In the second case the heat transfer is involved in the enclosure formed by two concentric cylinders in horizontal position, the correlation given by Raithby and Holland,

(5.17)

is the modified Rayleigh number given by,

where, d_i and d₀ are the outer and inner diameter of the inner and outer cylinders, respectively. The enclosure characteristic length l is defined as (d₀ - d_i).

The applicability of the eq. 5.17 is 10² <   > 10⁷.

It should be noted that the rate of heat flow by natural convection per unit length is same as that through the annular cylindrical region having effective thermal conductivity k_e for the case,

where, T₁ and T₂ are the temperatures of the inner and outer cylindrical walls, respectively.