An experimental investigation was performed on Marangoni effect of cracked liquid film of aqueousNa 2 SO 4 flowing over a vertical heated plate by using a sensitive infrared imaging technique. The results show that the thermal and solutal Marangoni effects, which result from the non-uniform distributions of surface temperature and concentration of the film, respectively, occur in the streamwise and transverse directions of the film, generating different influences on the film heat transfer. Taking account of the Marangoni number (Ma) and the solution concentration (c 0 ), a correlation of the Nusselt number (Nu) for the cracked liquid film is proposed. aqueous electrolyte, Na 2 SO 4 , Marangoni effect, cracked falling film, infrared imaging technique, heat transferHeat and mass transfers of falling film of aqueous electrolyte solution are popular processes in industrial production. During falling film apparatus runs, the liquid film breakdown is frequent due to the intense fluctuations of its operation conditions, remarkably reducing the efficiencies of the heat and mass transfers, and affecting the normal run of the apparatus [1,2] . At present, the information on the flow characteristics of falling film of aqueous electrolyte is still inadequate, causing the imprecision on the design and operation of falling film apparatus. Therefore, for enhancing the heat and mass transfers of falling film and optimising the design and operation of falling film apparatus, it is of practical significance to specifically understand the flow characteristics of cracked falling film of aqueous electrolyte and their influences on the heat and mass transfers of the film.In the work on falling film of aqueous electrolyte, most of researchers tried to enhance the heat transfer of falling film evaporation by adding different surfactants to create Marangoni effect [3][4][5] or changing the surface structure of falling film wall [6,7] . Whereas the work on the Marangoni effect for the aqueous electrolyte itself in heat and mass transfer processes is scarcely reported.Podgorski [8] and Holland [9] theoretically studied the variation of dry patch formed following the falling film breakdown. In the present work, aqueous Na 2 SO 4 solution, a common electrolyte solution in industrial production, was selected as the working liquid. Using a sensitive infrared imaging technique, a series of experiments were performed for the cracked liquid film of the solution falling down a vertical heated plate to determine the influences of Marangoni effect on the flow and heat transfer of the film. Moreover, the correlation of the intensity of heat transfer of the falling film (Nusselt number (Nu)) was obtained and expected to be used in the design of falling film apparatus.
