Froude number dominates condensation heat transfer of R245fa in tubes: Effect of inclination angles

Condensation heat transfer of R245fa in a tube with the diameter of 14.81 mm and length of 1200 mm was investigated. The R245fa mass fluxes, inlet vapor mass qualities and inclination angles covered the ranges of 191.3-705.4 kg/m2 s, 0.191-0.947 and −90° to 90°, respectively. The non-dimensional analysis yielded the importance of inertia force and gravity force to influence condensation heat transfer. Surface tension force played unimportant role. Condensation heat transfer coefficients (h) showed the general increase trend versus inclination angles (θ) and the non-monotonic behavior in the full inclination angle range. The maximum h occurred at θ = 30° and −15°. The h values were sensitive to θ near the horizontal positions. The main flow patterns in the condenser tube were stratified-smooth flow, stratified-wavy flow and intermittent flow. For inclined upflow, the liquid layer thickness and interface wave competed with each other to influence heat transfer. An optimal inclination angle existed at which heat transfer coefficients reached maximum. For inclined downflow, condensation heat transfer was balanced by the liquid layer thickness and buoyancy force. The flow was more stable. Heat transfer coefficients were well correlated with the general increase term multiplied by the non-monotonic variation term. The parameters in the correlation depended on the Froude number and vapor mass qualities. The correlation successfully explained the effects of mass fluxes, vapor mass qualities and inclination angles on condensation heat transfer coefficients. Based on the present finding, the condenser is recommended to operate at the inclination angle of −15° or 30°, at which heat transfer coefficients are maximum and pressure drops are minimum. The exactly horizontal flow weakens the condenser performance.