Heat transfer degradation during vaporization of non-azeotropic mixtures

Models to predict the heat transfer coefficient during flow boiling of pure fluids can be inaccurate when applied to non-azeotropic mixtures. Nevertheless, flow boiling of mixtures inside channels can be interesting in many applications, such as evaporators of air-conditioning systems, evaporators of ORC plants and many other industrial cases. Recently a strong reason for studying vaporization of mixtures comes from the replacement of greenhouse gases that are used in inverse cycle machines. The 28th Meeting of the Parties called for a reduction of hydroflourocarbons refrigerants in the next years. Some of the fluids that may be used as alternatives are zeotropic mixtures. The adoption of zeotropic mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures. The additional mass transfer resistance in this case leads to a degradation of the heat transfer performance. In the present paper, a flow boiling database encompassing non-azeotropic blends, such as R32/R1234ze(E) mixtures and the ternary mixture R455A, at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The corrections needed to account for the additional mass transfer resistance are discussed.