Low subcooling region

Ps > rjslP => Low subcooling region (flashing starts upstream of the nozzle)... 

Low subcooling region This region is defined when the flashing occurs upstream of the throat. 

Chapter four Overpressure protection Low subcooling region. 

Mass flux for low subcooling region. The mass flux depends on the type of flow. 

In the subcooled or low-quality region, the boiling crisis of a nonuniformly heated section can occur upstream of a boiling channel. The equation for predicting the local CHF can be written as... 

Sekoguchi, K., O. Tanaka, T. Ueno, M. Yamashita, and S. Esaki, 1982, Heat Transfer Characteristics of Boiling Flow in Subcooled and Low Quality Regions, 7th Int. Heat Transfer Conf. Paper FBI 2, Munich, Germany, Hemisphere, Washington, DC. (4)... 

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

It is important to note that arising naturally from the analysis, the instability region is bounded by the two roots of Equation (8), where the pressure drop versus flow rate is a minimum, maximum or point of inflection. The region of instability for a parallel channel system as bounded by these roots, which correspond to low (subcooled boiling) and high (saturated boiling) vapor qualities. This result has been rederived and confirmed by (Babelli et al, 1995), who retained the vapor to liquid density ratio, and the convective acceleration term, to derive... 

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). 

Figure 8 shows that increasing the heat flux at constant mass velocity causes the peak in wall temperature to increase and to move towards lower enthalpy or steam quality values. The increase in peak temperature is thus due not only to a higher heat flux, which demands a higher temperature difference across the vapor film at the wall, but to a lower flow velocity in the tube as the peaks move into regions of reduced quality. The latter effect of lower flow velocity is probably the dominant factor in giving fast burn-out its characteristically rapid and often destructive temperature rise, for, as stated earlier, fast burn-out is usually observed at conditions of subcooled or low quality boiling. 

We shall now consider subcooled liquid fed into the bottom of a vertical evaporator tube, that is uniformly heated along its entire length. The heat flux q is assumed to be low and the tube should be long enough such that the liquid can be completely evaporated. Fig. 4.53 shows, on the left, alongside the various heat exchange regions that have already been explained, the profiles of the liquid and wall temperatures. 

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... 

Effect of Channel Orientation. For the subcooled boiling region, a study of the effect of channel inclination on critical heat flux is reported by Brusstar and Merte [294]. The channel used by these authors was rectangular in cross section with one side heated. The orientation of this heated surface with respect to the horizontal could be varied between 0 and 360°. At low velocities, a sharp decrease in critical heat flux was observed when the heater surface was downward-facing, as exemplified by the results shown in Fig. 15.119. 

In general, the processes modeled in the prediction methods for subcooled and low-quality critical heat flux mentioned above are clearly important ones there seems scope for fundamental work on the precise mechanisms involved in the near-wall region. 

The implication is that correlations similar to those used for pool boiling can be used for multicomponent boiling at low-quality or subcooled conditions. For the annular flow region, predictions based on the models described in the preceding section should give reasonable results based on mean physical properties of the mixture. 

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