Heat Transfer 1970/6. [antikvár]

A PECULIAR TYPE OP BURNOUT IN EVAPORATIVE TUBES V.E.Doroshchuk, F.P.Lantsman, L.L.Levitaxi All-Union Heat Engineering Institute Moscow, USSR Abstract This paper discusses physical model burnout caused by drying out of wall liquid film. Investigation of conditions of drop falling out of dispersed steam-water-flow core on liquid film in the heated tube has been carried out. Experimental data on boundary steam quality in horizontal tubes and annulus are given. There are two types of burnout occuring in evaporative tubes. One of them (the burnout of first type) is due to conversion of nuclear boiling into film boiling. This type of burnout is caused by a high specific heat flux (q), which is usually called burnout heat flux and denoted as qb0* The burnout of second type we are interested in, may take place only at considerable steam-qualities and depends on hydrodynamics of the two-phase flow. It does not directly depend on "q" /V.% That is why the meaning of "burnout heat flux" has lost it*s sense in this case. For better understanding of the essence of the second type burnout one should consider some phenomena of annulus flow structure. Such a flow is characterized by steam current in the central part of tube and liquid current in the periphery of the cannel in the form of sufficiently thick wall liquid film. The film surface is disturbed by waves. Liquid drops are entrained from wave peaks by steam core. On the other hand, liquid drops fall out of the flow core on the wall of tube due to turbulence diffusion and are captured by a liquid film. As a result dynamic equilibrium between the drops entrained from the film and deposited on the film takes place. With the growth of steam-qualities and, consequently, linear steam velocity, more water comes into the flow core and wall liquid film becomes thinner. At some value of the film becomes so thin, that disturbance waves on its surface disappear and entrainment of drops stops. Conversion to smooth film may be checked experimentally by a sharp decrease in hydraulic resistance of the cannel ("crisis of hydraulic resistance") mentioned in a number of Russian investigations. B 6.1