Heat transfer of nonazeotropic mixtures in a falling film evaporator

Other conference contribution, 1985

The evaporative heat transfer coefficient for nonazeotropic mixtures of R12 and R114 has been studied in a falling film heat exchanger. The main aim with the project has been to study how the heat transfer coefficient is influenced by the additional mass transfer resistance that is introduced when a mixture is used instead of a pure refrigerant. The test apparatus consisted of one tube, which was heated from the outside and with the refrigerant forming a falling film on the inside. The heating was done by a liquid flowing in an annular space countercurrently to the falling film. From the results it can be concluded that the heat transfer coefficients for the mixtures at surface evaporation conditions are between the corresponding ones for the pure refrigerants at given Re numbers. This indicates that the additional mass transfer resistance when a mixture is used is small in a falling film at such conditions. This is probably due to the small film thickness. The heat transfer coefficients versus the heat flux were also measured. Again, the values for the mixtures at a given'heat flux fall between the corresponding for the two pure refrigerants, both at surface evaporation and nucleate boiling conditions.