Summary:

Boiling is an important phase change phenomenon that occurs in variety of applications such as electronic cooling, power generation, refrigeration, and cryogenics. It is defined as the process of phase change from liquid to vapor by heating the liquid past its saturation temperature and is characterized by nucleation, growth and detachment of the vapor bubbles. Explosive boiling is a special kind of boiling in which the phase transition from liquid to vapor occurs very rapidly. In this process the liquid is heated far beyond its saturation temperature so the phase transition from liquid to vapor is accompanied by a sharp pressure increase. It is encountered in nuclear and chemical industries and various emerging technologies such as ultra-fast laser materials processing. This phenomenon is still one of the least understood topics in heat transfer and no well-established theory exists for predicting the rate of heat transfer in explosive boiling.

Recently, the fast advancement of micro/nanofabrication technology enables us to manufacture different kinds of novel hierarchical nanopatterns on surfaces. Therefore, new opportunities to purse more efficient enhanced structures for boiling heat transfer can be explored. Surface effects in nanoscale significantly change the behavior of boiling due to the high surface-to-volume ratio in micro/nanofluidic system. Recent, experimental studies have showed that using nanostructure on flat surface provides significant enhancement in heat transfer coefficient and critical heat flux. Geometry and size of nanopatterns are two of the main factors affecting the behaviors of boiling and evaporation. Different shapes of nanostructures such as nano-cylinder, nano-sphere, mushroom-like, Y-shaped Nano-rods, nanowire, and nano-cone, can be manufactured for different applications. However, a deep understanding of physical phenomena during boiling of a liquid on different kinds of complicated nanostructures is still one of the challenging topics in nanoscale heat transfer.

With significant progresses in computer hardware and software, it is now possible to investigate transport phenomena in nanoscale via computer simulation. Molecular dynamics simulation is an attractive and powerful means for studying phenomena in nanoscale; hence, it is an ideal tool to study the behavior of the phase transition from liquid to vapor.  In this work, I studied the effect of various nanostructures on phase transition, especially explosive boiling, for the first time. I developed a methodology for predicting the enhancement in net evaporation of thin film on a flat surface with various nanostructures shapes and materials, using non-equilibrium molecular dynamics simulations based on Embedded Atom Method (EAM) potential.Both normal and explosive boiling over a nanostructured surface are investigated. The thickness of liquid was kept constant while the size of nanostructures on the surface varies which result in different longitudinal and transverse distances between particles and height of nanostructures.

(1) Conical Nanostructures

     Fig 1. Computational domain, initial atoms configuration and geometries of nanostructures.

Fig 2. Trajectories of atoms for surfaces – silver case.

Fig 3. Effect of nanostructures on temperature and pressure histories.

Fig 4. Number density: (a) t = 1.2 ns, (b) t = 2 ns.

Fig 5. Number of liquid and vapor atoms.

Fig 6. Non-evaporative thin film region.

(2) Spherical Nanostructures

Fig 7. Three-dimensional view of different size of nanoparticles on the surface

Fig 8. Temperature history of argon and solid wall—high temperature case 290K

Fig 9. Trajectory of atoms for different nanostructure surfaces—high temperature case 290K

Fig 10. Net evaporation number—high temperature case 290K

Fig 11. Net evaporation number—high temperature case 170