Convection liquid metals

Slug (plug) flow gas pocket in liquid convection (liquid metal fast bleeder reactor)... 

The thermal-convection loops are limited to flow velocities up to about 6 cm s . Where higher velocities are required, the liquid must be pumped, either mechanically or electromagnetically the latter is usually preferred as it avoids the problem of leakage at the pump seal. Basically, these forced-convection systemsconsist of (c) a hot leg, where the liquid metal is... 

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... 

It should be noted that without experimental data on the subcooled pool boiling crisis in liquid metals, the above equation cannot be verified. Another mechanism for estimating the subcooling contribution to the CHF was used for boiling with ordinary liquids (i.e., a conduction mechanism). The two mechanisms may operate simultaneously, along with the hydrodynamics and conduction-convection mechanisms (Dwyer, 1976). 

Sodium superheat experiments were performed in a forced-convection facility employing system parameters in the range of interest for application to loop- and pot-type liquid metal-cooled fast breeder reactors (LMFBRs). The test section was... 

Chen, J. C., 1963b, A Proposed Mechanism and Method of Correlation for Convective Boiling Heat Transfer with Liquid Metals, BNL-7319, Brookhaven Natl. Lab., Brookhaven, NY. (4)... 

Chen, J. C., 1970, An Experimental Investigation of Incipient Vaporization of Potassium in Convective Flow, in Liquid-Metal Heat Transfer and Fluid Dynamics, J. C. Chen and A. A. Bishop, Eds., Winter Annual Meeting, p. 129, ASME, New York. (4)... 

Tippets, F. E., J. A. Bond, and J. R. Peterson, 1965, Heat Transfer and Pressure Drop Measurements for High Temperature Boiling Potassium in Forced Convection, Proc. Conf. on Applied Heat Transfer Instrumentation to Liquid Metal Experiments, ANL-7100, p. 53-95, Argonne National Lab., Argonne, IL. (3)... 

Whalley, P. B., P. Hutchinson, and G. F. Hewitt, 1973, The Calculation of Critical Heat Flux in Forced Convective Boiling, Rep. AERE-R-7520, European Two Phase Flow Group Meeting, Brussels. (5) Whalley, P. B., P. Hutchinson, and G. F. Hewitt, 1974, The Calculation of Critical Heat Flux in Forced Convection Boiling, Heat Transfer 1974, vol. IV, pp. 290-294, Int. Heat Transfer Conf., Tokyo. (5) Wichner, R. P, and H. W. Hoffman, 1965, Pressure Drop with Forced Convection Boiling of Potassium, Proc. Conf. on Applications of Heat Transfer Instrumentation to Liquid Metals Experiments, ANL-7100, p. 535, Argonne National Lab., Argonne, IL. (3)... 

Specific correlations of individual film coefficients necessarily are restricted in scope. Among the distinctions that are made are those of geometry, whether inside or outside of tubes for instance, or the shapes of the heat transfer surfaces free or forced convection laminar or turbulent flow liquids, gases, liquid metals, non-Newtonian fluids pure substances or mixtures completely or partially condensable air, water, refrigerants, or other specific substances fluidized or fixed particles combined convection and radiation and others. In spite of such qualifications, it should be... 

Forced convection in air Forced convection in water Forced convection in liquid metals Free convection in air Free convection in water Boiling in water Condensing in steam... 

Under conventional casting conditions (without ultrasonic cavitation), the supercooling of a melt arising at the solidification front can spread into the liquid metal with a simultaneously decrease of the degree of supercooling. This process is due to convective streams and, particularly, due to forced movement of the melt with electromagnetic or mechanical stirring. 

These results prompted a number of experiments in subsequent missions designed to measure diffusion coefficients of liquid metals. Some of these experiments found a power law dependence on temperature, with the exponent varying around 2 for different materials, while others found a better fit with an Arrhenius model. However, all of the microgravity experiments consistently measured diffusion coefficients that were 30-50% lower than the accepted values, suggesting that all existing transport data for liquid metals may be contaminated by unwanted convection. A recent analysis demonstrated the difficulty of eliminating convective transport in such measurements in normal gravity.f ... 

The Peclet number reflects the ratio of heat transferred by convection to that transferred by conduction and is most commonly found in applications in laminar flow or with liquid metals. 

The both considered limit situations can be encountered in numerous problems of convective heat transfer they are schematically shown in Figure 3.1. One can see that in the case Pr — 0, which approximately takes place for liquid metals (e.g., mercury), one can neglect the dynamic boundary layer in the calculation of the temperature boundary layer and replace the velocity profile v(x, y) by the velocity v<, (x) of the inviscid outer flow. As Pr-)- oo, which corresponds to the case of strongly viscous fluids (e.g., glycerin), the temperature boundary layer is very thin and lies inside the dynamic boundary layer, where the velocity increases linearly with the distance from the plate surface. 

The temperature dependence of thermal conductivity for liquids, metal alloys, and nonconducting solids is more complicated than those mentioned above. Because of these complexities, the temperature dependence of thermal conductivity for a number of materials, as illustrated in Fig. 1,11, does not show a uniform trend. Typical ranges for the thermal conductivity of these materials are given in Table 1.1, We now proceed to a discussion of the foundations of convective and radiative heat transfer. 

Experimental data associated with gases, water, and viscous oils may be correlated with Eq. (5.150) as shown in Fig 5.22(a), where Rec denotes the critical Reynolds number at which the laminar flow is unstable. Beyond Rec the forced convection eventually becomes turbulent. Equation (5.150) does not correlate the liquid metal data. For liquid metals, viscous forces are small, the momentum equation degenerates to a limit of uniform velocity, and the importance of the Reynolds number diminishes. Consequently, as shown in Fig. 522(b),... 

Figure 5.22 Correlation of forced convection data on (a) gases, water, and viscous oils, (b) liquid metals.

For forced convection of liquid metals (Pr 1) over a horizontal flat plate subject to a uniform heat flux qm, evaluate the Nusselt number based on a uniform velocity and linear temperature profiles (Eg. 5P-2). 

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