Posted By: Kalash Sinha

June 18, 2016
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Overview

The natural circulation is based on thermo siphon principle. As the heat flux increases, the water temperature near the surface increases and reaches saturation temperature. At this point a change from liquid to vapour occurs locally, But since the bulk of water does not reach saturation temperature the steam bubbles collapse giving up their latent heat to raise the temperature of water. When the bulk of water reaches saturation temperature the bubbles do not collapse. This condition is known as nucleate boiling. Nucleate boiling regimes are characterized by high heat transfer coefficients. Beyond nucleate boiling region (i.e. at still higher heat fluxes) the bubbles coalesce to form a film of superheated steam over part or all the heating surfaces. This condition is known as film boiling. The point, beyond which film boiling occurs is known as Departure from Nucleate Boiling (DNB). Till the occurrence of DNB, metal temperature is slightly above the water temperature. When water starts boiling the metal temperature is slightly above the saturation temperature. But when DNB occurs, the metal temperature increases much higher than the saturation temperature. For high heat flux, the DNB point is reached at lower steam quality and peak metal temperature is higher.

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64x64

vinayreddy

February 23, 2018
What is DNB
64x64

vinayreddy

February 23, 2018
Kkk I got departure from nucleate boiling
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