Computational fluid dynamics (CFD) investigation of flame propagation and reflections of pressure waves through the fixed bed in a detonation tube (DT) is presented. A partially premixed combustion model involving no adjustable parameters is developed based upon the Flamelet Generated Manifold model to simulate the explosion of methane-air mixtures. A fixed bed with the detailed porous structure is generated using the discrete element method. Experiments are conducted to collect benchmark data to extensively validate the combustion model. After propagating into the fixed bed, the flame was sheared apart and locally engulfed the unburnt methane, which in turn resulted in rapid combustion. Due to reflections and resistance from the bed, the upstream pressure kept increasing until the flame passed the bed; this phenomenon was found to be more pronounced for longer propagation or bed length. Introducing the fixed bed into DT thus to induce rapid combustion might be an effective means to reach the deflagration-to-detonation transition.