The pool boiling curve is considerably altered when the fluid being evaporated is a binary mixture rather than a pure single-component liquid. The principal changes are shown diagramatically in Figure 1. First, the onset of boiling is delayed to higher wall superheats as a result of the temperature gradients set up in the pool to accommodate the corresponding gradients in liquid composition. Heat transfer coefficients in the nucleate boiling region are sharply reduced. The critical heat flux may be increased or reduced depending on the extent of the contribution from convection in the pool. The minimum heat flux and the corresponding wall superheat are increased. Finally, heat transfer rates in the film boiling region are also somewhat higher.

A. Nucleate boiling

Even small amounts of a second component cause considerable reductions in the heat transfer rate under nucleate pool boiling conditions compared with that measured for the pure liquid. The reason for this reduction can be traced back to the influence of the second component on the bubble growth rate. The minimum bubble growth rate, the minimum heat transfer coefficient, and the occurrence of a maximum critical heat flux all occur at the same liquid composition corresponding to a maximum value of |ỹ - x̃|. Starting with the early work of Bonilla and Perry (1941) and Cichelli and Bonilla (1944), many experimental studies of pool boiling of binary mixtures have been published. A good deal of useful experimental data for nucleate pool boiling of binary liquids have been presented by Sternling and Tichacek (1961). They used 14 binary systems with components ranging from water to light alcohols to heavy oils. All the mixtures had a wide boiling range, at least 90 °C. For all the systems the heat transfer coefficient for a given heat flux was less than would be expected for an "ideal" single-component fluid with the same physical properties. Extensive reviews on multicomponent pool boiling are given by Shock (1982) and Collier and Thome (1994). Here, the effects are illustrated by citing a few examples. It is helpful to define a heat transfer coefficient for nucleate pool boiling of a binary mixture as follows:

\[\label{eq1} \alpha=\dfrac{\dot{q}}{(T_{\rm{w}}-T_{\rm{bub}})}\tag{1}\]

where q̇ is the heal flux, T_w the wall temperature and T_bub the bubble point temperature. For a binary mixture, one may define an "ideal" heat transfer coefficient α_id as follows: