Abstract
This study aims to provide insights into the cavitation and bubble dynamics in liquid CO2 near the critical point. It is inspired by a previous work that reports the absence of dysfunctional behavior during the operation of a test compressor in the two-phase region of CO2. First, several characteristic parameters in the literature have suggested that thermal effects have significant impact on the dynamics of a CO2 bubble. These effects lead to the change of vapor pressure inside the bubble, impeding the motion of the bubble interface. As a consequence, the CO2 bubble collapse should feature a slow contraction of the bubble interface and the absence of noticeable pressure rise. In addition, a dynamic model has been proposed to quantitatively study the bubble collapse in liquid CO2 near the critical point. Simulation results have confirmed the qualitative prediction given by characteristic parameters. They have also revealed that the thermal layer inside the bubble has an important contribution to the bubble dynamics, in addition to the one outside the bubble, by altering the rate of phase change at the interface. These predicted results appear to be in line with the aforementioned experimental observations.
Highlights
Cavitation – the formation of vapor cavities in the bulk liquid – is the consequence of a rapid depressurization of the liquid to a sufficiently low pressure
The supercritical CO2 (sc-CO2) cycle was claimed to avoid most of problems of the Rankine steam and Brayton gas cycles and yet retain many of their advantages [6,2,5,4]: (i) potentially high efficiency thank to the low compression work in the reduced compressibility region near critical point; (ii) smaller size of the turbomachinery resulting from the high density working fluid; (iii) simpler system layout; (iv) less sensitivity of the thermodynamic performance to pressure losses; and (v) better match of temperature profile to that of the heat source provided by the fluid in the supercritical region
A numerical application of the characteristic parameters of the thermal effects has been performed at the beginning of this study
Summary
Cavitation – the formation of vapor cavities in the bulk liquid – is the consequence of a rapid depressurization of the liquid to a sufficiently low pressure. This experimental program aimed to investigate the operation of a compressor near the critical point in view of developing the innovative supercritical CO2 (sc-CO2) power cycle [37] Note that such operation is conservatively thought to lead to the growth and shrink or collapse of vapor pockets during the interaction of the two-phase flow with the pressure gradients caused by the impeller blade loading. There was no dysfunctional behavior observed during the tests, neither pressure instability nor notable increase in vibration and audible noise Such observation promisingly suggests that effects of cavitation in liquid CO2 near the critical point (lc-CO2) could be much less severe than those traditionally experienced in hydraulic devices. A model for the bubble dynamics has been proposed to study the bubble collapse in lc-CO2 under the increase in the liquid pressure
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