Inclination sensitivity of R32 condensation in smooth tubes: Heat transfer, film redistribution, and pressure gradient

Tube orientation is an important consideration in the design of compact heat exchangers because condensation performance in inclined tubes depends on the interplay of gravity, interfacial shear, and liquid-film redistribution. Although inclination effects on condensation heat transfer have been reported, few studies relate the gravity-to-shear transition to changes in film distribution, HTC, and pressure gradient for R32 condensation at intermediate orientations. The present study numerically examines the condensation of R32 at 11 inclination angles (β = −90° to +90°, where negative β represents upward flow and positive β represents downward flow) in a 5 mm smooth tube using the VOF method along with the SST k−ω turbulence model. Simulations are performed at a saturation temperature of 50 °C for mass fluxes of 200 and 400 kg/(m².s), and the modified Froude number (Fr so ) is used as a gravity–shear regime indicator to interpret the inclination sensitivity of the simulations. The inclination effect is most pronounced at low vapor qualities and mass fluxes: HTC decreases by about 37% from β = −90° to +90° at G = 200 kg/(m².s) and χ = 0.3. Increasing mass flux reduces HTC sensitivity to inclination and weakens film redistribution and pressure-gradient variation, though strong downward inclinations can still cause pressure recovery at low vapor quality. The results of the dimensionless Froude-number analysis indicate that inclination sensitivity decreases as mass flux and vapor quality increase, consistent with the the existing Dobson–Chato criterion (Fr so ≈ 20) for the transition from gravity-influenced to shear-dominated condensation.