Flow boiling characteristics in a minichannel with porous ribs forthermal management of agricultural equipment

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.