention and control of accidental infectious diseases outbreak.

Hazardous gas dispersion: A CFD model accounting for atmospheric stability classes

Nowadays, thanks to the increasing CPU power the use of Computational Fluid Dynamics (CFD) is rapidly imposing also in the industrial risk assessment area, replacing integral models when particular situations, such as those involving complex terrains or large obstacles, are involved. Nevertheless, commercial CFD codes usually do not provide specific turbulence model for simulating atmospheric stratification effects, which are accounted of by the integral models through the well-known stability-class approach. In this work, a new approach able to take account of atmospheric features in CFD simulations has been developed and validated by comparison with available experimental data.

Source identification for unsteady atmospheric dispersion of hazardous materials using Markov Chain Monte Carlo method

This paper is to study source inversion and identification of hazardous gas dispersion in a three-dimensional urban area. An unsteady adjoint transportation model was adopted, and an advanced numerical scheme based on adaptive mesh refinement was used. A time-dependent concentration database over the entire parameter space was generated. Markov Chain Monte Carlo sampling based on the Bayesian inference was used to invert the parameters such as source location and its strength obtained from the database at different sampling time of sensor readings and simulation results. The probability distributions of source parameters were calculated and predicted source location and strength agree well with actual values which indicates the feasibility of the proposed method and procedure. Numerical studies also show that the computational scheme is efficient when using unsteady adjoint transportation equation with MCMC methods. The unsteady inversion method proposed here can also improve the accuracy of source location in the wind direction compared with the steady inversion method for the case of atmospheric release of hazardous materials.

High concentrations of a passive scalar in turbulent dispersion

In problems involving the dispersion of hazardous gases in the atmosphere, the distribution of high concentrations is often of particular interest. We address the modelling of the distribution of high concentrations of a dispersing passive scalar at large Péclet number, concentrating on the case of steady releases. We argue, from the physical character of the small-scale processes, and from the statistical theory of extreme values, that the high concentrations can be fitted well by a Generalized Pareto Distribution (GPD). This is supported by evidence from a range of experiments. We show, furthermore, that if this is the case then the ratios of successive high-order absolute moments of the scalar concentration are linearly related to the reciprocal of the order. The linear fit thus obtained allows the GPD parameters to be determined from the moments. In this way the moments can be used to deduce the properties of the high concentrations, in particular the maximum possible concentration θmax = θmax(x). We argue, on general physical grounds, that θmax/C0 (where C0 = C0(X) is the centreline mean concentration, and X is the downstream distance from the source) decreases to zero very far from the centreline, but that the decrease takes place on a length scale much larger than the mean plume width (because it is controlled by the relatively slowly acting molecular diffusion, rather than the fast turbulent advection). Thus, over the distances for which accurate measurements can be made, we expect θmax/C0 to be approximately constant throughout the plume cross-section. On the centreline, we argue that θmax/C0 increases downstream from the source, reaches a maximum and then decreases, ultimately tending to 1 far downstream. In support of these deductions we present results for some high-quality data for a steady line source in wind tunnel grid turbulence. Finally, we apply to this problem some existing models for the relationships between moments. By considering the behaviour far from the centreline in these models, and linking the moments to the high concentrations, we derive relationships between the model parameters. This allows us to derive an expression for θmax/C0 which depends on a total of 5 parameters, and (weakly) on C/C0 (where C = C(x) is the local mean concentration). Comparison with the data is encouraging. We also discuss possible methods for modelling the spatial variation of these 5 parameters.

