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?

a) The convection heat flux for the air stream is 1400 W/m²

b) The convection heat flux for the water stream is 18000 W/m²

Explanation:

a) The convection heat flux for the air stream is:

Where

h = 40 W/m²K

Ts = 30°C = 303.15 K

T∞ = -5°C = 268.15 K

Qair = 40(303.15 - 268.15) = 1400 W/m²

b) The convection heat flux for the water stream is:

Where

h = 900 W/m²K

Ts = 30°C = 303.15 K

T∞ = 10°C = 283.15 K

Qwater = 900(303.15 - 283.15) = 18000 W/m²

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