| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 102 === This research investigates the mechanism of CO2 snow formation and the application of CO2 snow jet. The CO2 snow particles are produced by the process of flash-atomization. The particles are agglomerated to clusters in the formation chamber. The cryogenic CO2 snow can be used in cryosurgery.
First, the influences of superheat on flashing spray characteristics and on the snow formation of liquid CO2 are investigated. Results show that liquid, two-phase flow or even three-phase flow can be found upon the release of liquid CO2 from high pressure to atmospheric pressure. This is due to complicated phase transition processes that involve hydrodynamic instabilities and thermal non-equilibrium conditions. Results also show that the spray pattern transfers from jet spray to cone spray, and then to a bowl spray configuration with the increase of superheat. The drastic changes in spray angle and mass flow rate indicate onset conditions from external-flashing to internal-flashing atomization mode. This is due to bursts of bubbles inside the nozzle chamber that result in the choking of the two-phase flow. Under internal-flashing atomization mode, a critical spray angle resulting from two competing phenomena is also found. Moreover, a few microns of CO2 snow size are measured in this research, which is consistent with records from literature on this topic. As superheat increases, the standard deviation of particle size distribution decreases, reaches its lowest value and remains constant as the spray turns to the internal-flashing mode. This is due to gradual changes in the double layer shattering configuration of the spray caused by transition from heterogeneous to homogeneous nucleation in the upstream flow.
The influences of ambient pressure and length-to-diameter ratio (L/D) of the orifice on CO2 flash-spray patterns are also investigated. Results show that flash-evaporation is one of major factors on expansion of CO2 flash-spray angle. The spray angle reduces with ambient pressure. This is due to the decrease of flow expansion and flash-evaporation of CO2 flash-spray. The interaction between length-to-diameter ratio of the orifice and liquid superheat on the transition of flash-mode is observed.
Secondly, the agglomeration mechanisms of CO2 primary particles inside a tube type formation chamber are experimentally investigated. The results show that a complicated particle motion in the upper portion of the formation chamber is responsible for the formation of large snow particles. The high speed and complicated motion of the snow particles inside the tube provide both the opportunities and time for the collision of particles, which implies that only particle deposition and re-entrainment cannot completely describe the phenomenon of particle agglomeration. The results also show the mechanisms of particle agglomeration inside the formation chamber, which include primary particle agglomerate in jet vortexes, agglomerated particles flowing upward into the recirculation region, particle clusters growing in the recirculation flow, and finally particles being released with the jet flow. A minimum tube length (30mmin this case) is needed to ensure the complete formation of the agglomeration mechanisms with recirculation flow, and thus the formation of considerable amounts of agglomerated particles. The results of this study thus improve current understanding of the agglomeration process and mechanisms of CO2 snow formation inside the formation chamber.
Finally, the effects of injection pressure, formation chamber and bypass flow on spray performances and cryogenic characteristics of CO2 snow on cryosurgery guided by endoscopic are experimentally investigated. Results show that CO2 snow has a superior freezing capability in the application of cryosurgery. The length of formation chamber is an effective design parameter to control spray performances and cryogenic characteristics. Increase in the length of formation chamber can increase snow size and conversion ratio, as well as the cooling rate and impact area of the tissue. Thus, it is suitable for the application on wider range nidus. Moreover, the bypass flow can efficiently modulate the cooling effect of the main flow with CO2 snow particles, and it can not only be a control parameter, but also extend the operation time of cryosurgery. The experimental results also show that decrease the cylinder pressure, decrease the length of formation chamber, and increase the diameter of bypass apertures can slow down the flying velocity of CO2 snow which are effective methods to control the jet velocity and prevent the risk of penetration.
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