Validation and model sensitivity of the interfacial area transport equation in condensing flows

Abstract

The interfacial area transport equation (IATE) in condensing flows is thoroughly reviewed and validated with available experimental data. The IATE provides a dynamic prediction of flow structure consistent with the two-fluid model and has been studied extensively for adiabatic two-phase flows. However, the IATE still requires attention in flows with phase change. The modeling of the condensation interfacial area term and volume sink term is reviewed along with the void fraction prediction since the IATE is closely related to the void fraction development. The emphasis in this work is to expand the validation of the IATE in condensing flows to higher pressure and higher flow rate conditions as well as evaluating the modeling sensitivity in both the decoupled IATE and the IATE coupled with the void transport equation. Two datasets have been used for validation, both consisting of condensing two-phase flow data in the unheated section of a vertical annulus with conditions spanning a range of pressure, inlet subcooling and mass flux. The analysis shows that the current form of the condensation modeling results in an over-prediction of both condensation mass transfer and interfacial area concentration loss across all conditions. From the decoupled IATE simulation, it is established that the boundary diameter must be used in the Nusselt number correlation in the condensation model to get good predictions of the interfacial area concentration. Moreover, the convection term in the IATE is found to contribute to interfacial area concentration at high mass flux conditions. The coupled IATE simulation model results in better agreement in the predictions of the void fraction and the interfacial area concentration under nearly all conditions with the proposed modifications to the condensation model. Sensitivity studies show the impact of Nusselt number correlation and gas velocity on the prediction of interfacial area concentration. Overall, a coupled simulation is recommended for benchmarking the IATE in phase change flows.