Week 5 Q&A lecture 2022, S1
1. The heat transfer coefficient h
h is a concept derived from the Newton¡¯s law of cooling
A deceptively simple equation. Convective heat transfer is defined as the rate of heat transfer through a unit surface area by a unit temperature difference between a solid surface and a fluid in motion.
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A full theoretical treatment involves the understanding of the fluid velocity boundary layer and temperature boundary layer.
The flow region immediately next to the solid surface is called the boundary layer. In this layer, a significant velocity gradient is established; the fluid at the immediately adjacent location has zero velocity. Fluid ¡°sticks¡± to the solid surface, thus, this condition is known as the no-slip condition. Therefore, heat transfer from the solid surface to the fluid layer adjacent to the solid surface is purely by conduction.
A full empirical description of h must consider
(a) the intensity of the fluid motion (i.e. fluid velocity),
(b) the relative thicknesses of the velocity and temperature boundary layers, (c) the geometry of the solid object.
In the empirical modelling, we focus on quantitative description of fluid motion and heat transferred by fluid motion. We do not focus on heat transfer within the solid.
2. Difference between Bi and Nu
3. Prandtl number Pr
4. Reynolds number has been well explained in the lecture recording
5. Nu is empirically linked to Re, Pr in forced convection, and Gr, Pr in natural convection
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