Currently, methods to improve the phase change heat transfer efficiency in microchannels with two-phase flow include active, passive, and composite methods. Active enhancement techniques refer to the addition of external energy to enhance heat transfer (such as surface vibrations, electrodynamics, and magnetohydrodynamics). Passive enhancement techniques involve the use of additives (nanoparticles) in the working fluid, altering the properties of the heat transfer surface, or incorporating microstructures for enhanced heat transfer on the heat transfer surface. Composite enhancement techniques refer to the combined use of active and passive enhancement techniques. In passive enhancement techniques, hydrophobic porous membranes and capillary action from different microchannel structures are often used to achieve gas-liquid phase separation, addressing the issue of uneven gas-liquid distribution within microchannels. Studies by Priy et al. and Apreotesi et al. have shown that hydrophobic porous materials can effectively reduce pressure drops within microchannels. Luo Xiaoping et al. used a 30% glycerol aqueous solution as the working fluid and analyzed the pressure drop variations in two phase separation structures (porous/non-porous) and ordinary microchannels without exhaust holes. The results indicated that phase separation structures can significantly reduce the two-phase flow resistance within the channel, and that the ability to mitigate pressure drop increases with greater heat flux density and more open pores in the phase separation structure. In active enhancement techniques, external forces such as acoustic fields and electric fields are often applied to the microchannels to alter the heat transfer efficiency of the two-phase flow.

Tang et al. studied the effect of needle-shaped electrodes on the evaporation heat transfer of working fluids in heat pipes and microchannels. They found that the application of needle-shaped electrodes can promote an increase in capillary wetting length, reduce the wall temperature of microchannels, and effectively enhance heat transfer performance. Li et al. adopted a counterflow stepped microchannel to address the main issues faced by two-phase flow boiling in microchannels. Jing et al. used a counterflow diverging microchannel and found that compared to traditional co-flow designs, it significantly improved temperature uniformity, with a heat transfer coefficient increase of 45.1%, a pressure drop reduction of 73.8%, and a performance coefficient increase of 123.1%. From these studies, it can be observed that gas-liquid two-phase flow in direct contact with a liquid-repellent porous membrane or capillary structure can achieve a certain degree of separation between gas and liquid phases, effectively reducing local gas phase content, resulting in a more uniform working fluid within the channel, more stable heat transfer performance, and reduced pressure drop. However, the other side of such phase separation structures is in direct contact with the external environment, and during the phase separation process, there may be a certain degree of working fluid loss. Additionally, if the pressure within the channel becomes too high, the capillary structure or porous membrane may be damaged, posing safety hazards. The electric field applies a force to the gas-liquid interface, affecting the motion characteristics and morphology of bubbles, particularly having a significant impact on the growth and detachment processes of bubbles. However, the electric field mainly exerts a strong enhancement effect during the nucleate boiling stage, and as the heat flux density continues to increase, leading to a large number of confined bubbles in the channel, the enhancement effect of the electric field gradually diminishes. In terms of microchannel structure, counterflow microchannels can reduce the overheating region near the downstream outlet by facilitating heat exchange between hot and cold fluids, thereby improving the overall heat transfer performance of microchannels and enhancing temperature uniformity and pressure drop stability.

To address the aforementioned issues, this paper proposes a parallel counterflow microchannel structure, which features a gas-liquid phase separation structure with modified polyvinylidene fluoride (PVDF) porous membranes at its ribs. Additionally, needle-shaped electrodes are integrated into the phase separation structure, allowing for the synergistic effects of electric fields and phase separation to further enhance flow boiling heat transfer performance. The study investigates the two-phase pressure drop and bubble behavior in counterflow microchannels (PSP00, PSP10, PSP06, PSP04) with varying densities of gas-permeable pores under the absence of an electric field. Furthermore, the research visualizes the two-phase pressure drop and bubble behavior under different voltages (0, 200V, 400V, 600V) when the electric field (needle-shaped electrodes) collaborates with the modified PVDF membrane phase separation structure. Performance evaluation criteria (PEC) are introduced to examine the uniformity of surface temperature distribution and comprehensive heat transfer performance of PSP10 under no electric field and PSP00 and PSP10 under a voltage of 600V, providing references for the study of enhanced heat transfer in microchannels.

1.1.1 Preparation of Phase-Separated Porous Membranes and Selection of Working Fluids

In the formula, γsv is the surface tension of the gas-solid interface, N/m; γsl is the surface tension of the liquid-solid interface, N/m; γlv is the surface tension of the gas-liquid interface, N/m.

When θ < 90°, the solid surface exhibits wettability; when 90° ≤ θ ≤ 150°, the solid surface is considered non-wettable; when θ > 150°, the solid surface is superhydrophobic and non-wettable.

Currently, the commonly used porous hydrophobic membranes for microchannel phase separation include polytetrafluoroethylene (PTFE), polypropylene (PP), and polyvinylidene fluoride (PVDF). Among them, the porous membrane made of PVDF has excellent hydrophobicity and stability, allowing for long-term use at temperatures up to 120°C, and it is easy to process and shape. Therefore, this paper selects PVDF porous membranes to achieve gas-liquid phase separation. To enhance the hydrophobicity of the PVDF membrane, this paper employs a method used by Wu et al., utilizing a solution of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) to immerse and modify the PVDF membrane, followed by standing for 48 hours and then drying, resulting in the modified PVDF membrane. The contact angles of three commonly used working fluids (water, mineral oil, and ethanol) on the modified PVDF membrane were measured using a contact angle goniometer. It was found that the contact angle for water was 152°, for mineral oil 117°, and for ethanol 105°. The contact angles for all three working fluids were greater than 90°, with water having the largest contact angle and ethanol the smallest. Among them, the conductivity of ethanol solution is weak, allowing its liquid phase not to pass through the membrane, while the gas phase can pass through the membrane, and an electric field can be applied to it. Therefore, this paper uses industrial ethanol as the experimental working fluid.

1.1.2 Reverse Flow Structure Layout Plan

In the formula, Qloss is the heat loss power, W; Ta is the ambient temperature, °C; Ts is the average wall temperature, °C.

The calculation method for ΔPsp is shown in equation (6).