Numerical Simulation of Ultrasound-enhanced Calcium Mass Transfer Behavior during the Calcium Treatment of Molten Steel

Considering the problems of calcium low yield and instability during the calcium treatment of molten steel, the introduction of ultrasound into the secondary refining process is proposed to enhance the mass transfer coefficient of calcium (MTCC). A computational model including acoustic streaming and thermal effects is established to quantitatively characterize the mass transfer behavior and its influence on mixing time, calcium yield, and vaporization rate, and the effects of acoustic streaming and thermal effects on the MTCC are discussed. The results show that ultrasound significantly improves the local velocity and temperature of the molten steel, effectively promoting calcium transfer from the high-concentration zone to the low-concentration zone. Acoustic streaming plays a dominant role in enhancing MTCC, and the MTCC is two orders of magnitude higher than without ultrasound, corresponding to 7.061×10−5 and 5.230×10−7 m·s−1, respectively. The increase in MTCC with ultrasound radiation time is attributed to the thermal effect, increasing from 7.061×10−5 to 7.816×10−5 m·s−1. In addition, the MTCC positively correlates with ultrasonic power, while ultrasonic frequency and probe radius have a negative effect on MTCC. Consequently, the mixing time, calcium yield, and vaporization rate are 780 s, 6.32%, and 0.584%·s−1 without ultrasound applied, and those of are 250 s, 26.64%, and 0.295%·s−1 at a power of 96 kW and a frequency of 24 kHz.