Ill SUMMARY The variation of the heat transfer coefficients in a falling film evaporator at which evaporation takes place from the surface of the liquid films flowing down the inside surface of the vertical tube was investigated in this study. The experimental data were compared with the existing heat transfer data in the literature. As test fluids, distilled water and sodium hydroxide solutions at different concentrations (10% and 20%) were used. Test section which has a length of 500 mm. was heated by means of high current transformatör which supplies an alternative current of 275 A. Surface temperatures were measured at 8 different locations by means of `NiCr -Ni ` tliermocouples. hi the experiments performed by water and 10% sodium hydroxide solution the flow regime was turbulent and in experiments performed by 20% sodium hydroxide solution flow regime was laminar. The Reynolds Number ranged from 1 800 to 10 300, PrandÖ Number ranged from 1.76 to 4.625 and the heat flux applied to the tube wall was at the range of 12 255 to 12.690 W/m2. From the results it was seen that the thermal development length persisted up more than one half of the heated length for all the liquids tested, heat transfer coefficients were compared to each other and new correlations were developed. According to this comparison it was determined that for water and 10% sodium hydroxide solution the following correlation was used : h* = 0.003 Re 038Pr092 This is very close to the correlation that was recommended by Mudawwar. For the experiments performed by 20% NaOH which was in laminar regime, developed correlation was close to that was obtained by Chun and Seban and it was found to be, h*= 0.66 (Re/4)-023

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