Numerical and experimental study of inclined open thermosyphons

A numerical model of the inclined open thermosyphon has been developed using a finite difference algorithm to solve the vorticity vector potential form of the Navier‐Stokes equations. The model simulates flow in an inclined cylinder whose bottom end is sealed and whose top is connected to uniform temperature reservoir, a configuration typical of evacuated tubular solar absorbers. The solution domain includes the cylinder only without the reservoir; therefore a special set of boundary conditions has been derived for the vector potential at the top end which is a flow‐through surface. Steady flow is simulated at various combinations of Rayleigh number, aspect ratio and mode of heating. An experimental set‐up has also been developed in order to investigate the development of different flow patterns previously predicted by analytical and numerical workers, as well as to observe more closely the behaviour of the fluid at the orifice. Velocity profiles were measured at the orifice using laser doppler anemometry, and compared with predictions from the numerical model.