Experimental study of the forces associated with mixed convection from a heated sphere at small Reynolds and Grashof numbers. Part II: Assisting and opposing flows

The present paper addresses the issue of mixed convection from a small heated sphere in assisting and opposing flow configurations. The sphere is suspended in an electrodynamic chamber (EDC), where it is heated by a focused laser beam up to several hundred degrees above room temperature. As a result, free convection is induced from the sphere, with the Grashof number smaller than 0.02. A vertical forced flow is then applied and gradually increased in a quasi-static manner. The forced flow velocities are in the range 0–0.1 m/s, providing very low particle Reynolds numbers, usually less than 0.5. The effects of the free convection on the drag force experienced by the particle, and of the forced flow on the free convection are assessed quantitatively, measuring continuously the vertical forces experienced by the sphere. A similarity law developed in the previous study is successfully applied to determine the behavior of the drag force at various particle heating levels.