Chapter 12, Problem 12.123P

Growers use giant fans to prevent grapes from freezing when the effective sky temperature is low. The grape, which may be viewed as a thin skin of negligible thermal resistance enclosing a volume of sugar water, is exposed to ambient air and is irradiated from the sky above and ground below. Assume the grape to be an isothermal sphere of 15-mm diameter, and assume uniform blackbody irradiation over itstop and bottom hemispheres due to emission from the sky and the earth, respectively.

(a) Derive an expression for the rate of change of the grape temperature. Express your result in terms of a convection coefficient and appropriate temperatures and radiative quantities.

(b) Under conditions for which $T_{sky}=235K,T_{\infty }=273K$ , and the fan is off $\left(V=0\right)$ , determine whether the grapes will freeze. To a good approximation, the skin emissivity is 1 and the grape thermophysical properties are those of sugarless water. However, because of the sugar content, the grape freezes at $-5^{\circ }C$ .

(c) With all conditions remaining the same, except that the fans are now operating with $V=1m/s$ , will the grapes freeze?