Transient Heat Transfer in a Tube during Pressure Reduction from Supercritical to Subcritical Pressure

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For the development of a supercritical pressure water cooled power reactor, experiments were carried out to clarify the characteristics of the transient heat transfer in a tube during rapid pressure reduction from supercritical to subcritical pressure, using HCFC22 as the test fluid, and transient heat transfer characteristics were clarified comparing with data under steady state conditions. The test section was a circular tube of 4.4 mm I.D. The pressure was in the range of a supercritical pressure 5.5 MPa (a reduced pressure of 1.1) down below to a subcritical pressure 4.0 MPa (a reduced pressure of 0.8), and the mass velocity was 400 and 700 kg/(m^2s).Following results were obtained.1.Under steady state conditions, a reduction in the heat transfer due to the occurrence of critical heat flux condition of film boiling was found in the region of pressure slightly below the critical pressure of 5.0 MPa, even when the heat flux was such a low value at which no heat transfer deterioration was observed at a supercritical pressure of 5.5 MPa. The heat transfer reduction was found at lower pressure with increasing heat flux.2.In the transient conditions of reducing pressure, the same reduction of heat transfer due to the occurrence of film boiling was also recognized in the pressure region slightly below the critical pressure, even when no heat transfer deterioration was observed at the initial supercritical pressure condition. The heat transfer reduction and resulting tube wall temperature rise, however, were noticed to continue down to the pressure region much lower than the critical pressure. During the pressure reduction, the maximum tube wall temperature in the film boiling was kept almost constant independently of pressure reduction rate. Furthermore, the maximum temperature became higher with an increase of heat flux.