Forced-convective boiling coefficient

The forced-convective boiling coefficient will be estimated using Chen s method. 

The forced-convective boiling coefficient will be estimated using Chen s method. With 5% vapor, liquid velocity (for liquid flow in tube alone)... 

The detail experimental study of flow boiling heat transfer in two-phase heat sinks was performed by Qu and Mudawar (2003b). It was shown that the saturated flow boiling heat transfer coefficient in a micro-channel heat sink is a strong function of mass velocity and depends only weakly on the heat flux. This result, as well as the results by Lee and Lee (2001b), indicates that the dominant mechanism for water micro-channel heat sinks is forced convective boiling but not nucleate boiling. 

In forced-convective boiling the effective heat-transfer coefficient hcb can be considered to be made up of convective and nucleate boiling components h fc and h nb. 

The convective boiling coefficient lijc can be estimated using the equations for singlephase forced-convection heat transfer modified by a factor fc to account for the effects of two-phase flow ... 

Chen s method was developed from experimental data on forced convective boiling in vertical tubes. It can be applied, with caution, to forced convective boiling in horizontal tubes, and annular conduits (concentric pipes). Butterworth (1977) suggests that, in the absence of more reliable methods, it may be used to estimate the heat-transfer coefficient for forced convective boiling in cross-flow over tube bundles using a suitable cross-flow correlation to predict the forced-convection coefficient. Shah s method was based on data for flow in horizontal and vertical tubes and annuli. 

What is the order of magnitude for the convection heat transfer coefficient in free convection Forced convection Boiling ... 

Kenning and Cooper [233] report data for forced convective boiling of water in a vertical tube. At high qualities, the heat transfer coefficient becomes independent of heat flux and varies approximately linearly with quality as illustrated in Fig. 15.95. 

FIGURE 15.111 Variation of heat transfer coefficient with composition in the forced convective boiling of R134a/R123 mixtures (from Fujita and Tsutsui [279], with permission from Taylor Francis, Washington, DC. All rights reserved). 

In Table 4.1-2 some order-of-magnitude values of h for different convective mechanisms of free or natural convection, forced convection, boiling, and condensation are given. Water gives the highest values of the heat-transfer coefficients. 

The variety of regimes during the forced convection boiling in tubes or ducts requires different correlations in order to determine the heat transfer coefficient related to the respective boiling mechanisms. The well-established correlations have been developed for nucleate boiling controlled heat transfer - when evaporation occurs at the inner tube surface - and convective boiling heat transfer - when evaporation occurs at the liquid film interface. 

The heat transfer coefficient of boiling flow through a horizontal rectangular channel with low aspect ratio (0.02-0.1) was studied by Lee and Lee (2001b). The mass flux in these experiments ranged from 50 to 200 kg/m s, maximum heat flux was 15 kW/m, and the quality ranged from 0.15 to 0.75, which corresponds to annular flow. The experimental data showed that under conditions of the given experiment, forced convection plays a dominant role. 

Single-phase flow region at the inlet the liquid is below its boiling point (sub-cooled) and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat-transfer coefficient in this region. 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

For liquid metals the superiority of nucleate boiling heat transfer coefficients over those for forced-convection liquid-phase heat transfer is not as great as for ordinary liquids, primarily because the liquid-phase coefficients for liquid metals are already high, and the bubble growth period for liquid metals is a relatively short fraction of the total ebullition cycle compared with that for ordinary fluids. In the case of liquid metals, the initial shape of the bubbles is hemispheric, and it becomes spherical before leaving the heating surface. This is because of very rapid... 

Ivashkevitch, A. A., 1961, Critical Heat Flux and Heat Transfer Coefficient for Boiling Liquids in Forced Convection, Teploenergetika, October, Vol. 8. (5)... 

No, H. C., and M. S. Kazimi, 1982, Wall Heat Transfer Coefficients for Condensation Boiling in Forced Convection of Sodium, Nuclear Sci. Eng. 57 319-324. (4)... 

For the forced convective region, only limited data are available on the effects of the different variables involved, since the existence of this region has only been recognized recently. As previously mentioned, the velocity required for the suppression of nucleate boiling increases with pressure further, an increase in pressure reduced the specific volume of the vapour and hence the linear velocity of the two phase mixture at a given quality will be reduced. Thus higher velocities and steam qualities would be required for the forced convective region to be entered at the same heat flux. The effect of diameter is, as far as can be seen from the work of previous experiments and from these experiments, that to be expected with convective heat transfer, namely, that the coefficient is proportional to the diameter or the equivalent diameter to the power —02. 

It is interesting to note that the heat transfer coefficients due to this forced convection effect, when it suppresses nucleation, may be higher than the eoeflicients for nucleate boiling with the same heat flux and no forced convection effect. 

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