Thermal performance improvement in ultra-thin pulsating heat pipes

Abstract

In this study, a criterion of the channel width for normal operation is proposed to improve the thermal performance of the ultra-thin pulsating heat pipes (PHP). To determine the reason for the poor thermal performance of the ultra-thin PHPs, the ultra-thin PHP with the channel height of $0.2 mm$ is fabricated using MEMS techniques. The overall dimensions of the ultra-thin pulsating heat pipe are $19 \times 50 \times 1 mm^3$, with a volume of $15 \times 44 \times 0.2 mm^3$ available for the channel design. The evaporator section is located at the end of the channel design area, with dimensions of $15 \times 7 mm^2$. R-236fa is used as the working fluid, with a filling ratio of 50%. Experiments are conducted under vertical orientation, and the flow visualizations are conducted using a high-speed camera. The ultra-thin PHP with the inverse aspect ratio of 1.6, calculated by dividing the channel width by the fixed channel height, exhibits significant temporal variation of the evaporator temperature, exceeding $10^\circ C$ over time. And its effective thermal conductivity is about $320 W/m \codt K$ , demonstrating a deteriorated thermal performance similar to the level of that of the ultra-thin PHPs reported so far. Through flow analysis, it was confirmed that abnormal operation, in which the intermittent oscillating motion occurs, is the reason of the poor thermal performance of the ultra-thin PHPs. This abnormal operation occurs when the liquid supply rate to the evaporator section is less than the evaporation rate. To solve this problem, the criterion of the channel width for normal operation of the ultra-thin PHPs is proposed. The liquid supply rate and the evaporation rate are theoretically represented as correlation to the inverse aspect ratio. As the inverse aspect ratio of the channel increases, the increase in the liquid supply rate becomes greater than that of the evaporation rate. The inverse aspect ratio at which the liquid supply rate begins to surpass the evaporation rate is established as the criterion of the channel’s inverse aspect ratio for normal operation. In this experimental case, the ultra-thin PHP can have normal operation when the channel’s inverse aspect ratio is 4.7 or higher. To experimentally validate the criterion of the channel width for normal operation and to optimize the thermal performance of the ultra-thin PHPs, the ultra-thin PHPs with a channel height of $0.2 mm$ and various channel widths are fabricated. The ultra-thin PHPs with the inverse aspect ratio greater than 4.7 have the improved thermal performance. The temporal variation of the evaporator temperature is very stable, remaining below $0.7^\circ C$. Through flow analysis, it was confirmed that this improved thermal performance is occurred due to normal operation with the continuous oscillating motion resulting from the sufficient liquid supply rate greater than the evaporation rate. Among the ultra-thin PHPs with normal operation, the effective thermal conductivity of the ultra-thin PHP with the inverse aspect ratio of 5 has highest effective thermal conductivity about $1420 W/m \codt K$, which is 4.4 times higher than that of the ultra-thin PHP with the inverse aspect ratio of 1.6 and up to 9 times higher than that of the ultra-thin PHP reported so far. And, the thermally optimized channel design of the ultra-thin PHP can be obtained by considering both the figure of merit for the thermal performance and the criterion of the channel width for normal operation.