Effect of atomizer geometry on particle formation in dry-ice sprays

Abstract

Dry-ice particle sprays for thermal management imply the formation of large particles from agglomeration phenomena inside diffusers. This work investigates the effect of the atomizer geometry (relation between height and exit diameter, H/D) on the mechanisms of micro-particles’ agglomeration using divergent diffusers after the expansion nozzle. High-speed visualization characterizes the flow inside the diffuser, and simultaneous measurements of the static pressure along the atomizer diffuser’s length allow correlating this parameter with the flow structures. The results presented evidence a minimum of H/D=2 to produce larger dry-ice particles through the agglomeration process depending on two flow structures. The main recirculation zone in the upper part of the diffuser increases the residence time of micro-particles inside the turbulent flow. This change in the residence time promotes agglomeration phenomena forming larger particles. Besides, a stable backward flow is essential to enhance the production of larger particles, increasing their concentration. There is a clear relationship between the static pressure and the intensity of the mechanisms of inter-particle collisions occurring inside the diffuser. Namely, for H/D > 2, divergent cross-section diffusers appear to enhance the agglomeration of microparticles when compared to diffusers with a constant cross-section. However, a noticeable increase in the number of agglomerate particles for H/D > 3 points this as the minimum geometric threshold to produce dry-ice sprays with a higher concentration of larger particles, which is useful for cooling purposes.