naturalcirculation

Natural circulation refers to fluid motion driven by buoyancy forces that arise from density differences within the fluid, caused by heating, cooling, or compositional changes, without the input of mechanical pumps. When part of a loop is heated, the fluid becomes less dense and rises, while cooler, denser fluid sinks in the lower region, establishing a self-sustaining circulation. The flow results from buoyancy overcoming viscous and frictional losses and is governed by hydrostatic pressure differences between the hot and cold legs. The geometry, vertical extent, and heating power determine whether sufficient circulation occurs; the process depends on dimensionless groups such as the Prandtl, Grashof, and Rayleigh numbers.

Natural circulation systems are used where passive cooling or heating is advantageous. Examples include thermosiphon solar water heaters, which move water without pumps using vertical risers and condensers, and certain passive cooling schemes in nuclear reactors where core heat is removed by natural convection in coolant loops. Other applications appear in process heat exchangers and district heating networks, especially where reliability and simplicity are priorities.

Design considerations include loop height, pipe diameter, orientation, and the temperature difference between heat source and sink. Limitations include relatively low peak heat transfer capacity, sensitivity to orientation and geometry, potential for flow instabilities or reversal, and performance degradation from gas entrainment or fouling. Natural circulation is often analyzed with dimensionless groups such as Grashof and Rayleigh numbers to predict onset and stability of buoyancy-driven flow.