To develop an appropriate falling film evaporation device for organic fluid cogeneration,a numerical study on the gas-liquid two-phase counter-current flow characteristics of R113 inside a vertical tube under different struc-tural conditions was conducted using the Fluent software.The effects of the tube length,tube diameter,and an-nular gap on the falling film flow characteristics were determined,respectively.The results indicated that under a certain spray density,the falling film thickness in the region of the steady section was almost constant with dif-ferent structural parameters for the tube diameter,tube length,and annular gap.In addition,a smaller tube di-ameter resulted in a steadier film flow.When the tube diameter decreased to a specific value,the film thickness showed a uniform distribution along the wall surface.This indicated that a best falling tube diameter exists.Meanwhile,the film fluctuation was enhanced with an increase in the tube length.When the tube length was greater than 1.2 m,the falling film splashed and could not completely wet the wall surface.The film fluctu-ation was enhanced by decreasing the annular gap,and the film could not be formed when the annular gap was smaller than 1.2 mm.