Pool boiling enhancement via micro ratchets

Abstract

Nucleate boiling is an attractive method for achieving high heat flux at low superheat temperatures. It is frequently used for industrial applications such as heat exchangers and is being considered to cool advanced central processing units (CPU) which produce heat fluxes on the order of 1 MW/m2 and are becoming increasing less efficient to cool via forced conduction of air. The issues with implementing nucleate boiling as a cooling mechanism lies in the difficulty of quantifying the complex and numerous mechanisms which control the process. A comprehensive nucleate boiling model has yet to be formulated and will required in order to safely and reliably cool high performance electronics. Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. It was hypothesized that ratchets may enhance pool boiling heat transfer by aiding in the removal of vapor which forms on the heated surface. Therefore, preliminary experiments on pool boiling using asymmetric micro ratchets with de-ionized (DI) water and various concentrations of alumina particles and DI water as the working fluids were investigated. Three brass surfaces were tested for comparison, one surface was manually polished, the second was composed of asymmetric ratchets with 30 μm height and 150 μm period and the third 75 μm height and 375 μm period asymmetric ratchets. Small test aquariums were fabricated and tested on a hot plate. Heat flux and heat transfer coefficient (HTC) were measured with thermocouple arrays and video and photographic images were taken at various set temperatures. Results show that 150 μm ratchets can improve heat flux by 61 % using DI water and by 210 % using a 0.03 wt.% microparticle (MP) solution over a polished surface with DI water. Varying alumina nanoparticle (NP) solution concentrations on 375 μm ratchets shows an increase in heat transfer performance over pure DI water. The acquired data was used to show proof of theory and to design a more complex test apparatus which will allow for the testing of many surfaces and working fluids.