Experimental study on flow boiling of fluorocarbon fluid in the pin-ribbed micro-channel with rigid tails

In this work, a novel type of pin-ribbed micro-channel with rigid tails is designed based on the conventional pin-ribbed micro-channel. A comparative experimental study is carried out using fluorocarbon fluid Novec649. The flow boiling heat transfer and pressure drop characteristics are investigated under horizontal/vertical placement at inlet subcooling temperature of 25 °C. The ranges of inlet mass flux and heat flux are 519–1818 kg·m⁻²·s⁻¹ and 50–500 kW·m⁻², respectively. The results show that at a given mass flux, and heat flux of 300–500 kW·m⁻², the vertical pin-ribbed micro-channel with rigid tails has the lowest average wall temperature, the largest average heat transfer coefficient, and the smallest total pressure drop, therefore it has the largest j-factor, more than 60% higher than that of the horizontal/vertical channel without rigid-tails. With the aid of a high-speed camera, the localized bubble movement phenomena in horizontal/vertical micro-channel with rigid tails was photographed at a mass flux of 519 kg·m⁻²·s⁻¹ and a heat flux of 100 kW·m⁻². Three types of bubble detachment modes in the wake region at the boiling stable state are observed: natural shedding, one-sided shear and two-sided shear. The bubble detachment diameter under the one-sided shear is more than 23% smaller than that of two-sided shear, and that of the horizontal channel is more than 44% higher than that in the vertical channel, whereas its bubble detachment frequency is only 50% of that in the vertical channel. Therefore, the vertical placement is more helpful for the rigid tails playing a role in the breakup and detachment of bubbles. The experimental results confirm the advantages of adding rigid tails behind the square-rib to enhance the comprehensive heat transfer performance of flow boiling, which can provide a guidance for the structural design of two-phase miniature heat sinks.