Film Temperature
Encyclopedia
In heat transfer
and fluid dynamics
, the film temperature (
) is an approximation to the temperature of a fluid inside a convection
boundary layer
. It is calculated as the arithmetic mean
of the temperature at the surface of the solid boundary wall (
) and the free-stream temperature (
):
The film temperature is often used as the temperature at which fluid properties are calculated when using Prandtl number, Nusselt number
, Reynolds number or Grashof number to calculate a heat transfer coefficient, because it a reasonable first approximation to the temperature within the convection boundary layer.
Somewhat confusing terminology may be encountered in relation to boilers and heat exchangers, where the same term is used to refer to the limit (hot) temperature of a fluid in contact with a hot surface.
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
and fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
, the film temperature (
) is an approximation to the temperature of a fluid inside a convectionConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
. It is calculated as the arithmetic mean
Arithmetic mean
In mathematics and statistics, the arithmetic mean, often referred to as simply the mean or average when the context is clear, is a method to derive the central tendency of a sample space...
of the temperature at the surface of the solid boundary wall (
) and the free-stream temperature ():The film temperature is often used as the temperature at which fluid properties are calculated when using Prandtl number, Nusselt number
Nusselt number
In heat transfer at a boundary within a fluid, the Nusselt number is the ratio of convective to conductive heat transfer across the boundary. Named after Wilhelm Nusselt, it is a dimensionless number...
, Reynolds number or Grashof number to calculate a heat transfer coefficient, because it a reasonable first approximation to the temperature within the convection boundary layer.
Somewhat confusing terminology may be encountered in relation to boilers and heat exchangers, where the same term is used to refer to the limit (hot) temperature of a fluid in contact with a hot surface.