Free convection heat transfer is sometimes quantified by writing Equation 4.20 as qcony = Skeff AT1-2, where keff is an effective thermal conductivity. The ratio kefk is greater than unity because of fluid motion driven by buoyancy forces, as represented by the dashed streamlines. An experiment for the configuration shown yields a heat transfer rate per unit length of q'conv = 110 W/m for surface %3D temperatures of T1 = 53C and T2 = 15C, respectively. For inner and outer cylinders of diameters d = 20 mm and D = 60 mm, and an eccentricity factor of z =10 mm, determine the value of kef. The actual thermal conductivity of the fluid is k = 0.255 W/m- K.