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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.

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

[tex]Q_A = h(T_s-T_{\infty})[/tex]

Our values are,

[tex]h=40W/m^2K[/tex]

[tex]T_s = 30\°c[/tex]

[tex]T_{\infty} = -5\°c[/tex]

A) Substituting the values in our equation,

[tex]Q_A = (40)(30-(-5))[/tex]

[tex]Q_A = 1400W/m^2[/tex]

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

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

[tex]Q_W = h (T_s - T_{\infty})[/tex]

Our values here are,

[tex]h= 900W/m^2K[/tex]

[tex]T_s=30\°c[/tex]

[tex]T_{\infty}=10°c[/tex]

Substituting the values in our equation,

[tex]Q_W = (900)(30-10)[/tex]

[tex]Q_W = 18000W/m^2[/tex]

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

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