The value of Biot number is very small (<0.01) when __________.
A body cools from 90 °C to 80°C in 5 minutes. Under the same external conditions to cool from 80 °C to 70 °C the body will take __________.
Consider the following statements regarding fins:
(A). A fin is more effective if Biot number is less than one.
(B). A fin is less effective if Biot number is less than one.
(C). Fin will act as an insulation for a surface if h > mk.
(D). fin will enhance heat transfer if h > mk.
Which of the following statements are correct _________?
A 1 cm diameter, 30 cm long fin made of aluminum (k = 237 W/moC) is attached to a surface at 80oC. The surface is exposed to ambient air at 22oC with heat transfer coefficient of 11 W/m2 °C. If the fin can be assumed as infinitely long, its efficiency is ___________.
A steel ball of 10 mm diameter at 1000 K is required to be cooled to 350 K by immersing it in a water environment at 300 K. The convective heat transfer coefficient is 1000 W/m2‐K. Thermal conductivity of steel is 40 W/m-K. The time constant for the cooling process 𝜏is 16 s. The time required (in s) to reach the final temperature is __________
A steel ball of diameter 60 mm is initially in thermal equilibrium at 1030 °C in a furnace. It is suddenly removed from the furnace and cooled in ambient air at 30 °C, with convective heat transfer coefficient h = 20 W/m2K. The thermo-physical properties of steel are: Density ρ = 7800 kg/m3. Conductivity k = 40 W/mK and specific heat c = 600 J/kgK. The time required in seconds to cool the steel ball in air from 1030 °C to 430 °C is