Heat transfer of nonazeotropic mixtures in a falling film evaporator
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
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.