In recent years, reducing environmental pollution caused by combustion in traortation and industrial burners systems has been a major concern. Predicting accurate values of the temperature and products in a combustion field is essential for specifying the pollutants concentrations. Therefore, modeling and simulating turbulent combustion flows have been the focus of many researches. In order to simulate turbulent reacting flows, an appropriate combustion model is required. Due to separating chemical reactions calculations from the solution of turbulent flow field, flamelet model provides the possibility to consider chemical kinetic with any level of complexity. Therefore, different chemical mechanisms can be analyzed through this model. The purpose of this research is to study the application of steady and unsteady flamelet model in simulating several cases such as bluff body, pilot, and duct burner turbulent diffusion flames. The most important feature of this research is a precise prediction of pollutants such as NO and CO by using unsteady flamelet model as the most appropriate model represented in this field. Developing Eulerian particle unsteady model from one particle to several particles is another aim of this research. For this purpose, a database entitled flamelet library has been primarily created for steady state combustion, from which temperature and mass fraction quantities of chemical components can be extracted. This flamelet library has been obtained through solving steady flamelet equations for non-premixed flames in mixture fraction space. The effects of turbulent flow on these quantities have been considered by the means of probability density functions. The influences of time have been considered in a post-processing way using Eulerian particle flamelet model. In this model, for creating unsteady flamelet library, a parameter known as surface averaged conditional scalar dissipation rate is required which has been obtained by considering a virtual (unreal) index particle in order for finding unsteady flamelets. The probability of finding flamelet or unsteady flamelets can be determined by solving Eulerian transfer equation which includes unsteady, diffusion, and convection terms. The results indicate that the predictions of temperature field and mass fraction of main components using steady flamelet model retain an acceptable concordance with empirical results. However, the simulation with steady flamelet model overestimates the mass fraction of NO and CO components, and using unsteady solution provides better results for them. Furthermore, using several unsteady flamelet in pilot flame leads into a noticeable development in the results. Contrary to CO, NO expresses sensitivity toward chemical mechanism. Considering Lewis number 1 for flamelet equations is an acceptable assumption regarding the turbulent flow, and would provide better results compared to differential diffusion effect. Keywords Pollutant, Turbulent combustion, Flamelet model, probability density function, NO and CO components
