Forced convection boiling heat transfer and dryout characteristics in helical coiled tubes with various axial angles

In this paper, the forced convection boiling heat transfer and dryout characteristics of water flowing in a helical coiled tube at 0.4 - 3.0Mpa pressure are experimentally studied, the axis of the helical coiled tube was arranged in four different inclination angles, that is, horizontal, 45deg upwardly inclined, upwardly vertical, and 45deg downwards inclined. Experimental results demonstrate that the convection boiling heat transfer of water-steam can be divided into three regions including nucleate boiling region, forced convection region and post-dryout region, some equations for the calculations of heat transfer coefficients in the three regions and for the transition boundaries between two regions were proposed respectively. Based on experimental data and observations, we found that dryout points usually occur at the front and back sides of the cross-section of outlet of test section, a correlation for predicting the critical quality of dryout condition was proposed. The secondary flow induced by centrifugal force and the wail conduction around the dryout points were considered as the dominant factors affecting the critical conditions of dryout.