Thermal Perfomances of Flexible Low Bond Number Loop Two Phase Thermosyphons

If for high power applications the two-phase thermal loop has been highly investigated in the literature, thermal loops for power less than 1 kW has been scarcely studied. They could be divided into two main groups: high Bond number loops, where the capillary force can be neglected, and low Bond number loops, where the capillary forces cannot be neglected at all (Filippeschi & Franco, 2010). This paper deals with the effect of the geometry of the sections connecting the evaporator and the condenser, on the heat transfer rate in a low Bond number two-phase thermosyphon made of flexible pipes. A low Bond number loop two-phase thermosyphon is also referred in the literature as single turned Pulsating Heat Pipe. These loops are characterized by a natural circulation of mass flow inside. They consist of: an evaporator, a condenser and two adiabatic sections (the riser and the downcomer) which connect the two exchangers. In many applications like as PV solar cells cooling the condenser and the evaporator can be out of alignment, so that the use of the flexible pipes seems to be necessary. Several papers in the literature have experimentally shown that the heat transfer rate, which can be transferred from the evaporator to the condenser section, mainly depends on few parameters like as: the diameter of the tube, the working fluid, the adiabatic section length, the number of the turns and the operative orientation with respect to the gravity. However the thermal and hydraulic oscillations in these devices can not be accurately predicted and the heat and mass transfer phenomena is still unsolved problem. The use of flexible pipes for the adiabatic section introduce additional problems because the evaporator and the condenser sections cab be placed in any configuration, even out of alignment. In this configuration additional forces and additional pressure drops can decelerate or accelerate the motion. In addition the use of unconstraint pipes could develop damping pressure oscillations with different frequencies. In particular in this paper the authors wish to understand the effect of the geometry of these sections on the slugs and plugs motion inside the loop, and in particular of the bends and of the extension of the horizontal section. The experimental facility consists of a single closed loop of capillary dimensions. In particular the evaporator section is copper plate where a long circular hole has been realised (Internal Diameter: 2.0 mm, OD: 4.0 mm). The condenser section is a pipe made of brass (ID: 2.1 mm,OD: 3.0 mm). It is designed to facilitate parametric investigations as well as simultaneous flow-visualization. The condenser and the evaporator are connected with flexible pipes made of PTFE (ID: 2.0 mm,OD: 3.0 mm). The evaporator section is heated by two flat electrical heaters. The condenser section, made of a hollow copper chamber, is supplied with cooling water from a constant temperature bath. Adequate thermal insulation is provided on all sections. The working fluid is water. The motion of the plugs and the slugs is observed with a high-speed acquisition video-camera. The unstable two-phase flow of a single turned PHP has been qualitatively described in (Khandekhar et al, 2009) (Di Marco & Filippeschi, (2010). This paper shows the effect of the alignment distance on the thermal resistance in a upward heat transfer mode. Considerations on the increase of the instability and the reduction of the mass flow rate circulating with the flexible pipes have been drawn.