Novel flat plate pulsating heat pipe with ultra sharp grooves

Abstract

• Diffusion bonding was applied for the fabrication of flat plate PHPs. • Ultra-sharp grooves in the evaporator improved the heat transfer capacity of a PHP. • The proposed PHPs perform effectively for heat fluxes up to 1200 W (20.9 W/cm 2 ). • The gravity influence becomes negligible for powers beyond 600 W (10.4 W/ cm 2 ). • The PHPs appear to be a good alternative for temperature control of electronics. Pulsating heat pipe (PHP) is a very efficient solution for electronics cooling. Several strategies can be applied to improve the thermal performance of PHPs. In this context, the heat transfer enhancement of a flat plate pulsating heat pipe with a channel modification in the evaporator region, resulting in ultra sharp lateral grooves, was investigated experimentally. Chamfers were machined in the lateral walls of thirteen semi-circular cross section U-turn channels, drilled in flat copper plates. To form the sharp-grooved circular channels, two plates were faced against each other and diffusion bonded, resulting in a monolithic piece with high quality channels. The ultra sharp grooves had an angle of 29.1 ± 2.9°. The lateral grooves work as artificial nucleation sites, helping in the bubble formation, and act as a capillary medium, spreading the liquid over the evaporator region, delaying the dry-out. Therefore, the device could be less dependent on gravity, enabling it to be considered for applications in microgravity environments. To ascertain the efficiency of the proposed device, its performance was compared with another similar PHP with the same external geometry and with round ordinary cross-section channels of the same 2.5 mm channel diameter. Distilled water was selected as the working fluid, which, as predicted by literature models, worked at confinement conditions. As usual, the thermal behaviors of PHPs were characterized by their temperatures and pressures, depending on the operation conditions. The best filling ratio for each PHP was experimentally determined, considering heat loads from 20 up to 350 W (from 0.35 up to 6.1 W/cm 2 ). The influence of the ultra sharp grooves on the thermal performance of the PHP was investigated for a large range of power inputs, reaching up to 1200 W (20.9 W/cm 2 ), for the best filling ratio. The gravity influence in the PHP operation was evaluated by tests in three orientations: gravity-assisted, horizontal and against-gravity. Both cross-section profile PHPs performed effectively well for heat fluxes up to 20.9 W/ cm 2 , even in the against-gravity position, showing that the devices are suitable for temperature control of electronics, including those with high heat fluxes. Besides, the gravity effect could be neglected for heat powers beyond 600 W (10.4 W/ cm 2 ), which make them adequate for microgravity applications. The presence of ultra sharp grooves in the evaporator section of the PHP reduced by 2.1 °C the average evaporator temperature, decreased the temperature variations among sections and improved the thermal performance by 12% in the horizontal and gravity-assisted orientation.