A pressure coupled Representative Interactive Linear Eddy Model (RILEM) for heavy-duty truck engine combustion simulations

Internal combustion engines (ICE) are frequently debated due to their environmental consequences. Although the switch to electromobility is currently happening for light and medium-duty vessels, the transition for ships and heavy-duty trucks is more complex. The primary problem in the shift is the incapability of matching the range of ICE. Maintaining the ICE for large vessels is inevitable, requiring intensive research to improve the combustion engine technology to reduce their impact on the environment. Operating engines in nonstandard conditions, such as low-temperature ranges or utilizing partially premixed mixtures, reduces emissions such as NOx or soot. Simulating these scenarios requires special modeling techniques that can advance finite rate chemistry and simulate mixed combustion modes. The Linear Eddy Model (LEM) is a potential candidate to simulate these methods. LEM is a regime and mode-independent mixing model initially developed for nonreactive flows [1] and later extended to include reactive flows [2]. LEM advances finite rate chemistry, solves turbulence on a one-dimensional line, and advances molecular diffusion and heat convection. It offers the possibility of capturing the turbulence-chemistry interaction directly, which is essential for predicting pollutants formation. In this thesis, the coupling of LEM to a CFD code that simulates the spray chamber geometry is referred to as the Representative Interactive LEM (RILEM). RILEM describes the combustion chamber key parameters and interacts with the CFD in real-time as a combustion model to update its chemical state. RILEM shares similarities with the Representative Interactive Flamelet model (RIF) [3]. RIF is essentially the coupling of a CFD domain to laminar flamelets embedded in a turbulent flow; the coupling is based on the scalar dissipation rate χ for non-premixed [4], and on combustion progress variable c or level set approach for premixed cases [5]. RILEM features distinct differences compared to RIF, mainly the possibility of simulating partially premixed combustion modes and the ability to represent combustion on a physical one-dimensional line. A previous version of RILEM was implemented where the CFD and LEM were coupled based on a volume constraint. The volume-coupled RILEM was successfully validated against experiments [6]. A recent version of RILEM using pressure coupling is the objective of the presented thesis. utilizing the species mass fractions and the solution of the energy equation in its total enthalpy form. RILEM and mRILEMs were tested on a single cylinder case of a heavy-duty truck engine. The pressure trace was compared against the experimental data and reached a reasonable agreement. The two models allowed the quantification of intermediate species such as CO and OH. RILEM provided accurate results by manipulating the probability density functions of the turbulent scalars combined with different initialization techniques, where utilizing mRILEMs ensured that the LEM side exclusively controls the combustion process. Abstract - As a result of the increase of the use of combustion engines, more restrictive emission standards are applied. New combustion technologies are being constantly developed to enhance the internal combustion engine, this is motivated by overcoming issues that are relevant for both engine efficiency and the environmental aspects. To do that, turbulent combustion modelling is needed. In this paper, an updated version of the Linear Eddy Model (LEM) will be presented. LEM is capable of simulating premixed, nonpremixed and mixed mode combustion, and it is a regime independent model that runs under the assumption of finite-rate chemistry that is sensitive to turbulent chemistry interactions, which makes it suitable for prediction of pollutant formation. A New coupling scheme to the CFD-RANS simulation is proposed, the coupling is based on linking the two models via pressure. The benefits of pressure coupling are that the effects of wall heat losses and latent heat of evaporation that are modelled on the CFD side are an intrinsic part of the pressure term that is linking the two models, in contrast to volume coupling in which these relevant phenomena need supplementary modelling on the LEM side. The pressure coupling results in a radially uniform dilatation of the LEM domain reflecting the combined effects of pressure change, fuel addition, and cylinder volume change during the engine cycle. Consistency of the LEM cone volume and the CFD domain volume is maintained by adopting a split operator strategy involving a volume correction that adjusts the cone angle. A Spherical Stand-Alone Linear Eddy Model (SSALEM) has been created to conduct relatively fast simplified code development and parameter studies. SSALEM input parameters were drawn from a WSR-RANS simulation and a 1D slider-crank model that calculates the combustion chamber volume that corresponds to a given crank-angle. Model results show the capability to physically track the evolution of several scalars that are solved on the LEM line such as temperature, fuel, and intermediate species in the combustion process.