Lesson 5 - Convection SAQ
This lesson allows you to test yourself on the material covered in the lesson on Convection, and to determine the level to which you have met the learning outcomes.
Suggested Response - Q2
* Provide short answers to the following:
a) Compare and contrast natural and forced convection.
b) Explain the difference between film boiling and nucleate boiling. Which is more desirable?
c) Why is it important that utility steam remains free from air?
d) Explain why turbulent flow in a heat exchanger is more desirable than laminar flow.
Response:
a) Convection is the movement of molecules within a fluid due to temperature differences, which results in differences in densities between the warmer and cooler localities in the fluid. Natural convection occurs when there is no outside force causing the movement of the molecules. Forced convection is when there is an outside force like a pump or fan to generate movement of the molecules.
b) In nucleate boiling, bubbles form and create turbulence. As the bubbles move away from the surface, heat travels with the bubbles, allowing for homogenous heating and increased efficiency. Film boiling occurs at high temperature differences when bubbles cling to the surface and create a vapour film. The film acts as a layer of insulation inhibiting homogenous heating and loss of efficiency. Nucleate boiling is desired over film boiling.
c) Air is a non-condensable gas and therefore inhibits utility steam from condensing and transferring heat, which therefore causes a decline in boiler efficiency.
d) Turbulent flow in a heat exchanger is desirable over laminar flow because it promotes a thinner boundary layer thickness. This results in less resistance for heat transfer and better heat transfer.