An experimental study of upward and downward flow critical heat flux in a vertical round tube

In this paper, the experimental study of the influence of flow direction on critical heat flux in a vertical round tube is introduced. The test section is a stainless steel tube with 8.0 mm inner diameter and 1.0 mm wall thickness, heated uniformly by electric current. Freon-12 is used as the working fluid. Upward and downward flow test data of CHF are obtained at various mass flow rates (80 - 2700 kg/m/sup 2/ s), pressures (10.4-22.5 bar), and inlet qualities (-0.30 - -0.05). At very low mass flow rates, the CHF data of downward flow, lower in values, differs slightly from that of the upward flow. As mass flow rate increases, the difference becomes greater. However, when the mass flow rate is increased to greater values, the difference between them decreases and finally disappears. The greatest difference is about 25%