Visualization study on nucleate pool boiling of ethane, isobutane and their binary mixtures

Visualization experiments were carried out to investigate the nucleate pool boiling heat transfer characteristics of ethane, isobutane and their binary mixtures on a horizontal flat surface. Extensive experimental heat transfer coefficient data were obtained under various pressures of 0.1 MPa, 0.3 MPa and 0.5 MPa for pure substances and 0.3 MPa for mixtures. The measurements were performed under heat fluxes from 20 kW/m2 to 150 kW/m2 at a descending heat flux procedure. For binary mixtures, the pool boiling data were measured covering the whole concentration range. Obvious heat transfer coefficient reduction for mixtures was found. Various empirical correlations were screened out for comparison. The accuracy for those correlations varied considerably with mixture concentrations and measurement conditions. The bubble departure diameter and frequency for pure substances and mixtures were also carefully observed. Increasing heat flux can obviously increase the bubble departure diameter and frequency for both pure refrigerants and mixtures. But the mixture effect shows non-linear influence on the bubble size and departure frequency, which varies distinctly with mixture concentrations.