Horizontal rolls in convective flow above a partially heated surface

Horizontal rolls, generated in convective flow above a partially heated bottom in a rectangular box are studied experimentally for a wide range of the Prandtl number (7 ≤ Pr ≤ 1020), the Rayleigh number (300 ≤ Ra ≤ 2.8 × 107) and the aspect ratio (0.08 ≤ a ≤ 0.7). Experimental studies are supported by direct numerical simulations, which made possible the examination of the regimes inaccessible in the experiment, and also to investigate in detail the heat transfer in the convective flow. A variety of regimes with longitudinal helical rolls, with transverse rolls and with mixed structures has been observed. The structure of secondary flows is defined by the level of convective supercriticality in the boundary layer (Rayleigh number) and the intensity of the throughflow, defined by the Reynolds number, which depends itself on the heating and size, i.e. on the Rayleigh number. Most of the studied regimes were characterized by the appearance of longitudinal rolls. The transverse rolls appear in the flow only under the conditions of the large vertical drop in the temperature and weak large-scale flow (that is possible only at large values of the Prandtl number). Both longitudinal and transverse rolls lead to remarkable heat transfer enhancement. The formation and characteristics of horizontal rolls are described in details.