ABSTRACT
Natural circulation is a simple phenomenon which occurs in a fluid in presence of temperature and density gradients in a force field. In natural circulation systems there are a heat source and a heat sink, with the former placed lower than the latter, both in contact with a portion of fluid. As consequence of the heat fluxes, the heated part of the fluid becomes lighter and rises, while the cooled part becomes denser and is dropped down by gravity. These combined effects establish circulation.
The dynamic behaviour of natural circulation systems depends on different parameters: power transferred to the fluid, boundary conditions at the heat sink, intensity of gravity force, thermo-physical properties of the fluid, pipe material, etc. In this paper it is presented an experimental activity concerning the effect of the loop inclination, defined as the the rotation around the horizontal axis, which is parallel to heater and cooler. The inclination angle modifies the boundary forces acting on the system, and consequently the dynamic behaviours of the fluid flow. As the vertical loop displacement was already deeply investigated in previous works [1-2], its dynamic behaviour is briefly summarized, while this paper is focused on the results of the experimental campaigns performed with almost horizontal displacement (α=60° and α=75°). Two different working fluids were employed (water and FC43), and, as additional parameter, the coolant temperature at the heat sink varied between –10°C and +20°C.
The loop inclination modifies drastically the stability fields of the system. With the angle α=60° the dynamic behaviour is mostly unstable for both fluids, while with a position close to the horizontal one (α=75°) stability is easily achieved, in particular with water. Moreover, when the loop is inclined, two flow regimes occur (mono- and bi-directional pulsing) which never take place with the vertical displacement.
Considering that the inclined loop could simulate reduced gravity (α=75° corresponds to 0.26g), the good thermal performance of the loop with almost horizontal inclination suggests the possibility of using natural circulation systems also for space applications.
