CHEE2695 – Lecture 13
Convection:
- There is a layer near a surface boundary, at which there is a temperature and
velocity gradient.
- There are two types of convection:
o Natural (free),
o Forced.
- The behaviour governing each type of convection is very different.
- Both types of convection can be either:
o Internal (e.g., steam flowing through a pipe),
o External (e.g., steam flowing over a pipe).
Nusselt number:
- Consider a point on a flat plate at which V =0 m s−1, i.e., at y=0. Heat transfer
at this point is by conduction only (no bulk fluid motion).
- The Nusselt number is defined as:
- The Nusselt number is a function of position, flow and thermal fluid properties,
and fluid flow conditions.
Reynolds number:
- This had to do with fluid dynamics and is independent of heat transfer
considerations.
- It is related to the velocity boundary layer properties and is used to define a
system as having laminar or turbulent flow.
- The transition from laminar to turbulent flow depends on several factors.
, - If the Reynolds number is large, the flow is turbulent, if it is small, it is laminar.
- The critical Reynolds number, when the flow changes, depends on the
5
surface. For flat plates, RCR =5 ×10
Prandtl number:
- This takes the velocity and thermal boundary layer into consideration.
Nusselt number on a flat plane:
- For a fixed point, x , on a flat plane:
Convection:
- There is a layer near a surface boundary, at which there is a temperature and
velocity gradient.
- There are two types of convection:
o Natural (free),
o Forced.
- The behaviour governing each type of convection is very different.
- Both types of convection can be either:
o Internal (e.g., steam flowing through a pipe),
o External (e.g., steam flowing over a pipe).
Nusselt number:
- Consider a point on a flat plate at which V =0 m s−1, i.e., at y=0. Heat transfer
at this point is by conduction only (no bulk fluid motion).
- The Nusselt number is defined as:
- The Nusselt number is a function of position, flow and thermal fluid properties,
and fluid flow conditions.
Reynolds number:
- This had to do with fluid dynamics and is independent of heat transfer
considerations.
- It is related to the velocity boundary layer properties and is used to define a
system as having laminar or turbulent flow.
- The transition from laminar to turbulent flow depends on several factors.
, - If the Reynolds number is large, the flow is turbulent, if it is small, it is laminar.
- The critical Reynolds number, when the flow changes, depends on the
5
surface. For flat plates, RCR =5 ×10
Prandtl number:
- This takes the velocity and thermal boundary layer into consideration.
Nusselt number on a flat plane:
- For a fixed point, x , on a flat plane:
