Critical flow mass flux

Critical flow mass flux. Critical flow mass flux is estimated using Equation 4.80. 

Available data sets for flow boiling critical heat flux (CHF) of water in small-diameter tubes are shown in Table 6.9. There are 13 collected data sets in all. Only taking data for tube diameters less than 6.22 mm, and then eliminating duplicate data and those not meeting the heat balance calculation, the collected database included a total of 3,837 data points (2,539 points for saturated CHF, and 1,298 points for subcooled CHF), covering a wide range of parameters, such as outlet pressures from 0.101 to 19.0 MPa, mass fluxes from 5.33 to 1.34 x lO kg/m s, critical heat fluxes from 0.094 to 276 MW/m, hydraulic diameters of channels from 0.330 to 6.22 mm, length-to-diameter ratios from 1.00 to 975, inlet qualities from —2.35 to 0, and outlet thermal equilibrium qualities from -1.75 to 1.00. 

A vacuum chamber is vented using argon at 20 °C and standard atmospheric pressure. It flows into the chamber via a nozzle of 2 mm diameter. Calculate (i) the critical pressure (ii) the critical velocity (v ) and (iii) the mass flux of the gas. 

In forced convective systems, the bubble departure diameter can be critically affected by the presence of a velocity field. Studies of bubble departure diameters in forced convection include those of Al-Hayes and Winterton [66], Winterton [67], Kandlikar et al. [68], and Klausner et al. [69]. The results obtained by Klausner et al. for the probability density function of departure diameter as a function of mass flux and heat flux, respectively, are shown in Figs. 15.28 and 15.29. The most probable departure diameter decreases strongly with increasing flow rate and decreases (less strongly) with decreasing heat flux at a fixed flow rate. It is clear that these velocity effects have to be taken into account in predicting forced convective boiling systems. 

FIGURE 15.118 Effect of mass flux on critical heat flux in upward water flow in a vertical tube (calculated from the Bowring [293] correlation for a tube length of 1 m, a tube diameter of 0.01 m, and zero inlet subcooling) (from Hewitt [291], with permission from The McGraw-Hill Companies). 

Fines are migrated in the medium if the salinity of the aqueous phase, CSjW, is below a critical value or if a critical flow velocity is surpassed (u > iqc). The critical salinity concept does not apply to fines migration in the oleic phase, therefore ki0dl = 0. It is assumed that the rate at which fines are trapped at pore throats is proportional to the mass flux of the particles, Furthermore, for fines that... 

The heat flux at this condition is referred to as the critical heat flux, or CHF. It depends on the flow conditions, channel geometry, local quality, fluid properties, channel material, and flow history. Bergles and Kandlikar [8] discuss the CHF in microchannels from a systems perspective. It is important to establish CHF condition as a function of the mass flux and quality for a given system to ensure its safe operation. Qu and Mudawar [9] presented CHF data with water in 21 parallel minichannels of 215 x 821 pm cross-section over a range of G = 86—268 kg/(m s) and q" = 264-542 kW/m r = 0.0-0.56, and Fi = 121.3—139.8 kPa. Kosar et al. [5] present low-pressure water data in microchannels enhanced with reentrant cavities. Also, the correlation by Katto [10] developed for large channels may be applied for approximate CHF estimation in the absence of an established CHF correlation for microchannels. 

To determine the maximum possible flow through an actuated pressure relief device, use of the phenomenon that a maximum mass flux exists which only depends on the state in the vessel (see Figure 14.4) is made. For a given pressure Po in the vessel, the mass flow is determined by the size of the narrowest cross-flow area A] in the line. When the outlet pressure Pi is decreased, the mass flow increases. However, if Pi falls below a certain value, no further increase of the mass flow takes place (Figure 14.5). The maximum mass flow is called the critical mass flow merit ... 

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. 

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