Critical mass flux

Figure 3.59 Critical mass flux-homogeneous, equilibrium steam-water systems. (From Moody, 1975. Copyright 1975 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

They concluded that the critical mass flux is given by... 

This molar value compares to 0.16 for correlation of the data in Figure 9.16. This follows from the correlation at zero external flux and for the critical mass flux at extinction, 5 g/m2 s. 

The critical burning mass flux at extinction is between 2 and 4 g/m2 s when burning in air, as seen in the theoretical plot of Figure 9.27. A curious minimum mass flux at extinction appears to occur for the oxygen associated with air for nearly all of the fuels shown. The critical mass flux at exinction is difficult to measure accurately, but experimental literature values confirm this theoretical order-of-magnitude for air. The asymptote at 0.12 has not been verified. These theoretical renditions should be taken as qualitative for now. 

The critical mass flux G is given by the dimensionless form... 

Other criteria can be used to establish the extinction condition and that are partially equivalent to the critical Damkohler number. Such criteria are a critical mass transfer numbers (BCI) [21,32], critical mass flux of fuel [2,6,28] or critical temperatures (Ta) [2,5,29-31], The critical mass transfer number has a direct influence over the flame temperature, and thus, represents the link between the condensed phase (i.e., production of fuel) and the chemical time. The critical mass flux operates under the same principle, but assumes a consistent heat input. Combustion reactions generally have high activation energy, therefore, the reaction can be assumed to abruptly cease when the temperature reaches a critical value (Tcr). 

For isentropic expansion of two-phase mixture in thermal equilibrium, the expression for critical mass flux is obtained from enthalpy as... 

The critical mass flux is thus = G/hg, Pg, S). The Moody model (1965) is based on maximizing specific kinetic energy of the mixture with respect to the slip ratio whereas the Fauske model (1961, 1965) is based on the flow momentum with respect to the slip ratio. In Figure 22.18, the critical discharge rate of water at various stagnation pressure and enthalpy with Fauske slip model is shown. 

Figure 13.2 shows the FTS pressure loss versus LH2 mass flux for the same screens used in Figure 13.1. The mass flux is the mass flow rate of LH2 through the screen per unit cross-sectional area of screen. A reference state of LH2 saturated at 100 kPais defined to evaluate density and viscosity. The FTS pressure loss predictions were plotted over a wide enough range in mass flux so that the plots included the critical mass flux, which will be discussed in Section 13.3.

Table 13.3 List of Screen Meshes Ranked by Critical Mass Flux...

Ranking Screen Type Weave Type Critical Mass Flux (kg/(m s))... 

Safety Analysis Methods Critical mass flux ... 

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