Residence time of particles

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

The turbulent fluctuation frequency can be estimated by means of turbulent measurements. Mockel 124] found that the wave number k = 2Tlft/u in the interesting dissipation range is k>ko with the limiting value ko= (0.1. ..0.2)qL-From this becomes the frequency to ft> (0.016...0.032)u/qL. An important measure should be the related number of turbulent fluctuation z/zp which occur during the residence time of particles ti=Vi/qp inside the fictive impeller volume Vj at one circulation. It follows to ... 

Figure 24. Drawing of the continuous leaching model used by Vigier et al. (2001) to estimate the residence time of particles in the soils of the Mackenzie Basin, and the related equations. This model assumes that particles are continuously leached in the soil before leaving to the river. Dissolved load of the river integrates the present leaching of the whole soil profile. 238, 234, 230 and 226 are the leaching coefficients of h and Tla nuclides, respectively, and x is the duration of the chentical...

Americium released to the atmosphere will be associated with particulate matter and will be deposited on land or surface water by dry deposition or wet deposition (Essien et al. 1985). Dry deposition results from gravitational settling and impaction on surfaces, and wet deposition returns americium to earth in precipitation. Radionuclides resulting from atmospheric weapons tests are often injected into the lower stratosphere, while other atmospheric releases are into the troposphere. The residence time of particles in the atmosphere will depend on the altitude, latitude, season, and hemisphere because of atmospheric... 

With regards to solid phase, most of the estuaries are characterized by a "turbidity maximum". The fresh water flow generates an entrainment from the bottom water, the sea water which is entrained from the lower layer by this flow being compensated by a residual landward flow along the bottom. The river-borne particles which settle in this bottom layer are taken up by the residual landward current and trapped in the middle part of the estuary. This process greatly enhances the residence time of particles, especially in tidal estuaries. 

With a typical value of Vdp = 0.2 cm s 1 for particles of this size range, the calculated residence time of particles and particle-associated chemicals due to dry deposition in the well-mixed troposphere is 30 days. Modeling of the transport of particle-associated 210Pb leads to an estimated residence time of particles in the atmosphere of 5-15 days due to combined wet and dry deposition (Balkanski et al., 1993), consistent with the above calculations. 

Despite possible error(s), it is still helpful and of significance to get some rough or semi-quantitative understanding about the residence time of particles in the impingement zone from the analysis using the equations given above. The results estimated with Eq. (2.31) can be considered as being about the possible maximum residence time of particles. 

Figure 2.6 Residence time of particle in the impingement zone vs. the length of accelerating...

The total mean residence time of particles in the whole device is between 4.8 and 6.0 s and... 

The concentration of 210Po in seawater is generally close to what would be expected from radiochemical equilibrium with its grandparent, 210Pb. Below 400 m, Po activities are very similar to those of Pb. At shallower depths, the concentrations of Po and Pb are somewhat decoupled. Po is more readily adsorbed by particles than is Pb in the surface ocean (Bernat and Church, 1989) and, due to the short residence times of particles in the upper ocean, the relative concentrations of Po and Pb are controlled, in part, by particle fluxes and differences in polonium and lead solution and surface chemistries. The chemistry of... 

The chemical reactivity of particle-associated polycyclic aromatic hydrocarbons (PAHs) under real or simulated atmospheric conditions is receiving greatly increased attention (1-4). The residence times of particle-adsorbed PAHs in the atmosphere obviously depend upon the susceptibility of the compounds to chemical transformation. 

Be 53 d Residence time of particles and particle-sorbed pollutants, particle mixing, and short-term sedimentation rates Aaboe et al. (1981) Baskaran and Santschi (1993) Baskaran et al. (1997)... 

---