Applying dynamic mesh to examine evolution of effective thermal conductivity in porous medium undergoing macrostructure change

Abstract

Heat transfer dynamics in packed bed appears to be vital, particularly with respect to thermochemical conversion of solid fuels. However, due to complex bed structure varying under high temperatures the prediction of its thermal behavior is very demanding. Heating of coke oven charge is one example, regarded as being considerably affected by radiation through fissures and cracks that cross cut coke zone as the carbonization proceeds. The investigation and quantification of such impact encounters difficulties due to irregularity of fissure network that depends on a parent coal type and the process temperature regimes. Unfortunately, the analyses of this issue are very limited, meanwhile an uncertainty in the assessment of radiation contribution might be a source of notable discrepancy in effective thermal conductivity of coal/coke bed reported to date in the literature. In this respect, the CFD simulations of heating of a coal/coke bed while taking into account the crack propagation were performed to estimate an influence of structure alteration on heat transfer process. The computations were carried out using Ansys Fluent package and the dynamic mesh was implemented to simulate the crack development. The numerically-derived temperature profile at the coke oven center plane is in good agreement with the experimental temperature history. The predicted thermal conductivity values in the near-wall zone are significantly higher than for the case excluding fissure presence, and achieved ∼6.2 W(m K)-1 at the end of process (for 1373 K), showing additionally a clear increase from 1.4 to 2.9 W(m K)-1 within 1070–1090 K. The computation results also revealed that slower developing cracks, contrary to faster propagating ones, lead to higher thermal conductivity.