Film dryout behavior in obstructed annular two-phase flow

• Cylindrical obstructions generate parabolic dry patches far downstream in annular thin films. • The immediate wake of an obstruction has a thinner film without large disturbance waves. • Dry patches reduce the local heat transfer coefficient but do not result in thermal runaway. Flow boiling is utilized in many energy systems, including vapor compression and nuclear power cycles. Although there has been significant past work on boiling heat transfer and dryout characteristics in smooth tubes, many systems have obstructions on the heated surface that alter the flow. This study investigates the effect of obstructions under annular two-phase flow conditions flowing through a vertical, upward channel. A refrigerant with similar properties to high-pressure steam, R-245fa, is used as the working fluid. The flow around a cylindrical obstruction, tested with two different diameters, is characterized using high-speed video, temperature measurements, and optically based liquid film thickness measurements. The experiments investigate the flow phenomena over a range of operating conditions: mass fluxes of 130–170 kg/m 2 -s, inlet qualities of 0.63–0.95, and heat fluxes of 0–29 kW/m 2 . Unlike prior literature focusing on relatively high mass fluxes or obstructions separated from the heated surface - which have shown enhanced heat transfer coefficients downstream and delayed critical heat flux - this work examines an obstruction in direct contact with the heated wall to relate the movement of the liquid film to heat transfer performance. The results indicate that the obstruction induces sustained film dryout in its wake without a severe reduction in heat transfer coefficient at heat flux values less than that of unobstructed flows. This behavior is attributed to reduced wave activity and thinner liquid films compared to unobstructed flows at similar conditions.