Experimental Investigation on Dryout in Upflow Vertical Tube

The critical heat flux (CHF) experiment was performed in a tube with inner diameter of 8.2 mm and heated length of 2.4 m. The pressure is 3.2-19.7 MPa, mass flux is 963-2 707 kg/（m²•s）, inlet subcooling is 34-213 ℃ and outlet quality is 0.11-0.78. The results show that the CHF almost goes up linearly while mass flux or inlet subcooling increases, and falls down rapidly while the outlet quality rises. Based on the two-phase parameter analysis, it is found that within the experiment parameter range, the CHF is much related to vapor velocity. Criticality occurs when the liquid near the wall vanishes due to vapor entrainment after reaching the critical vapor velocity. As a result, the surface tension decreases with the increase of the pressure, the liquid film tends to tear out and the critical vapor velocity falls down while pressure rises. Based on the critical vapor velocity U_cr from experimental data of high pressure and the initiation entrainment velocity U₀ proposed by Steen and Wallis, a prediction model for the critical vapor velocity was presented. The predicted results by this model agree well with the experimental data of low pressure.