A unified approach to simulate thermal hydraulic instabilities and the mutual interactions using transient distributed model

Abstract

In the present work, a 1-D thermal-hydraulic model is developed to simulate two phase flow boiling for a working coolant R134a circulating inside a single horizontal channel which includes a surge tank. The TH model is the first of its kind which solves the transient conservation equations based on distributive formulations without the incorporation of quasi-steady state approximation and still can simulate pure pressure drop oscillations (PDOs), Ledinegg excursion and density wave oscillations (DWOs), and the co-existence of those instabilities as well using a unified approach by employing appropriate external characteristic curves. The model is validated with the available experimental data and then, further used for intended purpose as mentioned above. Finally, the notable conclusions are drawn based on the simulations conducted. Results indicate that the initial equilibrium position in the negative slope region of the pressure drop vs. mass flux curve is unstable, leading to Ledinegg excursion or PDOs based on the steepness of the external pump characteristic curve. Increased surge tank volume extends PDOs. For Ledinegg excursion, the unstable position shifts to positive slope regions, and gets stabilized or destabilized depending on the occurrence of DWOs. DWOs are more likely at low mass flux and positive slope region. With a constant mass flow rate from an external pump, DWOs occur with a surge tank upstream of the heated section but not without it. Simulations demonstrate coexistence of PDOs, Ledinegg excursion, and DWOs in certain situations.