Effects of superheat on characteristics of flashing spray and snow particles produced by expanding liquid carbon dioxide

• Liquid phase is found upon the release of liquid CO2 outside the nozzle orifice.
• With increases in ΔT(⁎), jet spray, cone spray and bowl spray appear sequentially.
• Onset conditions from external-flashing to internal-flashing spray are observed.
• The size distribution of CO2 particles is within a few microns.
• Mechanisms for external-flashing and internal-flashing are deduced.

This research experimentally investigates influences of superheat on flashing spray characteristics and on the snow formation of liquid CO2. 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. Moreover, a few microns of CO2 snow size are measured in this research, which is consistent with records from literature on this topic. The narrower size distribution of snow particles and a critical spray angle resulting from two competing phenomena under internal-flashing atomization mode are also discussed in this research.