İ. Bedii Özdemir* and Cengizhan Cengiz Use of Modified Temperature-Composition PDF Formulation in Modeling of Flame Dynamics in Diesel Engine Combustion https://doi.org/10.1515/ijnsns-2018-0023 Received January 23, 2018; accepted May 21, 2018 Abstract: In the present work, the modified temperature- composition (MT-C) PDF formulation was embedded in the KIVA to study the characteristics of flame development and emissions in a diesel engine. The model uses a time scale defined by an energy balance on the flame surface and a new normalization scheme exploiting the maximum attain- able mass fractions of progress variables. Development of the latter in the T - ξ parameter space regulates the flame progress in the physical space and, thus, the approach presents some potential to capture the local flame extinc- tion. The interactions of the swirl and spray penetration and their influence in the mixing process, combustion and emis- sions are also evaluated. Analyses of the temporal evolution of mixture fraction and temperature show that the swirl motion forms a homogeneous mixture on the lee sides of the spray jets where the ignition actually starts. Since the local time scales are considered in the model, the chemis- try-controlled premixed combustion developing there is well predicted. Keywords: flame dynamics, diesel combustion, presumed pdf, nonpremixed turbulent combustion modeling, ILDM PACS ® (2010). 47.70.Pq, 47.70.Fw, 47.11.Df, 47.27.E-, 47.27.T- 1 Introduction Over the last decade, it has been recognized that emis- sions from vehicles can have detrimental consequences on the environment and pose a severe threat to public health. Therefore, the regulations on emissions become very stringent and impose restrictions on the particulate matters [1, 2]. Diesel engines, operating on nonpremixed combustion mode [3] with fuel-lean equivalence ratios, provide a better energy efficiency and, hence, become an attractive choice to cut down carbon dioxide (CO 2 ) emis- sions from vehicles. High temperatures occurring in die- sel processes, however, increase the thermal–NO x and, when they coincide with the fuel-rich zones, lead to fuel pyrolysis and particulate emissions in the exhaust gas [4]. Strategies [5] that have been used to comply with the emission targets, however, bring more challenges to the modeling of diesel combustion process. The turbulent combustion in a diesel engine is a multi- physics phenomenon, including the coexistence of liquid and gas phases, all modes of heat transfer, evaporation, formation and depletion of pollutants, and acoustics. In addition, further complexity is introduced by the chemical kinetics with perhaps thousands of species and elementary reactions. Hence, the turbulent combustion in a diesel engine is an intricately stiff problem comprising a wide spectrum of length scales from chamber dimensions to scales associated with turbulence–chemistry interactions [6]. The increased concerns of particulate matter further require the inclusion of even smaller scales appearing in the process of soot inception [7, 8]. The diesel combustion process is largely influenced by the nonstationary in-cylinder motion, which is mainly due to intake flow and spray injection. The flow rotation around the cylinder axis is known as the swirl motion, which is one of the most important flow features contri- buting to the cycle-to-cycle variations and formation and oxidation of soot [9–11]. The tumble motion is another rotational motion, which is also closely related to the cycle-to-cycle variations [12–14] and emerges around an axis perpendicular to the centerline of the cylinder. The third most important large-scale in-cylinder flow is the squish, which occurs at the end of compression and, depending on the swirl, develops either along the cylin- der head or along the piston bowl. These structures are well understood in isolation, but the changes with their mutual interactions are still not thoroughly explored. The fine-scale turbulence is mainly generated by the injection process of liquid fuel into the hot dense air, which initiates the atomization process by peeling off *Corresponding author: İ. Bedii Özdemir, Faculty of Mechanical Engineering, Istanbul Technical University, Gumussuyu 34437, Istanbul, Turkey, E-mail: bozdemir@itu.edu.tr http://orcid.org/0000-0002-4126-2029 Cengizhan Cengiz, P/T 3D CFD & Combustion, Ford OTOSAN, B1-S7/ 16 Sancaktepe, Istanbul 34885, Turkey IJNSNS 2018; aop Brought to you by | McMaster University Authenticated Download Date | 6/20/18 12:53 PM