Investigation On The Steady State And Trans-Critical Transients Heat Transfer Of Supercritical Water

Supercritical water-cooled reactor(SCWR)is the only water-cooled reactor in the six advanced Generation-IV concept reactors.However,the heat transfer characteristics of supercritical water behave peculiarly due to the dramatically varied properties in the vicinity of pseudo-critical temperature.In the simple channel,there are three typical heat transfer phenomena: normal heat transfer,enhanced heat transfer and deteriorated heat transfer.In addition,the heat transfer behavior will be more complicated due to the effect of spacers.The SCWR will undergo a trans-critical transient process in the start-up and shut-down of reactor or loss pressure accident.The boiling crisis may occur again,threatening the safety of the cladding.Thus,there are great challenges in the steady-state and trans-critical transient thermal-hydraulic analysis.Relevant researches are urgent needed.This paper mainly includes the following four aspects.(1)The experimental study of steady-state heat transfer to supercritical water in a 2×2 rod bundle without spacer is carried out,to gain the circumferential and global heat transfer characteristics of the rod bundle.Four heating rods with out-diameter of 10 mm are arranged in a 2×2 square array,and the pitch-to-diameter ratio is 1.18.Non-uniformity of circumferential wall temperature distribution around the heating rod is observed.It is evenest near the pseudocritical region.The effects of heat flux,mass flux and pressure on global heat transfer of the rod bundle are discussed in detail.It is found that the heat transfer in the rod bundle is the same as that in the circular tube.There is also heat transfer deterioration in the rod bundle.The findings show that the dimensionless factor,which characterizes the effect of buoyance,has a good regularity with the heat transfer coefficient ratio.Based on the result,an empirical heat transfer correlation for rod bundle is proposed.(2)The experimental studies of steady-state heat transfer to supercritical water in vertical annular channel equipped with spacers and 2×2 rod bundles with wrapped wire are carried out.The effects of two kinds of typical spacers,namely,the grid spacer and the wrapped wire,on heat transfer are studied.The results show that the influence of grid spacer on the downstream heat transfer under supercritical conditions is significantly different from that under subcritical conditions,and the influence distance is also obviously larger.Seven factors on the grid spacer effect are analyzed,and four are dominant.The relationships between dimensionless factors and maximum heat transfer enhancement ratio are analyzed.A grid spacer heat transfer correlation suit for supercritical pressure is proposed.At a certain location downstream from the grid spacer,the further heat transfer impairment may occur.The corresponding heat transfer development at the downstream of the spacer is quite complex rather than in simple exponential decaying law.It is stabilized with oscillating regression.Based on the CFD numerical analysis,a reasonable explanation is given.Adopting the buoyancy dimensionless factor Bob,the criteria for the occurrence of further deterioration is obtained.The experimental study on the effect of wrapped wire shows that the circumferential heat transfer behavior of rod bundle is complicated.In general,the wrapped wire can enhance the global heat transfer,but cannot eliminate the heat transfer deterioration.(3)The experimental studies on the heat transfer of supercritical water flowing in 2×2 rod bundle with wrapped wire during the depressurizing and pressurizing transients are conducted,simulating the loss pressure accident and sliding pressure start-up.The corresponding wall temperature behaviors along with the flow direction are obtained.The results show that boiling crisis may occur in both depressurizing and pressurizing transients,but the behavior of the two is different.In the depressurization,there is a moving quenching front,which causes the wall temperature to rise at the same time but fall in turn.Though,the wall temperature is roughly the same rise and fall during the pressurization.In addition,the maximum temperature and influence range of boiling crisis in the depressurization are larger than those in the pressurization.The effects of mass flux,heat flux and pressure gradient on transient heat transfer are also studied.(4)For the trans-critical depressurizing transient process,the code is developed.By simulating the typical cases and experimental validation,to find out the critical conditions of boiling crisis in trans-critical transient.The code adopts steady-state one-dimensional flow equation and transient heat conduction equation of cladding,selects appropriate wall heat transfer model,realizes the moving of quenching front by giving velocity,regards the critical point temperature of water as the boundary of dry and wet wall at the critical pressure,and the trans-critical depressurizing transient is successfully simulated.Based on the experimental data and code calculated results,the critical conditions for the occurrence of boiling crisis during trans-critical depressurizing transient is obtained,which provides an important reference for the safety analysis of SCWR.