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Based on the previous discussion, the following conclusions can be drawn:
The Homogeneous-Equilibrium Model (HEM) underpredicts the critical flow rates for short pipes and near liquid saturation or subcooled upstream conditions.
The equilibrium slip models of Fauske, Moody and others, although successful for long tubes, underpredict the critical flow rates for short pipes. This is particularly true if the upstream condition is subcooled or near saturation.
Effects of thermal nonequilibrium must be taken into account for short pipes. However, it is not clear whether the pipe length, L, or the pipe length-to-diameter ratio, L/D, or both are important in determining the effects of thermal nonequilibrium.
More mechanistic models do exist for critical flow that can account for the nonequilibrium effects of velocity differences and temperature differences between the phases; but require iterative or numerical solution.
At present, there is no general model or correlation for critical flow which is valid for a broad range of pipe lengths, pipe diameters, and upstream conditions including subcooled liquid. The more sophisticated the model used for a more precise design, the more prudent it is to consider some data benchmarking under similar conditions.
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All contents © Michael L. Corradini
Tue Sep 2 15:06:55 CDT 1997