Lin Chen

The pressure-driven flow of supercritical CO2 through a planar-slit microchannel is numerically investigated as its thermodynamic trajectory crosses a pseudo-critical line defined by the local maximum of the isobaric specific heat. A fully compressible Navier–Stokes framework is coupled with the Peng–Robinson equation of state and state-dependent thermodynamic and transport properties. The results reveal a localized pseudo-critical response region in which density and thermodynamic properties vary rapidly but continuously, without formation of a liquid–vapor interface or latent-heat transport. Within this region, the approximately linear streamwise pressure decrease is accompanied by a pronounced nonlinear increase in velocity. The local density-gradient magnitude reaches 2330 kg m−4, while the streamwise velocity gradient increases to approximately 8 s−1, substantially exceeding the upstream and downstream values. This localized acceleration enhancement, referred to descriptively as critical acceleration, is associated with the nonlinear coupling among streamwise depressurization, rapid density reduction, strong thermal expansivity, and mass conservation. A localized negative temperature deviation also develops near the pseudo-critical crossing despite the isothermal inlet and wall conditions, reflecting the coupled pressure-dependent real-fluid response and wall heat transfer. Parametric calculations further show that the thermofluid response is strongly dependent on the imposed pressure conditions. Increasing the inlet pressure at fixed outlet pressure shifts the response region downstream and intensifies the density gradient, velocity gradient, temperature depression, and thermodynamic-property variation. Across the investigated cases, the maximum density-gradient magnitude and velocity gradient reach 3194 kg m−4 and 19.1 s−1, respectively, while the peak cp increases to 127.8 kJ kg−1 K−1. These results clarify the continuum-scale coupling between pressure-driven acceleration and strong near-pseudo-critical property variation in supercritical CO2 microchannel flow.

Utilizing microfluidic models, this review synthesizes experimental and simulation insights into the pore-scale behavior of hydrates during formation and decomposition in porous media. It outlines the fundamental characteristics of CO₂ hydrates and their significance in porous media, with a focus on major advancements in hydrate nucleation, growth, distribution, and decomposition kinetics. This study details various porous media systems, visualization experimental setups, and observation techniques employed in experimental research. Key factors, including temperature, pressure, salinity, and pore characteristics, are analyzed to determine their influence on hydrate formation (nucleation, growth kinetics, and phase equilibrium) and decomposition (dissociation kinetics and efficiency) behaviors. In terms of numerical simulation, we distinguishes multiscale numerical simulation methods for the molecular scale, the pore scale, and then the reactor scale, including molecular dynamics simulation, the phase field model, the pore network model, and the macroscopic kinetics model, and discusses the role of simulation in revealing the micro-mechanisms and predicting macroscopic behaviors. This article also summarizes the application of relevant numerical simulation methods (such as MD, CFD, and LBM) in revealing the micro-mechanism of hydrates. Therefore, this review offers critical insights into the micro-mechanisms of carbon dioxide hydrate behavior in porous media, thereby undergirding the theoretical basis for optimizing related engineering designs.