| OCR Text |
Show condensation alone does not suffice to balance the heat exchange. This occurrence, which favors the formation of a thin surface ice layer, can take place only when Q+ B < 0, that is, when the surface loses heat by radiation or to a cold underlayer. 5. The Influence of Air Pressure and Wind Flow on Ablation The numerical calculations leading to Fig. 2 were carried out for p = 760 torr. Required magnitudes for other p- values can easily be determined with the formulae. These are given in Fig. 3 for p = 525 torr, corresponding approximately to 3000 m altitude. It is seen that in this case the boundary line between domains of melting and non- melting surfaces makes a smaller angle with the abscissa than in Fig. 2. Because the point a^+ CQ^^'/^ L = o° C, e, = 4.58 torr, common to all domains, is independent of p, the dividing line intersects the abscissa at 15.3° C. The lower the air pressure is, the higher can the air temperature be and evaporation still take place. Ablation by evaporation can also be larger at lower pressure, as shown in Fig. 3 by the curve for - U/ iX^ = 0.01 mm h~ 1/ / m eal cm" 9z min~ l deg" l ( absent in Fig. 2) which intersects the lower right part of the domain for a non- melting surface. The lines with constant values of - M/ 0L, make a smaller angle with the abscissa in Fig. 3 than in Fig. 2, just as does the boundary line between melting and non- melting surfaces. An increase in - M effected by growth of n/" or Q+ B is, as shown by Equation ( 15), also independent of air pressure. Thus it can be said that while individually the quantities are more or less dependent on p, the overall picture is influenced relatively little by p. By using ( Q+ B)/£., and M/ dL in place of Q+ B and M in Figs. 1 to 3, it was possible to depict the overall relationship in a single diagram. The influence of 0L. on M is clearly included in the figures, but it is difficult to recognize, for d. enters in the expression ( Q+ B)/ 5CL as well as in M/ a" L . The heat transfer coefficient ' i, naturally plans a significant role in the ablation, - M, itself. |
