High subcooling region

Determine the transition saturation pressure ratio for low or high subcooling region ... 

Ps < r/slP, => High subcooling region (flashing starts at the throat of nozzle)... 

High subcooling region This region is defined when the flashing occurs at the throat. 

Mass flux for high subcooling region. For this region also, the mass flux depends on the type of flow. 

This is a high subcooling region and the flow is critical.)... 

So far the pressure drop in two-phase flow in pipes and rod bundles has often been predicted by empirical correlations, despite the development of analytical models as described in the previous sections. Thus, in the highly subcooled boiling region,... 

When the boiling crisis occurs, the surface temperature rises. Because of the fairly good transfer coefficient of a fast-moving vapor core in an annular flow, the wall temperature rise after a dryout in the high-quality region is usually smaller than that in a subcooled boiling crisis. It is even possible to establish steady-state conditions at moderate wall temperatures, so that physical burnout may not occur immediately. Thus dryout is also described as slow burnout. 

The graphical correlation shown in Figure 5.30 was originally developed for a steam-water mixture of high quality. Coffield et al. (1967) extended its validity into the subcooled region by comparing their subcooled Freon-113 DNB data with... 

For the second characteristic line at high quality, we have N is 0(3), from both theory and experiment for the first characteristic line in the low quality subcooled region, stability is possible when N is 0(1). 

Comparisons to a wide range of existing stability data points in tubes, channels and rod bundles show naturally grouping along the low and high quality limits. Analytically and empirically the maximum (second) unstable line has a value of N/Ns) = 5 and in highly subcooled flows the minimum (first) unstable line has a value of Np/Ns) = 0.7. The region of static instability effectively corresponds to the onset of CHF or DNB in all parallel channel systems with a constant pressure-drop boundary condition. 

Figure 30 shows the main characteristics of the effect of pressure. It can be seen that a maximum heat flux occurs generally below 500 psia. The most interesting feature of Fig. 30, however, is a secondary maximum in the region 1500-2000 psia which tends to occur with moderate Ljd ratios, large inlet subcooling, and high mass velocity. 

In the case of the subcooled liquid, which involves an extrapolation into the solid region, the vapor pressure is usually so low that the fugacity coefficient is. close to unity, and the fugacity of this hypothetical liquid is equal to the extrapolated vapor pressure. For the supercritical liquid, however, the extrapolation is above the critical temperature of the liquid and yields very high vapor pressures, so that the fugacity of this hypothetical liquid is equal to the product of the extrapolated vapor pressure and the fugacity coefficient (which is taken from the corresponding-states plot of Fig. 

The onset of nucleate boiling (line XX in Fig. 15.88) occurs above the x = 0 line at low heat flux (i.e., there is a net bulk superheat of the liquid) but at qualities less than 0 for high heat fluxes, corresponding to the region of subcooled nucleate boiling. For heat transfer to a single-phase liquid, the wall superheat (ATsat)w can be calculated from... 

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