Energy consumption is ever increasing in engineering systems, including agricultural machinery. Excessive heat generation has posed a serious threat to sustainable agriculture. It is an urgent demand for high-efficiency thermal-management technologies. Minichannel flow boiling has emerged as a highly promising solution, owing to its high heat-transfer efficiency and compact structure. However, its practical application has been severely constrained by flow instabilities, vapor accumulation, flow maldistribution, and local dryout under high heat-flux conditions. In this study, a porous rib minichannel (PRMC) was proposed to incorporate three-dimensional interconnected open-cell copper foam into parallel minichannels, aiming to simultaneously enhance boiling heat transfer and improve flow stability. A PRMC heat sink was designed and fabricated. A closed-loop experimental system was established using R141b as the working fluid. Systematic experiments were conducted under mass fluxes ranging from 63.5 to 381 kg/(m²·s) and effective heat fluxes up to 61.6 kW/m². High-speed visualization and local temperature measurements were employed to investigate two-phase flow patterns, heat transfer characteristics, temperature distribution, and pressure drop behavior. The performance of the PRMC was quantitatively compared with a conventional solid rib minichannel (SRMC). The results revealed that the porous rib structure significantly altered two-phase flow behavior. PRMC promoted earlier bubble nucleation, nucleate boiling, and the bubbly and slug flow regimes, while delaying the transition to churn and annular flow, compared with SRMC. A dynamic mass compensation was identified, in which vapor penetration into the porous ribs induced transient pressure fluctuations that drove cross-channel liquid redistribution on a millisecond timescale. This mechanism effectively suppressed intermittent dryout for flow uniformity among parallel channels. In terms of heat transfer performance, although the onset of nucleate boiling (ONB) showed negligible difference between the two configurations, the boiling curve slope of PRMC was significantly steeper, indicating the superior heat transfer capability. The heat transfer enhancement factor was maintained within 1.3-1.7. Furthermore, PRMC effectively delayed heat transfer deterioration in downstream regions under high heat-flux conditions. Temperature measurements demonstrated that PRMC provided remarkable thermal uniformity. The average wall temperature was reduced by up to 5.5 ℃, compared with SRMC, whereas the temperature rise along the flow direction was more gradual. Under boiling conditions, the maximum temperature difference across the channel was controlled to approximately 1 ℃ for PRMC, significantly lower than the 2-2.5 ℃ observed in SRMC. This improvement was attributed to the synergistic effects of capillary-driven liquid replenishment, enhanced nucleation, and cross-channel fluid redistribution from the porous structure. Additionally, despite the complex geometry, PRMC exhibited a lower pressure drop than SRMC, due to increased effective flow area and reduced mainstream velocity. The performance evaluation criterion (PEC) reached 1.6, indicating a favorable balance between heat transfer enhancement and flow resistance reduction. Overall, porous copper ribs and minichannels were integrated to serve as a highly effective passive strategy for the boiling heat transfer, thus suppressing flow instabilities for thermal uniformity. The findings can provide important insights into the underlying mechanisms of porous-structure-assisted flow boiling. Valuable guidance can also be offered for the design and optimization of next-generation high-performance thermal management.

