Flow stability and distribution in horizontal serpentine tube bundles with vertical headers.

Particle-based systems have immense potential for combining thermal energy storage (TES) with renewable energy sources. The so-called sandTES system, which is an active TES system, utilizes sand or other small particles as a storage material and consists of a hot tank, a cold tank, and a reversible fluidized bed heat exchanger. In the preferred design, the tubes are arranged in horizontal serpentine tube bundles; thus, the headers are positioned vertically, for one phase subcritical, two-phase and supercritical water/steam conditions. So far, no design principles have been published for macro-sized vertical headers and horizontal tube bundles. This paper investigates the unbalance in flow distribution and other flow instabilities of such an unusual heat exchanger and its scale-up using APROS simulations. The steady-state simulations of the tallest investigated heat exchanger show a strong unbalance in the mass flows between individual heat exchanger tubes: the smallest mass flow was only 55% of the largest one when generating steam. Smaller heat exchangers experience less pronounced unbalance factors in the order of 88%. When steam is condensed, the unbalance is insignificant. Dynamic simulations of start-up and shutdown revealed instabilities and that the flow maldistributions even lead to flow reversals in most investigated cases although neither last for stationary operation. Part-load cases show higher unbalances than their respective full-load case by 9 to 17 percentage points. Header design modifications - secondary headers with submodules and orifices - improve the unbalance factor from 55% to 88%.