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This paper reports on the rates of condensation heat transfer for weak ammonia–water mixtures in a horizontal,
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shell and tube condenser. It is shown that for inlet ammonia concentrations in the range 0.2–0.9 wt.% the average
condensation heat transfer for the condenser was enhanced by up to 14%. Furthermore, local enhancement of the
condensation heat transfer of up to 34% occurred at local bulk vapour concentrations between 0.2 and 2 wt.%
ammonia. This enhancement was caused by the Marangoni effect which produced a disturbed, turbulent banded
condensate film and a corresponding drop in the thermal resistance of the condensate film.9324
2004 Elsevier Ltd. All rights reserved.1. Introduction
The general conclusion of previous binary mixture
condensation studies has been that binary mixture con-
densation occurs at a lower heat transfer rate than pure
vapour condensation. This lower heat transfer rate has
been attributed to the development of a mass diffusion
layer between the bulk vapour and the surface of the
condensate film that acts as an additional resistance to
heat transfer [1]. Although this deleterious effect can be
reduced by high thermal gradients, large vapour veloc-
ities and finned tubes [2], the reduced heat transfer
associated with binary mixture condensation has, until
recently, been accepted as largely unavoidable.
It is notable, however, that this conclusion was
reinforced by numerical studies where the condensate
film was assumed to be smooth and laminar. While such
a condition readily facilitates modelling of the binary
mixture condensation problem, it ignores the potential
of binary mixtures to exhibit condensation behaviour
not possible with pure vapours. In particular, the con-
densation of binary mixtures can be significantly influ-
enced by the Marangoni effect. This effect describes the
influence of surface tension gradients, which can develop
in liquid mixtures from local perturbations in concen-
tration and temperature.
The Marangoni effect has been observed in several
heat and mass transfer processes such as distillation and
condensation [3–6]. In condensation the commonly ac-
cepted criteria used to determine the stability of the
condensate film to surface tension driven effects was
developed by Ford and Missen [7] who concluded that
the condition for instability could be expressed by Eq.
(1). The criterion states that if the change in surface
tension with respect to film thickness is positive, the film
will tend to be unstable. This conclusion is self evident
when a small area of disturbed film is considered. If a
region of this film that has the greatest depth also has
the highest surface tension, then condensate will be
drawn from adjacent thin film regions with lower surface
tension and the original disturbance to the film will be
reinforced. It was also shown that the inequalities ex-
pressed in Eq. (1) could be evaluated as a product of two
terms derived from the properties of the mixture and the
nature of the process. Two forms of this relation are
given in Eqs. (2a) and (2b).
*
Corresponding author. Tel.: +64-9-373-7599; fax: +64-9-
373-7479.
E-mail address: j.deans@auckland.ac.nz (J. Deans).
0017-9310/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
and if the mixture surface tension does not exhibit some
minimum or maximum at an intermediate concentra-
tion, then the condensate film will tend to become
unstable. The presence of such a maximum or minimum
would serve to complicate the description of the system
because they imply the possibility of both positive and
negative behaviour in the same system. The perturbation