There are 2 modes of heat transfer from the hot surfaces of the stove.
Natural convection
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It is caused by density difference produced by temperature gradient of fluid. Heat exchange by convection occurs between solid surface and fluid flowing over it. The Newton’s heat convection equation Q = h*A*DT
The study of h involves the fluid mechanics and its value depends on many factors including geometry of surface, the velocity and physical properties of the fluid and also the temperature difference. |
Where, Q = average rate of heat transfer by convection (watt) h = convective heat transfer A = heat transfer area, (m2) DT = difference between the temperature of the surface and that of the fluid at some specified location, (C) |
Nusselt number
It is a dimensionless quantity, which is a convenient measure of the convective heat transfer coefficient.
Nu
=
Where,
h = convective heat transfer
L =
k = thermal conductivity
Free convection heat transfer may be correlated by an equation of the form
Where,
Gr = A new dimensionless parameter. It is known as Grashof number.
=
Pr = Prandtl
parameter; 
(Pr is a property parameter)
Empirical equations for free convection based on experiment results
A) For horizontal plate
a) Laminar flow
For a 2-dimensional horizontal plate of length L, Fujii and Imura recommend for heated surface facing up,
(for
5x108
< Gr Pr)
b) Turbulent flow
For horizontal heated plates facing up or cooled plates facing down in air or gas Fischenden and Saunders suggest
(for
Gr Pr = 2x107
to 3x1010)
B) Verticle plates and cylinders
a) Laminar flow
Churchill and Chu have proposed the following correlation
(10-1<Ra<109)
Where, Ra = (Gr) (Pr) is called the Rayleigh number.
b) Turbulent flow
McAdams recommends;
(For Gr Pr = 109 to 1012)
Radiation
The heat exchange by radiation was studied experimentally by Stefan, in 1879 who proposed that the energy radiated by a body is proportional to the fourth the power of its absolute temperature. Boltzmann confirmed it in 1884 by theoretical studies resulting in the Stefan Boltzmann law that may be stated as:
q = sAT4;
Where q = maximum rate of heat emission by a body at the given temperature (black body emission), (watt)
A = area of the body, (m2)
T = absolute temperature, (K)
s = a constant known as Stefan-Boltzmann constant
= 5.667 x 10-8 W/m2 K4
Net rate of heat radiation from a black body of area A1 at absolute temperature T1 to another black body at a lower temperature T2 will therefore be given by
q = sA (T14- T24)
For the case of real bodies this equation becomes,
q = sA e (T14- T24)
Where, q = maximum rate of heat emission by a body
e = emissivity
A program in a excel sheet for the heat transfer is developed for the calculation of heat transfer from the stove surface.
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