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Tiberious
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Homework Statement
(a) Explain how heat is lost from a hot surface to the surrounding air.
The Attempt at a Solution
We will assume as there has been no other stipulation that the effect of radiation is negligible and thus we are taking purely about conduction.
We will also assume as there has been no othe stipulation that the fluid 'air' is in motion.
The image below depicts the flow of a fluid over a hot surface.
Image shows the flow profile and Temperature profile.
Figure 2a, Teesside University, 'Overall Heat Transfer Rates', unknown, 8nd April 2018.
Firstly, the fluid in contact with the hot surface has no velocity. Seen in the diagram above the velocity of the fluid increases with distance γ until it reaches the mainstream velocity of the fluid flowing of U_f. This region is known as the Boundary Layer.
The fluid that is in contact with the surface has the same temperature as the surface itself. This then falls through the Boundary Layer to the mainstream temperature T_f.
Further to our prior stipulation that we are to assume the radiation effect is negligible. Consider the fluid in contact with the surface is stationary, heat is transferred by conduction thus the Fourier's Law applies.
ϕ=-kA(dT/dy)_s
We can see the subscript s has been attached to the base of the temperature bracket indicating that the temperature gradient at the surface determines the heat Loss ϕ.
Can anyone let me know if this is a reasonable answer?