Heat transfer liquid-deficient

Film Boiling and Heat Transfer in Liquid-Deficient Regions 274... 

FILM BOILING AND HEAT TRANSFER IN LIQUID-DEFICIENT REGIONS... 

Era, A., G. P. Gaspari, A. Hassid, A. Milani, and R. Lavattarelli, 1966, Heat Transfer Data in the Liquid Deficient Region for Steam-Water Mixtures at 70 kg/cm2 Flowing in Tubular and Annular Conduits, Rep. CISE-R-184, Milan, Italy. (4)... 

Macbeth (M5) has recently written a detailed review on the subject of burn-out. The review contains a number of correlations for predicting the maximum heat flux before burn-out occurs. These correlations include a dependence upon the tube geometry, the fluid being heated, the liquid velocity, and numerous other properties, as well as the method of heating. Sil-vestri (S6) has reviewed the fluid mechanics and heat transfer of two-phase annular dispersed flows with particular emphasis on the critical heat flux that leads to burn-out. Silvestri has stated that phenomena responsible for burn-out, due to the formation of a vapor film between the wall and the liquid, are believed to be substantially different from phenomena causing burn-out due to the formation of dry spots that produce the liquid-deficient heat transfer region. It is known that the value of the liquid holdup at which dry spots first appear is dependent on the heat flux qmi. The correlations presented by Silvestri and Macbeth (S6, M5) can be used to estimate the burn-out conditions. 

The problem of burn-out prediction is a difficult one, and one on which a great deal of experimental work is being carried out, particularly in connection with nuclear-reactor development. Much of the earlier literature is rather confused, due to the fact that the mechanics of the burn-out were not carefully defined. Silvestri (S8) has discussed the definitions applicable to burn-out heat flux. It appears possible to define two distinctly different kinds of burn-out, one due to a transition from nucleate to film boiling, and one occurring at the liquid deficient point of the forced-convection region. The present discussion treats only the latter type of burn-out fluxes. The burn-out point in this instance is usually determined by the sudden rise in wall temperature and the corresponding drop in heat flux and heat-transfer coefficient which occur at high qualities. 

Urethanes are processed as rubber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The liquid prepolymers are cast using automatic metered casting machines, and the thermoplastic pellets are processed like all thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensile, and resistance to many oils is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

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