Cold energy transport and release characteristics of CO2+TBAB hydrate slurry flow with hydrate dissociation

Abstract

The CO2 hydrate slurry is an ideal medium for cold energy storage and transport due to its large latent heat and good fluidity. To moderate the phase change conditions of CO2 hydrate, the thermodynamic promoter of tetrabutylammonium bromide (TBAB) is employed by forming CO2+TBAB hydrate, and the phase change conditions can be moderated to suit specific refrigeration application by adjusting TBAB solution concentration and system pressure. The CO2+TBAB hydrate slurry is generated and transported to user sides where cold energy is released through hydrate dissociation. During cold energy release, CO2+TBAB hydrate crystals in slurry absorb heat and dissociate to gas CO2 and TBAB solution, resulting in variations of flow regime, TBAB solution concentration and heat transfer characteristics. Comprehensive understanding of the flow and heat transfer behaviors of CO2+TBAB hydrate slurry with dissociation is crucial for interpreting its cold energy transport and release characteristics. In this study, a multiphase flow model under Eulerian-Eulerian framework, coupled with hydrate dissociation and variation of TBAB solution concentration, is established to study the flow and heat transfer characteristics of CO2+TBAB hydrate slurry flow in a horizontal pipe. Numerical results indicate that dissociated gas CO2 first distributes near the pipe wall region and then tends to aggregate at the top region of pipe, which impairs heat transfer performance of CO2+TBAB hydrate slurry due to the low thermal conductivity of gas CO2. The local heat transfer coefficient of CO2+TBAB hydrate slurry undergoes a rapid decrease in the inlet developing region and keeps a slow decreasing trend in the fully developed region. Increasing wall heat flux and reducing the outlet pressure can promote the dissociation of CO2+TBAB hydrate, but this leads to an increase in gas volume fraction, further reducing the local heat transfer coefficient in the fully developed region. Increasing flow velocity can improve the heat transfer performance and reduce the aggregation of gas CO2, thereby mitigating the adverse effect of gas CO2 on heat transfer.