A two-dimensional and two-phase numerical model is presented for the smolder propagation in a horizontal polyurethane foam. The chemical processes considered include endothermic pyrolysis and exotherrnic oxidation degradation of polyurethane foam and exothermic oxidation of char. The governing equations are discretized in space using the finite element method and solved by the software package FEMLAB. Predicted profiles of solid temperature as well as evolutions of solid compositions （including foam, char and ash） are presented at an airflow velocity of 0. 28 cm/s. The computed average smoldering velocity is 0. 021 4 cm/s, and the average maximum temperature is 644. 67 K. Based on the evolutions of solid compositions, the packed bed can be obviously divided into four zones： unreacted zone, fuel pyrolysis and oxidation zone, char oxidation zone and fuel burned-out zone. Simultaneously, the effects of inlet air velocity and fuel properties （including thermal conductivity, specific heat, density and pore diameter） are studied on the smoldering propagation. The results show that the smoldering velocity and temperature have a roughly linear increase with increasing inlet air velocity; the fuel density is the most important factor in determining smoldering propagation; radiation has a non-negligible role on the smoldering velocity for larger pore diameters of porous material. The computational results are compared with the experimental data and a general agreement is reached.
To understand the working mechanism of the porous medium(PM)internal combustion engine,effects of a porous medium heat regenerator inserted into a combustion chamber on the turbulent flow char-acteristics and fuel-air mixture formation are studied by numerical simulation.The cylindrical chamber has a constant volume,in which a disk-shaped PM insert is fixed.A simplified model for the random structure of the PM is presented,in which the PM is represented by an assembly of a great number of randomly distributed solid units.To simulate flows in the PM a microscopic approach is employed,in which computations are performed on a pore-scale mesh and based on the standard k-ε turbulence model.A spray model,in which the effects of drop breakup,collision and coalescence are taken into account,is introduced to describe spray/wall interactions.Numerical computations are performed for the turbulent flows induced by a fuel spray outside and inside of the PM with different structure parameters.Calculation results show that the spray/PM interaction has substantial and positive influ-ences on the fuel-air mixture formation and homogenization in the combustion chamber,which could be very advantageous in engine applications.
The Senkin code of package is used to simulate the the Chemkin chemical kinetics combustion process of a porous medium（PM） engine fueled by n-heptane. The code is modified to incorporate the Woschni heat transfer correlation and heat transfer model within a porous medium. A detailed chemistry mechanism of NOx formation is coupled with the detailed chemical kinetics mechanism of n-heptane. The code is applied to a zero- dimensional single-zone model of engine combustion. Influences of operating parameters on the performance of the PM engine are discussed. With the increase in the intake temperature and compression ratio, or with the decrease of the excess air ratio, the ignition timing of the PM engine obviously advances. It is found that the porous medium acting as a heat recuperator can considerably preheat the fuel-air mixture, which promotes the ignition and combustion in the cylinder. And the initial PM temperature is a critical factor controlling the compression ignition of the mixture.
