Numerical study of patterns and influencing factors on flow boiling in vertical tubes by thermal LBM simulation

Heat transfer and flow pattern of flow boiling in vertical tube are investigated numerically based on the phase change Lattice Boltzmann method (LBM) which includes an improved pseudo -potential LB model and a thermal LB model. Two-dimensional numerical simulations are carried out under constant heat flux conditions for the first time. The processes of growth, slippage, detachment and coalescence of the bubbles are captured to verify the correctness of the model. The effects of gravity, contact angle and wall superheat on bubble departure diameter and nucleation waiting time are illustrated. The multiple flow patterns, single-phase flow, bubble flow, slug flow and DNB have been illustrated with the behaviors of bubble nucleation, growth, departure, and coalescence. Some basic features of flow boiling have been clearly observed in the simulation. The influences of several factors such as heat flux, Reynolds number, the width of flow channel, and the width of nucleation point on flow boiling especially on the point of DNB are investigated. The numerical results show that the DNB could be avoided by reducing the heating density, increasing the Reynolds number, increasing the width of the tube and reducing the heating concentration.