Micro/Nano-Structured Microchannel Heat Exchangers for Advanced Cooling

Technical Objectives

The U.S. military is highly interested in system energy management and cooling to enhance mission effectiveness for a range of increasingly complex systems and missions. Various military agencies seek technologies and design techniques to cool ultra-high heat fluxes and thereby increase system energy efficiencies in future advanced lasers, radars and power electronics. There is a general requirement to develop compact, light-weight and low-cost thermal control and heat exchange devices. This project is developing technologies and pathways to achieve cooling heat fluxes of 200-1000 W per sq cm with cost-effective manufacturing technologies.

Results to Date

We have obtained enhanced pool boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured Al, Cu and Si substrates that are commonly used in advanced electronics cooling applications. Nanostructuring was realized by using a low-temperature nanomaterial deposition process, Microreactor-assisted-nanomaterial-deposition [MAND(TM)]. Using this technique we deposited ZnO nano-structures on Al and Cu substrates and zeolite structures on Si. We are attempting to control a number of parameters such as micro-/nano-structure morphologies, pore sizes, densities and their inter-connectivity to identify optimal morphologies. These surfaces displayed hydrophilic/super-hydrophilic characteristics (Figure 1) with measured contact angles as low as 0°. We have demonstrated the capability to control MAND(TM) processes.

We have focused to date on water boiling in our experiments (Figure 2, left) but have future plans to investigate the boiling effects with other fluids. We observed pool boiling critical heat fluxes (CHF) of 80-82.5 W per sq cm for nanostructured ZnO on Al surfaces versus a CHF of 23.2 per sq cm on a bare Al surface with a wall superheat reduction of 25-38 C. These new CHF values on nanostructured surfaces represent a boiling heat coefficient over 20,000 W per sq meter-K (Figure 3 below). This represents an increase in some cases of almost 4X on nano-textured surfaces compared to uncoated surfaces, and the boiling phenomenon exhibited are contrary to conventional boiling heat transfer theory. We are currently investigating these surfaces under forced convection to assess their flow boiling capabilities. We have investigated methods to apply this technology to microchannel flow configurations to enhance heat transfer phenomenon in that application.

Implications

We have demonstrated a controllable surface modification technology that evidences enhanced heat transfer characteristics applicable to cooling high-heat flux electronic devices. This technology provides a technical pathway to surfaces that can sustain high-heat flux with low-cost manufacturing techniques in advanced cooling applications, such as advanced radar and lasers, high-performance computing systems, advanced power electronics and military avionics. The potential applications span a wide spectrum of commercial and military systems.

Collaborators

• Terry J. Hendricks
• Chih-hung Chang
• Brian Paul

Acknowledgments

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement number W911NF-07-2-0083 (ARL/SEDD Program Manager: Stephen Kilpatrick). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.

For Additional Information

Contact Program Manager Terry J. Hendricks.