This study aims to explore the heat transfer characteristics of flow boiling in the minichannels under the action of phase separation structure. A test section of parallel counterflow minichannel was also fabricated with the different structures of phase separation. Furthermore, the pressure difference between the downstream and counterflow channel confined the bubbles to discharge the gas phase from the high- to the low-pressure channel through the phase separation film. The gas phase separation was realized under the high- and low-pressure switching between adjacent channels. Two kinds of structure channels were fabricated: Type 1 phase separation structure channel (SPS1 channel) (Structure of Phase Separation, SPS) with few vents, and type 2 phase separation structure channel (SPS2 channel) with multiple vents, compared with the SPS3 channel without phase separation structure. The aqueous glycerol solution with a mass fraction of 30% was used as the test working medium. The flow boiling test was performed on the rectangular minichannel with a cross-section of 2 mm×2 mm under the effective heat flux density is 151.43 kW/m², the mass flow rate of 121.25 kg/(m²·s), and inlet temperature of 70°C. A systematic investigation was made to clarify the effects of high- and low-pressure switching cycles on the comprehensive performance and phase separation structures on the flow boiling heat transfer and temperature uniformity in minichannels. A high-speed camera was used to determine the length-to-diameter ratio of confined bubbles and the gas phase separation. An analysis was made to explore the heat transfer enhancement of the minichannel flow boiling under the action of the phase separation structure. The results show that the local saturated boiling heat transfer coefficient was the highest, and the total pressure drop was the lowest when the high- and low-pressure switching cycle was 120 s under the experimental conditions. An optimal value was achieved in the high- and low-pressure switching cycle. There was little difference in the boiling curve before the ONB point, while after the ONB point, the wall superheat of SPS2 and SPS3 channels was lower at the same heat flux. The maximum coefficients of local saturation boiling heat transfer in the SPS2 and SPS3 channels increased by 18.87% and 26.65%, respectively, compared with the SPS3. More importantly, the temperature uniformity of the SPS2 channel was the best in the two-phase region, SPS1 was the second, and SPS3 was the worst. The wall temperature standard deviations were reduced by 10.81% and 18.91%, respectively, along the SPS1 and SPS2 channels. The visual analysis results show that the phase separation structure reduced the length-to-diameter ratio of the confined bubbles, leading to the flow pattern transformation in the channel. Enhanced heat transfer was achieved in the first half cycle of the high- and low-pressure switching cycle. The length-to-diameter ratios of confined bubbles in the SPS1, SPS2, and SPS3 downstream channels were −118.71%, −158.16%, and 122.45% in the unit time, respectively. The phase separation structure can be expected to effectively enhance the heat transfer performance of flow boiling, and then improve the temperature uniformity of minichannels. The finding can provide new ideas for the application of the phase separation structure in minichannel heat exchangers.annel heat exchangers.

The purpose of this study is to explore the boiling heat transfer performance of microchannel flow under the combination of two enhanced technologies of pin electrodes and phase separation structure. Therefore, a parallel bidirectional microchannel heat sink was proposed to realize the gas-phase separation by switching high and low pressure for the downstream and countercurrent microchannels. Ethanol was taken as an experimental working fluid, while the modified polyvinylidene fluoride (PVDF) was as a gas-liquid phase separation tool. Then the flow boiling experiment was carried out under the conditions of inlet supercooling at 10 °C and effective heat flux range of 48.08 - 87.29 kW/m², where other experimental conditions remained the same. Furthermore, the heat transfer enhancement factor of saturated boiling heat transfer coefficient (F_ht) was introduced to investigate the boiling heat transfer mechanism in the microchannel under the combination of the number of pores in different phase separation structures (4-hole PSS-1, 6-hole PSS-2, 10-hole PSS-3) (phase separation structure, PSS) and different electric field strengths (200, 400 and 600 V). The results show that the heat transfer performance of the microchannel under the action of the single-phase separation structure and the pin electrodes was effectively improved under the same working conditions, compared with the ordinary microchannel (0 V without electric field and 0-hole PSS-0 without phase separation structure). The number of opening holes shared a strong correlation with the enhanced heat transfer of the phase separation structure. The maximum Fht of the phase separation structure was 1.14 at a heat flux of 80 kW/m², with an increase in the number of opening holes. There was the strengthening effect of the electric field on small or confined bubbles, with small lengths and diameters. An outstanding strengthening was found under the condition of low and medium heat flux, such as the maximum Fht under pin electrodes was 1.20 at a heat flux of 65 kW/m². The heat transfer performance of the microchannel was further improved under the combined role of phase separation structure and pin electrodes, in which the maximum F_ht was 1.29. In addition, the electric field was introduced to improve the strengthening effect of the phase separation structure at low heat flux density. The phase separation structure also effectively enhanced the strengthening effect of the electric field at high heat flux density, indicating the synergistic effect. In addition, the comparison was made on the effects of phase separation structure and electric field enhancement on the heat transfer of micro-channel boiling under the condition of mass flow rate (86.11 and 172.11 kg/(m²·s) of the two groups. The results show that the strengthening effect of phase separation during the boiling heat transfer of the microchannel was weakened at the high mass flow rate, compared with the low one. The strengthening effect of the electric field was enhanced, according to the influence of fluid flow rate. And there was little difference in the enhanced effect of boiling heat transfer under the two mass flow rates. The finding can provide a strong reference for the phase separation structure combined with the electric field.