The Nusselt number is a unitless comparison rate for heat transfer between materials with convection or conduction. It can be used for fluids or solids per gas flow. The equation is Nu = hL/λ. A Nusselt correlation close to one indicates laminar flow, while values between 100 and 1000 indicate convection as the dominant form of heat transfer. High Nusselt numbers are desirable for efficient heat transfer in engineering.
A Nusselt number is the product of a comparison of ratios for heat transfer between two fluids in which the convective heat transfer is divided by the thermal conductivity of the material at rest. It is a unitless number that essentially provides a comparison rate for the rate at which heat is transferred between materials where convection or a flow of a gas or liquid occurs, versus the basic heat transfer by conduction where internal movement of matter is happening. Although the Nusselt number is often used to measure heat transfer in fluids, it can also be used to compare convection and conduction heat transfer rates between solids per gas flow. The basis for comparisons of heat transfer in materials was established by Ernst Wilhelm Nusselt, a German mechanical engineer who taught in Dresden in the early 20th century. His pioneering work in fluid mechanics at the time led to Nusselt’s number being named after him.
A typical convection Nusselt number equation is Nu = hL/λ. Nu represents the Nusselt number, h is the heat transfer coefficient which can be measured in watts per square meter for the Kelvin temperature (W/m2K). L equals the boundary layer length in meters of a material into which heat is transferred and lambda (λ) represents the material’s stated thermal conductivity in watts per meter Kelvin or W/mK. The Nusselt number measurement of heat is a measurement across surface boundaries only and is directly related to Newton’s law of cooling from which the number can also be derived.
When the Nusselt correlation is close to a value of one, this indicates a type of heat transfer flow known as laminar flow or slug flow, which indicates that very little fluid motion is occurring and that the heat loss conductive is important. As flows increase, they are described as turbulent and can often lead to Nusselt number values ranging from 100 all the way up to 1,000. Any value for a Nusselt number between 100 and 1.000 is indicative that the dominant form of heat transfer that occurs is by convection, with conduction playing only a minor role. Such numbers are important values to know in various aspects of engineering, where heat transfer can be a desirable or undesirable trait. A high Nusselt number cylinder transfer rate is considered turbulent and an efficient method of heat transfer, as in fluid flow through piping where little thermal energy is lost in the material through conduction to the outside environment.
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