Molecular Dynamics Simulation of Bubble Nucleation Near Nanometer-Sized Surface Defects

1. Molecular Dynamics Simulation of Bubble Nucleation Near Nanometer-Sized Surface Defects Committee Members: Adviser: Dr. R. Panneer Selvam Member: Dr. Gregory Salamo Member: Dr. Juan Balda Member: Dr. Douglas Spearot Joseph Johnston Ph.D. student in Microelectronics-photonics University of Arkansas PhD Proposal – Joseph Johnston

2. Overview • Introduction to Thermal Management • Introduction to Spray Cooling • Surface Modifications • Need for Molecular Modeling of Liquid-Vapor Nucleation • Research Objectives • Preliminary Work • Time Schedule • Conclusion PhD Proposal – Joseph Johnston

3. Need for Thermal Management • High power electronic devices dissipate large amounts of thermal energy. • Heat flux is a measure of thermal energy density. Typical Heat Fluxes Light Bulb(1 W/cm2) Intel Pentium IV CPU(20 – 30 W/cm2) Car Engine(30 – 60 W/cm2) • High-heat flux thermal management solutions are necessaryto maintain lower operating temperature of devices, which can increase the device reliability and performance. Cray Supercomputer CPU(80 – 170 W/cm2) High Power Laser Diode(400 W/cm2) High Power Electronics(100 -1000 W/cm2) • Two-phase liquid cooling technology is the only option for high power electronics PhD Proposal – Joseph Johnston

4. Spray Cooling Over Other Two-Phase Liquid Cooling Technologies Spray cooling is one of the best option for thermal management of high power electronics PhD Proposal – Joseph Johnston

5. Liquid film Droplet y Vapor bubble Flow in a Nozzle x Heat in Heat input, q Droplet Vapor Thin liquid film Vapor bubble Heated surface Heat input, q Introduction to Spray Cooling • Advantages of Spray Cooling • High heat transfer • Uniformity of heat removal • Small fluid inventory • Low droplet velocity • Physical Interactions in Spray Cooling • Conduction • Convection • Phase change • Surface tension • Gravity • Liquid droplet impact on vapor bubbles & liquid film PhD Proposal – Joseph Johnston

6. Surface Modifications • Recent research has focused on using porous structures and microchannels (~10-100 µm) to enhance boiling by increasing the nucleation site density. • It is generally thought that sub-micron surface structures (i.e., nano-cavities) will not enhance boiling performance. • Recent experiments showed that a nano-structured copper surface with nanorods (50 nm diameter) enhance boiling by increasing the bubble release frequency, decreasing bubble departure diameter, and increasing the nucleation site density at a low superheat. Nearly an order of magnitude higher heat flux was reported at 7-12 K superheat range using nanorods compared to a flat surface. • Other experiments showed that nanofins increased heat flux by 30-40%. Nanofins were deposited on the surface by the settling of nanoparticles that were doped in a liquid. PhD Proposal – Joseph Johnston

7. Need for Molecular Modeling of Liquid-Vapor Bubble Nucleation • Previous numerical model assumed a bubble as initial condition. • Continuum-based approximations of fluid flow and heat transfer break down at the nano-scale. • Need a model that explicitly accounts for intermolecular forces. • Nano-scale cavities are not thought to participate in the nucleation process, but recent experiments suggest that nanometer-sized cavities and defects could promote enhanced boiling performance. • The mechanism of observed boiling enhancement is not well understood. Better understanding of surface cavities and defects can lead to improved heat removal system designs. • Measurements and investigations of bubble nucleation at the nano-scale (< 1 µm & < 1 ns) is difficult without computer simulation. PhD Proposal – Joseph Johnston

