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This color schlieren image reveals thermal convection from a human hand (in silhouette form) to the surrounding still atmosphere.. Two types of convective heat transfer may be distinguished:
The Nusselt number may be obtained by a non-dimensional analysis of Fourier's law since it is equal to the dimensionless temperature gradient at the surface: q = − k A ∇ T {\displaystyle q=-kA\nabla T} , where q is the heat transfer rate , k is the constant thermal conductivity and T the fluid temperature .
Earth's longwave thermal radiation intensity, from clouds, atmosphere and surface.. Heat transfer is the energy exchanged between materials (solid/liquid/gas) as a result of a temperature difference.
In very sticky, viscous fluids (large ν), fluid motion is restricted, and natural convection will be non-turbulent. Following the treatment of the previous subsection, the typical fluid velocity is of the order of g Δ ρ L 2 / μ {\displaystyle g\Delta \rho L^{2}/\mu } , up to a numerical factor depending on the geometry of the system.
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.
For air with a pressure of 1 bar, the Prandtl numbers in the temperature range between −100 °C and +500 °C can be calculated using the formula given below. [2] The temperature is to be used in the unit degree Celsius.
Where and are the velocities in the x and y directions respectively normalized by the free stream velocity, and are the x and y coordinates non-dimensionalized by a relevant length scale, is the Reynolds number, is the Prandtl number, and is the non-dimensional temperature, which is defined by the local, minimum, and maximum temperatures:
This mechanism is found very commonly in everyday life, including central heating and air conditioning and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example).