Flow Visualization in the Evaporator and Condenser of a Three-Dimensional Pulsating Heat Pipe

Heat sinks are widely used in electronics for ground and aeronautical applications. Conventionally, heat sinks are made of bulk metal with high thermal conductivity, such as copper or aluminum. However, bulk metal heat sinks can be too heavy in applications with strict mass reduction requirements. A plate-fin heat sink where a three-dimensional pulsating heat pipe (3D-PHP) was embedded was proposed to improve heat spreading in the heat sink and to reduce the weight of the bulk heat sink. To observe the fluid behavior, a glass window was mounted on the evaporator and condenser, i.e., at the cross-section of the fins. An aluminum heat sink with six fins was additively manufactured. The embedded PHP consisted of a 2.4mm x 2.4mm channel formed into six loops, i.e., one loop per fin. A refrigerant, R-1233zd(E), was charged as the working fluid at two filling ratios, 55 % and 70 %. The fluid flow was observed and recorded at 500 fps with a low-noise and high-light sensitive high-speed camera with a CMOS sensor. The camera captured different fluid behaviors depending on heat load and test cases. First, the oscillation of the meniscus was observed, and then nucleate boiling was started where a bubble lifted to the meniscus, and the bubble flow transitioned to the slug flow. The outcomes of the present study will contribute to a deeper understanding of the PHP’s chaotic fluid behavior with a 3-dimensional configuration effect.