Experimental and theoretical results are reported about the stability of pressure-driven flow in a vertical U-shaped loop to whose descending leg heat is supplied. The stability of the flow depends on a nondimensional number B, which represents the ratio between the buoyancy and the pressure heads. As long as B<Bcr, the flow is stable and the buoyancy assists the pressure in driving the flow. When B>Bcr, the flow ceases to be stable, the flow rate oscillates with increasing amplitude and then the flow reverses direction. The end result consists of stable ascending flow in the heated section. That is, the flow is driven by the buoyancy force and opposed by the pressure head. Two types of loops are considered: one containing single-phase fluid; and the other, saturated porous media (packed bed). A marginal stability curve is constructed using linear stability analysis. Finite difference numerical simulation suggests that the same curve is also valid for finite amplitude disturbances. The theoretical results are compared with experimental data.

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