Flow microbubble emission boiling (MEB) in open microchannels for durable and efficient heat dissipation

Abstract

Microbubble emission boiling (MEB) has been reported to be an advanced heat transfer mechanism due to its significant heat dissipation capacity at high heat flux. Microchannel heat sinks are considered to be effective heat transfer carriers, but MEB is difficult to be triggered in conventional microchannels due to insufficient mainflow subcooling and restriction of narrow channel walls. In this work, we conducted flow MEB experiments in plain and lasered open microchannels with various inlet temperatures (T_inlet) and open gap height (H_g). The open configuration can provide adequate mainflow subcooling and extra flow area to trigger MEB. The results showed that MEB occurred in plain open microchannels as a transition flow pattern between bubbly flow and flow reversal at T_inlet ≤ 25 °C and H_g = 0.3 mm, with a significant temperature drop after a one-time flow reversal, providing abundant vapor composition as MEB evaporation cores; MEB did not happen at higher T_inlet and larger H_g (1 mm). However, in lasered open microchannels, MEB was triggered at the beginning of flow boiling at T_inlet ≤ 65 °C with H_g = 0.3 and 1 mm, without temperature drop or flow reversal. This demonstrated that the required inlet subcooling, heat flux and temperature gradient in vertical direction for MEB initiation were simultaneously reduced in lasered microchannels. The two-phase heat transfer coefficient (h_tp) of MEB was significantly increased (up to 77.8 %) compared to conventional bubbly flow, due to the faster bubble nucleation frequency and rapid impact from the subcooled mainflow to channel walls, and was further enhanced in lasered microchannels. The durability of MEB in plain microchannel was unsatisfactory, as the persistent flow reversal dominated the flow pattern after ∼3750 s at G = 300 ml/min, T_inlet = 25 °C, H_g = 0.3 mm and q_eff ∼1450 kW/m². However, in lasered microchannels, MEB ran steadily for 22500 s at the same working condition. This study provided an effective and accessible method to achieve durable MEB in microchannels with excellent heat dissipation capacity, offering valuable insights for further thermal management engineering applications.