New methodology for modeling pressure drop and thermal hydraulic characteristics in long vertical boiler tubes at high pressure

Flow boiling in long vertical tubes in evaporators or boilers is the combination between hydrodynamics and boiling heat transfer processes. Precise modeling of these flow boiling processes is necessary to design evaporator/boiler and to avoid complexities such as flow instabilities, critical heat flux etc. Present study involves development of new methodology to model full range flow boiling phenomenon which covers all flow boiling regimes in long vertical tubes at high pressure. Computational fluid dynamics (CFD) simulations are performed on tubes of lengths 7 m, 13.4 m and 23 in at pressure range of 86 to 172 bar. The entire tube has been divided into four sections based on the value of the vapor fraction at the end of the section and different modeling strategies are applied to each section. Eulerian-Eulerian two fluid model along with appropriate wall boiling models and phase interaction models are used to model different sections. It has been found that the developed methodology shows more accuracy in predicting the pressure drop in the boiler tube (1-9 % error) than single section approach. For 23 m long tube full range flow boiling simulations, a reduction in two phase flow boiling heat transfer coefficient is observed due to the presence of instability at 20 % design heat duty. Axial distribution profiles of vapor fraction, heat flux, heat transfer coefficient, water/steam temperature have been plotted to get insights of flow boiling phenomenon occurring in 23 in long evaporator tube. Length of the single phase liquid region reduces with increase in design heat duty. In addition, shifting of length from nucleate to film boiling with respect to change in magnitude of flow rate was observed. Comparison of predictions obtained by the CFD model (with developed simulation strategy) have been done with one dimensional correlations/models. CFD approach shows advancement over one dimensional (1-D) approach in prediction of thermal hydraulic characteristics. (author)