Detailed visualization experiments on the start-up process and stable operation of double-layered pulsating heat pipes under vertical and horizontal orientations

The thermal characteristics in a double-layered 3D-CLPHP are investigated by visualization experiments. The results are compared with those of a single-layered CLPHP under the vertical and horizontal orientations and a filling ratio (FR) of 35%, 50%, or 65%. The tube's inner diameter (ID) is 6 mm, slightly over the limiting value for water. The orientation is found highly determinative to the flow pattern of liquid slug trains in each tube layer. The flow behavior appears similar for the two layers under the vertical orientation but apparently different under the horizontal orientation. When horizontally placed, the liquid slug trains tend to drain down to the lower layer, thereby not only triggered but sustained continuous pulsation motion. Intense cross-layered flow motion, attributed to the interaction between the downward gravity and upward gaseous expansion or buoyancy effect, is recorded during the operation. Sufficient working fluid distribution inside each layer for the overall FR of 65% is recommended. Instead, the single-layered CLPHP fails to maintain a stable oscillation motion without the assistance of gravity. The double-layered CLPHP outperformed the single-layered CLPHP by 12.8 - 15.1% in thermal resistance even under the vertical orientation.