What is the Nusselt number?

Nusselt number is the product of a comparison of the ratio for heat transfer between two liquids, where the heat transfer of convection is divided by thermal conductivity of the material at rest. It is a number without a unit that basically provides a comparative speed for how fast heat is transmitted between materials where convection or gas or liquid flow is carried out compared to the basic heat transfer, where there is a low internal movement of matter. Although the Nusselt number is often used to measure heat transfer in liquids, it can also be used to compare the convection with the heat transfer rate between the solids of the gas flow. The basis for comparison of heat transfer in the materials was founded by Ernst Wilhelm Nusselt, a German mechanical engineer who taught in Dresden during the early 20th century. His pioneering work in the Fluid mechanics at that time led to the name of Nusselt.

Typical convection Nusselt Number Equation is NO = hl/λ. nu represents a nussel number, h is a heat transfer coefficient that can be measured in Watts per square meter of Kelvin temperature (w/m 2 k). The l equals the length of the limit layer in meters of the material where heat is transmitted, and lambda (λ) represents established thermal conductivity of the material in watts per meter of Kelvin or w/mk. Measurement of Nusselt's heat number is only over the surface boundaries and is directly related to Newton's cooling law, from which the number can also be derived.

When the Nusselt correlation is closer to one, it shows the type of heat transfer known as laminar flow or slug flow, indicating that very small movement of fluid is ongoing and conductive heat loss is important. As the flow rates increase, they are depreciated as turbulent and can often lead to nusselt numbers that range from 100 to 1,000. Any value for the nussel number between 100 and 1,000 shows that the dominant form of heat transfer is convection and leads only a minor role. Such numbersThey are important values ​​that need to be known in various aspects of engineering, where heat transfer can be desirable or undesirable. The speed of cylinders with a high number of nussel numbers is considered to be a turbulent and efficient heat transfer method, for example, in the flow of fluids by pipeline, where little thermal energy is lost in the material leading to the external environment.

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