5.2.5 Natural convection in enclosure

It is another class of problems for which there are many cases and their corresponding correlations are also available in the literature. Here two cases will be discussed, (i) in which a fluid is contained between two vertical plates separated by a distance d, (ii) the other where the fluid is in an annulus formed by two concentric horizontal cylinders.

In the case first, the plates are at different temperature, T₁ and T₂. Heat transfer will be from higher temperature (T₁) to lower temperature (T₂) through the fluid.

The corresponding Grashof number will be

McGregor and Emery proposed the following correction for free convection heat transfer in a vertical rectangular enclosure, where the vertical walls are heated or cooled and the horizontal surfaces many be assumed adiabatic,

(5.15)

Applicability conditions for the above equation are,

or,

(5.16)

Applicability conditions are,

Here L/d is known as the aspect ratio.

At steady state condition, the heat flux (q_x) is equal thus,

	qx = h(T1 - T2)
or,
or,
or,
or,

where, k_c(= Nu_xk) is known as the apparent thermal conductivity.