Short-range vertical dispersion from a ground level source in a turbulent boundary layer

The short-range vertical dispersion of a passive containment in a neutrally stratified turbulent boundary layer can be usefully characterised by a number of approaches, such as a vertical diffusivity, K z , a vertical Gaussian dispersion parameter, σ z , and an entrainment velocity, w e . σ z and w e are approximately related by w e ∝d σ z /d t . While the Gaussian model is widely used in most air quality applications, the entrainment velocity is widely used in wind tunnel studies and models for the dispersion of hazardous gases. This paper summarises experiments in three simulated rough-wall atmospheric boundary layers (two in the US and one in the UK), which lead to the conclusion that for passive releases the entrainment coefficient α in the formula w e =αu ∗ is α =0.65±0.05. This value is consistent with a recent analysis of the Prairie Grass data (out to x =100 m, where vertical profiles were obtained) that gave α =0.63±0.05. Somewhat surprisingly we were not able to locate a theoretical analysis that could reproduce these results without recourse to some empirical input.

Modelling and control of the dispersion of hazardous heavy gases

Dispersion of several common `heavy' gases (ethylene, propylene, ammonia, and chlorine) has been modelled on the basis of modifications in plume path theory. The model takes into account, among other things, the variations in temperature, density, and specific heat during the movement of the heavy gas plume. The effects of wind speed, density of the gas, and venting speed on the plume dispersion have been simulated. Based on the simulations a set of empirical equations has been developed. The equations have been validated by theoretical as well as experimental studies. Studies have also been carried out to simulate the effect of venting speed (manipulated by injecting hot air with the released gas) on the plume dispersion. The study reveals that the effect of venting speed on dispersion is very pronounced and can be used to reduce the risk posed by the accidental luxurious release of toxic/flammable gases. For example an increase of 20% in venting speed of chlorine (54.1 m/s) can reduce the distance up to which toxic concentration would occur by about 1100 meters.

Estimation of point-source emissions of hydrogen sulfide and ammonia using a modified Pasquill-Gifford approach

Estimation of point-source emissions based on measured downwind concentrations and known meteorology is a challenging task. A facility to study atmospheric dispersion of hazardous gases has been designed and implemented at the Wind Engineering Research Field Laboratory (WERFL) at Texas Tech University in Lubbock, Texas. The field lab features a 48.5 m (160 ft) meteorological tower which can monitor wind speed at six heights, wind direction and vertical temperature gradients using temperature measurements at two elevations. The relative humidity and barometric pressure can also be monitored. A description of the facility and its features are provided in this paper. In this work, the Pasquill-Gifford (PG) model was modified to provide predictions of emission rates for two hazardous gases; hydrogen sulfide (H 2S) and ammonia (NH 3). Two mathematical manipulations of the PG model were used to refine estimates of emission rates of H 2S and NH 3 based on experimental field data. The first modified PG approach was based on an empirical correction to the Gaussian model, and was evaluated only for direct downwind distances and neutral atmospheric stability conditions. The second approach was based on an improved procedure for estimation of the specific dispersion coefficients for H 2S and NH 3 required in the PG model. These new dispersion coefficients were estimated from experimental data obtained in the field under both neutral and stable atmospheric conditions.

Design of a system for real-time modelling of the dispersion of hazardous gas releases in industrial plants

Abstract This paper describes the hardware and software used in a system designed to model episodic or continuous releases of hazardous or toxic materials in industrial plants. The work has been divided into two parts; in this first part, the system's hardware and the algorithms used in the calculation of real time dispersion of pollutants released in the air from multiple industrial stacks are described. The hardware of the system comprises meteorological observations (vector wind, air temperature and relative humidity). A validation of the system for a particular case study is presented.

Design of a system for real-time modelling of the dispersion of hazardous gas releases in industrial plants

Abstract This paper is the second part of a research programme concerning the modelling capabilities of accidental releases of heavier-than-air toxic gases. The existing theory, which includes the strength of the source and the subsequent development of the released cloud under representative environmental conditions, is described. Comparison of the ZZB-2 system predictions with field data from the Desert Tortoise and Lyme Bay V, ammonia and chlorine releases, shows excellent agreement at distances between ≈ 200 m and a few kilometres from the source. The correlation between observed and predicted cloud concentrations, was in all cases significant at a confidence level better than 95%.