Trace with dynamic model for interfacial area concentration prediction

The present study implements the one-dimensional interfacial area transport equation into the TRACE being developed by the U.S. NRC This transport equation replaces the conventional flow regime dependent correlations and regime transition criteria for furnishing the interfacial area concentration in the two-fluid model by mechanistically modeling bubble coalescence and disintegration. This dynamic method eliminates potential artificial bifurcations or numerical oscillations stemming from the use of conventional static correlations. To implement the interfacial area transport equation, a three-field version of TRACE that can track a dispersed gas field is utilized. For simplicity, only adiabatic two-phase flow conditions are considered, and the interfacial area sources or sinks due to phase change phenomena are ignored. Data obtained from two vertical co-current upward air-water experiments performed in round pipes (25.4 mm and 48.3 mm) are used to help evaluate the implementation. In the experiments, local values of void fraction, interfacial area concentration (a_i), and bubble velocity measured with a multi-sensor conductivity probe are area-averaged to generate one-dimensional transport data. Results obtained from TRACE with and without the one-group interfacial area transport equation are compared to demonstrate the enhancement in prediction accuracy. It is found that predictions made by TRACE without interfacial area transport tend to underestimate a_i,-. Moreover, it is found that while TRACE without interfacial area transport generally does well at predicting the axial trend of a_i, it cannot predict non-linear axial increases found in several of the flow conditions. In total, 18 different flow conditions are evaluated using TRACE with the one-group interfacial area transport equation, with an average error to the a_i, experimental data of approximately ±8%.