AIR POLLUTION – MONITORING MODELLING AND HEALTH

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212 Air Pollution <strong>–</strong> Monitoring, Modelling and Health 26.8 26.4 26.0 25.6 25.2 24.8 24.4 24.0 23.6 8x10 -3 6 5 Hz 8 Hz 10 Hz 9 Hz 12 Hz 15 Hz Mass [kg] Hydrated alumina mass -dm/dt 6x10 -3 5 Alumina -dm/dt = 0.00122 (u .20 - 2.45) + 0.00052 (u .20 - 2.45) 2 -dm/dt [kg/s] 4 2 -dm/dt [kg/s] 4 3 2 -2 1 -4 0 0 500 1000 1500 Time [sec] Fig. 6. -Left, the mass loss and its negative time derivative from alumina test surface with respect to time at different air velocities. -Right, the threshold wind velocity at 0.20 m above the pile is 2.45 m/s. Assuming z 0 =30 μm, this gives u * t=0.11 m/s. 4.4 Comparison with results in the literature 2000 2500 Before quantifying and comparing the results for different fractions, it is important to note a few basics about the dynamics of the mass outflow. It can safely be assumed that the surface S exposed to erosion stays unchanged for a short while after the measurement begins and that the mass flux is governed by the equation dm/dt = - p S, where p is some erosion potential of the surface S. If the wind field, the shape of the surface and the composition of the surface material layer are independent of time, then the mass outflow is a constant and the mass loss will be obviously a linear function of time m(t) = m 0 - p S t. Such a linear drop is indeed observed in the starting time interval of each measurement. The “active” surface (affected by wind erosion) in the vessel inevitably diminishes with time and finally results in saturation (a zero mass outflow). This is reflected in the measured time dependence of the mass loss. The mass outflow diminishes with time because the wind friction velocity u * gradually approaches (from above) the local threshold velocity u * t. Consistent with measurements and according to the US EPA Eq. (4), p depends quadric-linearly on the friction velocity difference u * - u * t but the time evolution of u * is an unknown a priori since this is a non-local component that depends on the wind field that in turn depends on surface morphology. To predict the time dependence of the mass loss m(t) the coupled wind field - surface erosion problem must be solved. On the one hand, the erosion potential at a given point depends on the wind field, but the wind field itself depends on the erosion effect, which modifies the surface. If one is able to calculate the wind field for a particular geometry (given by function g of the coal surface z(x,y,t) in the vessel at certain time t), it is possible to estimate the alteration of the surface due to erosion for the small time interval. The wind field is then calculated again and with this knowledge a new shape of the surface is calculated, until u * drops below u * t everywhere at the surface. Formally, the equations, which should be propagated in time, starting from t = 0, when the wind field is turned on, are: 0 1 2 3 u .20 [m/s] 4 5 * * * t z(x,y,t Δt) z(x,y,t) p(u (x,y,t) u )Δt, u (x,y,t Δt) g(z(x,y,t Δt)) (10)
Fugitive Dust Emissions from a Coal-, Iron Ore- and Hydrated Alumina Stockpile 213 If friction velocity u * t at any given time is the same everywhere along the surface (as assumed above), a reasonable assumption for a homogeneous flat terrain composed of uniform particle sizes submitted to the uniform wind field, erosion will proceed infinitely long with the same speed resulting in a constant mass outflow signal. Any reduction in the mass outflow and finally the saturation is an effect of screening, which occurs for example, when a finite quantity of material is placed in the vessel, or even for the flat terrain situation if this is composed of different particle sizes. The aim of this work was not to follow up the mass outflow with substantial modifications of surface morphology. However, for the "finite" test surface, the erosion potential p can still be measured by changing the wind velocity u .20 above an initially flat test surface and considering only the first, linear part of the mass loss. This is true, if in this initial and relatively short period of time (so that surface morphological changes can be neglected), u * can be thought as being frozen to a value which is the same along the whole surface area (negligible edge effects and large scale variation of u * over the surface due to the localized wind source) and if the way that u * is related to the measurable quantity, in our case u .20 , is known. According to US EPA (1995), for a flat terrain with no obstacles, the wind friction velocity is related to u 10 , the wind velocity measured at 10 m above the terrain (Eq. 1). The wind speed profile in the surface boundary layer follows a logarithmic distribution and becomes zero at the aerodynamic surface roughness height z 0 (Eq. 2). Accordingly, u * t is the wind speed just above the surface at z-Z =z 0 exp(0.4)~ 1.5 z 0 . Even in the case of a finite flat terrain (as in the vessel), the surface boundary behaviour is similar to that of the flat terrain if the vertical profile is limited to the region close to the surface and away from the edges. The idea is to determine the actual wind friction velocity from the simulations of the wind field for the described experimental set-up. First, the input parameters of the wind field simulation are chosen in such a way to obtain the wind velocity u .20 value at 0.20 m above the test surface as actually measured. Then the vertical wind velocity profiles close to the surface is extracted in order to identify the region with logarithmic z/z 0 dependence and their possible differences at different parts of the surface. By comparing this local behaviour to the behaviour of the flat terrain close to the surface, the relation between the local and flat terrain situation can be established. Let it be assumed for the moment that 0.20 m is sufficiently close to the 0.46 x 0.46 m 2 surface so that the wind profile is defined solely by the surface characteristics, i.e. the central point 0.20 m above the surface is still in the boundary layer. As for the aerodynamic surface roughness, this is usually smaller than the particle diameter. The experimental parametrisation (Byrne, 1968) formula is z Qexp(d/ ) (11) 0 It applies for a uniform bed of quartz particles with diameter d and parameters Q = 2.0 x 10 -6 m and λ = 6.791 x 10 -4 m. According to Eq. (11), the average d=4 mm of FR3 coal corresponds to a z 0 = 0.72 mm. If the vertical profile of the wind velocity obeys the large flat terrain law (Eq. 1 and 2) and z - Z ≈ z, then the relation between the measured u .20 and wind friction velocity for FR3 is given by * Ku.20 u 0.070 u.20 (12) ln(286)
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AIR POLLUTION - MONITORING, MODELLI
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Contents Preface IX Chapter 1 Urban
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Preface This book provides a compre
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2 Air Pollution - Monitoring, Model
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10 Air Pollution - Monitoring, Mode
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AIR POLLUTION – MONITORING MODELLING AND HEALTH

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