Hot steel sphere is submerged into the cup of cold water. Calculate the equilibrium temperature.
Axisymmetric problem of heat transfer.
Steel sphere volume Vsteel = 65.4·cm3, water volume Vwater = 3600 cm3.
Tsteel= 85°C, Twater=20°C;
Volume density of steel ρsteel = 7800 kg/m3,
Volume density of water ρwater = 1000 kg/m3,
Specific heat of steel Csteel = 460 J/kg·K,
Specific heat of water Cwater = 4200 J/kg·K.
Calculate the equilibrium temperature.
We have to divide two bodies in order to prevent heat exchange between them at initial stage. In the transient problem the bodies are connected through the periodic boundary condition (T1=T2).
According to the 1st law of thermodynamics, the energy of the isolated system is preserved*:
(CρV)water·(Twater - T) + (CρV)steel·(Tsteel - T) = 0, where
T is the equilibrium temperature.
(CρV)water = 4200·1000·3600·10-6 = 15120 [J/K]
(CρV)steel = 460·7800·65.4·10-6 = 235 [J/K]
15120·(20 - T) + 235·(85 - T) = 0
T = 20.99°C
Temperature distribution calculated in QuickField (animation):
*A textbook of Engineering Thermodynamics, R.K.Rajput, Luxmi publication (P) Ltd, Fourth edition, 2010.