An experimental study of forced convective heat transfer from smooth, solid spheres

Forced convective heat transfer from smooth, solid and isothermal spheres of various diameters has been studied experimentally in air flows with various free-stream velocities. The average heat transfer coefficient has been determined from the steady measured power input to a heating element inside the spheres and the steady measured temperatures of the flowing air and of the surface of the spheres, employing corrections to account for heat transfer due to thermal radiation and due to natural convection. The current data for the average heat transfer coefficient, expressed as a relationship between the Nusselt number and the Reynolds number, complement data in literature with respect to the range of large Reynolds numbers that have been considered: here the Reynolds numbers were between 7.8×103 and 3.3×105. The experimental results show a sudden increase in the Nusselt number above a critical Reynolds number of approximately 2.9×105, analogous to the “drag crisis” for the drag force on the sphere. A correlation for the Nusselt number as a function of the Reynolds number has been formulated for air flows that describes these data well for Reynolds numbers below the critical Reynolds number.