Experimental and numerical study on density wave oscillations of supercritical water in parallel water wall channels of an ultra-supercritical circulating fluidized bed boiler

Density wave oscillations (DWOs) of supercritical water in the parallel water wall channels of an ultra-supercritical circulating fluidized bed (CFB) boiler were investigated experimentally and numerically. The variation ranges are presented as follows: pressure from 23 MPa to 30 MPa, mass flow rate from 0.06 kg.s(-1) to 0.22 kg.s(-1), inlet water temperature from 200 degrees C to 370 degrees C. Wall temperatures increase rapidly after DWOs occur. As the period decreases, the flow departs significantly from the quasi-steady conditions and the penetration of the variation of the flow toward the wall is significantly reduced. A general calculation model based on the time-domain method was established to analyze DWOs in parallel channels. Four pulsation patterns were found during calculation, that is, pulseless oscillation, damped oscillation, self-sustaining periodic oscillation with constant amplitude, and divergent oscillation. The mechanism of DWOs was investigated through experimentally and numerically results. Results show that mass flow rate of each channel displays a reverse-phase pulsation during the oscillations. DWOs are intertube oscillations with a short period and small amplitude. Increases in inlet pressure drop coefficient or decreases in outlet pressure drop coefficient are conducive to system stability.