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

为研制超临界压力水冷动力堆，以HCFC22为试验流体，对超临界至亚临界压力快速降压过程中管内的瞬时换热特性进行了实验研究，并与稳态条件下的数据进行了对比。试验段为直径4.4 mm的圆管，压力范围为超临界压力5.5 Mpa(减压1.1)至亚临界压力4.0 Mpa(减压0.8)，质量速度分别为400和700 kg/(m^2s)。1.在稳态条件下，在略低于5.0 Mpa的临界压力范围内，由于出现膜沸腾的临界热流条件，导致换热降低。即使当热流密度很低时，在5.5兆帕的超临界压力下也没有观察到换热恶化。在降低压力的瞬变条件下，即使在初始超临界压力下没有观察到换热恶化，在略低于临界压力的压力区，也可以发现由于膜沸腾的发生而导致的换热减少。然而，换热减少和由此引起的管壁温升一直持续到远低于临界压力的压力区。在减压过程中，膜沸腾的最高管壁温度几乎保持不变，与减压速率无关。随着热流密度的增加，最高温度也随之升高。

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.