This review highlights recent developments in improving thermal-hydraulic performance through two phase heat transfer facilitated by microstructured and nanostructured surfaces.
Flow
boiling and evaporation in tubes and channels occur in a wide
variety of energy systems, such as refrigeration, air conditioning,
power generation, electronics cooling, distillation, and purification.
In this work, we demonstrate remarkably increased heat transfer coefficients
of 270% during refrigerant flow boiling in scalable microstructured
(∼40 μm), industrial-scale (∼1 m long) aluminum
(Al) tubes, when compared to smooth unstructured Al tubes. To achieve
scalable nanomanufacturing, we create highly conformal and durable
structured surfaces by relying on hydrochloric acid Al etching. Flow
boiling tests were conducted in 6.35 mm diameter Al tubes using R134a
refrigerant as the working fluid. To benchmark our approach and to
elucidate the effect of the structure length scale, we also fabricated
ultrascalable boehmite (AlO(OH)) nanostructured (∼300 nm) Al
tubes, showing that etched microscale features are necessary and key
to enhancement. Durability tests conducted using a 28 day long continual
flow boiling experiment demonstrated negligible degradation of the
etched surfaces. The scalable and cost-effective techniques used to
create these durable, etched-Al microstructures may significantly
reduce manufacturing cost when contrasted with current enhancement
approaches such as extrusion, drawing, and welding.
Incorporation of micro- and nanostructures on metals can improve thermal performance in a variety of applications. In this work, we demonstrate two independent highly scalable and cost-effective methods to generate micro- and nanostructures on copper and stainless steel, two widely used metals in energy and thermal applications. The performance of the developed structures, fabricated using scalable chemical etching techniques, is compared against their respective base metals. Our results demonstrate significant flow boiling heat transfer coefficient improvements up to 89% for etched copper and 104% for etched stainless steel. Mercury porosimetry is used to demonstrate that the varying pore-size distributions and presence of micro/nanoscale channels help to regulate heat transfer mechanisms, such as nucleate and convective flow boiling. Furthermore, structure integrity after 7-day flow boiling tests demonstrate surface structure resiliency to damage, a key challenge to implementation. This work combines advances in thermal performance with surface structure durability to provide guidelines for broader application of similar chemical etching methods to scalably create micro- and nanosculptured surfaces.
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