Experimental investigations on single-phase convection and steam-water two-phase flow boiling in a vertical rod bundle

Single-phase convective heat transfer and steam-water two-phase flow boiling heat transfer experiments have been carried out with a vertical seven-rod bundle at low flow conditions near atmospheric pressure. With respect to single-phase experimental results, new correlations were developed to predict convective heat transfer coefficients of laminar flow and Reynolds numbers at the transition from the laminar region to the turbulent region. Two-phase experimental results show that the rod bundle geometry enhanced heat transfer compared with circular tubes. An obvious mass velocity effect on the flow boiling heat transfer coefficient was observed. This conclusion indicates that forced convection, instead of nucleate boiling heat transfer, dominated two-phase flow boiling heat transfer under the experimental conditions. In addition, two-phase flow boiling heat transfer coefficients in the vertical rod bundle increased first and then decreased with the quality increases for a given mass velocity. By taking the enhancement factor of forced convection and the suppression factor of nucleate boiling into account, a new correlation in terms of Boiling number, Reynolds number and Martinelli number was developed to be successful in predicting predict two-phase flow boiling heat transfer coefficients.