Investigation on Flashing Atomization and Snow Formation of Carbon
Dioxide Spray
Student: Yi-Jun Shen
Advisor: Muh-Rong Wng, Tzong-Shyng, Leu
Abstract
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
xi 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
xii 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
xiii 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.
Keywords: Carbon dioxide, Flashing spray, Superheat, Snow formation, Size distribution
xiv