It makes sense that a glass lid on top of a thermally mixed vessel would produce a better CS.
In a cylindrical vessel heated at its lower end, the steady-state convection will be an updraft vertically along the center axial region, and a downdraft along the periphery of the cylinder except for cooling losses at the top surface. A glass lid will diminish the latter loss, thereby increasing the convection down the inside periphery of the vessel. The result is more stirring without additional heating. Best wishes, Matthew
