Characteristics of the critical heat flux for downward flow in a vertical tube at low flow rate and low pressure conditions

The characteristics of the critical heat flux (CHF) for downward flow were studied experimentally with an Inconel 600 circular tube test section in a water test loop at low-flow rate (0 200 kg/m²s) and low-pressure (0.1 0.7 MPa) conditions. The attention was given to the effects of upstream conditions—upper plenum and inlet throttling. Two totally different kinds of CHF behaviors were observed. It seems appropriate to interpret them as flooding-type CHF and dryout in annular flow. The CHF in downward flow may vary from extremely unstable flow CHF as low as near the flooding CHF value to stable flow CHF as high as that of upflow, depending on the upstream conditions of the test section. The CHF correlation by Mishima and that by Weber were proposed for the presentation of the lower and upper limits of the CHF for downward flow in a vertical tube at low-flow rate and low-pressure conditions.