Post-CHF axial-and swirl-flow heat transfer in small horizontal tubes

Axial- and swirl-flow heat transfer in the post-critical heat flux (CHF) region in small diameter horizontal tubes under low wall superheat conditions was investigated experimentally. For this reason, a horizontal, concentric-tube, counterflow, test section was designed. The boiling fluid [refrigerant 113 (R-I 13)] flows in the inner brass tube of 4.74 mm inside diameter while the heating fluid (water) flows in the outer copper tube of 10.9 mm in a counterflow mode. The heated length of the test section is 1.66 m. The test section was designed, fabricated and connected to the existing University of Illinois at Chicago How boiling facility. Four tapes were used with tape twist ratios of Y = 4, 5.27, 7.63, and 9.15. A total of 25 experiments in an open tube and 109 experiments in swirl flow were conducted. In both types of experiments (axial and swirl flow), the refrigerant mass flux was in the range 51-518 kg/m(2) - s with a pressure of 0.18 MPa to simulate once through and recirculating steam generators. The liquid heating method that was adopted in our experiments permits the study of the whole post-CHF region for a wide range of variables. In all experiments, the R-113 entered the test section as subcooled liquid and exited as superheated vapor, and a heat balance was calculated. Wall superheats were maintained below 60 degrees C for heat exchanger/steam generator application. On the other hand, existing correlation to predict the swirl-flow, post-CHF heat transfer in vertical tubes with larger diameters (8-10 mm) was modified to account for the small diameter, horizontal configuration and the increased drop-wall interaction at the high mass fluxes. The experimental data are presented in graphical form.