Boiling, flow departure from,

It is concluded that thermodynamic equilibrium cannot exist in film boiling and transitional regimes during cool-down. Although this is generally expected, the extent of the departure from vapor-liquid equilibrium was previously unknown. This departure from equilibrium and the occurrence of mist and slug flows during cool-down have provided a means for the determination of vapor fractions under transient, nonisothermal conditions. 

The MCP is located at the side of the RPV. Since the SPWR has only one MCP, an MCP seizure accident was considered to be severe in view of the danger of DNBR (Departure from Nuclear Boiling Ratio). A preliminary calculation shows that a burn-out of the fuel never occurs because of the low core power density and a high natural circulation capability due to the low flow resistance and high water head of the primary circuit. 

VIPRE-01 is a finite-volume sub-channel analysis code enable of three-dimensional modelling of reactor cores and other similar geometries in steady-state and transient conditions. VIPRE-01 calculates the detailed steady-state and operational transient core flow distributions, coolant conditions, fuel rod temperature and departure from nucleate boiling ratio (DNBR). 

The reactor coolant pumps provide an adequate core cooling flow rate for sufficient heat transfer to maintain a departure from nucleate boiling ratio greater than the limit established in the safety analysis. 

Demineralization Removal of minerals from water. The term is generally restricted to ion exchange process. Departure from nucleate boiling The rate of bubble formation exceeds the rate of bubble collapse. As a consequence, sodium hydroxide would concentrate and lead to the dissolution of magnetite film. Downcomer Cold water flows in the tubes after separation of steam in the steam drum. The water flows in the downward direction. 

The calculation results of the total loss of reactor coolant flow at 15 MPa are shown in Fig. 6.56 as an example of the loss of flow events. Due to the coast-down and hence the decrease in the core coolant flow rate, departure-from-nucleate-boiling (DNB) occurs at 3 s and the cladding temperature quickly increases. However, the peak value is much lower than the criterion. Although the water source effect of the water rods is small at subcritical pressure, the core coolant flow can be kept by natural circulation in the recirculation loop before the start of the AFSs. 

A spirally wound water wall construction is applied to the furnace to have sufficient mass flow velocity in the water wall tubes under variable loads to prevent departure from nucleate boiling (DNB) and to achieve uniform water temperature distribution at the furnace outlet when operating below critical pressure and without pseudo DNB when operating above critical pressure. All heated water walls will be arranged to have upward fluid flows. 

The static limit is the non-linear limit of conditionally instability, where departure from nucleate boiling or critical heat flux will occur at low and high qualities, respectively. There are sufficient data in the literature which show that instability in multiple channels precedes the limit of classic single channel (mass-flow controlled) dryout (Mathison, 1967) (D Arcy, 1967). This differs from the result for the zero frequency condition, which can only be written as a cubic in, (Ns / Np), and does not give a critical subcooling number. The condition of static instability in parallel channels is the Ledinegg condition (Saha et al, 1976) (Duffey and Hughes, 1991),... 

The flow friction of a channel having subcooled local boiling behaves in a manner similar to the flow friction of a channel having a rough surface. From air-water data obtained at low void fractions (Malnes, 1966), the two-phase flow friction factor without bubble departure was found (Tong, 1968b Thorgerson, 1969)to be... 

When nucleate boiling occurs in narrow channel the flow regime could be different from it s in conventional tube. In conventional tube the bubble departure size is typically less that the tube diameter, so bubble flow pattern and nucleate boiling are preferable in wide range of vapor content. In case of narrow channel the bubble departure size could be comparable with the channel size, it causes strong interaction between bubbles and change both the flow pattern and heat transfer rate. 

Models for bubble departure in flow boiling are given by Klausner et al. [69] and Al-Hayes and Winterton [66]. The Al-Hayes and Winterton model was developed for gas bubble detachment from a surface but has been extended to the case of vapor bubble detachment (in boiling) by Winterton [67], When inertial forces are negligible, the stability of a bubble on the surface is governed by a balance between three forces on the bubble, resolved parallel to solid surface, namely the buoyancy force Fb, the drag force Fd, and a surface tension force Fs. Al-Hayes and Winterton give the buoyancy force as... 

FIGURE 15.28 Probability density function of bubble departure diameter in flow boiling as a function of mass flux for a constant heat flux (from Klausner et al. [69], with permission of Elsevier Science). 

FIGURE 15.109 Typical departure diameters and burst diameters for bubbles formed in nucleate boiling in annular flow (from Sun et al. [272], with permission from Taylor Francis, Washington, DC. All rights reserved). 

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