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Show The latter authors have incorporated in their model a number of processes especially relevant to ash deposition (Hajduk, 1987), and are able to reproduce some interesting and complicated effects, such as the variation of deposition rate with changes in gas velocity. In the elastic-plastic model with adhesion, the temperature dependence of the deposition rate arises primarily through the effects of temperature on yield stress and surface energy. An alternative is to consider the ash as liquid and supercooled liquid droplets. A justification for this approach is that most, although by no means all, ash arriving at the furnace exit has been fused in the flame. According to this picture, the viscosity of a liquid droplet determines the extent to which it is deformed on collision, and therefore the area over which contact is established with particles on the surface. An advantage of this approach is that models are available with which to estimate the temperature and composition dependences of droplet viscosity (Riboud, Roux, Lucas, and Gaye, 1981; Urbain, Cambier, Deletter, and Anseau, 1981; Mills, 1986; Vorres, Greenberg, and Poeppel, 1986). A disadvantage is that the theory of the collision of viscous droplets with surfaces is less well developed than that describing elastic-plastic impacts. The collision of an inviscid drop with a surface has been analyzed by Savic and Boult (1955). Hartley and Brunskill (1958) reported the results of experiments showing the influence of viscosity and surface tension on sticking probabilities. The effects of droplet size and velocity were investigated by Gallily and LaMer (1958), Engel (1955, 1960), Gillespie (1955), and Gillespie and Ride a 1 ( 1955) . For two liquids with similar surface energies per unit area and small equilibrium contact angle, the larger the contact area 5 |