A slender rod of length L, diameter d (L >> d) and thermal conductivity k1 is joined with another rod of identical dimensions, but of thermal conductivity k2, to form a composite cylindrical rod of length 2L. The heat transfer in radial direction and contact resistance are negligible. The effective thermal conductivity of the composite rod is
1
k1 + k2
2
\(\sqrt {{k_1}{k_2}}\)
3
\(\frac{{{k_1}{k_2}}}{{{k_1} + {k_2}}}\)
4
\(\frac{{2{k_1}{k_2}}}{{{k_1} + {k_2}}}\)