Direct numerical simulation of methane hydrate dissociation in pore-scale flow by using CFD method

摘要

The objective of this work is to establish a new pore-scale (m-μm) model for estimating the dissociation rate of methane hydrate (MH) synthesized in laboratory-scale sediment samples. Finite volume method (FVM) with unstructured mesh were constructed in a representative face-centered cubic unit. The surface model of MH reported by Sean et al. (2007) has been employed. In the bulk flow, concentration of methane in water flow was analyzed by computational fluid dynamics (CFD) method. However, only water flow and solid MH are simulated without considering ice or gas phase. In this study, tentative cases with porosity 0.74,0.66, and 0.49 is individually considered as representative cubic unit of MH array. The initial temperature, 253.15 (K) of MH pellets inside the cubic unit dissociated due to the driving force of fugacity variation, ex. 0.56 and 0.54 (MPa) while warm water of 282.15 and 276.15 (K) flow in. In the calculation, periodic conditions are imposed at surfaces of inlet/right/front sides updated every time step. The flux of methane at the surface are all regarded as being dissolved into the water in this high pressure state, and compared to Kim et al. (1987)'s correlation at Reynolds no. of about 50. Results of dissociation flux in cases 5 and 6 with porosity 0.49 show good agreements with Kim's correlation. However, as the porosity increases, flux increases due to the fast transport in bulk flow such as cases 1-4 in this study. If the transport process in bulk flow is faster than dissociation rate, then the surface flux becomes saturated as Reynolds no. greater than 100 in this work.