A thermos flask consists of two thin-walled coaxial cylinders with a narrow gap such that the radius

A thermos flask consists of two thin-walled
coaxial cylinders with a narrow gap such that the radius ratio R1/R2
= 0.95. The surfaces that face each other are silvered and have emissivity of
0.05 each. The annulus is evacuated. The length of flask is L = 0.3 m and the
radius R1 = 0.05 m. Calculate the heat loss when the thermos flask
is filled with hot water such that T1 = 373 K while the outer
cylinder at a temperature of T2 = 300 K. Also calculate the
instantaneous rate of change of temperature of hot water.

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Hints: Eq. (7.69) is used to find

Initial cooling rate of hot water at 100ºC
is obtained by finding the properties of water at 373 K like  and cp.
Then mass of water in the flask m =  kg and the rate of
cooling is

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