Cross flow and pool boiling of ethanol-water mixtures on the outside of horizontal enhanced tubes

The influence of two-phase cross flow on shell-side boiling of ethanol-water mixtures on recently developed Gewa-B tubes(1) has been experimentally investigated. A series of single phase forced convection, pool boiling and cross flow boiling experiments with pure water, pure ethanol and several mixtures of the two is presented. In pool boiling of the pure components Gewa-B heat transfer characteristics differ considerably from common smooth and finned surfaces. The Gewa-B surface outperforms the smooth tube pool boiling heat transfer up to factors of 11, if compared at the same heat duty per tube length. Mixture pool boiling enhancement factors are less, but not lower than 5. Although in the literature it is often proposed that boiling of high performance surfaces is not significantly influenced by cross flow, we find heat transfer improvement up to 60% in the case of the Gewa-B surface. When compared to a smooth tube, Gewa-B cross flow boiling heat transfer augmentation is significantly lower than in pool boiling, but still ranging from 2.5 to 7. This is because heat transfer augmentation due to cross flow is more efficient with the smooth surface than with the Gewa-B surface. Possible mechanisms of heat transfer augmentation are discussed.