Water flow boiling heat transfer and pressure drop in smooth, etched, and herringbone aluminum tubes

Designing next generation heat exchangers for efficient heat transfer and minimal material consumption is required for sustainable development of society. While several heat transfer augmentation techniques are available, extended surfaces have been the most widely adopted due to their relative ease of integration and high heat transfer enhancement. However, since extended surfaces are mainly applicable for soft metals, and require significant capital investment for manufacturing, we propose scalably fabricated microstructures as an alternative and scalable heat transfer augmentation technique. Our etching-based structures are cost-effective, scalable, and applicable to a wide array of metallic tubing. The conformal microstructures, which have length scales ranging between similar to 1 to 12 mu m, are introduced through chemical etching of internal tube walls using hydrochloric acid. To test the water flow boiling performance of our etched surfaces, experiments were conducted on 0.25-inch diameter aluminum tubes over a range of mass and heat fluxes spanning 200 kg/(m(2)<middle dot>s) < G < 380 kg/(m(2)<middle dot>s) and 60 kW/m(2) < q '' < 125 kW/m(2), respectively. Using deionized water as the working fluid, an enhancement of up to 104 % in the tube-averaged heat transfer coefficient with a concurrent increase of up to 65 % in pressure drop was observed. The increase in the heat transfer coefficient and pressure drop is attributed to the microstructures increasing the surface roughness and creating additional nucleation sites for bubble entrapment. When compared to commercially available finned herringbone tubes, we observed that for certain operating conditions, the enhancement in heat transfer due to use of microstructures in smooth tubes can exceed that of the enhancement observed using un-structured herringbone tubes. However, the pressure drop due to the increased surface area and fluid flow disruption of the chevron shaped herringbone fins always exceeded the etched aluminum round tube. This work provides insights and understanding related to how chemical etching and surface structuring can be utilized as an alternative passive heat transfer enhancement technique in flow boiling applications.