It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid.
The heat transfer coefficient has SI units in watts per square meter per kelvin (W/(m2K)).
Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale.
Experimental assessment of the heat transfer coefficient poses some challenges especially when small fluxes are to be measured (e.g. < 0.2 W/cm2).
is referred to as the difference of two radii where the inner and outer radii are used to define the thickness of a pipe carrying a fluid, however, this figure may also be considered as a wall thickness in a flat plate transfer mechanism or other common flat surfaces such as a wall in a building when the area difference between each edge of the transmission surface approaches zero.
Architects and engineers call the resulting values either the U-Value or the R-Value of a construction assembly like a wall.
Each type of value (R or U) are related as the inverse of each other such that R-Value = 1/U-Value and both are more fully understood through the concept of an overall heat transfer coefficient described in lower section of this document.
Although convective heat transfer can be derived analytically through dimensional analysis, exact analysis of the boundary layer, approximate integral analysis of the boundary layer and analogies between energy and momentum transfer, these analytic approaches may not offer practical solutions to all problems when there are no mathematical models applicable.
Recommendations by Churchill and Chu provide the following correlation for natural convection adjacent to a vertical plane, both for laminar and turbulent flow.
It is observed that a transition from a laminar to a turbulent boundary occurs when RaL exceeds around 109.
For cylinders with their axes vertical, the expressions for plane surfaces can be used provided the curvature effect is not too significant.
This represents the limit where boundary layer thickness is small relative to cylinder diameter
For cylinders of sufficient length and negligible end effects, Churchill and Chu has the following correlation for
[7] For heat flow between two opposing vertical plates of rectangular enclosures, Catton recommends the following two correlations for smaller aspect ratios.
: where H is the internal height of the enclosure and L is the horizontal distance between the two sides of different temperatures.
See main article Nusselt number and Churchill–Bernstein equation for forced convection over a horizontal cylinder.
Sieder and Tate give the following correlation to account for entrance effects in laminar flow in tubes where
[7] For fully developed laminar flow, the Nusselt number is constant and equal to 3.66.
This correlation is applicable when forced convection is the only mode of heat transfer; i.e., there is no boiling, condensation, significant radiation, etc.
In analyzing the heat transfer associated with the flow past the exterior surface of a solid, the situation is complicated by phenomena such as boundary layer separation.
Various authors have correlated charts and graphs for different geometries and flow conditions.
is the height of the boundary layer, a mean Nusselt number can be calculated using the Colburn analogy.
[7] There exist simple fluid-specific correlations for heat transfer coefficient in boiling.
This correlation is useful for rough estimation of expected temperature difference given the heat flux:[12]
In this case, the pipe wall can be approximated as a flat plane, which simplifies calculations.
This assumption allows the heat transfer coefficient for the pipe wall to be calculated as: where However, when the wall thickness is significant enough that curvature cannot be ignored, the heat transfer coefficient needs to account for the cylindrical shape.
is a measure of the overall ability of a series of conductive and convective barriers to transfer heat.
The equation takes into account that the perimeter of the heat exchanger is different on the hot and cold sides.
The overall heat transfer coefficients will adjust to take into account that a different perimeter was used as the product
The product of the average thickness and thermal conductivity will result in the fouling resistance on a specific side of the heat exchanger.