Hybrid Molecular Dynamics Modeling of Interfacial Phenomena in Boiling Processes

Dr. Van P. Carey
Mechanical Engineering Department
University of California at Berkeley

The thermophysics of liquid-vapor interfaces has long been recognized as playing a key role in the   physical mechanisms of boiling processes. This seminar will describe results of recent molecular dynamics  (MD)  simulation  studies  that  explore  the  structure  and  stability  of  liquid-vapor  interfacial regions  using  a  hybrid  analysis  scheme  that  combines  new  formulations  of  capillarity  theory  with  MD simulations that use similar interaction potentials.  Two forms of this type  of hybrid scheme have  been developed:  one  for  nonpolar  fluids  based  on  a  Lennard-Jones  interaction  potential,  and  a  second  specifically  for  water  using  a  modified  treatment  of  the  SPC/E  interaction  potential  that  accounts  for water  dipole  interactions.  The  hybrid  model  has  the  advantage  that  the  capillarity  theory  provides theoretical  relationships  among  parameters  that  govern  interfacial  region  structure  and  thermophysical behavior,  while  the  companion  MD  simulations  allow  more  detailed  molecular  level  exploration  of  the interfacial  region  thermophysics.    Predictions  of  interfacial  region  structure  indicated  by  this  kind  of hybrid modeling will be described for pure non-polar and water liquid-vapor interfaces, and for water with dissolved  ionic  solutes  (i.e.,  salts).    Extension  of  the  methodology  to  thin  liquid  films  will  also  be described.    Rupture  of  a  free  liquid  film  dictates  merging  of  adjacent  bubbles,  which  is  particularly important  in nucleate  boiling heat transfer,  bubbly two-phase flow  in small tubes, and the mechanisms that   dictate   the   Leidenfrost   transition.      To   understand   the   mechanisms   of   bubble   merging   in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and onset of rupture at   the   molecular   level.      Results   obtained   with   the   hybrid   model   indicate   that   wave   instability predominates as an onset of rupture mechanism for free liquid films of macroscopic extent, but for free liquid  films  with  nanoscale  lateral  extent  (in,  for  example,  nanostructured  boiling  surfaces),  lack  of  film core stability is more likely to be the mechanism.  Predictions of the hybrid models will be compared to results of experimental studies of the effects of ionic solutes on interfacial tension and bubble merging. The  implications  of  the  molecular  dynamics  model  predictions  for  boiling  processes  in  microchannels and boiling in nanostructured surfaces will also be discussed.

Professor  Carey  is  widely  recognized  for  his  research  on  near-interface  micro-scale  phenomena, thermophysics  and  transport  in  liquid-vapor  systems,  and  computational  modeling  and  simulation  of energy  conversion  and  transport  processes.    Since  joining  the  Berkeley  faculty  in  1982,  Professor Carey’s  research  has  spanned  a  variety  of  applications  areas,  including  high  heat  flux  cooling  of  electronics,  heat  transfer  in  porous  burners,  data  center  energy  efficiency,  energy  sustainability  of information processing, fuel cell thermal management, building and vehicle air conditioning, forging and casting  of  aluminum,  phase  change  thermal  energy  storage,  Rankine  cycle  power  for  manned  space missions,  heat  pipes  for  aerospace  applications,  advanced  concentrating  solar  absorber  designs,  and turbomachinery technologies for green energy conversion applications.