Thermal-decomposition-induced radial-force imbalance in molten salt pumps: Multiscale modal characterization of Gas–Liquid two-phase flow

For the NaNO 3 –NaNO 2 molten salt used in this study, thermal decomposition above 450°C produces NaNO 2 and O 2 , generating dispersed gas bubbles that threaten the safe and stable operation of molten salt pumps. To investigate the influence of this gas on hydraulic performance and unsteady flow, the Type 16ENH-2 high-temperature molten salt pump in the Dunhuang 100 MW CSP project was studied. A gas–liquid two-phase CFD model was established, and Multi-Resolution Dynamic Mode Decomposition (MRDMD) and Dynamic Mode Decomposition (DMD) were used to analyze the transient radial force and pressure field. Thermally decomposed gas significantly degrades pump hydraulic performance: the maximum efficiency reduction is about 7% at the design condition (1.0 Q d ), while the maximum head reduction is about 11% at low flow (0.2 Q d ). MRDMD shows that thermal decomposition effects on radial-force frequency depend strongly on operating condition: at low flow, spectral energy shifts to low frequencies; at design flow, additional high-frequency components appear, with distinct peaks in the 8 f n –16 f n range. DMD further indicates that at low decomposition rates, dominant pressure modes concentrate at low frequencies related to impeller rotation. As the decomposition rate increases, dominant modal frequencies shift to higher bands, with enhanced pressure perturbations and intensified flow disturbances. These findings provide insights into thermal-decomposition-induced unsteady flow and guidance for reliable design of high-temperature molten salt pumps.