The characterization of obstructed laminar channel flows for local heat transfer enhancement in microfluidic cooling applications

Some of the largest heat fluxes that can be found in contemporary engineering applications (O ∼ 10^3 W/cm2) are generated by laser-bar arrays within Photonics Integrated Circuits (PICs) – the backbone of optical telecommunication networks. A combination of μthermoelectrics and chip-embedded μfluidics have been pro- posed to cool these devices within their temperature tolerance of ±0.1K, enabling next generation PICs with significantly denser laser-bar arrays, and consequently higher data throughput. The chip level heat flux is non-homogeneous due to the laser-bar scale (O ∼ μm) relative to the device (O ∼ mm). Consequently, hot spots exist which require greater local heat transfer coefficients than the surrounding regions. To cool on-chip hot-spots, a passively actuated structure could be placed within the μfluidic channel and, deploying as necessary, control the local heat transfer coefficient by disturbing flow in a target location. To demonstrate this concept, a passive smart structure device was developed at the macro scale using the NiTi Shape Memory Alloy (SMA). This passive device was observed to actuate in response to changes in fluid temperature. The objective of this thesis is to hydrodynamically and thermodynamically characterize channel flow around this demonstrator geometry. In doing so, the relationship between local heat transfer and fluid flow has been investigated in detail for two novel channel obstructions. The first was a curved plate, modeled on the design of the SMA demonstrator device. The second was a curved orifice-plate, a variant of the demonstrator designed to produce a channel-confined jet. Empirical correlations describing head loss and heat transfer performance have also been developed for these channel obstructions. Three experiments were performed to non-invasively measure the head loss coefficient, velocity field and local heat transfer in a square miniature channel for a range of Reynolds numbers (channel Re = 100 – 200) and obstruction opening area ratios (β = 0.2 – 0.5). This was achieved using pressure-flow measurements, particle image velocimetry, and combined infrared thermography and Joule-heated thin foil techniques respectively. It was found that the presence of a channel obstruction significantly en- hanced local heat transfer at the target locations. Maximum improvements in the area-averaged heat transfer coefficient of 495% relative to a channel without an obstruction were measured. The curved plate geometry showed Nusselt number (Nu) scaling with Re^0.44, similar to that of an array of pillar geometries. The curved orifice-plate geometry showed the greatest enhancement in heat transfer, and successfully generated an inclined jet within the channel. The correlation showed Nu∼Re^0.59 scaling, similar to the performance of an array of normally impinging jets. These physical relationships have beneficial use in the design and modeling of μfluidic systems. The findings highlight the impact that complex fluid flows have on spatial heat transport, and demonstrate the potential for controlled heat transfer enhancement using unconventional obstructions within laminar channel flows.