# Heat Transfer and Combustion -- Explain the mechanisms of natural and forced convection

Tiberious

## Homework Statement

(a) The Grashof number and the Reynolds number appear in most correlations of experimental data for convective heat transfer. Explain, in a maximum of 150 words, the mechanisms of natural and forced convection with particular reference to the above non- dimensional groups.

SOLUTION

Heat transfer when referring to the forced or free movement of fluids is called convection. In natural (Free) convection, the fluid moves by difference in fluid densities owing due to changes in temperature gradient’s and can be represented by the Grashof number.

The Grashof number (Gr), approximates the ratio of buoyancy to viscous forces acting on a fluid.

In forced convection, the fluid is moved via influence of a pump; fan or external force. This is represented by the Reynolds number.

The Reynolds number (Re), is a nondimensional number that provides an indication whether fluid flow will be laminar or turbulent. The below image, Figure 1a, taken from Gary Settles PhD, ‘Laminar-turbulent transition’, unknown, 2ndApril 2018. Shows the change of flow from a candle from laminar too turbulent flow.

Is this a reasonable answer ?

Gold Member
I wonder why you put "combustion" in the title...

Anyway this is a very big argument and you can't explain it in less than 150 words. What you said is true but you can add a lot of more stuff. Just to mention a few:
- Reynolds number represents the ration of inertial and viscous force
- How do you get Reynolds and Grashof numbers? From which equations ?
- What relation exist between dimensional number and the heat transfer coefficient ? What they look like?
- In natural convection we usually observe "circular" (closed) motions (i.e hot air goes up, it cools down and falls back). Why ?
exc...

Is this a reasonable answer ?
Yes, but it depends on what level you want to answer the question.