A novel visualization method was developed and implemented in a refrigerant-flooded evaporator tube bundle. Specifically, a method to visualize the two-phase flow inside plain tube bundles was developed using a dummy tube with a tubular glass section for visual access with a digital high-speed video camera and a laser light/photodiode. These were used to observe and characterize the two-phase flow structure and improve understanding of the physical phenomena taking place. In particular, a dryout line was objectively identified in a flow pattern map separating high-and low-performance heat transfer data measured locally on the bundle. The database was recorded for vertical flow of R134a and R236fa over smooth tubes at saturation temperatures of 5-15 degrees C. The tests span low mass velocities applicable in flooded evaporators, and heat fluxes include adiabatic and diabatic tests of 7-25 kW/m(2). A new prediction method for onset of dryout on smooth tube bundles is proposed.