Koichi Sada

Numerical Calculation of Flow and Stack-gas Concentration Fluctuation around a Cubical Building

A numerical simulation model was developed to predict the instantaneous concentration fluctuation of a plume and applied to stack-gas diffusion around a cubical building. The flow field, including an instantaneous velocity component, was predicted using the large eddy simulation (LES) method in the developed numerical model. Then, the instantaneous concentration fluctuation was predicted using the obtained unsteady flow field. Concentration was calculated using the finite difference method, in which the LES is expanded for concentration, and the puff method, in which small volumes of the tracer gas are divided and combined according to the calculation mesh sizes. In order to avoid numerical viscous effects, a puff method and finite difference method were applied separately in the regions near and far from the stack-gas release point, respectively. Then, the flow field around a cubical building and the diffusion of stack-gas, emitted from an elevated point source at an upstream position of the building, were calculated using the model mentioned above. Numerical calculation results were compared with those obtained in wind tunnel experiments in which concentration fluctuation was measured using high-response flame ionization detectors. Although there were some discrepancies in the flow field between the calculated results and those of wind tunnel experiments, e.g., the calculated windward length of a cavity region behind the building, the calculated mean velocity and turbulent intensity showed good agreement with those of the wind tunnel experiments. Furthermore, the calculated concentration fluctuation showed good agreement with that in the wind tunnel, not only regarding the features of fluctuating concentration signals, but also statistic quantities, viz., mean concentration, fluctuation intensity and high-concentration values.

An atmospheric dispersion model for the environmental impact assessment of thermal power plants in Japan--a method for evaluating topographical effects.

Abstract An atmospheric dispersion model was developed for the environmental impact assessment of thermal power plants in Japan, and a method for evaluating topographical effects using this model was proposed. The atmospheric dispersion model consists of an airflow model with a turbulence closure model based on the algebraic Reynolds stress model and a Lagrangian particle dispersion model (LPDM). The evaluation of the maximum concentration of air pollutants such as SO2, NOx, and suspended particulate matter is usually considered of primary importance for environmental impact assessment. Three indices were therefore estimated by the atmospheric dispersion model: the ratios (α and β, respectively) of the maximum concentration and the distance of the point of the maximum concentration from the source over topography to the respective values over a flat plane, and the relative concentration distribution [γ(x)] along the ground surface projection of the plume axis normalized by the maximum concentration over a flat plane. The atmospheric dispersion model was applied to the topography around a power plant with a maximum elevation of more than 1000 m. The values of α and β evaluated by the atmospheric dispersion model varied between 1 and 3 and between 1 and 0.4, respectively, depending on the topographical features. These results and the calculated distributions of γ(x) were highly similar to the results of the wind tunnel experiment. Therefore, when the slope of a hill or mountain is similar to the topography considered in this study, it is possible to evaluate topographical effects on exhaust gas dispersion with reasonable accuracy using the atmospheric dispersion model as well as wind tunnel experiments.

Effects of emissions from a coal-fired power plant on surface soil trace element concentrations

Abstract Increase in trace element (As, Cd, Cr, Cu and Hg) concentrations in surface soils caused by deposition of emissions from a power plant was evaluated through field studies and model calculations. The enrichment discrimination factor (EDF) was used as an indication of the effects. Model calculations estimated that the maximum deposition of the elements would be located at a site 3 km north of the plant. The increase in EDF, ΔE, at the ssite due to cumulative depositions over 25 years was evaluated to be As:0.06, Cd:0.20, Cr:0.02, Cu:0.13 and Hg:0.08, respectively. In contrast, observed EDFs of each element at 30 sites within a 10 km radius from the plant fluctuated within the range far beyond the ΔE, and there was no statistically significant correlation among the EDFs as a function of distance or direction from the plant, suggesting that the effects of emissions lie concealed in normal ambient fluctuation of the element concentrations.

Numerical Simulation of Tracer Gas Diffusion from Continuous Point Source. Calculations of Tracer Gas Concentration in Turbulent Boundary Layer on a Flat Plate.

The numerical simulation results of tracer gas diffusion from a continuous point source were obtained and compared with wind tunnel experiment results. The flow fields were predicted by the k-e model, and the turbulent kinetic energy gained here was divided into directional turbulence intensities by using algebraic equations and an eddy viscosity concept. Using estimated turbulence intensities, tracer gas diffusion was simulated by a Lagrangian particle diffusion model. Predicted mean concentration profiles showed good agreement when turbulence intensities were obtained from algebraic equations. The Lagrangian time scale was estimated from the flow calculation results by means of the k-e model. It was apparent that the optimum value of model constant C1ψ for simulating the tracer gas diffusion was 2.0.

Coal particle dispersion monitoring system

Abstract A monitoring system for coal panicle dispersion at coal storage yards is necessary to meet environmental standards for the operation of coal fired power plants. We have developed a system having three functions as follows: 1. (1) To measure coal particle concentrations at the first connecting part of belt conveyors. 2. (2) To predict occurrence of coal particle dispersion during stacking. 3. (3) To moisten the coal at the second connecting part of belt conveyors when coal particle dispersion occurs. The generation of coal dispersion is determined by whether or not the hygroscopic moisture is greater than the critical hygroscopic moisture value for each coal type. Coal particle concentrations are measured using dust meters. In this report, the dust meter readings were related to emission rate of coal particle and dustiness index of coarse dust defined by ASTM D547-41 in order to predict occurrence of coal particle dispersion using dust meters. The adaptability of the system to real coal handling facilities was examined by a model which consisted of belt conveyors, a stacker, connecting parts and the monitoring system. As a result of these experiments, it was demonstrated that coal particle dispersion can be suppressed by means of wetting coal in response to dust meter readings.