Convective condensation in three enhanced tubes with different surface modifications

An experimental investigation took place in order to determine the convective condensation characteristics of R410A in: two enhanced heat transfer tubes with novel surface modifications; a micro-fin tube and a plain tube. Conditions included: mass flux values ranging from 70 to 330 kg m−2 s−1; saturation temperature is fixed at 313 K; with an inlet quality of 0.9 and outlet quality of 0.2. All tubes evaluated have an outer diameter of 9.52 mm; the heat transfer surface of the 1EHT tube consists of dimples and petal arrays; and the 4EHT tube is produced using trapezoidal dimples in a grid-like arrangement. The experimental results were analyzed using available flow pattern maps. Results indicate that several flow regimes are involved over the range of conditions, including stratified wavy, intermittent and annular. The micro-fin tube exhibits the best enhanced performance with heat transfer coefficients 1.8-2.2 times higher than those of the plain tube; while the 1EHT tube produces a heat transfer coefficient that is 1.4-1.65 times larger than those of the plain tube. Enhancement of the 1EHT tube is achieved by: disruption of the boundary layer; secondary fluid flow generation; fluid mixing caused by a transverse fluid transport; increased fluid turbulence; and increased heat transfer area. When considering enhanced tube frictional pressure drop, the 1EHT tube produces the largest pressure drop and the 4EHT tube is the smallest. Finally, a performance factor comparison was performed in order to evaluate the enhancement effect of the enhanced tubes. This analysis details the superior performance of the 1EHT tube under condensation conditions.