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    This paper reports on the rates of condensation heat transfer for weak ammonia–water mixtures in a horizontal,
    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
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