Effect of inclination on pressure drop and flow regimes in large flattened-tube steam condensers

This paper presents an experimental study of the inclination effect on pressure drop and flow regime during condensation of steam in a large flattened tube used in air-cooled condensers (ACC) for power plants. Steam with mass flux of about 7 kg m⁻² s⁻¹ was condensed inside a 10.7 m long, flattened test tube with inclination angle varied from horizontal up to 70°. The original full-sized steel tube was cut in half along the centerline, and the removed part was replaced by a polycarbonate window to enable simultaneous flow visualization in situ with heat transfer and pressure drop measurements. A uniform velocity profile of 2.03 ± 0.12 m s⁻¹ was imposed on the air side to extract heat from the steam in a cross flow direction. The experimental results showed that increasing the inclination angle led to reductions of pressure drop due to the improvement in the gravity-assisted drainage of condensate inside the test tube. At such low mass fluxes, tube inclination significantly influenced the flow pattern which was observed to be a well-separated stratified flow throughout the tube at all downward inclination angles. The separated flow pattern enabled the direct measurement of void fraction, and the traditional void fraction models using the newly-defined superficial quality successfully predicted the measurements within ±10%. The experimental data were converted to reflect pressure drop in a full tube based on the model that was developed to account for the differences in tube geometry between the full and test tube at the same operating condition. A prediction of pressure drop performance of the same steam condensing system under vacuum condition was also discussed. The negative dependence of total pressure drop on inclination angle also prevailed in both converted results in atmospheric condition and the predicted ones in vacuum condition.