Experimental investigation of heat transfer to supercritical pressure R410a in a U-tube

To properly guide the design of vapor generators in transcritical organic Rankine cycles, heat transfer charac-teristics of supercritical pressure R410a in a horizontal U-tube was experimentally investigated. The pipe diameter d and bend diameter D of the U-tube are 4.35 mm and 43.5 mm, respectively. Measurements were performed at mass flux of 300-1600 kg/(m2s), heat flux of 25-80 kW/m2, inlet temperature of 283-343 K, and pressure of 5-5.88 MPa. In all cases, local enhancement of heat transfer was observed within the bend. The extent of bend-induced enhancement is related to Reynolds numbers and property variations of fluid. Up to 25 % in-crease in the enhancement was observed in the case of dramatic property changes when compared to case with little property change. The bend-affecting upstream length is within 5.75d, and the bend effect diminishes at 11.5d downstream of the bend and further. Moreover, it was shown that a few existing correlations perform reasonably well in the straight sections, but not in the bending section. A new correlation was developed by introducing an enhancement factor to account for the bend-induced centrifugal effect. The newly proposed equation correlates 85 % of the experimental data with an absolute error less than 10 %.