Boiling in free flow. The Nukijama curve

Lower values n 0.5 have only been found for substances with low boiling points, such as helium. The quanitity c is strongly dependent on the material properties of the boiling liquid and the structure of the heating surface. If, for example, c is known for a particular liquid that boils at a given pressure on a flat, polished, steel tube, then the heat transfer coefficient calculated with this quantity may not be used if the same liquid was boiling on a rough steel tube or a copper tube. 

Equation (4.89) or (4.91) is only valid in regions of intensive nucleate boiling. However, as Nukijama [4.67] was the first to show there are further regions of vaporization. Fig. 4.37 illustrates the individual regions that appear in boiling water at atmospheric pressure and in free flow. The heat flux q is plotted against 

The A-shaped curve, also called the Nukijama curve, which at first glance seems rather strange, is physically plausible With increasing wall temperature more vapour bubbles form at the heated wall, which set the liquid close to the wall into motion. The result of this is that bubble formation promotes heat transfer from the wall to the liquid. 

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