Heat transfer deterioration

Fig. 1.8 Comparison of heat transfer deterioration at supercritical pressure and dryout at subcriti-cal pressure...

Analysis of the heat transfer deterioration mechanism by numerical simulation using the k-s turbulence model is in ref. [112]. Transient and accident analysis code for fast reactors, SPRAT-F, and calculation of the Oka-Koshizuka heat transfer correlation for the safety analysis at supercritical pressure are described in ref. [113]. 

N. Takano, A Numerical Investigation of Heat Transfer Deterioration Mechanism at Supercritical Water Cooling, Master s thesis, the University of Tokyo (1994) (in Japanese)... 

Fig. 2.5 Heat transfer deterioration ratio at various flow rates, a heat trcuisfer coefficient ao ideal heat transfer coefficient at q = 0.

Fig. 2.6 Map of heat transfer deterioration, (a) Temperature and (b) Prandtl number...

Heat Transfer Deterioration at High Flow Rates... 

Figures 2.5 and 2.6 reveal that deterioration is caused by a different mechanism at low flow rates. The calculation results at G = 39 kg m s and 7 = T, which gives the Reynolds number 10,000, are rearranged in terms of the Grashof number and the Nusselt number in Fig. 2.8. Nu has a minimum value at Gr = 2 x 10. Nu is constant when Gr is lower than it, which means forced convection is dominant. On the other hand, Nu increases linearly when Gr is larger than the minimum point, which implies that natural convection is dominant. The minimum point emerges at the boundary between the two convection modes. Flow velocity and turbulence energy profiles are depicted in Fig. 2.9. When the heat flux is enhanced, the flow velocity increases near the wall and the profile becomes flat. Since turbulence energy is produced by the derivative of flow velocity, it is reduced. Hence, heat transfer is deteriorated. When the heat flux is enhanced above the minimum point, the flow velocity profile is more distorted and turbulent heat transfer is then enhanced. This type of heat transfer deterioration is attributed to acceleration as well as buoyancy. In the present analysis, buoyancy force is dominant. The computational results without the buoyancy force term in the Navier-Stokes equations are...

Generally speaking, the conventional numerical analysis with a k-e turbulence model and accurate treatment of thermophysical properties can successfully explain the unusual heat transfer phenomena of supercritical water. Heat transfer deterioration occurs due to two mechanisms depending on the flow rate. When the flow rate is large, viscosity increases locally near the wall by heating. This makes the viscous sublayer thicker and the Prandtl number smaller. Both effects reduce the heat transfer. When the flow rate is small, buoyancy force accelerates the flow velocity near the wall. This makes the flow velocity distribution flat and generation of turbulence energy is reduced. This type of heat transfer deterioration appears at the boundary between forced and natural convection. As the heat flux increases above the deterioration heat flux, a violent oscillation of wall temperature is observed. It is explained by the unstable characteristics of the steep boundary layer of temperature. 

More recent research studies on the heat transfer deterioration have revealed the following characteristics. Generally, the heat transfer deterioration phenomenon occurs only around the critical point (for water, the critical point is at 374.2°C and 22.1 MPa) or the pseudocritical temperature. The mechanisms of the heat transfer deterioration differ from those of the boiling crises of the subcritical pressure. Compared with the boiling crisis, the temperature rise of the heated surface wall is milder. The post deterioration heat transfer rate can be predicted by numerical analyses based on turbulence models and the occurrence of the heat transfer deterioration can be suppressed by promoting the turbulence. 

Therefore, in the core design of the Super LWR, it is possible to eliminate the CHF from the core design criteria. In this case, the occurrence of the heat transfer deterioration may be permitted as long as the fuel cladding temperature is kept below its limit. If the core design of the Super LWR were limited by the CHF to prevent the heat transfer deterioration, the core outlet average coolant temperature... 

Design Considerations with Heat Transfer Deterioration... 

The core design criteria of the Super LWR are significantly different from those of PWRs there is no criterion like the minimum departiue from nucleate boiling ratio (DNBR) and the heat transfer deterioration at supercritical pressure is not such a violent phenomenon as DNB at subcritical pressiue. 

The fuel rod cladding material of the Super LWR is being developed and tested. For fuel rod design of the Super LWR in the cmicept development phase, typical austenitic stainless steels or Ni-alloys are applied as described in Chap. 2. The principle of the safety criteria for fuel rod integrity is shown in Table 6.6. Since heat transfer deterioration is a much milder phenomenon than boiling transition, the minimum deterioration heat flux ratio was eliminated from the transient criterion related to fuel rod heat-up [7]. The types of abnormalities are separated into loss of cooling and overpower . 

The B-500SKDIRPV weight is heavier than that of the VVER-1(X)0, but the specific metal expenditures are close to those for VVER-1000. Titanium alley is used for the SG tubes. It was described in reference [8] that the large amount of heat transfer experimental data at supercritical pressure water flow in large bundles were obtained in Kurchatov Institute, and that there was no heat transfer deterioration in the experiments with multi rod bundles within the same test parameters range at which heat transfer deterioration occurred in tubes. It is said that the B500-SKDI concept... 

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