Heat Transfer Correlations
The heat transfer coefficient may be approximated using Bromley's equation,
Where, is the outside diameter of the tube. The correlation constant C is 0.62 for horizonatal cylinders and vertical plates and 0.67 for spheres. Vapor properties are evaluated at film temperature.
For stable film boiling on a horizontal surface, Berenson has modified Bromley's equation to yield,
For vertical tubes, Hsu and Westwater have correlated the following equation,
Where, m is the mass flow rate in at the upper end of the tube
At excess temperatures above that at the minimum heat flux, the contribution of radiation becomes appreciable and becomes dominant at high excess temperatures. The total heat transfer coefficient can be is thus a combination of the two. Bromley has suggested the following equations for film boiling boiling from the outer surface of horizontal tubes.
If ,
The effective radiation coefficient, can be expressed as,
Where, is the emissivity of the solid and is the Stefan-Boltzmann constant.
Read more about this topic: Leidenfrost Effect
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