This work investigates the steam condensation phenomena in an air-cooled condenser. The considered horizontal flattened tube has a 30 mm hydraulic diameter, and its length is a function of the steam quality with a limit value between 0.95 and 0.05. The mass flow rate ranges from 4 to 40 kg/m 2 .s with a saturated temperature spanning an interval from 40°C to 80°C. A special approach has been implemented using the Engineering Equation Solver (EES) to solve a series of equations for the two-phase flow pattern and the related heat transfer coefficients. A wavy-stratified structure of the two-phase flow has been found when the mass rate was between 4 and 24 kg/m 2 .s. In contrast, an initially annular flow is gradually converted into a wavy stratified flow (due to the condensation process taking place inside the flattened tube) when the considered range ranges from 32 to 40 kg/m 2 .s.
KEYWORDSCondensation; flow pattern structure; heat transfer rate; flow in horizontal pipe; flow behaviour; EES modelling Nomenclature A Tube cross-section area, [m 2 ] A l Liquid cross-section area, A l = A (1-ε), [m 2 ] A ld Dimensionless cross-sectional area occupied by liquid, A ld = A l /d 2 A V Vapor cross-sectional area, Av = A ε, [m 2 ] A vd Dimensionless cross-sectional area occupied by vapor, A vd = A v /d 2 c p Specific heat, [J/(kg.K)] D h Hydraulic diameter, [m] Fr l Liquid Froude number, G 2 /(q 2 l gD h ) g Acceleration of gravity, [m/s 2 ] G Total mass flow rate (mass velocity) of liquid and vapor [kg/(m 2 .s)] G strat Transition mass velocity of stratified flow [kg/(m 2 .s)] G wavy Transition mass velocity of wavy flow [kg/(m 2 .s)] h Heat transfer coefficient [W/(m 2 .K)]