Enhancement of Pool Boiling Heat Transfer Using Nanostructured Surfaces on Aluminum and Copper

Abstract

System energy management and cooling for future advanced lasers, radars, and power electronics is gaining importance, resulting in an intense search for technologies and design techniques to dissipate ultra-high heat fluxes, reduce system energy usage, and increase system efficiencies. We report enhanced pool boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured Al and Cu substrates, which are commonly used in advanced electronics cooling applications. Nanostructured surfaces were realized by using a low temperature nanomaterial deposition process, Microreactor-assisted-nanomaterial-deposition (MAND™). Using this technique we deposited ZnO nano-structures on Al and Cu substrates. We varied a number of parameters such as micro-/nano-structure morphologies, pore sizes, densities, and their inter-connectivity to identify optimal morphologies. These surfaces displayed typical pore sizes of 50–100 nm with pore densities of about 100–200 per μm2 and hydrophilic/super-hydrophilic characteristics with measured contact angles as low as 0°. We have demonstrated the capability to control MAND™ processes to create static contact angle from about 58° to near 0° for ZnO on Al surfaces and 80° to about 40° for zeolite texturing on Si surfaces. Average roughness measured using Atomic Force Microscopy (AFM) was in the range of 200–600 nm. Pool boiling refers to boiling under natural convection and nucleate boiling conditions, where the heating surface is submerged in a large body of stagnant liquid and the relative motion of the vapor bubble and its surrounding liquid is primarily due to buoyancy effect. We have focused to date on water as test liquid, but have future plans for investigating HFE 7100. We observed pool boiling CHF of 80–82.5 W/cm2 for nanostructured ZnO on Al surfaces versus a CHF of 23.2 W/cm2 on a bare Al surface with a wall superheat reduction of 25–38 °C. These new CHF values on ZnO nanostructured surfaces correspond to a boiling heat transfer coefficient as high as ∼23000 W/m2K. This represents an increase of almost 4X in CHF on nano-textured surfaces, which is contrary to conventional boiling heat transfer theory. We will discuss our current data, compare the behavior with conventional boiling theory, and compare with similar recent boiling heat transfer data on other nano-scale surfaces by other researchers. We are currently investigating these surfaces under forced convection conditions in microchannels to assess their flow boiling capabilities.