Critical heat flux regimes

FIGURE 15.86 Map of critical heat flux regimes for boiling in tube bundles in cross flow (from Jensen and Tang [222], with permission from ASME). 

Bergles, A. E., J. P. Ross, and J. G. Bourne, 1968, Investigation of Boiling Flow Regimes and Critical Heat Flux, NYO-3304-13, Dynatech Corp., Cambridge, MA. (3)... 

The boiler will operate in the nucleate boiling regime, and the design heat flux will not exceed 60 percent of the critical heat flux to provide an adequate safety margin,... 

Pribyl, D.J., Bar-Cohen, A. and Bergles, A.E., An Investigation of Critical Heat Flux and Two-Phase Flow Regimes for Upward Steam and Water Flow, Proc. of the International Conference in Boiling Heat Transfer, May 4-8, 2003, Jamaica. 

FIGURE 15.122 Tentative map of regimes of operation of various forced convective critical heat flux mechanisms (from Hewitt and Semaria [300], with permission from Taylor Francis, Washington, DC. All rights reserved). 

The relationship between the critical heat flux and various system parameters depends on the specific flow conditions. Four different CHF regimes (referred to as V-, /-, L-, and /// -regimes) have been identified for free-surface jets [110]. In each regime, the critical heat flux depends on parameters such as the jet velocity at the nozzle exit (K ), density ratio (pf/pg), heater diameter (D), and has been shown to be markedly different in the different CHF regimes. To date, however, specific demarcations between the respective regimes have not been proposed. The following expression for the critical heat flux of a circular, free-surface jet ... 

For practical spray heat transfer calculations, a summary of empirical and interpolation correlations for each regime of spray boiling curve, ranging from single-phase to film boiling, and of transition conditions between these regimes has been provided [139,140]. As an illustration of how the hydrodynamic and other parameters affect heat transfer to sprays, the empirical equation for the critical heat flux q"cw is provided as an example [139] ... 

However, the enhanced heat transfer regime of nucleate boiling at high pressures does not imply an increase of the critical heat flux As with the increasing pressure, the density differences between vapor and liquid phases decrease the buoyancy force of bubbles drop rapidly when the pressure approaches the critical pressure pc- Thus, an increase of is only to be observed at reduced pressures up to about 0.3. Figure 2.24 illustrates the data for water. 

J.G. Denten and M. Ishii, Flow Visualization Study of Post Critical Heat Flux Region for Inverted Bubbly, Slug and Annular Flow Regime, NUREG/CR-5171, ANL-88-27, (1988). 

The effect of confinement on the heat transfer coefficient before dry-out was found to be an increase of 74% when the hydraulic diameter decreased from 2 to 0.77 mm. The effect of confinement on dry-out was found to be a decrease in the critical quality from 0.3-0.4 to 0.1-0.2 for the same reduction of the hydraulic diameter. Heat flux dependent boiling prevailed in the 2 mm hydraulic diameter tube while quality dependent boiling prevailed in the 0.77 hydraulic diameter tube because of the difference in boiling and confinement numbers. The transition from one regime to another occurred for Bo - (1 - x) si 2.2-10 regardless of the heat and mass velocity. Moreover it was found that dry-out could even be the dominant boiling mechanism at low qualities. The results obtained with the 2 mm hydraulic diameter tube were in total agreement with Huo et al. (2004) s work. Finally frictional pressure losses seem to dominate up to mass velocities of 469 kg/m s. 

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