Convection

Discussion

the basics

Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid that transfers the internal energy along with its mass. Although the heat is initially transferred between the object and the fluid by conduction, the bulk transfer of energy comes from the motion of the fluid. Convection can arise spontaneously (or naturally or freely) through the creation of convection cells or can be forced by propelling the fluid across the object or by the object through the fluid.

spontaneous convection

Spontaneous convection is driven by buoyancy for the most part and surface tension to a lesser extent.

Examples

forced convection

The heat loss due to the forced convection is given by the following equation called Newton's Law of Cooling …

P =  dQ  = hA (T − T0)
dt

where …

P = dQ/dt is rate at which heat is transferred
h = convection heat-transfer coefficient (or film coefficient or film conductance)
A = exposed surface area
T = temperature of the immersed object
T0 = temperature of convecting fluid

There is currently no general theoretical model for analyzing forced convection problems. The heat-transfer coefficients h are can only be described by equations based on empirical analysis. For example, the h of air is approximately equal to …

h = 10.45 − v + 10√v

where v is the relative speed of the object through the air; that is, the speed of the object through the air or the speed of the air around the object. This equation is valid for speed from 2 to 20 m/s. Outside of this range h is … ?

wind chill

Which leads us to the topic of wind chill. Q. What is it and why do we need it? A. To insipe fear and get you to watch the weather report on the local news.