Confinement on thermal convection from thermal plumes perspective

Despite the widespread occurrence of thermal convection in nature and artificial systems, we still lack a unified approach that integrates system geometry, fluid properties, and thermal forcing to characterize the transition from free to confined convective regimes. This transition is crucial for understanding energy transport and mixing across various environments, including convection in ample environments, like oceans and lakes, as well as in narrow systems, such as hydrothermal flows through fractures and engineering applications like heatsinks and microfluidics. In this work, we discuss the effect of confinement on flow structure and heat transfer from the standpoint of the smallest convective structure, asking: how tight the confinement is from a thermal plume perspective? Here, we introduce the degree of confinement Λ—the ratio of a thermal plume's thickness in an unbounded domain to the system's lateral extent—as a simple and universal metric to characterize convective regimes differing in flow dimensionality and time dependency, showing via laboratory experiments that transitions between these regimes are well-defined by Λ, offering a unified view of convection in closed systems from the plume's perspective.