Thermo-Fluid Dynamics of Flash Atomizing Sprays and Single Droplet Impacts

Department of Mechanical Engineering
Advisor: Professor Guillermo Aguilar

Spray  atomization  and  droplet  dynamics  are  research  topics  that  have existed  for  many  decades.  Their prevalence  in  manufacturing,  energy  generation  and  other  practical applications  is  undeniable,  though researchers have often overlooked the importance of understanding the physics of
atomization or droplet impact characteristics in the ongoing effort to improve efficiency. In this talk, I will address the atomization of  thermodynamically  unstable  “flashing”  sprays and  the  splashing  mechanisms  of  single  droplets impinging on flat, smooth surfaces. The related heat transfer phenomena for cooling applications are also addressed. These topics are motivated by efforts to improve the thermal protection provided by cryogenic spray  cooling  in  laser  dermatological  procedures, increasing  the  throughput  of the  spray  production  of nano  and  micro-scale  particulates  used  as  dyes  and catalysts,  and  in  modeling  of  the  release  and dispersion of flammable or hazardous chemicals through large-scale collisions with storage containers.

Through  the  use  of  high-speed  video  imaging,  phase Doppler  interferometric  measurements  and numerical  modeling  of  the two-phase  flow  taking  place  within  spray  nozzles,  a  detailed  picture  of  the processes  involved  in  flash  atomization  are  attained. Results  reveal  that  flashing fluid jets  under  low superheats  undergo  many  dynamic  processes  leading  to eventual  droplet  formation,  including  the nucleation  of  vapor  bubbles  within  the  nozzle  interior  and  their  subsequent  expansion  and  explosion.  At  high superheats, a stable “flare flashing” regime is attained resulting in very fine atomization. These insights may lead to improved nozzle designs to better control the atomization process.

High-speed  imaging  of  droplet  impacts also  reveals  new  insights  into the  mechanisms  of  splashing.  The surrounding  ambient  air  pressure,  fl
uid  viscosity,  and  fluid-surface  affinity  are  found  to  profoundly influence   the   initiation   and   characteristics   of   splashing.   A   new   analytical   model   explaining   the mechanisms of crown splashing is developed along with correlations predicting the threshold of splashing.