A Numerical Investigation on the Rapid Turbulent Filtration Combustion under High Background Pressure

Abstract

ABSTRACT A numerical investigation of the rapid turbulent filtration combustion within a packed pebble bed is conducted at a fixed superficial velocity of 0.5 m/s, equivalence ratio of 1.0 (methane/air) under two background pressures of 1.0 and 0.5 MPa. Turbulence is modeled with the detached eddy simulation (DES) method. The interaction between turbulence and chemical reaction is considered through the fast reaction method, i.e., burning velocity model (BVM). To validate the DES method, the relative turbulence intensities in a 3D staggered cylinders are verified previously which are in excellent agreement with the experimental data (no reaction). With the confidence in the DES method, the turbulent combustion characteristics in a 3D packed pebble bed are studied in detail, including pressure fluctuation, spatial inhomogeneities of velocity and temperature fields, and evolution of the flame surface. Numerical results show that a strong pressure fluctuation arises after ignition due to thermal expansion contributing significantly to the velocity and temperature fluctuations for the case of 1.0 MPa, whereas it is relatively moderate for the case of 0.5 MPa. Though the evolution of the flame surface both can be divided into two stages for the two cases, i.e., initial-rapid spreading stage, and stable propagating stage, there is a slight difference between them. Specifically, it closely resembles to an atomic blast for the former, while it is similar to a volcanic eruption for the later. Undergoing this stage, about 20 ms, the wrinkle and fragmented flames or flamelets creep over the pebbles through the thin layers close to the walls. Statistical stability is identified in the process of flame propagating upstream. Turbulent flame speeds obtained in this study agree well with the experimental data, especially for the case of 1.0 MPa.