Condensation heat transfer of R245fa in a shell-tube heat exchanger at slightly inclined angles

Condensation heat transfer of R245fa was investigated in a shell-tube condenser. The copper tube had an inner diameter of 14.70 mm with a 1600 mm heat transfer length. Mass fluxes and vapor mass qualities covered ranges of 198.8-504.7 kg/m2s and 0.291-0.976, respectively. Condensation heat transfer coefficients are increased with increases of mass fluxes and vapor mass qualities. The horizontal position yielded minimum condensation heat transfer coefficients. Deviating from the horizontal position enhances condensation heat transfer. The condenser is suggested to operate at weakly inclined flow to reach better thermal performance. Flow patterns and liquid height signals explored the condensation heat transfer mechanisms. The flow is stable for inclined down-flow but becomes unstable for inclined up-flow, which was caused by the vapor-liquid interface wave. Non-dimensional parameter analysis identified the competition between inertia force and gravity force. The vapor-liquid interface wave is the mechanism to enhance condensation heat transfer for inclined up-flow, while the decreased liquid film thickness on the tube bottom enhances heat transfer enhancement for inclined down-flow. A correlation based on the Froude number of vapor phase successfully matched the experimental data. This study is useful for the condenser design and operation such as applied in Organic Rankine Cycles.