A visualization comparison of convective flow boiling heat transfer augmentation devices

Abstract

The qualitative effects of inset-table heat transfer phics. augmentation devices on vertical in-tube convective flow boiling flow regimes, transition mechanisms, and heat transfer are presented in this study. Three twisted tapes with twist ratios of 9.50, 7.2 1, and 3.72, a static mixer, and a circumferentailly wrapped wire screen were compared with bare tube data to ascertain the qualitative effects of these devices. Visualization experiments using high resolution video techniques were conducted in a transparent 58.4 cm long counterflow test section with water as the heating fluid and ethanol as the test 2 d heat fluxes from 0.8 to 12.3 kW/m2 were fluid. Mass fluxes from 10 to 100 kg/m s an examined for pressures ranging from 80 to 135 kl-a. Flow regime definitions and commentary are presented regarding contemporary flow regime transition models. Additionally, an empirical flow regime map by Fair (1960) and theoretical flow regime maps by Taitel et at. (1980) and Mcouillan and Whalley (1985) are compared with the experimental data. The Fair (1960) map is found to be inadequate in predicting the experimental data. Both the Taitel et al. (1980) and Mcouillan and Whalley (1985) fail to accurately predict the churn to annular transition. Average saturated boiling heat transfer coefficients are presented for all but the static mixer data. For these data, the second twisted tape (twist ratio of 7.21) exhibits the highest heat transfer coefficients. The Chen (1966) flow boiling correlation predicted 63 percent of the heat transfer data.