Flow transition from one liquid phase to vapour phase and vice versa is encountered in almost all types of power generation systems. As a result, there is certain equipment as well as vessels that are designed for handling such two-phase mixtures of handling fluid. The design of these devices has become critical in order to account for and incorporate the phenomenon of such instabilities. Perhaps the most intriguing aspect is that, despite significant advances in the field of fluid and thermal science research, it is still impossible to predict the arrival and oscillation of these instabilities accurately. Understanding of it, as well as identifying future work to prevent such instabilities in heat. Any mathematical model cannot fully explain the variation in amplitudes of flow parameters caused by such instabilities. This is possibly why these undesirable effects cannot be completely eliminated, but can be minimized so that they do not interfere with system operation. So, the goal here is to categorize flow instabilities, consolidate basic knowledge about them, and identify future work to prevent such instabilities in heat transfer equipment by using numerical simulation. Due to rapid changes in the main variables, the flow excursion instability could cause severe damage in thermo-hydraulic components. This phenomenon is investigated in this paper by simulating the system's transient behavior during the process by using simulation in CFD tool.