Effect of the Virtual Mass Force Term on the Critical Flow

Abstract

Abstract Critical (choked) flow is a highly concerning phenomenon in safety analysis for nuclear energy. During an accident, the radioactive aerosol in nuclear power plant (NPP), which is hazardous for the environment, may be released accompanying the fluid discharge. Consequently, the discharge mass flow rate prediction is crucial for engineering design and emergency response in case of nuclear accidents. Unfortunately, the critical flow is difficult to predict especially when two-phase flow exists. Developing a more accurate model for critical flow is an essential requirement to the system thermal-hydraulic (STH) codes (e.g., RELAP5, TRACE, and ATHLET, etc.) for nuclear safety analysis. The accuracy is based on a deeper understanding of the complex phenomenon of critical flow. One of the difficulties concerned with the mathematical modeling of the two-phase critical flow is the complexity of the transfer phenomena at the interface. Normally, an interfacial drag law and a virtual mass force were used to quantify the momentum transfer between the phases in the STH codes. The present study is concerned with the effect of virtual mass force on phase separation during the acceleration of a two-phase mixture. Virtual mass represents real physical effects and it can be regarded as induced inertia on the dispersed phase which is accelerating relative to the continuous phase. It has been verified that the virtual mass acceleration is objective, and the consideration of it in the two-phase flow simulation can improve the numerical stability and efficiency. Therefore, a variety of objective forms of virtual mass acceleration were derived in the last fifty years. But the influence of virtual mass force on the two-phase critical flow was seldom concentrated on owing to the lack of suitable critical flow models for studies in detail. This study is based on a developed two-phase critical flow model based on the separate conservation equations of mass, momentum, and energy for each phase. Several typical models of virtual mass force are selected and their influences on the accuracy of critical flow predictions will be studied in some detail. Furthermore, the critical flow that happened in the NPP accident scenarios may go through several flow regimes such as subcooled flow, bubbly flow, slug flow, and annular flow, etc. The impact of virtual mass force on these different stages of critical flow is another topic of this study. The results of this study can benefit a further understanding of virtual mass force, especially its influence on critical flow, and will contribute to the development of a more accurate two-phase critical flow model.