Condensation heat transfer in annuli outside horizontal stainless steel enhanced tubes

Abstract

The condensation heat transfer of R410A outside several enhanced heat transfer (EHT) tubes [dimple/herringbone (DIM/HB), herringbone (HB), and dimple (DIM)] and smooth tube were evaluated. All tubes are made of stainless steel; the test length is 2 m; the outer diameter is 12.7 mm; the inner diameter is 11.5 mm. Experimental conditions were set at a saturated temperature of 45 °C; condensed vapor mass decreased from 0.8 to 0.2; mass flux ranged between 75 and 225 kg m−2 s−1. As a result, the HB tube effectively improved flow condensation performance outside tubes; while the dimple-only structure is not effective since it produces a lower heat transfer coefficient (HTC) than a smooth tube (at some low flow rate conditions). Substantial enhancement is produced by the DIM/HB composite surface produced from dimple and herringbone microfin patterns, which is due to the combination of the liquid drainage effects by herringbone structure to remove condensate and causing strong turbulence of dimple structure. With the increase of mass flux and average vapor quality, the condensation heat transfer coefficient and pressure drop of each test tube increase as well. Additionally, an improved Nusselt equation for external condensation on a smooth tube was developed to predict the heat transfer coefficient for the experimental conditions considered here. Finally, correlations of enhanced heat transfer (EHT) tubes (microfin – HB, dimple - DIM, and composite surface – DIM/HB) to predict condensation heat transfer coefficient were developed and evaluated.