  In this paper, we present a theoretical study aimed at investigating the
rate-limiting factors in thin-film evaporative heat transfer processes,
considering the finite-rate evaporation kinetics. The problems of evaporation
of a flat thin-film in either pure vapours or vapour-inert-gas mixtures are
analysed based on the non-dimensionalised macroscopic transport equations for
continuum fluids, coupled with out-of-equilibrium kinetic boundary conditions.
Both the full numerical solutions and asymptotic analytical solutions at slow
evaporation limit are provided and applied to analyse thin water film
evaporation. Existing solutions, assuming negligible heat transfer in the gas
domain, or negligible temperature jump across the non-equilibrium kinetic
layer, or more boldly a thermodynamically equilibrial interface (i.e. its
temperature is at the saturation temperature), can be fully recovered from the
more general solutions presented here. Our results show that while these
assumptions hold in special cases, they can lead to significant errors in many
conditions, especially when the film thickness $\delta$ is reduced to a few
micrometers or thinner. We show that the conventional views that the
rate-limiting factors in thin-film evaporative heat transfer is either the heat
diffusion through the liquid film or the mass transfer in the gas domain only
apply to thick film. As $\delta$ decreases to a few micrometer or smaller, the
interfacial thermal resistance due to the evaporation kinetics can be on the
same orders of magnitude as the thermal resistance in the liquid film. The
analysis also allows us to compare the heat transfer processes during the
evaporation of a thin-film in pure vapours to those in inert gases, providing
deeper insight into the effectiveness of various strategies for exploring the
evaporation process in practical thermal management.
