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Microreactors for chemical synthesis and combustion have already attracted significant attention. Exothermic catalytic activity features heavily in these devices and thus advective-diffusive transport is of key importance in their analyses. Yet, thermal modelling of the heat generated by catalytic reactions on the internal surfaces of porous microreactors has remained as an important unresolved issue. To address this, the diffusion of heat of catalytic reactions into three phases including fluid, porous solid and solid walls is investigated by extending an existing interface model of porous media under local thermal non-equilibrium. This is applied to a microchannel fully filled with a porous material, subject to a heat flux generated by a catalytic layer coated on the porous-wall boundary. The finite wall thickness and viscous dissipation of the flow kinetic energy are considered, and a twodimensional analytical model is developed, examining the combined heat and mass transfer and thermodynamic irreversibilities of the system. The analytical solution is validated against the existing theoretical studies on simpler configurations as well as a computational model of the microreactor in the limit of very large porosity. In keeping with the recent findings, the wall thickness is shown to strongly influence the heat and mass transport within the system. This remains unchanged when the symmetricity of the microchannel is broken through placing walls of unequal thicknesses, while deviation from local thermal equilibrium is significantly intensified in this case. Importantly, the Nusselt number is shown to have a singular point, which remains fixed under various conditions. NomenclatureInterfacial area per unit volume of porous media, (m -1 ) Prandtl number Biot number Wall heat flux ratio ′ Modified Brinkman number ′′ Total catalytic heat flux (W m -2 ) Mass species concentration (kg m -3 ) 1 ′′ Lower wall heat flux (W m -2 ) 0 Inlet concentration (kg m -3 ) 2 ′′ Upper wall heat flux (W m -2 ) Specific heat capacity (J K -1 kg -1 ) Reynolds number Mass diffusion coefficient (m 2 s -1 ) Specific gas constant (J K -1 kg -1 ) Darcy number Shape factor of the porous medium Coefficient of thermal mass diffusion (m K -1 kg -1 s -1 ) ̇′ ′′ Volumetric entropy generation due to mass diffusion (W K -1 m -3 ) ℎ 1 Half-thickness of the microchannel to lower wall (m) ′ ′′ Volumetric entropy generation due to fluid friction (W K -1 m -3 ) ℎ 2 Half-thickness of the microchannel to upper wall (m) ̇′ ′′ Volumetric entropy generation in the fluid (W K -1 m -3 ) ℎ 3 Half-height of microchannel (m) ′ ′′ Volumetric entropy generation in the porous solid (W K -1 m -3 ) ℎ Interstitial heat transfer coefficient (W K -1 m -2 ) ̇1 ′′′ Volumetric entropy generation rate from lower wall (W K -1 m -3 )

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Direct injection of natural gas into the cylinder of spark ignition (SI) engines has shown a great potential to achieve the best fuel economy and reduced emission levels. Since the technology is rather new, in-cylinder flow phenomena have not been completely investigated. In this study, a numerical model has been developed in AVL FIRE software to perform an investigation of natural gas direct injection into the cylinder of spark ignition internal combustion engines. In this regard, two main parts have been taken into consideration aiming to convert a multipoint port fuel injection (MPFI) gasoline engine to a direct injection natural gas (NG) engine. In the first part of the study, multidimensional simulations of transient injection process, mixing, and flow field have been performed. Using the moving mesh capability, the validated model has been applied to methane injection into the cylinder of a direct injection engine. Five different piston head shapes have been taken into consideration in the investigations. An inwardly opening single-hole injector has been adapted to all cases. The injector location has been set to be centrally mounted. The effects of combustion chamber geometry have been studied on the mixing of air-fuel inside the cylinder via the quantitative and qualitative representation of results. In the second part, an investigation of the combustion process has been performed on the selected geometry. The spark plug location and ignition timing have been studied as two of the most important variables. Simulation of transient injection was found to be a challenging task because of required computational effort and numerical instabilities. Injection results showed that the narrow bowl piston head geometry is the most suited geometry for NG direct injection (DI) application. A near center position has been shown to be the best spark plug location based on the combustion studies. It has been shown that advanced ignitions timings of up to 50 degrees crank angle ( °CA) should be used in order to obtain better combustion performance.