Experiments of heat transfer and flow characteristics of subcooled water flowing in circular tubes, with and without swirl tapes, at high heat fluxes (ITER levels) were carried out at Xi’an Jiaotong University. Test sections of vertical tubes with an inner diameter of 9.0mm were electrically heated and water flow upwards. Two types swirl tapes were used with twist ratios of 2 and 4. Wall temperature distributions, critical heat fluxes and pressure drops were obtained during the experiment. The main attentions were paid to the effects of swirl tapes on enhancing the heat transfer and postponing the departure from nucleate boiling (DNB). The test parameters were as follows, heat flux (q)=10, 15MW/m2, pressure (P)=3, 4.2, 5MPa, mass flow rates (G)=6000, 8000kg/m2s, bulk temperature (Tb)=25–247 °C. Compared with the smooth tube, the experimental results showed that the swirl tapes can improve the heat transfer performance slightly while the pressure drops increase greatly. However, the DNB can be remarkably postponed with the swirl tapes, which is of vital practical significance to ITER. In addition, it was interesting to find that harsh boiling noise, which could be caused by microbubble emission boiling (MEB), and some inner surface of the tube was damaged, looks like cavitation erosion.

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