Annulus-Side Flow Boiling and Visualization of a Three-Dimensionally Enhanced Tube Available to Purchase

Both the flow boiling heat transfer and the associated flow patterns for R410A flowing within the annulus formed by an inside enhanced heat transfer (EHT) tube (arrays of large circular dimples and background arrays of small petals) and a smooth outside tube were investigated. An annulus where both surfaces were smooth was investigated to serve as the baseline. The effects that mass flux and quality had on heat transfer and flow pattern were investigated from 100 kg/(m²s) to 204 kg/(m²s), and from 0.2 to 0.8, respectively. The findings indicate that the EHT tube annulus exhibits a 35%–49% higher heat transfer coefficient (HTC) and 5%–10% higher frictional pressure drop compared to the smooth tube annulus. At low mass flux, the effect of quality on HTC is relatively small. However, at high mass flux, the impact of quality on HTC increases initially and then decreases as the quality increases. Furthermore, the HTC decreases as the quality increases when the refrigerant flow pattern transitions to annular flow. Flow pattern observations showed that the dimple structure encouraged slug flow as compared to the smooth surface, which favored annular flow for similar flow conditions. The heat transfer enhancement mechanism of the EHT tube is due to: (1) increased heat transfer surface area; (2) enhanced turbulence in the vapor core; and (3) increasing surface wetting, which reduces the dry zone area. The Gungor and Winterton correlation was modified and fitted to the heat transfer measurements to produce a new correlation for the EHT tube.