An electrically heated sphere of 1.5 cm diameter is cooled in a quiescent medium of air at 315 K. In order to maintain the surface temperature of the sphere at 385 K, estimate the amount of heat to be supplied by the electrical heater.
D = 1.5 $\times$10–2 m , $T_{\infty }$ = 273 + 20 = 293 K
The properties of air at 
= 350 K
Kinematic viscosity, n = 2.076 $\times$10–5 m2/sec
Prandtl number, Pr = 0.697
Thermal conductivity, k = 0.03 W/m-K
Coefficient of thermal expansion,
($\beta$) =2.86 $\times$10-3 K-1
Use the relationship = Nu = 2+0.43 $Ra^{\frac{1}{4}}_{D}$