Question

You've experienced convection cooling if you've ever extended your hand out the window of a moving vehicle or into a flowing water stream. With the surface of your hand at a temperature of 30 degree C. determine the convection heat flux for a vehicle speed of 35 km/h in air at -5 degree C with a convection coefficient of 40 W/m2 . K and a velocity of 0.2 m/s in a water stream at 10 degree C with a convection coefficient of 900 W/m2 K. Which condition would feel colder? Contrast these results with a heat loss of approximately 30 W/m2 under normal room conditions.

Answer 1

The Newton's law of cooling can determine the convection heat flux for the air stream. That is,

`Q_A = h(T_s-T_(\infty))`

Our values are,

`h=40W/m^2K`

`T_s = 30\°c`

`T_(\infty) = -5\°c`

A) Substituting the values in our equation,

`Q_A = (40)(30-(-5))`

`Q_A = 1400W/m^2`

Therefore the convection heat flux for the air stream is
`Q_A = 1400W/m^2`

B) The convection heat flux for the water can also be determined by Newton's law of cooling, i.e,

`Q_W = h (T_s - T_(\infty))`

Our values here are,

`h= 900W/m^2K`

`T_s=30\°c`

`T_(\infty)=10°c`

Substituting the values in our equation,

`Q_W = (900)(30-10)`

`Q_W = 18000W/m^2`

Therefore the connection heat flux for the water stream is 18000W/m^2

The hand feels colder in water stream when compared to air stream and uncomfortable in both the water and aire stream when compared to the hand in normal room condition