Experimental study of buoyancy and centrifugal effects on supercritical heat transfer in U-bend and contiguous straight tubes

Abstract

The buoyancy-induced thermal non-uniformity in heat transfer of supercritical fluids is a major issue that must be addressed in transcritical systems like organic Rankine cycle. In cases where U-bends are involved in heat exchangers, the introduction of centrifugal effect and its decaying nature makes the heat transfer process even more complex. In this work the buoyancy and centrifugal effects on local heat transfer to supercritical R410A were experimentally studied in a U-bend and its contiguous horizontal straight tubes (tube diameter d = 4.35 mm, bend diameter D = 43.5 mm). Measurements were performed at four circumferential positions along the heated length. Results show that bend-induced enhancement reached the maximum at the bend center, where heat transfer coefficients at the outside, top, bottom, and inside increase by 88%, 49%, 28% and 19% respectively. U-bend also caused a significant enhancement on contiguous horizontal straight tubes. In the upstream straight section, the affected distance can reach 11.5d, and the downstream affected range is more than 23d. The enhancement and the affected distance generally increased with heat flux, mass flux, and inlet temperature, but barely affected by pressure. Further analysis indicates that the centrifugal force dominated heat transfer in the bend section. Moving away from the bend, it gradually decayed, buoyancy force recovered and eventually played the major role. Existing dimensionless numbers failed to represent this transition, and a new parameter ψ* was proposed to evaluate the competition between the buoyancy effect and the decaying centrifugal effect. With a threshold value of 1, ψ* can quantitatively characterize the local heat transfer.