Numerical investigation of radiative heat transfer in internal combustion engines

The importance of radiative heat transfer in internal combustion engines is studied using Computational Fluid Dynamics (CFD) simulations. The Discrete Ordinates Method (DOM) is implemented and a Spherical surface Symmetrical equal Dividing angular quadrature with order of 2 is selected for the angular direction discretization with <4% relative error in prediction of radiative heat transfer. The radiative properties for a participating medium of CO2, H2O, CO and soot particles are considered by using a narrowband model. An Equilibrium Phase spray model together with detailed chemistry and a semi-detailed soot model are used to model the spray combustion and emission processes. The integrated model is validated extensively against available experiments for participating gas emissivity and radiative heat transfer, as well as against combustion characteristics in a constant volume spray combustion chamber, a conventional diesel engine, and a reactivity controlled compression ignition engine. The results show that the participating gases are the major source of radiative heat transfer in engine combustion, while soot is a secondary contributor. The radiative heat loss accounts for 9–18% of the total wall heat loss in the studied engine cases covering from low- to high-load conditions, and is found to be linearly correlated with the global equivalence ratio. The soot emission at exhaust valve opening is increased by as much as 50% due to the inclusion of thermal radiation, while the NOx emission is less effected. Additionally, the DOM model is used to demonstrate a more straightforward and accurate comparison between CFD simulations and radiation-based experiments. A suggestion is also made to consider infrared natural luminosity imaging to provide a better correlation between the measured luminosity and soot concentrations.