The effects of asymmetric configuration on the flow and heat transfer characteristics of pulsating heat pipes (PHPs) were experimentally studied. Asymmetric configurations promote circulation motion of working fluid inside PHPs. Circulatory flow is desirable for thermal performance enhancement because the circulation of the working fluid increases the capability of the working fluid to transport heat from the evaporation zone to the condensation zone. Generally, as inclination angle decreases from a vertical position to horizontal position, thermal performance of the PHP decreases. However, it is expected that performance degradation according to inclination angle can be minimized when circulating motion exists in the PHP. Varying channel diameter was used to effectively generate circulating flow in the PHP. Experiments were performed to investigate the effect of variation in channel diameter on the flow and heat transfer characteristics of PHPs made of capillary glass tubes which have different inner diameters. Inner diameters of the tubes which were used were 1.2, 1.7, 2.2 mm, respectively. Three of the studied PHPs were composed of two parallel tubes of different diameter connected with each other, and remaining were composed of two tubes of the same diameter. Experimental results show that flow patterns changes according to input heat power. Low amplitude oscillation motion turned to larger amplitude oscillation and then circulation flow in one direction as increasing input power. Visualization results present that asymmetric PHPs with varying channel diameter show circulatory flow over a wider range of heat input compared to normal PHPs. In addition, experimental results indicate that variation in channel diameter helps to enhance the thermal performance of the PHP, especially as inclination angle decreases. This paper proves that channel configuration with variation in diameter can be one effective way of improving the performance of the PHP.
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