Global and local heat transfer behaviour of a three-dimensional Pulsating Heat Pipe: combined effect of the heat load, orientation and condenser temperature

Pulsating heat pipes having non-planar layouts (i.e. three-dimensional layouts) are preferable to the planar geometries when both gravity-independency and compactness are needed. A multi-turn three-dimensional closed loop pulsating heat pipe is tested on ground with the aim of investigating the effects of gravity by means of a quantitative description of the working modes occurring during its operation at different orientations. The viscous effects are furthermore considered by varying the condenser temperature. The studied device is made of an annealed aluminium tube (inner/outer diameter: 3/5 mm), folded in 14 turns and partially filled with methanol (volumetric filling ratio: 0.5). The channels are externally coated with a black high-emissivity paint, thus allowing the measurement of the outer wall temperature in the whole adiabatic section by means of a high-speed medium wave infrared camera. The wall temperature and the fluid pressure are instead monitored by means of thermocouples and pressure transducers directly inserted in the fluid stream. The pulsating heat pipe global performance is first assessed by evaluating the equivalent thermal resistance for every test case, showing that the studied three-dimensional layout is independent of gravity at high power input levels, where the equivalent thermal resistance settles around 0.25 K/W for almost every orientation. The thermographic acquisitions are therefore post-processed to estimate the heat flux locally exchanged between the working fluid and the channels wall, and the resulting local wall-to-fluid heat fluxes are statistically reduced for the quantification of the observed working modes, i.e. pure conduction, start-up, intermittent flow and full activation. At high heat loads to the evaporator, the gravity-independent behaviour of the device is found to be promoted by full activations, i.e. when a regular fluid motion is perceivable in every branch within the adiabatic section. The flow modes are also identified, pointing out that the local heat transfer behaviour is capable of discerning fluid oscillations from net fluid circulations. All the qualitative and quantitative pieces of data are comprehensively presented to give further information regarding the device dependency on the orientation. To deeper investigate the link between local and global quantities, the equivalent thermal resistance is finally compared with the statistical coefficients provided by the local heat transfer analysis.