Sc-CO2 Phase Transition Evolution Migration and Heat–Mass Transfer Characteristics

• Focuses on the phase transition evolution and migration mechanism and heat and mass transfer characteristics of supercritical CO₂; the problems of limited improvement in the quality of phase transition and difficulty in accurately regulating the actuation performance were discussed. • Establishes a one-dimensional four-stage physical model to reveal phase transition stages: from liquid to supercritical state, thermal equilibrium in the supercritical state, from supercritical state to gas state, and energy relaxation in the gas phase. • The transition stage from the supercritical state to the gas state dominates the work performance, accounting for 80.75% of the total work. The energy relaxation in the gas phase will still continue to do work externally, but the intersection with the saturation curve will lead to the formation of dry ice or liquid CO₂, reducing the work potential. Supercritical carbon dioxide (sc-CO₂) phase transition expansion technology is a new type of driving technology that provides sc-CO₂ via physical or chemical interactions and enables external expansion. Compared with traditional gas expansion technologies, this technology offers advantages such as high power, low cost and environmental friendliness, thus demonstrating considerable application value in coal mining, road construction, and cold launch. However, the limited understanding of the evolution mechanisms of sc-CO₂ under transcritical, highly transient, and high-density conditions restricts technological advancement. In this study, a sc-CO₂ phase transition expansion experimental system was established, series evolution experiments were performed, and a one-dimensional four-stage physical model was developed to describe and reproduce migration behavior. By combining experiments with theoretical analysis, the heat and mass transfer characteristics were investigated. Key parameters, such as fluid pressure, temperature, mass flow rate, and work capacity at various stages, were obtained. CO₂ migration patterns were elucidated, and the effect of phase transition behavior on expansion performance was revealed. The results indicated that the phase transition process can be divided into the following four stages: liquid-to-supercritical phase transition, supercritical phase thermalization, supercritical-to-gas phase transition, and gas phase energy relaxation. Among these stages, the supercritical-to-gas transition stage more notably influenced work performance. The phase transition path revealed that at the fourth stage, the gas phase energy relaxation curve intersects the gas‒liquid and gas‒solid saturation curves, thereby yielding the formation of dry ice or liquid CO₂. The charge of the excitation tube considerably regulated the phase transition. With lower charge, the thermal energy from combustion was insufficient for fully gasifying all the CO₂, which resulted in considerable dry ice residue after depressurization. As the charge increased, the dry ice residue decreased greatly. Therefore, optimizing the charge could effectively increase the fluid utilization efficiency, substantially reduce dry ice formation, and increase the expansion work capacity.