DE-2534 -- An Experimental Investigation of Dry-Out with R-22 Evaporators PDF

An experimental investigation was made of the heat transfer characteristics in the region of transition to dry-wall for the evaporation of R-22 in a horizontal tube.

Three 10 ft lengths of 3/8-in. O.D. stainless tubing were heated by passing an electric current. The onset of dry-out was made to occur in the beginning of the third tube. In general, the heat flux in this tube was set lower than that in the preceding sections, to more nearly simulate the heating conditions of a fluid-to-fluid evaporator. Mass velocities from 116,000 to 616,000 lb/hr-ft2 and heat fluxes from 2000 to 10,000 Btu/hr-ft2 were investigated. The dry-out occurred at vapor mass fractions, xcr between 0.89 to 0.99. It started at the top of the tube, continued down the sides and finally to the bottom. The onset of transition was usually marked by a rapid and relatively small increase in local heat transfer coefficients, followed immediately by an order of magnitude decrease. The experiments showed very large fluctuations of local coefficients during transition, and to a small extent approaching it.

In the transition region, the ratio of local coefficient of heat transfer to that for pure gas flow was found to be a function of mass velocity and n defined as (x-xcr)/(l-xcr), and was independent of the heat flux.