CO2-expanded organic solvent is a kind of important fluid medium and has broad applications in chemical industry, environmental protection and other fields. Ultrasonic cavitation in gas expanded liquids (GXLs) is conducive to enhancing mass transfer and producing many exciting phenomena. In this paper, the ultrasonic cavitations and streaming in the saturated CO2-expanded liquid N, N-dimethylformamide (DMF) at 4.2 MPa and 5.2 MPa are observed by a high-speed camera. The cavitation intensity and time trace of pressure pulses are recorded using a PZT hydrophone. The influences of gas-liquid equilibrium pressure and ultrasonic power on the cluster dynamics of transient and stable cavitation are examined. The excess molar enthalpies required for CO2 dissociation from DMF are calculated by Peng-Robinson equations of state and the change of surface free energy of CO2-expanded DMF is predicted. The results show that the excess enthalpy of the mixture is one of the key factors to control ultrasonic cavitation at high pressurized conditions, while the surface tension is the key factor for low pressure. As the increase of applied ultrasonic power, the formation and collapsing frequency of bubble clusters increases, and the amplitude and cyclic frequency of pressure pulse are enhanced. The transient cavitation intensity increases as it reaches a maximum value at a certain ultrasonic power and then decreases. The change trends of stable cavitation intensity under different pressures are basically same. It can be concluded from the evidence that ultrasonic cavitation in CO2-expanded DMF is affected by the combined effect of compression and substitution: compression depresses the nucleation and growth of bubbles, while the high solubility of CO2 in DMF is conducive to the generation of bubbles in cavitation.
Keywords: CO(2)-expanded liquid; Cluster dynamics; Excess enthalpy; Pressure pulse; Surface free energy; Ultrasonic cavitation.
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