EXPERIMENTAL RESEARCH ON THE SIMILARITY OF ANNULAR FLOW MODELS AND CORRELATIONS FOR THE CONDENSATION OF R134A AT HIGH MASS FLUX INSIDE VERTICAL AND HORIZONTAL TUBES

摘要

This paper presents an experimental investigation on the usage of annular flow models and correlations valid especially for horizontal tubes to the downward annular flow in the vertical test section. Condensation experiments are performed at the mass flux of 340 kg m~(-2)V~(-1) during co-current downward condensation of R 134a in a vertical smooth copper tube having inner diameter of 8.1 mm and a length of 500 mm. The saturation temperatures are between 40-50°C, heat fluxes are between 12.8 and 45.36 kW m~(-2), average qualities are ranging between 0.76-0.95. The experimental apparatus are designed to capable of changing the different operating parameters such as mass flow rate, condensation temperature of refrigerant, cooling water temperature and mass flow rate of cooling water etc and investigate their effect on heat transfer coefficients and pressure drops. Considering Chen et al.'s annular flow theory on the heat transfer coefficients that are independent from tube orientation as long as annular flow exists along the tube length, the average predicted condensation heat transfer coefficient of the refrigerant is determined by means of the annular flow model of Kosky and Staub, and Von Karman universal velocity distribution correlations using interfacial shear stress proposed for horizontal and vertical tubes separately. Some well-known annular flow correlations generally used for horizontal tubes in the literature were compared with experimental condensation heat transfer coefficient obtained from vertical tube data during annular flow conditions in the test section.