Cross-scale mechanistic investigation of multistage carbon dioxide pool flash evaporation with vertical heterogeneity

Abstract

Pool flash evaporation is a canonical liquid–vapor phase-change process driven by rapid depressurization, yet the stage-dependent flow-pattern evolution and vertical thermodynamic heterogeneity remain poorly understood. Existing heat and mass transfer models treat the process as a single equilibrium stage with spatially uniform phases, contradicting experimental observations of pronounced vertical temperature gradients and stage-dependent flow morphologies. Here, a cross-scale investigation of carbon dioxide pool flash evaporation is conducted by combining high-speed visualization experiments, pseudopotential lattice Boltzmann method (LBM) simulations, and stage-resolved diagnostic macroscopic modeling. The complete four-stage evolution — adiabatic expansion, nucleation–cluster stratification, liquid–vapor stratification, and mist evaporation — is captured, with simultaneous measurement of three-point vertical temperature histories and chamber pressure. The detailed stage-resolved analysis is presented for a representative condition (initial pressure 6.5 MPa; initial temperature 20 °C; valve opening 5%), while the same four-stage sequence and vertical thermodynamic heterogeneity are consistently observed in all runs within the tested ranges (6.0–7.0 MPa; 14–23 °C; valve opening 5–70%). Mesoscale LBM simulations with phase-adaptive pressure outlet boundaries reveal that bubbles act as cold nuclei relative to the surrounding liquid, generating localized low-temperature cores through expansion cooling and interfacial phase-change heat absorption. In the continuous vapor phase, the dominant upward flow retains the cooling effect in a layered form, producing a vertical temperature gradient; in the continuous liquid phase, strong convective entrainment and high heat capacity homogenize the temperature field. Within bubble clusters, overlapping recirculation fields merge into cluster-scale upward channels that progressively accumulate cold fluid, establishing a cold-bottom, warm-top gradient absent in the discrete-bubble regime. Based on these mesoscale insights, stage-dependent macroscopic heat and mass transfer models are developed with explicit inter-regional heterogeneity terms, and validated against the experimental liquid-level histories within the error of 5% of the domain height. The models are diagnostic rather than predictive: the pressure and temperature histories measured in the experiments are imposed as inputs, so that the mass-related terms are the quantities resolved by the models and the liquid level is their experimentally measurable counterpart. A fully predictive formulation, in which the pressure and temperature fields are computed rather than imposed, would require additional closure laws for the depressurization dynamics and interfacial transfer rates and is left as future work. The models reveal that inter-regional mass transfer is retained within the vessel during the cluster-nucleation stage but is nearly fully discharged during the liquid–vapor stratification stage, clarifying the dominant mass-transfer pathways at each stage of the flash process.