Article

Heat and Mass Transfer and Physical Gasdynamics
2005. V. 43. № 6. P. 930–936
Ponomarev A.N.
An Investigation of Diffusion in Droplets and of Evaporation of Volatile Components into Vacuum
Annotation
A numerical investigation is performed of the dynamics of diffusion in droplets and of free-molecule evaporation into vacuum of volatile components of magnesium $(\text{Mg})$ and zinc $(\text{Zn})$ from aluminum alloys. The calculations are performed in application to the conditions of my experiments in high-frequency droplet formation under conditions of centrifugal dispersion in vacuum of rotating (overloads of $70$–$4000$) blanks by the melting method under the effect of a concentrated energy source. In view of analogy between mass transfer and heat transfer, available solutions are used in heat transfer under convection cooling of bodies with boundary conditions of the third kind. It is demonstrated that the numerical values of Biot diffusion criteria $(\text{Bi})$ are functions of the properties of alloy components and do not depend on the initial content of volatile components. At the droplet temperature $T = 1000$ K and radius $R = 0.25 \times 10^{-3}$ m, the Biot number is $\text{Bi} = 14.8$ for alloying systems $\text{Al}$–$\text{Mg}$ and $\text{Bi} = 74.4$ for $\text{Al}$–$\text{Zn}$. The solution demonstrates that the diffusion resistance within droplets causes a several-fold decrease (compared to the initial value) in the transfer of the mass of volatile component to the droplet surface (with a decrease in the flow of vapors) even for the given short time of droplet flight $\tau = 0.01$ s corresponding to the value of the Fourier criterion $\text{Fo} = D\tau/R^2 = 1.2 \times 10^{-3}$. The final depletion of the composition of droplets of magnesium, which is estimated relative to its permissible content in the alloy according to GOST (State Standard), is about $0.07\%$ for a D16 alloy with a wide range of variation of $\text{Mg}$ content from $1.2$ to $1.8\%$ by mass, which may be regarded as insignificant. For an AMg6 alloy with a smaller range of variation of Mg content from $5.8$ to $6.8\%$, the depletion is about $0.2\%$, which may be regarded as appreciable.
Article reference:
Ponomarev A.N. An Investigation of Diffusion in Droplets and of Evaporation of Volatile Components into Vacuum, High Temp., 2005. V. 43. № 6. P. 930