Simulation of Flow Boiling Transition Patterns by Using GENTOP Concept and a Mechanistic Bubble Dynamics Model

Flow boiling occurs when a subcooled liquid enters a vertical heating pipe and its temperature near the heating wall exceeds the boiling onset temperature. Bubbles are generated on the heating wall. Further, along the pipe the bulk fluid temperature increases and coalescence and evaporation create large bubbles which results in two-phase flow patterns formation including bubbly, slug and annular flows.

Flow patterns transition in flow boiling is simulated by using the recently developed concept of GEneralized-TwO Phase (GENTOP) and a developed bubble dynamics model. The GENTOP concept is an extension of the inhomogeneous MUltiple SIze Group (iMUSIG) by adding a continuous gas phase in the multi-field Euler-Euler (E-E) approach. Transitions between the fields result from coalescence and breakup of the gas bubbles.

A recently developed mechanistic bubble dynamics model, which is based on the balance of forces applied on a single bubble, is implemented in the GENTOP framework as a sub-model. This model also considers the evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble as well as the dynamic inclination and contact angles. In addition, it does not require a recalibration of parameters to predict the bubble dynamics and its implementation in the E-E framework needs an extension of the current nucleation site density and heat partitioning models. Finally, for a generic demonstration case, flow boiling morphological patterns in a vertical heated pipe is simulated by ANSYS CFX.