ISSN:
0001-1541
Schlagwort(e):
Chemistry
;
Chemical Engineering
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Werkstoffwissenschaften, Fertigungsverfahren, Fertigung
Notizen:
New data on the dendritic growth and microstructure of ice crystals in quiescent pure water are reported for small subcoolings of 0.035 K 〈ΔT〈1.000 K, where thermal or natural convection is prevalent. Accurate and systematic measurements of the growth velocity VG and the tip radii of the edge and basal planes R1 and R2 were made as functions of time.The central point of this work is that with the harmonic mean of the tip radii Rm as the lengthscale, the intensity of natural convection can be correlated accurately by using the thermal convection analogy, Gr = Re2. On this basis, natural convection has a crucial effect on dendritic growth of ice at ΔT〈0.35 K, the region of subcooling in which the tip of the dendrite splits consistently.The experiments show that the morphological stability parameter C* is independent of subcooling and equals 0.075, when the lengthscale is Rm. With the observed values of R1 and R2, the aspect ratio is 28, and the growth velocity for small ΔT is significantly higher than that predicted by the conduction theory of Horvay and Cahn (1961). Thus, the effect of convection on the growth of ice crystals is more important as the subcooling decreases.Moving boundary solutions of the three-dimensional Navier-Stokes and energy equations for the dendritic growth of an elliptical paraboloid were obtained here with the Stokes flow approximation. Experimental observations of the quantities, VG, R1, and R2, agree well with predictions of this theory when Gr=Rem2 is based on Rm. In contrast, if convection is neglected in the theory, it does not agree with the experiments and the difference increases significantly as the subcooling is decreased.
Zusätzliches Material:
17 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/aic.690380615
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