8. Research Objectives • To simulate liquid-vapor bubble nucleation along nanometer-sized cavities using a multiphase molecular dynamics model. • To validate the molecular model by comparing the calculated nucleation rate and size with other theoretical models and experimental research. • To better understand the vapor bubble nucleation process at the molecular level along solid surfaces and illustrate bubble nucleation along nanometer-sized cavities to design efficient and improved cooling systems. • To evaluate the effects of surface parameters (cavity size, cavity geometry, solid wall temperature, and system pressure) on nucleation rate, and thus, heat transfer. • To extend the model to more realistic materials in heat transfer applications (i.e., water instead of argon and a more realistic inter-atomic potential for copper). PhD Proposal – Joseph Johnston

9. Preliminary Work • Droplet evaporation • Only considered liquid Ar • Top view shows a time progression of a droplet heated to a subcritical temperature (below the evaporation temperature) • Bottom view shows a time progression of a droplet heated to a supercritical temperature (above the evaporation temperature) PhD Proposal – Joseph Johnston

10. Preliminary Work • Evaporation from a solid surface • Need to increase the simulation time (picture shown is only 6 ps) • Need to include more liquid and vapor layers • Need to include a nano-sized cavity • Red: Ar vapor • Green: Ar liquid • Blue: Cu solid PhD Proposal – Joseph Johnston

11. Time Schedule Major Research Milestones: Droplet evaporation (completed) Preliminary study w/ solid Cu layer (Est. Completion, Dec. ‘08) Verify model (Est. Completion, April ‘09) Preliminary study w/ defect layer (Est. Completion, April ‘09) Modify code for water (Est. Completion, July ‘09) Modify code for more realistic Cu potential (Est. Completion, Oct. ’09) Dissertation rough draft (Est. Completion, Nov. ‘09) Dissertation defense (Est. Completion, Feb. ‘10) Graduation (Est. Completion, May ‘10) PhD Proposal – Joseph Johnston

12. Impact of Research • Enable high-power electronic devices through more efficient thermal management using multiphase cooling techniques. • Decrease energy usage by industrial heat exchangers through enhanced boiling performance. PhD Proposal – Joseph Johnston

13. Publication List Selvam, R.P., Hamilton, M.T., Johnston, J.E. and Silk, E.A., “Spray Cooling Modeling: Droplet Impact and Vapor Growth Effects on Heat Transfer in Micro & Macro-Gravity,” submitted for review in AIAA Journal of Thermophysics and Heat Transfer, submitted on Jan. 18, 2008. Johnston, J.E.,Selvam, R.P. and Silk, E.A., “Spray Cooling Modeling: Droplet Sub-Cooling Effect on Heat Transfer,” AIP Conference Proceedings: American Institute of Physics, Ed. M.S. El-Genk, Proceedings: Space Technology and Applications International Forum (STAIF 2008), Conference on Thermophysics in Microgravity, February 10-14, Albuquerque, NM, 2008. Selvam, R.P., Johnston, J.E. and Sarkar, S., “3-D Multiphase Flow Modeling of Bubble Growth and Burst in Spray Cooling using Parallel Computing,” 2007 ASME-JSME Thermal Engineering Conference and Summer Heat Transfer Conference, July 8, 2007, Vancouver, B. C. Canada. Johnston, J.E., “Multi-Phase Numerical Simulation of Heat Transfer during Spray Cooling with Phase Change at the Micro-Scale”, Master’s thesis, University of Arkansas, August 2007. Selvam, R.P., Sarkar, S., Sarkar, M., and Johnston, J.E., “Modeling Multiphase Flow with Phase Change and Heat Transfer: Status Review, Challenges and Future Research Direction,” Invited article in review from Nova Publishers, submitted July 15, 2008. PhD Proposal – Joseph Johnston

14. Course Work and Dissertation Coursework Registered – 33 Credit Hrs. (Minimum Required : 27 Credit Hrs) GPA: 3.632 Dissertation Registered – 7 Credit Hrs. (Minimum Required: 21Credit Hrs.) PhD Proposal – Joseph Johnston

15. Thank you for your time.Questions? PhD Proposal – Joseph Johnston