Experimental and analytical investigation of void fraction and heat transfer during evaporation in horizontal tubes

Summary :
A new optical void fraction measurement system has been coupled to the existing LTCM flow boiling test facility to obtain dynamic and time-averaged void fractions, simultaneously with measurements of the local heat transfer coefficients. A series of evaporation tests have been run for R-22 and R-410A in 13.84 mm and 8.00 mm ID tubes. Using our newly developed image processing system for processing laser illuminated cross-sectional views of the flow, about 310 000 images have been analysed in this study to provide the same number of dynamic void fraction measurements. From these images, 238 and 87 time-averaged void fraction values have been obtained for the 13.60 mm and 8.00 mm diameter glass tubes, respectively. The same number of time-averaged dry angles has been obtained.

The measured void fractions have been compared to the principal prediction models, showing good agreement to the Steiner version of Rouhani-Aelsson drift flux model. Based on analysis of the void fraction evolution as a function of time, several modifications of the Kattan et al. flow patern map [30] (1998a) have been made to improve the heat transfer prediction model in stratified-wavy flow and to extend its application to vapour qualities velocities from 70 to 700 kg/m2s and heat fluxes from 2.0 to 57.5 kW/m2.

Based on the 368 heat transfer points obtained in dryout and mist flow conditions, new boundaries for the transitions (from/to) annular / dryout and dryout / mist flow have been defined and integrated into the flow pattern map of Kattan et al. A new heat transfer model for dryout and mist flow conditions has also been proposed, extending the flow pattern oriented model of Kattan et al. to these flow regimes.